Fueling Body Activities: Digestion€¦ · food for assimilation by cells. Types of Digestive Systems. Some invertebrates have a gastrovascular cavity, but vertebrates have a digestive

1017

51Fueling Body

Activities: Digestion

Concept Outline

51.1 Animals employ a digestive system to preparefood for assimilation by cells.

Types of Digestive Systems. Some invertebrates have agastrovascular cavity, but vertebrates have a digestive tractthat chemically digests and absorbs the food.Vertebrate Digestive Systems. The different regions ofthe gastrointestinal tract are adapted for different functions.

51.2 Food is ingested, swallowed, and transported tothe stomach.

The Mouth and Teeth. Carnivores, herbivores, andomnivores display differences in the structure of their teeth.Esophagus and Stomach. The esophagus delivers foodto the stomach, which secretes hydrochloric acid andpepsin.

51.3 The small and large intestines have very differentfunctions.

The Small Intestine. The small intestine has mucosalfolds called villi and smaller folds called microvilli thatabsorb glucose, amino acids, and fatty acids into the blood.The Large Intestine. The large intestine absorbs water,ions, and vitamin K, and excretes what remains as feces.Variations in Vertebrate Digestive Systems. Digestivesystems are adapted to particular diets.

51.4 Accessory organs, neural stimulation, andendocrine secretions assist in digestion.

Accessory Organs. The pancreas secretes digestiveenzymes and the hormones insulin and glucagon. The liverproduces bile, which emulsifies fat; the gallbladder storesthe bile.Neural and Hormonal Regulation of Digestion.Nerves and hormones help regulate digestive functions.

51.5 All animals require food energy and essentialnutrients.

Food Energy and Energy Expenditure. The intake offood energy must balance the energy expended by the bodyin order to maintain a stable weight.Essential Nutrients. Food must contain vitamins,minerals, and specific amino acids and fatty acids for health.

Plants and other photosynthetic organisms can producethe organic molecules they need from inorganic com-

ponents. Therefore, they are autotrophs, or self-sustaining.Animals are heterotrophs: they must consume organic mol-ecules present in other organisms (figure 51.1). The mole-cules heterotrophs eat must be digested into smaller mole-cules in order to be absorbed into the animal’s body. Oncethese products of digestion enter the body, the animal canuse them for energy in cell respiration or for the construc-tion of the larger molecules that make up its tissues. Theprocess of animal digestion is the focus of this chapter.

FIGURE 51.1Animals are heterotrophs. All animals must consume plantmaterial or other animals in order to live. The nuts in thischipmunk’s cheeks will be consumed and converted to bodytissue, energy, and refuse.

cialized in different regions for the ingestion, storage, frag-mentation, digestion, and absorption of food. All higheranimal groups, including all vertebrates, show similar spe-cializations (figure 51.3).

The ingested food may be stored in a specialized regionof the digestive tract or may first be subjected to physicalfragmentation. This fragmentation may occur through thechewing action of teeth (in the mouth of many vertebrates),or the grinding action of pebbles (in the gizzard of earth-worms and birds). Chemical digestion then occurs, break-ing down the larger food molecules of polysaccharides anddisaccharides, fats, and proteins into their smallest sub-units. Chemical digestion involves hydrolysis reactions thatliberate the subunit molecules—primarily monosaccha-rides, amino acids, and fatty acids—from the food. Theseproducts of chemical digestion pass through the epitheliallining of the gut into the blood, in a process known as ab-sorption. Any molecules in the food that are not absorbedcannot be used by the animal. These waste products are ex-creted, or defecated, from the anus.

Most animals digest their food extracellularly. Thedigestive tract, with a one-way transport of food andspecialization of regions for different functions, allowsfood to be ingested, physically fragmented, chemicallydigested, and absorbed.

1018 Part XIII Animal Form and Function

Types of Digestive SystemsHeterotrophs are divided into three groups on the basisof their food sources. Animals that eat plants exclusivelyare classified as herbivores; common examples includecows, horses, rabbits and sparrows. Animals that aremeat-eaters, such as cats, eagles, trout, and frogs, arecarnivores. Omnivores are animals that eat both plantsand other animals. Humans are omnivores, as are pigs,bears, and crows.

Single-celled organisms (as well as sponges) digesttheir food intracellularly. Other animals digest theirfood extracellularly, within a digestive cavity. In thiscase, the digestive enzymes are released into a cavity thatis continuous with the animal’s external environment. Incoelenterates and flatworms (such as Planaria), the diges-tive cavity has only one opening that serves as bothmouth and anus. There can be no specialization withinthis type of digestive system, called a gastrovascular cavity,because every cell is exposed to all stages of food diges-tion (figure 51.2).

Specialization occurs when the digestive tract, or ali-mentary canal, has a separate mouth and anus, so thattransport of food is one-way. The most primitive digestivetract is seen in nematodes (phylum Nematoda), where it issimply a tubular gut lined by an epithelial membrane.Earthworms (phylum Annelida) have a digestive tract spe-

51.1 Animals employ a digestive system to prepare food for assimilation by cells.

Gastrovascularcavity

Body stalkTentacle

Mouth Food

Wastes

FIGURE 51.2The gastrovascular cavity of Hydra, a coelenterate. Becausethere is only one opening, the mouth is also the anus, and nospecialization is possible in the different regions that participatein extracellular digestion.

Nematode

Earthworm

Salamander

Mouth

Mouth

Mouth

Pharynx

Pharynx

Esophagus

Intestine

Intestine

Intestine

Anus

Anus

Anus

Crop Gizzard

Liver

Pancreas

Stomach Cloaca

FIGURE 51.3The one-way digestive tract of nematodes, earthworms, andvertebrates. One-way movement through the digestive tractallows different regions of the digestive system to becomespecialized for different functions.

Vertebrate Digestive SystemsIn humans and other vertebrates, the digestive system con-sists of a tubular gastrointestinal tract and accessory diges-tive organs (figure 51.4). The initial components of thegastrointestinal tract are the mouth and the pharynx, whichis the common passage of the oral and nasal cavities. Thepharynx leads to the esophagus, a muscular tube that deliv-ers food to the stomach, where some preliminary digestionoccurs. From the stomach, food passes to the first part ofthe small intestine, where a battery of digestive enzymescontinues the digestive process. The products of digestionthen pass across the wall of the small intestine into thebloodstream. The small intestine empties what remainsinto the large intestine, where water and minerals are ab-sorbed. In most vertebrates other than mammals, the wasteproducts emerge from the large intestine into a cavitycalled the cloaca (see figure 51.3), which also receives theproducts of the urinary and reproductive systems. In mam-mals, the urogenital products are separated from the fecalmaterial in the large intestine; the fecal material enters therectum and is expelled through the anus.

In general, carnivores have shorter intestines for theirsize than do herbivores. A short intestine is adequate for acarnivore, but herbivores ingest a large amount of plantcellulose, which resists digestion. These animals have along, convoluted small intestine. In addition, mammalscalled ruminants (such as cows) that consume grass andother vegetation have stomachs with multiple chambers,where bacteria aid in the digestion of cellulose. Other her-bivores, including rabbits and horses, digest cellulose (withthe aid of bacteria) in a blind pouch called the cecum lo-cated at the beginning of the large intestine.

The accessory digestive organs (described in detail later inthe chapter) include the liver, which produces bile (a greensolution that emulsifies fat), the gallbladder, which storesand concentrates the bile, and the pancreas. The pancreasproduces pancreatic juice, which contains digestive enzymesand bicarbonate. Both bile and pancreatic juice are secretedinto the first region of the small intestine and aid digestion.

The tubular gastrointestinal tract of a vertebrate has acharacteristic layered structure (figure 51.5). The innermostlayer is the mucosa, an epithelium that lines the interior ofthe tract (the lumen). The next major tissue layer, made ofconnective tissue, is called the submucosa. Just outside thesubmucosa is the muscularis, which consists of a doublelayer of smooth muscles. The muscles in the inner layerhave a circular orientation, and those in the outer layer arearranged longitudinally. Another connective tissue layer, theserosa, covers the external surface of the tract. Nerves, in-tertwined in regions called plexuses, are located in the sub-mucosa and help regulate the gastrointestinal activities.

The vertebrate digestive system consists of a tubulargastrointestinal tract, which is modified in differentanimals, composed of a series of tissue layers.

Chapter 51 Fueling Body Activities: Digestion 1019

Salivary gland

Salivary gland

Liver

Esophagus

Gallbladder

Pharynx

Cecum

Appendix

Anus

Rectum

Smallintestine

Pancreas

Stomach

Colon

FIGURE 51.4The human digestive system. Humans, like all placentalmammals, lack a cloaca and have a separate exit from the digestivetract through the rectum and anus.

Blood vessel

Nerve

Myentericplexus

Submucosalplexus

Connective tissue layerSerosa

Gland insubmucosa

Longitudinal layer

Circular layerMuscularis

Gland outsidegastrointestinaltract

Mucosa

Lumen

Submucosa

FIGURE 51.5The layers of the gastrointestinal tract. The mucosa contains alining epithelium; the submucosa is composed of connectivetissue (as is the serosa), and the muscularis consists of smoothmuscles.

The Mouth and TeethSpecializations of the digestive systems in different kinds ofvertebrates reflect differences in the way these animals live.Fishes have a large pharynx with gill slits, while air-breathingvertebrates have a greatly reduced pharynx. Many verte-brates have teeth (figure 51.6), and chewing (mastication)breaks up food into small particles and mixes it with fluid se-cretions. Birds, which lack teeth, break up food in their two-chambered stomachs (figure 51.7). In one of these chambers,the gizzard, small pebbles ingested by the bird are churnedtogether with the food by muscular action. This churninggrinds up the seeds and other hard plant material intosmaller chunks that can be digested more easily.

Vertebrate Teeth

Carnivorous mammals have pointed teeth that lack flatgrinding surfaces. Such teeth are adapted for cutting andshearing. Carnivores often tear off pieces of their prey buthave little need to chew them, because digestive enzymescan act directly on animal cells. (Recall how a cat or doggulps down its food.) By contrast, grass-eating herbivores,such as cows and horses, must pulverize the cellulose cellwalls of plant tissue before digesting it. These animals havelarge, flat teeth with complex ridges well-suited to grinding.

Human teeth are specialized for eating both plant andanimal food. Viewed simply, humans are carnivores in thefront of the mouth and herbivores in the back (figure 51.8).The four front teeth in the upper and lower jaws are sharp,chisel-shaped incisors used for biting. On each side of theincisors are sharp, pointed teeth called cuspids (sometimesreferred to as “canine” teeth), which are used for tearingfood. Behind the canines are two premolars and three mo-lars, all with flattened, ridged surfaces for grinding andcrushing food. Children have only 20 teeth, but these de-ciduous teeth are lost during childhood and are replaced by32 adult teeth.

The Mouth

Inside the mouth, the tongue mixes food with a mucous so-lution, saliva. In humans, three pairs of salivary glands se-crete saliva into the mouth through ducts in the mouth’smucosal lining. Saliva moistens and lubricates the food sothat it is easier to swallow and does not abrade the tissue itpasses on its way through the esophagus. Saliva also con-tains the hydrolytic enzyme salivary amylase, which initi-ates the breakdown of the polysaccharide starch into thedisaccharide maltose. This digestion is usually minimal inhumans, however, because most people don’t chew theirfood very long.

The secretions of the salivary glands are controlled bythe nervous system, which in humans maintains a constant

flow of about half a milliliter per minute when the mouth isempty of food. This continuous secretion keeps the mouthmoist. The presence of food in the mouth triggers an in-creased rate of secretion, as taste-sensitive neurons in themouth send impulses to the brain, which responds by stim-ulating the salivary glands. The most potent stimuli are


51.2 Food is ingested, swallowed, and transported to the stomach.

Molars Premolars

Canines

Incisors

FIGURE 51.6Diagram of generalized vertebrate dentition. Differentvertebrates will have specific variations from this generalizedpattern, depending on whether the vertebrate is an herbivore,carnivore, or omnivore.

Mouth

Esophagus

Stomach

Gizzard

Intestine

Anus

Crop

FIGURE 51.7Birds store food in the crop and grind it up in the gizzard.Birds lack teeth but have a muscular chamber called the gizzardthat works to break down food. Birds swallow gritty objects orpebbles that lodge in the gizzard and pulverize food before itpasses into the small intestine.

Pharynx

Soft palate

Hard palateTongue

Epiglottis

Glottis

Larynx

Trachea

Esophagus

Air

acidic solutions; lemon juice, for example, can increase therate of salivation eightfold. The sight, sound, or smell offood can stimulate salivation markedly in dogs, but in hu-mans, these stimuli are much less effective than thinking ortalking about food.

When food is ready to be swallowed, the tongue movesit to the back of the mouth. In mammals, the process ofswallowing begins when the soft palate elevates, pushingagainst the back wall of the pharynx (figure 51.9). Elevationof the soft palate seals off the nasal cavity and prevents foodfrom entering it. Pressure against the pharynx triggers anautomatic, involuntary response called a reflex. In this re-flex, pressure on the pharynx stimulates neurons within itswalls, which send impulses to the swallowing center in thebrain. In response, electrical impulses in motor neuronsstimulate muscles to contract and raise the larynx (voicebox). This pushes the glottis, the opening from the larynxinto the trachea (windpipe), against a flap of tissue calledthe epiglottis. These actions keep food out of the respiratorytract, directing it instead into the esophagus.

In many vertebrates ingested food is fragmentedthrough the tearing or grinding action of specializedteeth. In birds, this is accomplished through thegrinding action of pebbles in the gizzard. Food mixedwith saliva is swallowed and enters the esophagus.


(a)

Cusp

Enamel

Gingiva

Dentin

Pulp cavity withnerves and vessels

Periodontalligaments

Root canalCementum

Bone

(b)

FIGURE 51.9The human pharynx, palate, and larynx. Food that enters the pharynx is prevented from entering the nasal cavity by elevation of the softpalate, and is prevented from entering the larynx and trachea (the airways of the respiratory system) by elevation of the larynx against theepiglottis.

FIGURE 51.8Human teeth. (a) The front six teeth on the upper and lower jawsare cuspids and incisors. The remaining teeth, running along thesides of the mouth, are grinders called premolars and molars.Hence, humans have carnivore-like teeth in the front of theirmouth and herbivore-like teeth in the back. (b) Each tooth is alive,with a central pulp containing nerves and blood vessels. Theactual chewing surface is a hard enamel layered over the softerdentin, which forms the body of the tooth.

Esophagus and StomachStructure and Function of the Esophagus

Swallowed food enters a muscular tube called the esopha-gus, which connects the pharynx to the stomach. In adulthumans, the esophagus is about 25 centimeters long; theupper third is enveloped in skeletal muscle, for voluntarycontrol of swallowing, while the lower two-thirds is sur-rounded by involuntary smooth muscle. The swallowingcenter stimulates successive waves of contraction in thesemuscles that move food along the esophagus to the stom-ach. These rhythmic waves of muscular contraction arecalled peristalsis (figure 51.10); they enable humans andother vertebrates to swallow even if they are upsidedown.

In many vertebrates, the movement of food from theesophagus into the stomach is controlled by a ring of circu-lar smooth muscle, or a sphincter, that opens in response tothe pressure exerted by the food. Contraction of thissphincter prevents food in the stomach from moving backinto the esophagus. Rodents and horses have a true sphinc-ter at this site and thus cannot regurgitate, while humanslack a true sphincter and so are able to regurgitate. Nor-mally, however, the human esophagus is closed off exceptduring swallowing.

Structure and Function of the Stomach

The stomach (figure 51.11) is a saclike portion of the diges-tive tract. Its inner surface is highly convoluted, enabling itto fold up when empty and open out like an expanding bal-loon as it fills with food. Thus, while the human stomachhas a volume of only about 50 milliliters when empty, itmay expand to contain 2 to 4 liters of food when full. Car-nivores that engage in sporadic gorging as an importantsurvival strategy possess stomachs that are able to distendmuch more than that.

Secretory Systems

The stomach contains an extra layer of smooth muscle forchurning food and mixing it with gastric juice, an acidic se-cretion of the tubular gastric glands of the mucosa (figure51.11). These exocrine glands contain two kinds of secre-tory cells: parietal cells, which secrete hydrochloric acid(HCl); and chief cells, which secrete pepsinogen, a weakprotease (protein-digesting enzyme) that requires a verylow pH to be active. This low pH is provided by the HCl.Activated pepsinogen molecules then cleave one another atspecific sites, producing a much more active protease,pepsin. This process of secreting a relatively inactive en-zyme that is then converted into a more active enzyme out-side the cell prevents the chief cells from digesting them-selves. It should be noted that only proteins are partiallydigested in the stomach—there is no significant digestionof carbohydrates or fats.

Action of Acid

The human stomach produces about 2 liters of HCl andother gastric secretions every day, creating a very acidic so-lution inside the stomach. The concentration of HCl inthis solution is about 10 millimolar, corresponding to a pHof 2. Thus, gastric juice is about 250,000 times more acidicthan blood, whose normal pH is 7.4. The low pH in thestomach helps denature food proteins, making them easierto digest, and keeps pepsin maximally active. Active pepsinhydrolyzes food proteins into shorter chains of polypep-tides that are not fully digested until the mixture enters thesmall intestine. The mixture of partially digested food andgastric juice is called chyme.


Epiglottis

EsophagusLarynx

Relaxed muscles

Contracted muscles

Stomach

FIGURE 51.10The esophagus and peristalsis. After food has entered theesophagus, rhythmic waves of muscular contraction, calledperistalsis, move the food down to the stomach.

The acidic solution within the stomach also kills most ofthe bacteria that are ingested with the food. The few bacte-ria that survive the stomach and enter the intestine intactare able to grow and multiply there, particularly in thelarge intestine. In fact, most vertebrates harbor thrivingcolonies of bacteria within their intestines, and bacteria area major component of feces. As we will discuss later, bacte-ria that live within the digestive tract of cows and other ru-minants play a key role in the ability of these mammals todigest cellulose.

Ulcers

Overproduction of gastric acid can occasionally eat a holethrough the wall of the stomach. Such gastric ulcers arerare, however, because epithelial cells in the mucosa of thestomach are protected somewhat by a layer of alkalinemucus, and because those cells are rapidly replaced by celldivision if they become damaged (gastric epithelial cellsare replaced every 2 to 3 days). Over 90% of gastrointesti-nal ulcers are duodenal ulcers. These may be produced whenexcessive amounts of acidic chyme are delivered into theduodenum, so that the acid cannot be properly neutralizedthrough the action of alkaline pancreatic juice (describedlater). Susceptibility to ulcers is increased when the mu-cosal barriers to self-digestion are weakened by an infec-tion of the bacterium Helicobacter pylori. Indeed, modern

antibiotic treatments of this infection can reduce symp-toms and often even cure the ulcer.

In addition to producing HCl, the parietal cells of thestomach also secrete intrinsic factor, a polypeptide neededfor the intestinal absorption of vitamin B12. Because this vi-tamin is required for the production of red blood cells, per-sons who lack sufficient intrinsic factor develop a type ofanemia (low red blood cell count) called pernicious anemia.

Leaving the Stomach

Chyme leaves the stomach through the pyloric sphincter (seefigure 51.11) to enter the small intestine. This is where allterminal digestion of carbohydrates, lipids, and proteins oc-curs, and where the products of digestions—amino acids,glucose, and so on—are absorbed into the blood. Onlysome of the water in chyme and a few substances such asaspirin and alcohol are absorbed through the wall of thestomach.

Peristaltic waves of contraction propel food along theesophagus to the stomach. Gastric juice contains stronghydrochloric acid and the protein-digesting enzymepepsin, which begins the digestion of proteins intoshorter polypeptides. The acidic chyme is thentransferred through the pyloric sphincter to the smallintestine.


Gastric pits

Mucosa

SubmucosaGastric glands

Chiefcell

Parietalcell

Mucouscell

Esophagus

Stomach

Mucosa

EpitheliumPyloricsphincter

Villi

Duodenum

FIGURE 51.11The stomach and duodenum. Food enters the stomach from the esophagus. A band of smooth muscle called the pyloric sphinctercontrols the entrance to the duodenum, the upper part of the small intestine. The epithelial walls of the stomach are dotted with gastricpits, which contain gastric glands that secrete hydrochloric acid and the enzyme pepsinogen. The gastric glands consist of mucous cells,chief cells that secrete pepsinogen, and parietal cells that secrete HCl. Gastric pits are the openings of the gastric glands.

The Small IntestineDigestion in the Small Intestine

The capacity of the small intestine is limited, and its diges-tive processes take time. Consequently, efficient digestionrequires that only relatively small amounts of chyme be in-troduced from the stomach into the small intestine at anyone time. Coordination between gastric and intestinal ac-tivities is regulated by neural and hormonal signals, whichwe will describe in a later section.

The small intestine is approximately 4.5 meters long in aliving person, but is 6 meters long at autopsy when themuscles relax. The first 25 centimeters is the duodenum;the remainder of the small intestine is divided into the je-junum and the ileum. The duodenum receives acidicchyme from the stomach, digestive enzymes and bicarbon-ate from the pancreas, and bile from the liver and gallblad-der. The pancreatic juice enzymes digest larger food mole-cules into smaller fragments. This occurs primarily in theduodenum and jejunum.

The epithelial wall of the small intestine is covered withtiny, fingerlike projections called villi (singular, villus; figure

51.12). In turn, each of the epithelial cells lining the villi iscovered on its apical surface (the side facing the lumen) bymany foldings of the plasma membrane that form cytoplas-mic extensions called microvilli. These are quite tiny and canbe seen clearly only with an electron microscope (figure51.13). In a light micrograph, the microvilli resemble thebristles of a brush, and for that reason the epithelial wall ofthe small intestine is also called a brush border.

The villi and microvilli greatly increase the surface areaof the small intestine; in humans, this surface area is 300square meters! It is over this vast surface that the productsof digestion are absorbed. The microvilli also participate indigestion because a number of digestive enzymes are em-bedded within the epithelial cells’ plasma membranes, withtheir active sites exposed to the chyme (figure 51.14).These brush border enzymes include those that hydrolyzethe disaccharides lactose and sucrose, among others (table51.1). Many adult humans lose the ability to produce thebrush border enzyme lactase and therefore cannot digestlactose (milk sugar), a rather common condition called lac-tose intolerance. The brush border enzymes complete the di-gestive process that started with the action of the pancre-atic enzymes released into the duodenum.


51.3 The small and large intestines have very different functions.

Mucosa

Submucosa

Muscularis

Small intestine Villi

Microvilli

Cell membrane

Nucleus

Epithelial cell

Capillary Villus

Lacteal

Vein

Artery

Lymphatic duct

FIGURE 51.12The small intestine.Cross-section of the smallintestine; theenlargements show villiand an epithelial cell withnumerous microvilli.


Lumen of duodenum

Epithelial cell of small intestine

EN

EN

EN

EN

EN

EN

EN

FIGURE 51.13Intestinal microvilli. Microvilli, shown in an electronmicrograph, are very densely clustered, giving the small intestinean enormous surface area important in efficient absorption of thedigestion products.

FIGURE 51.14Brush border enzymes. These enzymes, which are labeled “EN”in this diagram, are part of the plasma membrane of the microvilliin the small intestine. They catalyze many of the terminal steps indigestion.

Table 51.1 Digestive Enzymes

Location Enzymes Substrates Digestion Products

Salivary glands Amylase Starch, glycogen DisaccharidesStomach Pepsin Proteins Short peptidesSmall intestine Peptidases Short peptides Amino acids(brush border) Nucleases DNA, RNA Sugars, nucleic acid bases

Lactase, maltase, sucrase Disaccharides MonosaccharidesPancreas Lipase Triglycerides Fatty acids, glycerol

Trypsin, chymotrypsin Proteins PeptidesDNase DNA NucleotidesRNase RNA Nucleotides

Absorption in the Small Intestine

The amino acids and monosaccharides resulting from the di-gestion of proteins and carbohydrates, respectively, aretransported across the brush border into the epithelial cellsthat line the intestine (figure 51.15a). They then move to theother side of the epithelial cells, and from there are trans-ported across the membrane and into the blood capillarieswithin the villi. The blood carries these products of digestionfrom the intestine to the liver via the hepatic portal vein.The term portal here refers to a special arrangement of ves-sels, seen only in a couple of instances, where one organ (theliver, in this case) is located “downstream” from anotherorgan (the intestine). As a result, the second organ receivesblood-borne molecules from the first. Because of the hepaticportal vein, the liver is the first organ to receive most of theproducts of digestion. This arrangement is important for thefunctions of the liver, as will be described in a later section.

The products of fat digestion are absorbed by a differentmechanism (figure 51.15b). Fats (triglycerides) are hy-drolyzed into fatty acids and monoglycerides, which are ab-sorbed into the intestinal epithelial cells and reassembledinto triglycerides. The triglycerides then combine withproteins to form small particles called chylomicrons. In-

stead of entering the hepatic portal circulation, the chy-lomicrons are absorbed into lymphatic capillaries (seechapter 52), which empty their contents into the blood inveins near the neck. Chylomicrons can make the bloodplasma appear cloudy if a sample of blood is drawn after afatty meal.

The amount of fluid passing through the small intes-tine in a day is startlingly large: approximately 9 liters.However, almost all of this fluid is absorbed into the bodyrather than eliminated in the feces. About 8.5 liters areabsorbed in the small intestine and an additional 350 mil-liliters in the large intestine. Only about 50 grams of solidand 100 milliliters of liquid leave the body as feces. Thenormal fluid absorption efficiency of the human digestivetract thus approaches 99%, which is very high indeed.

Digestion occurs primarily in the duodenum, whichreceives the pancreatic juice enzymes. The smallintestine provides a large surface area for absorption.Glucose and amino acids from food are absorbedthrough the small intestine and enter the blood via thehepatic portal vein, going to the liver. Fat from foodenters the lymphatic system.


Lumen of small intestine

Protein Carbohydrate

Bile saltsEmulsification

droplets

Free fatty acids,monoglycerides

Resynthesisof triglycerides

Triglycerides + protein cover

Chylomicron

Fat globules(triglycerides)

Mono-saccharidesAmino

acids

Blood capillary Lymphatic capillary

Epithelial cell of intestinal villus

(a) (b)

FIGURE 51.15Absorption of the products of digestion. (a) Monosaccharides and amino acids are transported into blood capillaries. (b) Fatty acids andmonoglycerides within the intestinal lumen are absorbed and converted within the intestinal epithelial cells into triglycerides. These arethen coated with proteins to form tiny structures called chylomicrons, which enter lymphatic capillaries.

The Large IntestineThe large intestine, or colon, is much shorter than thesmall intestine, occupying approximately the last meter ofthe digestive tract; it is called “large” only because of itslarger diameter. The small intestine empties directly intothe large intestine at a junction where two vestigial struc-tures, the cecum and the appendix, remain (figure 51.16).No digestion takes place within the large intestine, andonly about 4% of the absorption of fluids by the intestineoccurs there. The large intestine is not as convoluted as thesmall intestine, and its inner surface has no villi. Conse-quently, the large intestine has less than one-thirtieth theabsorptive surface area of the small intestine. Althoughsodium, vitamin K, and some products of bacterial metabo-lism are absorbed across its wall, the primary function ofthe large intestine is to concentrate waste material. Withinit, undigested material, primarily bacterial fragments andcellulose, is compacted and stored. Many bacteria live andreproduce within the large intestine, and the excess bacteriaare incorporated into the refuse material, called feces. Bacte-rial fermentation produces gas within the colon at a rate ofabout 500 milliliters per day. This rate increases greatlyafter the consumption of beans or other vegetable matterbecause the passage of undigested plant material (fiber) intothe large intestine provides substrates for fermentation.

The human colon has evolved to process food with a rel-atively high fiber content. Diets that are low in fiber, whichare common in the United States, result in a slower passageof food through the colon. Low dietary fiber content isthought to be associated with the level of colon cancer inthe United States, which is among the highest in the world.

Compacted feces, driven by peristaltic contractions ofthe large intestine, pass from the large intestine into a shorttube called the rectum. From the rectum, the feces exit thebody through the anus. Two sphincters control passagethrough the anus. The first is composed of smooth muscleand opens involuntarily in response to pressure inside therectum. The second, composed of striated muscle, can becontrolled voluntarily by the brain, thus permitting a con-scious decision to delay defecation.

In all vertebrates except most mammals, the reproduc-tive and urinary tracts empty together with the digestivetract into a common cavity, the cloaca. In some reptiles andbirds, additional water from either the feces or urine maybe absorbed in the cloaca before the products are expelledfrom the body.

The large intestine concentrates wastes for excretion byabsorbing water. Some ions and vitamin K are alsoabsorbed by the large intestine.


Ascending portionof large intestine

Appendix

Last portionof small intestine

Ileocecal valve

Cecum

FIGURE 51.16The junction of the small and large intestines in humans. The large intestine, or colon, starts with the cecum, which is relatively smallin humans compared with that in other mammals. A vestigial structure called the appendix extends from the cecum.

Variations in Vertebrate DigestiveSystemsMost animals lack the enzymes necessary to digest cellu-lose, the carbohydrate that functions as the chief struc-tural component of plants. The digestive tracts of someanimals, however, contain bacteria and protists that con-vert cellulose into substances the host can digest. Al-though digestion by gastrointestinal microorganisms playsa relatively small role in human nutrition, it is an essentialelement in the nutrition of many other kinds of animals,including insects like termites and cockroaches, and a fewgroups of herbivorous mammals. The relationships be-tween these microorganisms and their animal hosts aremutually beneficial and provide an excellent example ofsymbiosis.

Cows, deer, and other ruminants have large, dividedstomachs (figure 51.17). The first portion consists of therumen and a smaller chamber, the reticulum; the secondportion consists of two additional chambers: the omasumand abomasum. The rumen which may hold up to 50 gal-lons, serves as a fermentation vat in which bacteria and pro-tozoa convert cellulose and other molecules into a varietyof simpler compounds. The location of the rumen at thefront of the four chambers is important because it allowsthe animal to regurgitate and rechew the contents of therumen, an activity called rumination, or “chewing the cud.”The cud is then swallowed and enters the reticulum, fromwhich it passes to the omasum and then the abomasum,where it is finally mixed with gastric juice. Hence, only theabomasum is equivalent to the human stomach in its func-tion. This process leads to a far more efficient digestion ofcellulose in ruminants than in mammals that lack a rumen,such as horses.

In horses, rodents, and lagomorphs (rabbits and hares),the digestion of cellulose by microorganisms takes place inthe cecum, which is greatly enlarged (figure 51.18). Be-cause the cecum is located beyond the stomach, regurgita-tion of its contents is impossible. However, rodents andlagomorphs have evolved another way to digest cellulosethat achieves a degree of efficiency similar to that of rumi-nant digestion. They do this by eating their feces, thuspassing the food through their digestive tract a secondtime. The second passage makes it possible for the animalto absorb the nutrients produced by the microorganisms inits cecum. Animals that engage in this practice of co-prophagy (from the Greek words copros, “excrement,” andphagein, “eat”) cannot remain healthy if they are preventedfrom eating their feces.

Cellulose is not the only plant product that vertebratescan use as a food source because of the digestive activities

of intestinal microorganisms. Wax, a substance indi-gestible by most terrestrial animals, is digested by symbi-otic bacteria living in the gut of honey guides, Africanbirds that eat the wax in bee nests. In the marine foodchain, wax is a major constituent of copepods (crus-taceans in the plankton), and many marine fish and birdsappear to be able to digest wax with the aid of symbioticmicroorganisms.

Another example of the way intestinal microorganismsfunction in the metabolism of their animal hosts is pro-vided by the synthesis of vitamin K. All mammals rely onintestinal bacteria to synthesize this vitamin, which is nec-essary for the clotting of blood. Birds, which lack these bac-teria, must consume the required quantities of vitamin K intheir food. In humans, prolonged treatment with antibi-otics greatly reduces the populations of bacteria in the in-testine; under such circumstances, it may be necessary toprovide supplementary vitamin K.

Much of the food value of plants is tied up in cellulose,and the digestive tract of many animals harbors coloniesof cellulose-digesting microorganisms. Intestinalmicroorganisms also produce molecules such as vitaminK that are important to the well-being of theirvertebrate hosts.


Esophagus

Rumen

Reticulum

OmasumAbomasum

Smallintestine

FIGURE 51.17Four-chambered stomach of a ruminant. The grass and otherplants that a ruminant, such as a cow, eats enter the rumen, wherethey are partially digested. Before moving into a second chamber,the reticulum, the food may be regurgitated and rechewed. Thefood is then transferred to the rear two chambers: the omasumand abomasum. Only the abomasum is equivalent to the humanstomach in its function of secreting gastric juice.


Stomach

Anus

Anus

Spiralloop

Esophagus

Rumen

Anus

Cecum

Cecum

Anus

Esophagus

Stomach

Stomach

Reticulum

Omasum

Abomasum

Cecum

Insectivore

Short intestine,no cecum

Carnivore

Short intestineand colon,small cecum

Ruminantherbivore

Four-chamberedstomach with large rumen;long small andlarge intestine

Nonruminantherbivore

Simple stomach,large cecum

FIGURE 51.18The digestive systems of different mammals reflect their diets. Herbivores require long digestive tracts with specializedcompartments for the breakdown of plant matter. Protein diets are more easily digested; thus, insectivorous and carnivorous mammalshave short digestive tracts with few specialized pouches.

Accessory OrgansSecretions of the Pancreas

The pancreas (figure 51.19), a largegland situated near the junction of thestomach and the small intestine, isone of the accessory organs that con-tribute secretions to the digestivetract. Pancreatic fluid is secreted intothe duodenum through the pancreaticduct; thus, the pancreas functions asan exocrine organ. This fluid containsa host of enzymes, including trypsinand chymotrypsin, which digest pro-teins; pancreatic amylase, which di-gests starch; and lipase, which digestsfat. These enzymes are released intothe duodenum primarily as inactivezymogens and are then activated bythe brush border enzymes of the in-testine. Pancreatic enzymes digestproteins into smaller polypeptides,polysaccharides into shorter chains ofsugars, and fat into free fatty acids andother products. The digestion ofthese molecules is then completed bythe brush border enzymes.

Pancreatic fluid also contains bi-carbonate, which neutralizes the HCl from the stomachand gives the chyme in the duodenum a slightly alkalinepH. The digestive enzymes and bicarbonate are producedby clusters of secretory cells known as acini.

In addition to its exocrine role in digestion, the pancreasalso functions as an endocrine gland, secreting several hor-mones into the blood that control the blood levels of glu-cose and other nutrients. These hormones are produced inthe islets of Langerhans, clusters of endocrine cells scat-tered throughout the pancreas. The two most importantpancreatic hormones, insulin and glucagon, are discussedlater in this chapter.

The Liver and Gallbladder

The liver is the largest internal organ of the body (see fig-ure 51.4). In an adult human, the liver weighs about 1.5kilograms and is the size of a football. The main exocrinesecretion of the liver is bile, a fluid mixture consisting ofbile pigments and bile salts that is delivered into the duode-num during the digestion of a meal. The bile pigments donot participate in digestion; they are waste products result-ing from the liver’s destruction of old red blood cells and

ultimately are eliminated with the feces. If the excretion ofbile pigments by the liver is blocked, the pigments can ac-cumulate in the blood and cause a yellow staining of thetissues known as jaundice.

In contrast, the bile salts play a very important role inthe digestion of fats. Because fats are insoluble in water,they enter the intestine as drops within the watery chyme.The bile salts, which are partly lipid-soluble and partlywater-soluble, work like detergents, dispersing the largedrops of fat into a fine suspension of smaller droplets. Thisemulsification process produces a greater surface area of fatupon which the lipase enzymes can act, and thus allows thedigestion of fat to proceed more rapidly.

After it is produced in the liver, bile is stored and con-centrated in the gallbladder. The arrival of fatty food inthe duodenum triggers a neural and endocrine reflex (dis-cussed later) that stimulates the gallbladder to contract,causing bile to be transported through the common bileduct and injected into the duodenum. If the bile duct isblocked by a gallstone (formed from a hardened precipi-tate of cholesterol), contraction of the gallbladder willcause pain generally felt under the right scapula (shoul-der blade).


51.4 Accessory organs, neural stimulation, and endocrine secretions assist indigestion.

From liverβ-cell

α-cell

Gallbladder

Pancreaticduct

Pancreas

Pancreatic islet(of Langerhans)

Commonbile duct

Duodenum

FIGURE 51.19The pancreas and bile duct empty into the duodenum. The pancreas secretes pancreaticjuice into the pancreatic duct. The pancreatic islets of Langerhans secrete hormones into theblood; α-cells secrete glucagon and β-cells secrete insulin.

Regulatory Functions of the Liver

Because the hepatic portal vein carries blood from thestomach and intestine directly to the liver, the liver is in aposition to chemically modify the substances absorbed inthe gastrointestinal tract before they reach the rest of thebody. For example, ingested alcohol and other drugs aretaken into liver cells and metabolized; this is why the liveris often damaged as a result of alcohol and drug abuse. Theliver also removes toxins, pesticides, carcinogens, and otherpoisons, converting them into less toxic forms. An impor-tant example of this is the liver’s conversion of the toxicammonia produced by intestinal bacteria into urea, a com-pound that can be contained safely and carried by the bloodat higher concentrations.

Similarly, the liver regulates the levels of many com-pounds produced within the body. Steroid hormones, forinstance, are converted into less active and more water-soluble forms by the liver. These molecules are then in-cluded in the bile and eliminated from the body in thefeces, or carried by the blood to the kidneys and excreted inthe urine.

The liver also produces most of the proteins found inblood plasma. The total concentration of plasma proteins issignificant because it must be kept within normal limits inorder to maintain osmotic balance between blood and in-terstitial (tissue) fluid. If the concentration of plasma pro-teins drops too low, as can happen as a result of liver dis-ease such as cirrhosis, fluid accumulates in the tissues, acondition called edema.

Regulation of Blood Glucose Concentration

The neurons in the brain obtain their energy primarilyfrom the aerobic respiration of glucose obtained from theblood plasma. It is therefore extremely important that the

blood glucose concentration not fall too low, as might hap-pen during fasting or prolonged exercise. It is also impor-tant that the blood glucose concentration not stay at toohigh a level, as it does in people with uncorrected diabetesmellitus, because this can lead to tissue damage.

After a carbohydrate-rich meal, the liver and skeletalmuscles remove excess glucose from the blood and store itas the polysaccharide glycogen. This process is stimulatedby the hormone insulin, secreted by the β ( beta) cells in theislets of Langerhans of the pancreas. When blood glucoselevels decrease, as they do between meals, during periods offasting, and during exercise, the liver secretes glucose intothe blood. This glucose is obtained in part from the break-down of liver glycogen to glucose-6-phosphate, a processcalled glycogenolysis. The phosphate group is then re-moved, and free glucose is secreted into the blood. Skeletalmuscles lack the enzyme needed to remove the phosphategroup, and so, even though they have glycogen stores, theycannot secrete glucose into the blood. The breakdown ofliver glycogen is stimulated by another hormone, glucagon,which is secreted by the α (alpha) cells of the islets ofLangerhans in the pancreas (figure 51.20).

If fasting or exercise continues, the liver begins to con-vert other molecules, such as amino acids and lactic acid,into glucose. This process is called gluconeogenesis (“newformation of glucose”). The amino acids used for gluco-neogenesis are obtained from muscle protein, which ex-plains the severe muscle wasting that occurs during pro-longed fasting.

The pancreas secretes digestive enzymes andbicarbonate into the pancreatic duct. The liverproduces bile, which is stored and concentrated in thegallbladder. The liver and the pancreatic hormonesregulate blood glucose concentration.


Pancreatic islets

Eating carbohydrate-rich meal

Insulin secretionGlucagon secretion

Formation ofglycogen and fat

Fasting or exercise

Metabolism

Increasing blood glucose

Pancreatic islets

Breakdown ofglycogen and fat

Decreasing blood glucose

Insulin secretionGlucagon secretion

FIGURE 51.20The actions of insulin and glucagon.After a meal, an increased secretion ofinsulin by the β cells of the pancreaticislets promotes the deposition of glycogenand fat. During fasting or exercising, anincreased glucagon secretion by the α cellsof the pancreatic islets and a decreasedinsulin secretion promote the breakdown(through hydrolysis reactions) of glycogenand fat.

Neural and Hormonal Regulation of DigestionThe activities of the gastrointestinal tract are coordinatedby the nervous system and the endocrine system. The ner-vous system, for example, stimulates salivary and gastric se-cretions in response to the sight and smell of food. Whenfood arrives in the stomach, proteins in the food stimulatethe secretion of a stomach hormone called gastrin (table51.2), which in turn stimulates the secretion of pepsinogenand HCl from the gastric glands (figure 51.21). The se-creted HCl then lowers the pH of the gastric juice, whichacts to inhibit further secretion of gastrin. Because inhibi-tion of gastrin secretion will reduce the amount of HCl re-leased into the gastric juice, a negative feedback loop iscompleted. In this way, the secretion of gastric acid is keptunder tight control.

The passage of chyme from the stomach into the duo-denum inhibits the contractions of the stomach, so thatno additional chyme can enter the duodenum until theprevious amount has been processed. This inhibition ismediated by a neural reflex and by a hormone secreted bythe small intestine that inhibits gastric emptying. Thehormone is known generically as an enterogastrone (enterorefers to the intestine; gastro to the stomach). The chemi-cal identity of the enterogastrone is currently controver-

sial. A hormone known as gastric inhibitory peptide(GIP), released by the duodenum, was named for thisfunction but may not be the only, or even the major, en-terogastrone. The secretion of enterogastrone is stimu-lated most strongly by the presence of fat in the chyme.Fatty meals therefore remain in the stomach longer thanmeals low in fat.

The duodenum secretes two additional hormones.Cholecystokinin (CCK), like enterogastrone, is secreted inresponse to the presence of fat in the chyme. CCK stimu-lates the contractions of the gallbladder, injecting bile intothe duodenum so that fat can be emulsified and more effi-ciently digested. The other duodenal hormone is secretin.Released in response to the acidity of the chyme that ar-rives in the duodenum, secretin stimulates the pancreas torelease bicarbonate, which then neutralizes some of theacidity. Secretin has the distinction of being the first hor-mone ever discovered.

Neural and hormonal reflexes regulate the activity ofthe digestive system. The stomach’s secretions areregulated by food and by the hormone gastrin. Otherhormones, secreted by the duodenum, inhibit stomachemptying and promote the release of bile from thegallbladder and the secretion of bicarbonate inpancreatic juice.


Table 51.2 Hormones of Digestion

Hormone Class Source Stimulus Action Note

Gastrin

Cholecystokinin

Gastric inhibitory peptideSecretin

Polypeptide

Polypeptide

Polypeptide

Polypeptide

Pyloric portionof stomach

Duodenum

Duodenum

Duodenum

Entry of foodinto stomach

Fatty chyme induodenum

Fatty chyme induodenumAcidic chymein duodenum

Stimulates secretion of HCland pepsinogen by stomach

Stimulates gallbladdercontraction and secretion ofdigestive enzymes by pancreasInhibits stomach emptying

Stimulates secretion ofbicarbonate by pancreas

Unusual in that it actson same organ that secretes itStructurally similar togastrin

Also stimulates insulinsecretionThe first hormone tobe discovered (1902)


Liver

Gallbladder

Duodenum

Pancreas

Stomach

Bile

Proteins

Pepsin

HCl

+

+ +

+ –

+

Enzymes

BicarbonateCCK

Secretin

Chiefcells

Parietalcells

Gastrin

Enterogastrone

Acinarcells

FIGURE 51.21Hormonal control of the gastrointestinal tract. Gastrin is secreted by the mucosa of the stomach and stimulates the secretion ofpepsinogen (which is converted into pepsin) and HCl. The duodenum secretes three hormones: cholecystokinin (CCK), which stimulatescontraction of the gallbladder and secretion of pancreatic enzymes; secretin, which stimulates secretion of pancreatic bicarbonate; and anenterogastrone, which inhibits stomach emptying.

Food Energy and EnergyExpenditureThe ingestion of food serves two primary functions: it pro-vides a source of energy, and it provides raw materials theanimal is unable to manufacture for itself. Even an animalthat is completely at rest requires energy to support itsmetabolism; the minimum rate of energy consumptionunder defined resting conditions is called the basal meta-bolic rate (BMR). The BMR is relatively constant for agiven individual, depending primarily on the person’s age,sex, and body size.

Exercise raises the metabolic rate above the basal levels,so the amount of energy that the body consumes per day isdetermined not only by the BMR but also by the level ofphysical activity. If food energy taken in is greater than theenergy consumed per day, the excess energy will be storedin glycogen and fat. Because glycogen reserves are limited,however, continued ingestion of excess food energy resultsprimarily in the accumulation of fat. The intake of food en-ergy is measured in kilocalories (1 kilocalorie = 1000 calo-ries; nutritionists use Calorie with a capital C instead ofkilocalorie). The measurement of kilocalories in food is de-termined by the amount of heat generated when the food is“burned,” either literally, or when the caloric content offood is measured using a calorimeter, or in the body whenthe food is digested. Caloric intake can be altered by thechoice of diet, and the amount of energy expended in exer-cise can be changed by the choice of lifestyle. The daily en-ergy expenditures (metabolic rates) of people vary between1300 and 5000 kilocalories per day, depending on the per-son’s BMR and level of physical activity. If the food kilo-calories ingested exceed the metabolic rate for a sustainedperiod, the person will accumulate an amount of fat that isdeleterious to health, a condition called obesity. In theUnited States, about 30% of middle-aged women and 15%of middle-aged men are classified as obese, which meansthey weigh at least 20% more than the average weight fortheir height.

Regulation of Food Intake

Scientists have for years suspected that adipose tissue se-cretes a hormonal satiety factor (a circulating chemical thatdecreases appetite), because genetically obese mice loseweight when their circulatory systems are surgicallyjoined with those of normal mice. Apparently, someweight-loss hormone was passing into the obese mice!The satiety factor secreted by adipose tissue has recentlybeen identified. It is the product of a gene first observedin a strain of mice known as ob/ob (ob stands for “obese”;the double symbols indicate that the mice are homozy-gous for this gene—they inherit it from both parents).The ob gene has been cloned in mice, and more recently

in humans, and has been found to be expressed (that is, toproduce mRNA) only in adipocytes. The protein productof this gene, the presumed satiety factor, is called leptin.The ob mice produce a mutated and ineffective form ofleptin, and it is this defect that causes their obesity. Wheninjected with normal leptin, they stop eating and loseweight (figure 51.22).

More recent studies in humans show that the activity ofthe ob gene and the blood concentrations of leptin are actu-ally higher in obese than in lean people, and that the leptinproduced by obese people appears to be normal. It hastherefore been suggested that most cases of human obesitymay result from a reduced sensitivity to the actions of lep-tin in the brain rather than from reduced leptin productionby adipose cells. Aggressive research is ongoing, as mightbe expected from the possible medical and commercial ap-plications of these findings.

In the United States, serious eating disorders have be-come much more common since the mid-1970s. The mostcommon of these disorders are anorexia nervosa, a conditionin which the afflicted individuals literally starve themselves,and bulimia, in which individuals gorge themselves andthen vomit, so that their weight stays constant. Ninety to95% of those suffering from these disorders are female; re-searchers estimate that 2 to 5% of the adolescent girls andyoung women in the United States have eating disorders.

The amount of caloric energy expended by the bodydepends on the basal metabolic rate and the additionalcalories consumed by exercise. Obesity results if theingested food energy exceeds the energy expenditure bythe body over a prolonged period.


51.5 All animals require food energy and essential nutrients.

FIGURE 51.22Injection of the hormone leptin causes genetically obesemice to lose weight. These two mice are identical twins, bothmembers of a mutant strain of obese mice. The mouse on theright has been injected with the hormone leptin. It lost 30% of itsbody weight in just two weeks, with no apparent side effects.

Essential NutrientsOver the course of their evolution, many animals have lostthe ability to synthesize specific substances that neverthe-less continue to play critical roles in their metabolism. Sub-stances that an animal cannot manufacture for itself butwhich are necessary for its health must be obtained in thediet and are referred to as essential nutrients.

Included among the essential nutrients are vitamins, cer-tain organic substances required in trace amounts. Hu-mans, apes, monkeys, and guinea pigs, for example, havelost the ability to synthesize ascorbic acid (vitamin C). If vi-tamin C is not supplied in sufficient quantities in theirdiets, they will develop scurvy, a potentially fatal disease.Humans require at least 13 different vitamins (table 51.3).

Some essential nutrients are required in more than traceamounts. Many vertebrates, for example, are unable to syn-thesize 1 or more of the 20 amino acids used in makingproteins. These essential amino acids must be obtained fromproteins in the food they eat. There are nine essentialamino acids for humans. People who are vegetarians must

choose their foods so that the essential amino acids in onefood complement those in another.

In addition, all vertebrates have lost the ability to syn-thesize certain unsaturated fatty acids and therefore mustobtain them in food. On the other hand, some essential nu-trients that vertebrates can synthesize cannot be manufac-tured by the members of other animal groups. For exam-ple, vertebrates can synthesize cholesterol, a keycomponent of steroid hormones, but some carnivorous in-sects cannot.

Food also supplies essential minerals such as calcium,phosphorus, and other inorganic substances, including awide variety of trace elements such as zinc and molybdenumwhich are required in very small amounts (see table 2.1).Animals obtain trace elements either directly from plantsor from animals that have eaten plants.

The body requires vitamins and minerals obtained infood. Also, food must provide particular essential aminoacids and fatty acids that the body cannot manufactureby itself.


Table 51.3 Major Vitamins

Vitamin Function Source Deficiency Symptoms

Vitamin A (retinol)

B-complex vitaminsB1

B2(riboflavin)B3(niacin)B5(pantothenic acid)B6(pyridoxine)B12(cyanocobalamin)Biotin

Folic acid

Vitamin C

Vitamin D (calciferol)Vitamin E (tocopherol)Vitamin K

Used in making visual pigments, maintenanceof epithelial tissues

Coenzyme in CO2 removal during cellularrespirationPart of coenzymes FAD and FMN, which playmetabolic rolesPart of coenzymes NAD+ and NADP+

Part of coenzyme-A, a key connection betweencarbohydrate and fat metabolismCoenzyme in many phases of amino acidmetabolismCoenzyme in the production of nucleic acids

Coenzyme in fat synthesis and amino acidmetabolismCoenzyme in amino acid and nucleic acidmetabolismImportant in forming collagen, cement of bone,teeth, connective tissue of blood vessels; mayhelp maintain resistance to infectionIncreases absorption of calcium and promotesbone formationProtects fatty acids and cell membranes fromoxidationEssential to blood clotting

Green vegetables,milk products, liver

Meat, grains, legumes

Many different kindsof foodsLiver, lean meats,grainsMany different kindsof foodsCereals, vegetables,meatsRed meats, dairyproductsMeat, vegetables

Green vegetables

Fruit, green leafyvegetables

Dairy products, codliver oilMargarine, seeds,green leafy vegetablesGreen leafy vegetables

Night blindness, flaky skin

Beriberi, weakening of heart,edemaInflammation and breakdownof skin, eye irritationPellagra, inflammation ofnerves, mental disordersRare: fatigue, loss ofcoordinationAnemia, convulsions,irritabilityPernicious anemia

Rare: depression, nausea

Anemia, diarrhea

Scurvy, breakdown of skin,blood vessels

Rickets, bone deformities

Rare

Severe bleeding


Chapter 51 Summary Questions Media Resources

51.1 Animals employ a digestive system to prepare food for assimilation by cells.

• The digestive system of vertebrates consists of agastrointestinal tract and accessory digestive organs.

• Different regions of the digestive tract displayspecializations of structure and function.

1. What are the layers thatmake up the wall of thevertebrate gastrointestinal tract?What type of tissue is found ineach layer?

• The teeth of carnivores are different from those ofherbivores

• The esophagus contracts in peristaltic waves to drivethe swallowed food to the stomach.

• Cells of the gastric mucosa secrete hydrochloric acid,which activates pepsin, an enzyme that promotes thepartial hydrolysis of ingested proteins.

2. How does tooth structurevary among carnivores,herbivores, and omnivores? 3. What normally prevents

regurgitation in humans, andwhy can’t horses regurgitate?4. What inorganic substance is

secreted by parietal cells?

51.2 Food is ingested, swallowed, and transported to the stomach.

• The duodenum receives pancreatic juice and bile,which help digest the chyme that arrives from thestomach through the pyloric valve.

• Digestive enzymes in the small intestine finish thebreakdown of food into molecules that can beabsorbed by the small intestine.

• The large intestine absorbs water and ions, as well ascertain organic molecules such as vitamin K; theremaining material passes out of the anus.

5. How are the products ofprotein and carbohydratedigestion absorbed across theintestinal wall, and where dothey go after they are absorbed? 6. What anatomical and

behavioral specializations doruminants have for making useof microorganisms?

51.3 The small and large intestines have very different functions.

• Pancreatic juice contains bicarbonate to neutralizethe acid chyme from the stomach. Bile contains bilepigment and bile salts, which emulsify fat. The livermetabolizes toxins and hormones that are delivered toit in the hepatic portal vein; the liver also helps toregulate the blood glucose concentration.

• The stomach secretes the hormone gastrin, and thesmall intestine secretes various hormones that help toregulate the digestive system.

7. What are the main exocrinesecretions of the pancreas, andwhat are their functions? 8. What is the function of bile

salts in digestion?9. Describe the role of gastrin

and secretin in digestion.

51.4 Accessory organs, neural stimulation, and endocrine secretions assist in digestion.

• The basal metabolic rate (BMR) is the lowest level ofenergy consumption of the body.

• Vitamins, minerals, and the essential amino acids andfatty acids must be supplied in the diet.

10. What is a vitamin? What isthe difference between anessential amino acid and anyother amino acid?

51.5 All animals require food energy and essential nutrients.

www.mhhe.com/raven6e www.biocourse.com

• Art Activity: Digestive tract wall

• Art Activities: Digestive systemMouthTeethSwallowingGlottis function

• Art Activities: Small intestine anatomyHepatic lobules

• Introduction todigestion

• Human digestion• Digestion overview

• Stomach to smallintestine

• Small intestinedigestion

• Art Activity: Digestive system

• formation ofgallstones

• Nutrition

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Fueling Body Activities: Digestion€¦ · food for assimilation by cells. Types of Digestive Systems. Some invertebrates have a gastrovascular cavity, but vertebrates have a digestive