CHAPTER 13: THE DIGESTIVE SYSTEMMS. LUACES – HONORS ANATOMY & PHYSIOLOGY

CSI CASE #13 – PG. 488READ & ANSWER QUESTION AT THE END

OVERVIEW▪ The digestive system allows the body to break down

complex molecules into simple ones needed for body energy, tissue and cell development

▪ Made up of 2 components:▪ The digestive tract (alimentary canal): Mouth, esophagus,

stomach, small intestine, large intestine and rectum▪ Accessory digestive organs (produce secretions to help with

digestion and food absorption): salivary glands, pancreas, liver, and gallbladder

OVERVIEW▪ The digestive system is one of the first organs to develop from the endoderm during

week 4 of development – gastrulation▪ Accessory organs don’t begin to develop until week 5 (liver and pancreas) up until week

40 (liver). The liver only produces blood cells for the fetus at this time▪ At birth, the digestive system is fully formed and releases meconium (waste product –

green/black, sticky substances made of bile, mucus, and epithelial cells)

MOUTH & PHARYNX▪ The mouth is involved in both chemical (saliva) and mechanical

(breakdown) digestion

▪ The anterior portion is made up of the lips (labia) which hold sensory receptors that detect temperature and texture of food

▪ Inside you find the oval (buccal) cavity, housing the teeth and where food is moistened

MOUTH & PHARYNX▪ The palate forms the roof of the buccal cavity ▪ Hard palate (roof – maxillary and palatine bones)▪ Soft palate (posterior back portion – contains uvula)

▪ At the base of the buccal cavity, we have the Oropharynx ▪ Contains the epiglottis – separates the respiratory system from the pharynx▪ Tongue (lingual membrane) – contains taste buds, 4 intrinsic muscles, and 3 extrinsic

muscles▪ Intrinsic muscles allow the tongue to take a variety of shapes, assist with speech and swallowing

while extrinsic muscles form most of the muscle mass and allows the tongue to retract, raise, and depress the tongue

MOUTH & PHARYNX▪ Lingual tonsils are found at the base of the tongue and are lymphatic tissues to fight

throat infections

▪ Salivary glands release saliva and moisten food to start chemical digestion▪ 3 major salivary glands

▪ Parotid: in cheeks; largest; watery section

▪ Sublingual: underneath the tongue; mucous section

▪ Submandibular: under the mandible; mixture of water and mucous

MOUTH & PHARYNX▪ Mechanical breakdown of food involves the 2 jaw bones, hard palate,

and the teeth▪ Permanent teeth consist of 16 teeth in each jaw – 32 total▪ 4 central teeth in each jaw bone – incisors (cut up food)▪ 4 canine (cuspid) teeth – hold and tear food▪ 8 bicuspids (premolars) – break down food into fine particles▪ 6 molars – grind food▪ Extra 2 molars (wisdom teeth) – farthest back ; may/may not develop

between 18-20 years old depending on space left

ESOPHAGUS & STOMACH▪ The esophagus:▪ Located behind the epiglottis▪ Muscular tube that carries food & liquids to the stomach▪ Has an upper sphincter (closes the larynx during swallowing) and a lower sphincter –

cardiac sphincter (closes entrance to stomach)▪ Has 4 distinct tissue layers: mucosa, submucosa, muscularis layer, and serosa/adventitia

ESOPHAGUS & STOMACH▪ Mucosa: layer of epithelium that secretes mucus▪ Innermost layer▪ Connected to lamina propria and has a boundary before

submucosa made of smooth muscle and connective tissue called muscularis mucosae layer – thicker in the esophages to assist with passage of food

▪ Submucosa: layer of connective tissue that supports the shape of the esophagus

▪ Muscularis Layer: layer of smooth muscles to contract and move food / liquid through the digestive tract

▪ Serosa / Adventitia: thick covering of connective tissue filled with nerves to send signals to the muscles

ESOPHAGUS & STOMACH▪ Sphincters prevent backflow of material into previous organs▪ Cardiac sphincter (at base of esophagus) prevents acid reflux (heartburn)▪ Pyloric sphincter closes off the bottom of the stomach

ESOPHAGUS & STOMACH▪ The stomach:▪ Large, sac-like organ with 4 layers that can store 3 pints of food for digestion▪ Stomach’s mucosa secretes acid and protein digestion enzymes and is designed to protect

the stomach from digesting itself▪ Divided into upper (cardiac), middle (fundic) and lower (pyloric) regions

ESOPHAGUS & STOMACH▪ Cardiac region:▪ Secretes mucus

▪ Fundic region:▪ Contains parietal cells (produce hydrochloric acid), chief cells (produce digestive

enzymes), mucous neck cells (produces mucus when stimulated) and gastric stem cells (replaces gland cells)

▪ Pyloric region:▪ Also secretes mucus

ESOPHAGUS & STOMACH▪ The stomach is composed of 3 thick layers of smooth

muscle:▪ Oblique: directly underneath the submucosa

▪ Stomach squeezing

▪ Circular: thick bands of muscle that mix food▪ Longitudinal: mix and move digested food out of the

stomach

SMALL INTESTINE▪ Long narrow tube that tightly loops back and forth

▪ Composed of the:▪ Duodenum: receives partially digested food; where most digestion takes place▪ Jejunum: where nutrients are absorbed▪ Ileum: remaining nutrients are absorbed▪ All areas contain fingerlike projections called villi and microvilli (increase surface area for

absorption) and lacteals (collections of lymphatic vessels)

SMALL INTESTINE▪ Most common cells found in the small intestine are the enterocytes (absorptive

cells) – found mostly in the jejunum

▪ Enteroendocrine cells produce hormones that regulate digestion

▪ Paneth cells found in mucosa areas called crypts produce antibacterial enzymes that kill harmful bacteria but promote beneficial microorganism growth

▪ Peyer’s patches (collections of lymphatic tissue) in the duodenum help fight infection and are involved in immune response to food allergies

SMALL INTESTINE▪ Composed of an inner circular layer (mixing food) and outer longitudinal layer

(transport of food) of muscle

▪ Contains many extensive networks of nerves to send signals

▪ Attached to mesenteries to hold it in place and are also places for blood vessels to innervate

▪ Separated from the large intestine by the ileoceccal valve

LARGE INTESTINE▪ Also known as the colon – much larger in diameter and

shorter than small intestine

▪ Contains:▪ Cecum: small, swollen region that contains the appendix

(common site of infection - appendicitis)▪ Ascending colon: travels vertically up on right side to hepatic

flexure (below liver)▪ Transverse colon: runs parallel and connects at the splenic flexure

(below spleen)▪ Descending colon: travels vertically down on left side▪ Sigmoid colon: s-shape end that spills into the anal canal

LARGE INTESTINE▪ Main functions: absorb electrolytes, vitamins and

water ; remove undigested materials

▪ Contains no villi but many absorptive cells that use active transport to move electrolytes into its many capillaries

▪ Contains many bacteria and a type of yeast to help break down waste – essential!

LARGE INTESTINE▪ Ends in the rectum, a muscular storage area for undigested wastes

▪ Anal canal is the transitional region to the anus, which is rich in apocrine and eccrine sweat glands

▪ The anal sphincter has internal and external bands of muscle

CSI – CASE BREAK▪ Knowing the major parts of the alimentary canal (digestive tract) now, think about

what area of this is malfunctioning in our patient. Turn to Pg. 503 and read the CSI Case Break, and respond in your notes

GLANDULAR STRUCTURES: PANCREAS▪ Lies posterior to the stomach and divided into glandular lobules surrounded by

connective tissue that is each connected to blood vessels, nerves and ducts

▪ Has both endocrine and exocrine functions▪ Endocrine: islets of Langerhans, which secrete glucagon – raises blood glucose (alpha

cells) and insulin – lowers blood glucose (beta cells)▪ Exocrine: saclike cluster of serous glands called acini – have granules with active and

inactive digestive enzymes

GLANDULAR STRUCTURES: PANCREAS▪ Exocrine (cont): ▪ Inactive enzymes: zymogens, converted into active enzymes

once they enter the digestive tract▪ Ductal system of the pancreas originates in the acini that feeds

into large duct called pancreatic duct common bile duct duodenum

▪ The yellow-green bile contains acids, cholesterol, glyceride fats, and salts to help in fat digestion▪ Cholesterol is secreted from the liver through bile (also contains

bilirubin, pigments, and broken down RBC’s)

GLANDULAR STRUCTURES: LIVER▪ Carries out the most complex functions using 4 lobes: left and right (make up most

of the mass) and quadrate and caudate lobe. ▪ Left & right lobe separated by the falicform ligament

GLANDULAR STRUCTURES: LIVER▪ Two blood vessels enter the liver at the hilum▪ Hepatic artery: provides blood for the liver cells▪ Hepatic vein: carries wastes from the liver lobes

to the inferior vena cava▪ Haptic portal vein transfer food from the small

intestine to the liver

GLANDULAR STRUCTURES: LIVER▪ Composed of hepatocytes: unusual cells with 2 or more nuclei that live about 5

months and are slow at carrying out mitosis

▪ Capable of regeneration (small pieces of liver have regrown lobes)

▪ Arteries going to the hepatocytes empty into sinusoids (exchange atmospheric gases, nutrients and wastes) and contain Kupffer cells that remove microorganisms

▪ Please refer to Pg. 507 for some of the many jobs the liver does

GLANDULAR STRUCTURES: GALLBLADDER

▪ Located underneath the right lobe of the liver

▪ Receives bile through the bile ductwork and releases it through the cystic duct, where it joins the common hepatic duct to form the common bile duct into the small intestine

▪ Main job: store and concentrate bile▪ If bile has too much bilirubin, cholesterol, or bile salts, bile

flow is blocked by the formation of gallstones (very painful!)

THE DIGESTIVE PROCESS▪ Pre-gastric factors (hunger and thirst) are released from the hypothalamus to signal

the beginning of digestion▪ Attitudes about eating and one’s upbringing help determine a person’s food intake

▪ Satiety center (also in the hypothalamus) will later signal to stop eating, depending on blood sugar levels, fat levels, appearance and taste of food▪ Abnormalities in this hypothalamus area can trigger certain eating disorders

THE DIGESTIVE PROCESS▪ Digestion begins with ingestion (taking in food through the mouth)▪ If taken by injection into muscles of veins, known as parenteral nutrition (IV

fluids or food)

▪ Mastication (chewing) is the first step in mechanical digestion, while chemical digestion also begins as saliva and salivary amylase enzyme begin to breakdown starch into glucose

▪ Foods and liquids travel down the esophagus by peristalsis (smooth-muscle contractions) and enter the stomach when the cardiac sphincter relaxes▪ Reverse peristalsis (vomiting) can occur if bitter receptors triggered

THE DIGESTIVE PROCESS▪ Pregastric factors and parasympathetic nervous system release the following

stomach secretions before and when food enters the stomach:▪ Protease zymogens (become proteases)▪ Hydrochloric acid▪ Proteases (digest protein – require the acidic environment of pH 1-3 in order the work)▪ Bicarbonate and mucus (to protect stomach from digesting itself)

THE DIGESTIVE PROCESS▪ Hormones released by the stomach and small intestine further stimulate the

digestive system:▪ Cholecystokinin (CCK): stimulates pancreas to produce pancreatic juice and gallbladder to

empty▪ Gastrin: produces hydrochloric acid in the stomach (assist with protein digestion)▪ Secretin: stimulates pancreas to secrete digestive juices including bicarbonate, stomach to

produce protease, and liver to produce bile

THE DIGESTIVE PROCESS▪ Chyme enters the duodenum after the pyloric sphincter

relaxes, and the duodenum mixes the food with bile (breaks down fat) and pancreatic secretions (breaks down carbs, lipids, and proteins).

▪ Enterokinases activate zymogens, which aid the absorption of nutrients into the blood and lacteals via the villi in the jejunum▪ Proteins and carbs go to the liver through the hepatic portal system

for further digestion into ammonia

THE DIGESTIVE PROCESS▪ Any substance not absorbed passes the ileocecal valve enters the large intestine,

where bacteria digest and absorb much of the material (this produces flatulence)

▪ Peristalsis continues to transport undigested material, while fluids and electrolytes are reabsorbed in the ascending and transverse colon

▪ The final waste is compacted into feces (made of bacteria, carbohydrate polymers, fat, etc) in the descending and sigmoid sections of the colon, where mucus is secreted to aid in the expulsion by muscular contractions through the rectal sphincter