Digestive and Excretory Systems The surface of your small intestine is covered with microvilli such as these (magnification: 100×)

Digestive and Excretory Systems

• The surface of your small intestine is covered with microvilli such as these (magnification: 100×)

Digestive and Excretory Systems

Food and Nutrition

• How important is food in your life?• Before you answer, think of two American holidays:

Independence Day and Thanksgiving Day• What comes to mind?• No matter where you live, chances are that meals are

the centerpieces of those special days• To most of us, food is more than just nourishment—it is

an important part of our culture• Human societies throughout the world organize meetings

and family gatherings around food

Food and Energy

• Have you ever wondered why you need to eat food?

• The most obvious answer is to obtain energy• You need energy to climb stairs, lift books, run,

and even to think• Just as a car needs gasoline, your body

needs fuel for all that work, and food is your fuel

• Cells convert the chemical energy stored in Cells convert the chemical energy stored in the sugar glucose and other molecules into the sugar glucose and other molecules into ATPATP

Food and Energy

• The energy available in food can be measured in a surprisingly simple way—by burning the food!

• When food is burned, the energy content of the food is converted to heat, which is measured in terms of calories

• The amount of heat needed to raise the temperature The amount of heat needed to raise the temperature of 1 gram of water by 1 degree Celsius is 1 calorieof 1 gram of water by 1 degree Celsius is 1 calorie

• Scientists refer to the energy stored in food as dietary Calories with a capital C

• One Calorie is equal to 1000 calories, or 1 kilocalorie One Calorie is equal to 1000 calories, or 1 kilocalorie (kcal)(kcal)

Food and Energy

• The energy needs of an average-sized teenager are about 2200 Calories per day for females and about 2800 Calories per day for males

• If you engage in vigorous physical activity, however, your energy needs may be higher

Food and Energy

• Chemical pathways in your body's cells can extract energy from almost any type of foodfrom almost any type of food

• Why then does it matter which foods you eat?• Although most of the food you eat is used as fuel, a certain

amount of the food you eat has other important functions• Food supplies the raw materials used to build and repair body

tissues– Some of these raw materials are used to manufacture new

biomolecules• These include the proteins that regulate cellular reactions, the

phospholipids in cell membranes, and DNA—your genetic material

• Food also contains at least 45 substances that the body needs but cannot manufacture

Food and Energy

• The science of nutrition—the study of food and its effects on the body—tries to determine how food helps the body meet all of its various needs

• Based on their research, nutritionists recommend balanced diets that include many different types of food

• They also plan diets for people with particular needs, such as diabetics

Nutrients

• Nutrients are substances in food that supply the energy and raw materials supply the energy and raw materials your body uses for growth, repair, and your body uses for growth, repair, and maintenancemaintenance

• The nutrients that the body needs are water, carbohydrates, fats, proteins, water, carbohydrates, fats, proteins, vitamins, and mineralsvitamins, and minerals

Water

• The most important nutrient is waterThe most important nutrient is water• Every cell in the human body needs water because many of the

body's processes, including chemical reactions, take place in take place in waterwater

• Water makes up the bulk of blood, lymph, and other bodily fluids

• On hot days or when you take part in strenuous exercise, sweat glands remove water from your tissues and release it as sweat on the surface of your body

• As the water in sweat evaporates, it cools the body• In this way, sweating helps maintain homeostasis• Water vapor is also lost from the body with every breath you exhale

and in urine

Water

• Humans need to drink at least 1 liter of water each day

• If enough water is not taken in to replace what is lost, dehydration can result

• This condition leads to problems with the circulatory, respiratory, and nervous systems

• Drinking plenty of clean water is one of the best things you can do to help keep your body healthy

Carbohydrates

• Simple and complex carbohydratescarbohydrates are the main source of energy for the body

• The sugars found in fruits, honey, and sugar cane are simple carbohydrates, or monosaccharides and disaccharides

• The starches found in grains, potatoes, and vegetables are complex carbohydrates, or polysaccharides

• Starches are broken down by the digestive system into simple sugars

• These molecules are absorbed into the bloodstream and carried to cells throughout the body

• Sugars that are not immediately used to supply energy are converted into the complex carbohydrate glycogen, which is stored in the liver and in skeletal muscles

Carbohydrates

• Many foods contain the complex carbohydrate cellulose, often called fiberfiber

• Although the human digestive system cannot break down cellulose, you need fiber in your diet

• The bulk supplied by fiber helps muscles to keep food and wastes moving through your digestive and excretory systems

• Foods such as whole-grain breads, bran, and many fruits and vegetables are rich in fiber

Fats

• Fats, or lipids, are an important part of a healthy diet• Fats are formed from fatty acids and glycerol• Your body needs certain fatty acids, called essential

fatty acids, to produce cell membranes, myelin sheaths, and certain hormones

• Fatty acids also help the body absorb fat-soluble vitamins

• When a person eats more food than is needed, the body stores the extra energy as fat– Deposits of fat protect body organs and insulate

the body

Fats

• Based on the structure of their fatty acid chains, fats are classified as saturated or saturated or unsaturatedunsaturated

• When there are only single bonds between the carbon atoms in the fatty acids, each carbon atom has the maximum number of hydrogen atoms and the fat is said to be fat is said to be saturatedsaturated

• Most saturated fats are solidssolids at room temperature—including butter and other animal fats

Fats

• Unsaturated fats have at least one double bond in a fatty acid chain

• Unsaturated fats are usually liquids at room temperature

• Because many vegetable oils contain more than one double bond, they are called polyunsaturatedpolyunsaturated

Fats

• People often consume more fat than they actually need

• The American Heart Association recommends a diet with a maximum of 30 percent of Calories from fat, of which only 10 percent should be from saturated fats

• The health consequences of a diet high in fat are serious– They include an increased risk of high blood

pressure, heart disease, obesity, and diabetes

Proteins

• Proteins have a wide variety of roles in the body

• Proteins supply raw materials for growth and repair of structures such as skin and muscle

• Proteins have regulatory and transport functions:– Example:

• Hormone insulin is a protein that regulates the level of sugar in the blood

• Hemoglobin, a protein found in red blood cells, helps the blood transport oxygen

Proteins• Proteins are polymers of amino

acids• The body is able to synthesize only

12 of the 20 amino acids used to make proteins

• The other 8, which are listed in the figure, are called essential amino essential amino acidsacids

• Essential amino acids must be Essential amino acids must be obtained from the foods that you eatobtained from the foods that you eat

• Meat, fish, eggs, and milk generally contain all 8 essential amino acids

• Foods derived from plants, such as grains and beans, do notdo not

• People who People who don't eat animal don't eat animal productsproducts must eat a combination of must eat a combination of plant foods, such as beans and rice, plant foods, such as beans and rice, to obtain all of the essential amino to obtain all of the essential amino acidsacids

Essential Amino Acids

• When plant foods are eaten in the right combination, they provide all of the essential amino acids

Essential Amino Acids

Vitamins • If you think of carbohydrates, fats,

and proteins as the fuel of an automobile, then vitamins are the then vitamins are the ignitionignition

• VitaminsVitamins are organic molecules that help regulate body processes, often working with enzymes

• As you can see in the table at right, most vitamins must be obtained from food

• However, the bacteria that live in the digestive tract are able to synthesize vitamin K

• The skin is able to synthesize vitamin D when exposed to sunlight

• A diet lacking certain vitamins can have serious, even fatal, consequences

Vitamins

Vitamins

• This table lists the food sources and functions of 14 essential vitamins

• The fat-soluble vitamins are listed in the blue rows, and the water-soluble vitamins in the white rows

• What is the function of vitamin K?

Vitamins

• There are two types of vitamins: fat-soluble and water-soluble

• The fat-soluble vitamins A, D, E, and K can be stored in the fatty tissues of the body– The body can build up small stores of The body can build up small stores of

these vitamins for future usethese vitamins for future use

Vitamins

• The water-soluble vitamins, which include vitamin C and the B vitamins, dissolve in water and cannot be stored in the body– Therefore, they should be included in the Therefore, they should be included in the

foods you eat each dayfoods you eat each day

• Eating a diet containing a variety of foods will supply the daily vitamin needs of nearly everyone

Vitamins

• Food stores and pharmacies sell vitamin supplements

• Taking extra-large doses of vitamin supplements does not benefit the body; and, in some cases, it may cause real harm

• Excessive amounts of vitamins A, D, E, Excessive amounts of vitamins A, D, E, and K can be toxicand K can be toxic

Minerals

• Inorganic nutrients that the body needs, usually in small amounts, are called mineralsminerals

• Some examples of minerals are calcium, iron, and magnesium– Calcium is a major component of bones and teeth– Iron is needed to make hemoglobin, the oxygen-

carrying protein in red blood cells– Calcium, sodium, and potassium are required for

normal functioning of nerves

Minerals

• Minerals are sometimes called trace elements because they are needed by the body in such small amounts

• Why do you think some cities and towns add fluoride to their water supplies?

Minerals

Minerals

• Although the body does not metabolize the does not metabolize the minerals it takes inminerals it takes in, it does lose many of them in sweat, urine, and other waste products

• How are these important chemicals replaced?• Many of these elements are found in the living

tissues of plants and other animals• By eating a variety of foods, you can meet

your daily requirement of minerals

Nutrition and a Balanced Diet

• It's no easy task to figure out the best balance of nutrients for the human diet, but nutritionists have tried to do exactly that

• The result is the Food Guide Pyramid

Food Guide Pyramid

• The Food Guide Pyramid illustrates the main characteristics of a balanced diet

• Foods at the base of the pyramid should make up the major portion of the diet, whereas foods containing fats and sugars should be eaten sparingly

• Why do you think nutritionists recommend that you limit your intake of fats, oils, and sweets?

Food Guide Pyramid


• The Food Guide Pyramid classifies foods into six groups• It also indicates how many servings from each group should be

eaten every day to maintain a healthy diet• Foods rich in complex carbohydrates carbohydrates are at the base of the

pyramid• At the top top of the pyramid are foods such as fats and sweets, fats and sweets,

which should be used sparinglywhich should be used sparingly• Some foods in the other groups also contain fats and sugars, so you

should keep this in mind when choosing foods from these groups• The basic idea behind the pyramid is sound and simple—you The basic idea behind the pyramid is sound and simple—you

should eat a variety of foods each day and limit your intake of should eat a variety of foods each day and limit your intake of fatty, sugary foodsfatty, sugary foods


• Food labels can also be used to choose healthful foods

• A food label provides some general information about nutrition, listing the Daily Values and the Calories per gram for protein, carbohydrates, and fats

• The daily value shows you how the particular food fits into the overall daily diet

• Keep in mind that daily values are based on a 2000-Keep in mind that daily values are based on a 2000-Calorie dietCalorie diet

• Nutrient needs are affected by age, gender, and lifestyle

• Rapidly growing adolescents and other groups of people need more nutrients than the daily values indicate


• When choosing foods, you should use the information on food labels to compare similar foods on the basis of their proportion of nutrients to Calories

• When you choose a food, it should be high in nutrition and low in Calories

The Process of Digestion

• Food presents every chordate with at least two challenges

• The first is how to obtain it• Once a chordate has caught, or gathered its food, it

faces a new challenge—how to break that food down into small molecules that can be passed to the cells that need them

• In humans and many other chordates, this is the job of the digestive system

• As food passes through the digestive system, it gets disassembled, distributing its nutrient value to the body along the way

The Process of Digestion

• The human digestive system, like those of other chordates, is built around an alimentary canal—a one-way tube that passes through the body

• The digestive system includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine

• Several major accessory structures, including the salivary glands, the pancreas, and the liver, add secretions to the digestive system

DIGESTION

• Process of breaking down food into molecules the body can use

• Food is mechanically and chemically broken down into smaller molecules that can diffuse into the circulatory system

• Occurs in the alimentary canal, or digestive tract, which begins at the mouth and winds through the body to the anus

The Mouth• As you take a forkful of food into your mouth, the work of the

digestive system begins• The teeth tear and crush the food into a fine paste until it is

ready to be swallowed• Chewing begins the process of mechanical digestion• Mechanical digestion is the physical breakdown of large pieces

of food into smaller pieces• But there is a great deal more to it than that• As you chew your food, digestive enzymes begin the breakdown

of carbohydrates into smaller molecules• This process is called chemical digestion• During chemical digestion, large food molecules are broken

down into smaller food molecules• The function of the digestive system is to help convert foods

into simpler molecules that can be absorbed and used by the cells of the body

CHEMICAL DIGESTION

• Chemical breakdown of large molecules into smaller molecules mainly by hydrolysis (splitting by addition of water)

• Proteins are hydrolyzed to amino acids• Carbohydrates are hydrolyzed to simple

sugars (mainly glucose)• Lipids (fats,oils) are hydrolyzed to fatty acids

and glycerol after emulsification by bile • After hydrolysis enzymes chemically react with

the food

The Digestive System

• The digestive system includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine

• Because the pancreas and most of the gallbladder are behind other organs, their locations are indicated by dotted lines

The Digestive System

MECHANICAL DIGESTION

• Physical breakdown of food into smaller particles• Chewing (mastication) and muscular churning

are physical means of breaking food into smaller particles

• Speeds up chemical digestion by exposing large amounts of food to digestive enzymes

• Smaller particles are easier to chemically digest than larger particles since only the molecules on the surface come into contact with the digestive enzymes

Teeth

• The teeth are anchored in the bones of the jaw

• The surfaces of the teeth are protected by a coating of mineralized enamelmineralized enamel

• Teeth do much of the mechanical work of digestion by cutting, tearing, and crushing food into small fragments

The Teeth

• Human teeth include sharp incisors; cuspids and bicuspids, which grasp and tear food; and large, flat molars

The Teeth

MOUTH(Oral Cavity)

• Digestion of all food begins • Mechanical Teeth:

– Inside each tooth is a pulp (soft,moist mass of tissue) cavity that contains nerves and blood vessels

– Surrounding the pulp cavity is a hard, bonelike material called dentin

– The dentin of the roots is covered by a thin layer of harder material called cementum

– The crown (top) of the tooth is covered by the hardest material in the body, enamel

– Cut:incisors– Pierce and tear:canine– Grind:premolar and molar

Saliva

• As the teeth cut and grind the food, the salivary glands secrete saliva, which helps to moisten the food and make it easier to chew– The release of saliva is under the control of

the nervous system and can be triggered by the scent of food—especially when you are hungry!

Saliva

• Saliva not only helps ease the passage of food through the digestive system but also begins the process of chemical digestion

• Saliva contains an enzyme called amylaseamylase that breaks the chemical bonds in starches and releases sugars– If you chew on a starchy food like a cracker long enough, it

will begin to taste sweet– This sweet taste is a sign that sugar has been released from

starch by the action of amylase

• Saliva also contains lysozymelysozyme, an enzyme that fights infection by digesting the cell walls of many bacteria that may enter the mouth with food

MOUTH(Oral Cavity)

• Chemical digestion of only carbohydrates in the mouth• Several secretions are added to food in the mouth• Mucus,which moistens the food, comes from cells lining

the mouth (lubricates the food making it easier to swallow)

• Three pairs of salivary glands (parotid, submandibular, sublingual) add saliva which also lubricates the food

• Salvia contains the enzyme (amylase) which chemically breaks down the polysaccharide carbohydrate starch into the disaccharide maltose

MOUTH(Tongue)

• Muscular organ• In mastication squeezes food (bolus) against the

roof of the mouth (hard palate)• Pushes food to the back of the mouth (soft

palate extension – uvula) directs the food to the Pharynx for swallowing

• Taste buds– Bitter: rear– Sour: sides– Sweet: tip– Salt: either side of tip

PHARYNX

• Air and food mix (large area at back of mouth)

LARYNX/TRACHEA(RESPIRATORY SYSTEM)

• In swallowing, the rear of the tongue presses on the epiglottis (cartilage) which directs food into the Esophagus thus preventing food (bolus) from entering the glottis opening to the larynx/trachea (windpipe)

• Vocal cords in the larynx

The Esophagus

• During swallowing, the combined actions of the tongue and throat muscles push the chewed clump of food, called a bolus, down the throat

• Recall that as you swallow, a flap of connective tissue called the epiglottisepiglottis closes over the opening to the trachea– This action prevents food from blocking the air

passageways to the lungs

The Esophagus• From the throat, the bolus passes

through the esophagus, or food tube, into the stomach

• You might think that gravity draws food down through the esophagus, but this is not correct

• The reason food travels through the esophagus into the stomach is that it is moved along by contractions of smooth muscle

• These contractions, known as peristalsisperistalsis, squeeze the food through the esophagus into the stomach

• The process of peristalsis is illustrated in the figure

Peristalsis

• Muscles in the walls of the esophagus contract in waves

• Each wave pushes the chewed clump of food, or bolus, in front of it

• Eventually, the bolus is pushed into the stomach

Peristalsis

The Esophagus

• A thick ring of muscle, called the cardiac sphinctercardiac sphincter, closes the esophagus after food has passed into the stomach and prevents the contents of the stomach from moving back up into the esophagus

• Have you ever suffered from “heartburn”?• HeartburnHeartburn is a painful, burning sensation that feels as if

it is coming from the center of the chest (by your heart), just above the stomach

• The sensation is usually caused by a backflow of stomach acid– Heartburn can be caused by overeating or drinking an excess of

caffeinated drinks

ESPOHAGUS

• As food is swallowed, it passes from the pharynx to the esophagus

• Connects the pharynx with the stomach• Food is moved through the esophagus by

peristalsis (rhythmic muscular contraction of smooth muscles of the esophagus) to the stomach

• Inner lining called mucosa contains mucus-secreting cells which lubricates food (throughout the alimentary canal)

• Hollow interior is called lumen

The Stomach

• Food from the esophagus empties into a large muscular sac called the stomachstomach

• The stomach continues the mechanical and chemical digestion of food

• Alternating contractions of the stomach's three smooth muscle layers thoroughly churn and mix the food you churn and mix the food you swallowswallow

STOMACH

• Mechanical digestion of all foods

• Chemical digestion of protein

• Sphincter muscles regulate the entrance (cardiac sphincter) and exit (pyloric sphincter) of food

• Mucosa lining is folded (rugae)

STOMACH

• Three kinds of cells in the stomach mucosa– One secretes mucus for lubrication of food– Second secretes enzymes that are mainly proteases

(principal protease is pepsin) which breaks down protein to polypeptides

– Third secretes hydrochloric acid (HCl) and water (breaks up connective tissue and cell membranes in the food and kills bacteria)

– Enzymes, water, and HCl combine to form gastric juice

– pH of stomach is acidic

STOMACH

• Food usually remains in the stomach for two to three hours

• Muscle contractions in the stomach walls churn (mechanical digestion) its contents

• Churning breaks up the food and mixes it with gastric juice (chemical digestion)

• Food is liquefied (chyme)• When chyme enters the small intestine, it mixes

with secretions from the liver and pancreas

Chemical Digestion • The lining of the stomach contains millions of

microscopic gastric glandsgastric glands that release a number of substances into the stomach– Some of these glands produce mucus, a fluid that

lubricates and protects the stomach wall– Other glands produce hydrochloric acid, which

makes the contents of the stomach very acidic• The acid activates pepsin, an enzyme that

begins the digestion of protein and is secreted by a third set of stomach glands

• Pepsin works best under the acidic conditions Pepsin works best under the acidic conditions present in the stomachpresent in the stomach

Chemical Digestion

• The combination of pepsin and hydrochloric acid begins the complex process of protein digestionprotein digestion

• Pepsin breaks proteins into smaller polypeptide fragments

• While pepsin requires the acidic environment of the stomach in order to function, other enzymes such as amalyse are denatured by the stomach acidamalyse are denatured by the stomach acid– As a result, chemical digestion of carbohydrates

stops when food enters the stomach and does not resume until the food passes into the small intestine

• Not all enzymes that aid in digestion are released by the stomach

Digestive Enzymes

• Digestive enzymes break down foods and make nutrients available to the body

Digestive Enzymes

Mechanical Digestion

• As digestion proceeds, stomach muscles contract to churn and mix stomach fluids and food, gradually producing a mixture known as chymechyme

• After an hour or two, the pyloric valve, which is located between the stomach and small intestine, opens and chyme begins to flow into the small intestine

The Small Intestine• As chyme is pushed through

the pyloric valve, it enters the duodenum

• The duodenum is the first of three parts of the small intestine, and it is where almost and it is where almost all of the digestive enzymes all of the digestive enzymes enter the intestineenter the intestine

• Most of the chemical digestion Most of the chemical digestion and absorption of the food you and absorption of the food you eat occurs in the small intestineeat occurs in the small intestine

• As chyme enters the duodenum from the stomach, it mixes with enzymes and digestive fluids from the pancreas, the liver, and even the lining of the duodenum itself

• The pancreas and liver are shown in the figure

Pancreas and Liver

• Accessory structures, including the liver and pancreas, add secretions to the digestive system

• The pancreas secretes The pancreas secretes enzymes that help enzymes that help break down break down carbohydrates, carbohydrates, proteins, and fats proteins, and fats

Pancreas and Liver

PANCREAS

• Next to stomach

• Functions with the endocrine and digestive systems

• Secretes pancreatic juice into the small intestine (duodenum) which contains digestive enzymes

Accessory Structures of Digestion

• Just behind the stomach is the pancreas• The pancreas is a gland that serves three important functions

– One function is to produce hormones that regulate blood sugar levels

• Within the digestive system, the pancreas plays two key roles– It produces enzymes that break down carbohydrates, proteins,

lipids, and nucleic acids– The pancreas also produces sodium bicarbonatesodium bicarbonate, a base that

neutralizes stomach acid so that these enzymes can be effective

• Why is this neutralization necessary?• Recall that enzymes are proteinsRecall that enzymes are proteins

– Stomach acid can change the shapes of protein molecules– If the shape of an enzyme's active site does not match the shape of

its substrate, the enzyme will not be effective

PANCREATIC JUICE

• Contains sodium bicarbonate which changes the pH of the chyme from acid to base

• Once the pH is changed the enzymes in the pancreatic juice are activated (chemical digestion) – Amylase: starch/glycogen broken down to

disaccharides – Lipase: breaks lipids into fatty acids and glycerol– Proteases (Trypsin, chymotrypsin,

carboxypeptidase) splits proteins into peptides

Accessory Structures of Digestion

• Assisting the pancreas is the liver, a large organ located just above and to the right of the stomach

• The liver produces bile, a fluid loaded with lipids and salts

• Bile acts like a detergentBile acts like a detergent, dissolving and dispersing the droplets of fat found in fatty foods– This action makes it possible for enzymes to reach

the smaller fat molecules and break them down

• Bile is stored in a small, pouchlike organ called the gallbladdergallbladder

LIVER

• Vital role in the digestion of lipids (fats/oils) by secreting bile (stored in the gallbladder)

• Bile is not a digestive enzyme but an emulsifying agent that breaks fat globules into small droplets exposing a greater surface area of fats to the action of digestive enzymes

SMALL INTESTINE

• Approximately 6-7m in length

• Three regions:– Duodenum: 25cm– Jejunum: 2.5m– Ileum: 4m

DUODENUM

• Secretions from the liver and pancreas enter and continue the chemical digestion of the chyme from the stomach

• Intestinal mucous glands release large quantities of mucus protecting the intestinal wall from protein-digesting enzymes and the acidic chyme

JEJUNUM/ILEUM

• Regions of the small intestine in which the majority of digestion and absorption of nutrients occurs

SMALL INTESTINE

• Glands in the mucous lining release the following enzymes– Peptidase completes protein digestion by

breaking down peptides into amino acids– Maltase, lactase, and sucrase split the

disaccharides maltose, lactose, and sucrose into monosaccharides (fructose, galactose, and glucose)

– Intestinal Lipase splits hydrolyzed lipids into glycerol and fatty acids

Absorption in the Small Intestine

• The duodenum is much shorter than the remaining parts of the small intestine—the jejunum and the ileum, which together average about 6 meters long

• By the time chyme enters these parts of the small intestine, much of the chemical digestion has been completed

• The chyme is now a rich mixture of medium and small nutrient molecules

The Small Intestine

• The lining of the small intestine consists of folds that are covered with tiny projections called villivilli

• Within each villus there is a network of blood capillaries and lymph vessels that absorb and carry away nutrients

• How do the folds in the small intestine help in absorption?

The Small Intestine


• The small intestine is specially adapted for the absorption of nutrients

• The folded surfaces of the small intestine are covered with fingerlike projections called villi ( singular: villus)

• The villi are illustrated in the figure at right

• The surfaces of the cells of the villi are covered with thousands of fingerlike projections known as microvillimicrovilli

• These folds and projections provide an enormous surface area for the absorption of nutrient molecules

• Slow, wavelike contractions of smooth muscles move the chyme along this surface


• Nutrient molecules are rapidly absorbed into the cells lining the small intestine

• Most of the products of carbohydrate and protein digestion are absorbed into the capillaries in the villicapillaries in the villi

• Molecules of undigested fat and some fatty acids are absorbed by lymph lymph vesselsvessels


• By the time food is ready to leave the small intestine, it is basically nutrient-free

• The complex organic molecules have been digested and absorbed, leaving only water, cellulose, and other undigestible substances behind

SMALL INTESTINE

• End products of digestion-amino acids, monosaccharides, glycerol, and fatty acids-are then absorbed into the circulatory system through blood and lymph vessels in the lining of the small intestine

• Highly folded inner lining is covered with millions of fingerlike projections called villi

• The cells covering the villi, in turn, have extensions on their cell membranes called microvilli

• The folds, villi, and microvilli give the small intestine a surface area of about 250 cm2, roughly the size of a tennis court

SMALL INTESTINE

• End products of digestion-amino acids, monosaccharides, glycerol, and fatty acids-are then absorbed into the circulatory system through blood and lymph vessels in the lining of the small intestine

• Highly folded inner lining is covered with millions of fingerlike projections called villi

• The cells covering the villi, in turn, have extensions on their cell membranes called microvilli

• The folds, villi, and microvilli give the small intestine a surface area of about 250 cm2, roughly the size of a tennis court

SMALL INTESTINE

• Nutrients are absorbed by diffusion and active transport

• Villus contains blood capillaries and tiny vessels called lacteals– Glycerol and fatty acids enter the lacteals, are carried through

the lymph vessels, and eventually reach the bloodstream through lymphatic ducts near the heart

– Amino acids and monosaccharides enter the capillaries and are carried to the liver (detoxifies the blood and removes any excess glucose,converting it to glycogen for storage) (filters the blood)

SMALL INTESTINE

• Most of the absorption of nutrients and water occurs– About 9L of water enters the small intestine,

but only about 0.5L of water is present in the material that leaves the small intestine entering the large intestine

LARGE INTESTINE

• Located at the junction between the small intestine and the ascending colon is the appendix– No known function in humans– In some herbivores, like rabbits, it functions in

cellulose digestion


• As the water, cellulose, and other undigestible substances leave the small intestine and enter the large intestine, they pass by a small saclike organ called the appendix

• In humans, the appendix appears to do little to promote digestion

• In other mammals, the appendix is used to store cellulose and other materials that the digestive enzymes cannot break down

• The only time you may pay attention to the appendix is when it becomes clogged and inflamed, causing appendicitis

• The only remedy for appendicitis is to remove the infected organ by surgery—as quickly as possible

LARGE INTESTINE

• Absorbs minerals and vitamins (K) produced by bacteria that live in the colon– Escherichia coli (E. coli): bacterial specie

that inhabits the colon• As long as they are in the colon, no harm• Any other body region , they could kill

• Absorbs any remaining water not absorbed by the small intestine solidifying the material into feces

The Large Intestine• When the chyme leaves the small

intestine, it enters the large intestine, or coloncolon

• The large intestine is shown in the figure• The primary function of the large

intestine is to remove water from the undigested material that is left

• Water is absorbed quickly across the wall of the large intestine, leaving behind the undigested materials

• Rich colonies of bacteria present in the large intestine produce compounds that the body is able to use, including vitamin K

– When large doses of antibiotics are given to fight an infection, they can destroy these bacteria and a vitamin K deficiency can occur

• The concentrated waste material that remains after the water has been removed passes through the rectum and is eliminated from the body

Large Intestine

• This barium X-ray shows the large intestine

Large Intestine

LARGE INTESTINECOLON

• Final organ of digestion (3 to 4 feet in length)

• Regions: – Ascending colon– Transverse Colon– Descending Colon– Sigmoid Colon– Rectum– Anal Canal

LARGE INTESTINE

• As feces solidifies, mucous secretions lubricate the wall making the passing of the feces less abrasive

• Mucus also binds the fecal matter together and neutralizes any acids that are released when bacteria act upon the feces

• Feces is eliminated through the anus by the of defecation

Digestive System Disorders

• The powerful acids released into the stomach sometimes damage the organ's own lining, producing a hole in the stomach wall known as a peptic ulcer– For years, physicians hypothesized that the primary cause of

ulcers was too much stomach acid– They prescribed drugs that suppressed acid production and

recommended bland, easily digested diets

• Scientists have since discovered that most peptic ulcers are Scientists have since discovered that most peptic ulcers are caused by the bacterium caused by the bacterium Helicobacter pyloriHelicobacter pylori– Doctors now know that many peptic ulcers are caused by an infectious

disease that can be cured– Thanks to powerful antibiotics, cure rates for peptic ulcers are as high

as 90 percent

Digestive System Disorders

• Other digestive system disorders include diarrhea and constipation

• When something happens that interferes with the removal of water by the large intestine, you usually become aware of it right away

• If not enough water is absorbed a condition known as diarrhea occurs

• If too much water is absorbed from the undigested materials, a condition known as constipation occurs

The Excretory System

• The chemistry of the human body is a marvelous thing• An intricate system of checks and balances controls

everything from your blood pressure to your body temperature

• Nutrients are absorbed, stored, and carefully released when they are needed

• However, every living system, including the human body, produces chemical waste products that are produces chemical waste products that are not useful to the bodynot useful to the body– In fact, some waste products are so toxic that In fact, some waste products are so toxic that

they will cause death if they are not eliminatedthey will cause death if they are not eliminated

Functions of the Excretory System

• You might think that homeostasis involves the body's efforts to respond only to changes in the external environment– However, homeostasis also requires the body to deal with internal However, homeostasis also requires the body to deal with internal

processes that might upset the internal cellular environmentprocesses that might upset the internal cellular environment

• For example, as a normal consequence of being alive, every cell in the body produces metabolic wastes, such as excess salts, carbon dioxide, and urea– Urea is a toxic compound that is produced when amino acids are Urea is a toxic compound that is produced when amino acids are

used for energyused for energy

• The process by which these metabolic wastes are eliminated is The process by which these metabolic wastes are eliminated is called excretioncalled excretion

• Excretion is one part of the many processes that maintain Excretion is one part of the many processes that maintain homeostasishomeostasis


• You have already learned about two organs of excretion—the skin and the lungs

• The skinThe skin excretes excess water and salts, as well as a small amount of urea, in the form of sweat

• The lungs excrete carbon dioxide, a gas produced when energy is captured from compounds in foods

ORGANS OF EXCRETION

• LUNGS: excretory organs that remove carbon dioxide and water vapor

• LIVER: excretory organ that– removes products of RBC breakdown by

means of the bile– Metabolizes amino acids producing urea

• Contains nitrogen• Poisonous to cells


• The liver, which we normally think of as a digestive organ, also plays a number of important roles in excretion

• When cells of the body break down proteins, excess amino When cells of the body break down proteins, excess amino acids are released into the bloodstreamacids are released into the bloodstream

• The liver takes up these amino acids and converts them into other useful compounds, producing nitrogen wastes in the process

• The liver quickly converts these potentially poisonous nitrogen compounds into ureaurea

• Urea, in turn, is removed from the bloodstream along with other Urea, in turn, is removed from the bloodstream along with other metabolic wastes by the body's principal organs of excretion, metabolic wastes by the body's principal organs of excretion, the kidneysthe kidneys

• The kidneys play an important role in maintaining homeostasisThe kidneys play an important role in maintaining homeostasis– They remove waste products from the blood; maintain blood pH; They remove waste products from the blood; maintain blood pH;

and regulate the water content of the blood and, therefore, blood and regulate the water content of the blood and, therefore, blood volumevolume

The Kidneys

• The kidneys are located on either side of the spinal column near the lower back

• A tube, called the ureter, leaves each kidney, carrying urine to the urinary bladder

• The urinary bladder is a saclike organ where urine is stored before being excreted

• The structures of the kidney are shown in the figure at right

ORGANS OF EXCRETION

• KIDNEYS:– Pair of bean shaped organs flanking the lower part of

the vertebral column– Filters urea and excess salts from the blood– Main excretory organ of the body– Approximately 180 liters of fluid is filtered through the

kidneys daily– Maintains proper water balance in the body– Excretes urea from the body in the form of urine

• Mostly water• Small amounts of dissolved urea, salts, and traces of other

substances

RENAL SYSTEM

• RENAL ARTERIES– Conducts blood into the kidneys

• Blood contains high amounts of urea and salts

• RENAL VEINS– Conducts purified blood away from the

kidneys• Blood contains less urea, salts, and water

KIDNEY

• INTERNAL STRUCTURE– RENAL CORTEX:

• Outermost layer• Filtration of blood and formation of urine

– RENAL MEDULLA:• Below the cortex• Filtration of blood and formation of urine

– RENAL PELVIS: • Large cavity• Collects urine which drains into ureter

Structure of the Kidneys

• Kidneys are made up of nephrons

• Blood enters the nephron, where impurities are filtered out and emptied into the collecting duct

• The purified blood leaves the nephron through the renal vein

Structure of the Kidneys

The Kidneys

• What does a kidney do?• As waste-laden blood enters the kidney

through the renal arteryrenal artery, the kidney removes urea, excess water, and other waste products and passes them to the ureter

• The clean, filtered blood leaves the kidney through the renal veinrenal vein and returns to circulation

Kidney Structure

• If a kidney is cut in half, two distinct regions can be seen

• The inner part is called the renal medulla• The outer part is called the renal cortex• The functional units of the kidney are called

nephronsnephrons– Each nephron is a small, independent processing

unit

• Nephrons are located in the renal cortex, except for their loops of Henle, which descend into the renal medulla

NEPHRON

• Millions of microscopic units in the renal cortex and renal medulla

• Structures:– Glomerulus: blood capillaries that are covered by a cup-like

structure called Bowman’s Capsule• Located in Renal Cortex

– Proximal Tubule:• Tube extending from Bowman’s Capsule to the loop of Henle

(Renal Medulla)– Distal Tuble:

• Tube returning from loop of Henle to the Renal Cortex connecting to the Collecting Duct

– Collecting Duct:• Runs from the Renal Cortex through the Renal Medulla and drains

urine into the Renal Pelvis

NEPHRON• FUNCTION:

– Membranes of the Glomeruli and Bowman’s Capsule are permeable to small molecules and ions

• Some water, ions, sugar, amino acids, and urea diffuse from the blood (glomerulus) into Bowman’s Capsule

– 180 liters of water and 600 grams of sodium filter into Bowman’s Capsule every 24 hours (body contains approximately 35-40 liters of water)

– As fluid travels from Bowman’s Capsule to the Collecting Duct, its composition changes

• Blood vessels surrounding the Tubules reabsorb many substances from the fluid in the tubules by active transport

– Requires energy– Glucose, amino acids, and sodium are returned to the blood– Much of the urea remains in the tubule– Water molecules follow the sodium by diffusion

» Body recovers over 99% of the water and sodium

– Renal Pelvis now contains urine which consists mainly of the water that has not been reabsorbed, some salts, and urea

Kidney Structure

• Each nephron has its own blood supply: an arteriole, a venule, and a network of capillaries connecting them

• In addition, each nephron releases fluids to a collecting duct, which leads to the ureter

• As blood enters a nephron through the arteriole, impurities are filtered out and emptied into the collecting duct

• The purified blood exits the nephron through the venule

• The mechanism of blood purification involves two distinct processes:– FiltrationFiltration– ReabsorptionReabsorption

Filtration

• Passing a liquid or gas through a filter to remove wastes is called filtration

• The filtration of blood mainly takes place in the glomerulusglomerulus

• The glomerulus is a small network of capillaries encased in the upper end of the nephron by a hollow, cup-shaped structure called Bowman's capsule

Filtration

• Because the blood is under pressure and the walls of the capillaries and Bowman's capsule are permeable, much of the fluid from the blood flows into Bowman's capsule

• The materials that are filtered from the blood are collectively called the filtratefiltrate– The filtrate contains water, urea, glucose, salts, The filtrate contains water, urea, glucose, salts,

amino acids, and some vitaminsamino acids, and some vitamins

• Because plasma proteins, cells, and platelets are too large to pass through the capillary walls, they remain in the blood

Reabsorption

• The kidneys filter all the blood in the body approximately every 45 minutes

• Needless to say, not all of the filtrate is excreted• Most of the material removed from the blood

at Bowman's capsule makes its way back into the blood

• The process in which liquid is taken back into a vessel is called reabsorptionreabsorption

Reabsorption

• A number of materials, including amino acids, fats, and glucose, are removed from the filtrate by active transport and reabsorbed by the capillaries

• Because water follows these materials by osmosis, almost 99 percent of the water that enters Bowman's capsule is reabsorbed into the blood– When the filtrate drains in the collecting ducts, most of

the water and nutrients have been reabsorbed into the blood

Reabsorption

• The material that remains, called urineurine, is emptied into a collecting duct

• UrineUrine, which contains urea, excess salts, and water, among other substances, is primarily concentrated in the loop of Henle

• The loop of Henleloop of Henle is a section of the nephron tubule in which water is conserved and the volume of urine minimized

URETER

• Two tubes (one on each side)

• Connects the kidney with the urinary bladder

• Urine passes through this tube from the kidney to the urinary bladder

URINARY BLADDER

• Storage organ for urine

Reabsorption

• As the kidney works, purified blood is returned to circulation while urine is collected in the urinary bladder

• Urine is stored in the urinary bladder until it can be released from the body through a tube called the urethraurethra

URETHRA

• Connects urinary bladder with the outside of the body

• Tube through which urine is voided from the body

Reabsorption • When blood is filtered through the kidneys, small molecules,

including salts, amino acids, sugars, and many drugs, are removed from the circulation

• Although many of these are reabsorbed into the bloodstream, drugs generally remain in the filtrate and are eliminated in the urine

• This is one of the principal reasons why the effects of many This is one of the principal reasons why the effects of many drugs, including antibiotics, wear off over timedrugs, including antibiotics, wear off over time

• This also means that drugs, legal and illegal, become concentrated in the urine, providing a quick and easy way to test for their presence

• Urine testing is now done routinely to check for the presence of prohibited drugs in athletes

• It has also become common for employers to use such tests to screen job applicants for illegal drug use

Kidney Stones

• Sometimes substances such as calcium, magnesium, or uric acid salts in the urine crystallize and form kidney stones

• When kidney stones block the ureter, they cause great pain

• Kidney stones are often treated using ultrasound waves– The sound waves pulverize the stones into smaller

fragments, which are eliminated with the urine

Control of Kidney Function

• To a large extent, the activity of the kidneys is controlled by the composition of blood itself

• In addition, regulatory hormones are released in response to the composition of blood

• These mechanisms combineThese mechanisms combine to ensure that the kidneys will maintain the proper composition of blood


• When you drink glass after glass of liquid, the liquid is quickly absorbed into the blood through the digestive system

• As a result, the concentration of water in the blood increases

• If it were not for your kidneys, this increased concentration of water in the blood would force water into cells and tissues by osmosis, causing your body to swell


• As the amount of water in the blood increases, the rate of water reabsorption in the kidneys decreases

• Less water is returned to the blood, and the excess water is sent to the urinary bladder to be excreted as urine


• If you eat salty food, your kidneys will respond to the increased level of salt in your blood

• When your kidneys detect an increase in salt, they respond by returning less salt to the blood by reabsorption

• The excess salt the kidneys retain is excreted in urine, thus maintaining the composition of the blood

HOMEOSTASIS

• All the cells of the body are surrounded by a watery tissue fluid which contains dissolved salts– Concentration of the salts must always be the same in order for

the body to function properly– Loss of excess water or salt can effect this concentration

• Body loses water in four ways/day– Exhalation: 1 liter– Feces: 100 ml– Perspiration: 50 ml– Kidneys: 1 liter

• Keeps the composition of tissue fluid more or less constant (kidneys are very important part of this process)

Homeostasis by Machine

• The kidneys are the master chemists of the blood supply

• If anything goes wrong with the kidneys, serious medical problems soon follow

• Fortunately, humans have two kidneys and can survive with only one

• If both kidneys are damaged by disease or injury, however, there are only two ways to keep an individual alive

• The first way is to transplant a healthy kidney from a compatible donor to the person in need of the kidney

Homeostasis by Machine• A second way is used when a donor is not available or surgery

is not advisable• In these instances, a kidney dialysis machine becomes a lifesaver• In a common form of dialysis, blood is removed from the body

through a tube inserted in the arm and pumped through special tubing that acts like nephrons

• Tiny pores in the tubing allow salts and small molecules, including nitrogen wastes, to pass through

• Wastes—urea and excess salts—diffuse out of the blood into the fluid-filled chamber, allowing purified blood to be returned to the body

• Dialysis is not only expensive, but it also is time-consuming, occupying several hours a day as often as three times a week

• The ideal solution, assuming a kidney transplant is not possible, would be the implantation of an artificial kidney

• Medical science is working toward developing such an artificial kidney

Kidney Dialysis

• For people with damaged kidneys, dialysis machines can perform many of the functions of the kidneys

• Why is dialysis such an important lifesaving technique?

Kidney Dialysis

Documents

Digestive and Excretory Systems The surface of your small intestine is covered with microvilli such as these (magnification: 100×)