Chapter 26 Digestive System

What are the two main parts of the digestive system?

Digestive tract (alimentary canal): about 30 feet long in adult cadaver

Accessory digestive organs: help with digestion but food does not pass through them directly (produce or store substances like bile, enzymes, or saliva)

What are the six main functions of the digestive system

Ingestion - food enters the body

Motillity - muscular contractions move food

Secretion - disgestive fluids released

Digestion - mechanical and chemical breakdown of food

Absorption - movement of nutrients into blood or lymph

Elimination - removal of indigestible materials as feces

What is muscularis mucosae

A thin layer of smooth muscle located in the mucosa

What does muscularis externa do

Handles peristalsis and segmentation; muscularis mucosae tweaks villi shape to aid absorptioin

What are the four tunics of the GI tract wall (inner → outer)

Muscosa: inner layer for absorption and secretion

Submucosa: connective tissue with blood vessels, lymph, and nerves

Muscularis: responsible for motillity (peristalsis and mixing)

Serosa and Adventitia: outermost layer; protection and structural support

What is found in the submucosa?

Areolar and dense irregular connective tissue containing blood vessels, lymph vessels, and the submucosal nerve plexus, which regualates local blood flow, absorption, and secretion

What GI tract layer contains Peyer patches and MALT

Submucosa

*Tip: “sub” like “support”— supports immune defenses and nutrient transport

What is MALT and what are Peyer patches

MALT: Mucosa-associated lymphatic tissue that protects against pathogens

Peyer patches: clusters of lymphoid tissue in the ileum that monitor intestinal contents for pathogens

What is the enteric nervous system (ENS)

Sensory and motor neurons in the GI wall (submucosal and myenteric plexuses) that control smooth muscle and glands — coordinates mixing and propulsion reflexes

How do the autonomic nervous system branches affect disgestion

Parasympathetic: promotes GI activity

Sympathetic: inhibits GI activity

Which receptors initiate digestive reflexes

Baroreceptors: detect stretch in GI wall

Chemoreceptors: monitor chemical contents in the lumen

What hormones regulate digestion

Gastrin, secretin, cholecystokinin (CCK), and motillin

What is peritoneum

A serous membrane lining in the abdominopelvic cavity; allows organs to move freely

• parietal peritoneum: lines abdominal wall

• visceral peritoneum: covers organ surfaces

• peritoneal cavity: space between them containing lubricating fluid

What is the difference between intraperitoneal and retroperitoneal organs

Intraperitoneal: completely surrounded by visceral peritoneum (stomach, small intestine)

Retroperitoneal: lie against posterior wall, only front covered (pancreas, duodenum, rectum)

Name the serous membrane folds (omentum and ligaments) and their roles

• falciform ligament: supports and anchors liver

• round ligament: provides structural support and helps blood flow regulation

• lesser omentum: supports and protects

• greater omentum: protects, insulates, stores fat, supports immunity, aids wound healing

What organs make up the upper GI tract

Oral cavity, salivary glands, pharynx, esophagus, and stomach (duodenum also considered part of upper GI)

What are the functions of oral cavity and salivary glands

• mechanical digestion begins

• saliva moistens food and starts starch digestion via salivary amylase

• forms bolus for swallowing

What are functions and composition of saliva

• 99.5% water + solutes (salivary amylase, mucin, lysozyme)

• moistens food, dissolves tastants, cleanses oral cavity, contains antibacterial agents (lysozyme, IgA)

How many deciduous teeth and permanent teeth are there

20 deciduous (baby) teeth, 32 permanent teeth replacing them

What is the function of teeth

Perform mechanical digestion by breaking down food into smaller pieces

What is the role of pharynx

Moves the bolus from the mouth to the esophagus; mucus secretion facilitates swallowing

What is the role of the esophagus

Transports bolus to the stomach using peristalsis; lubricated by mucus

What happens in the stomach

Bolus mixes with gastric secretions, forming chyme; smooth muscle contractions aid mixing

What structure controls chyme exiting the stomach

Pyloric sphincter

tip: pylorus= gateway between stomach and small intestine

What three segments of the small intestine and their functions

Duodenum: receives chyme and secretions from liver, gallbladder, and pancreas (NOT from large intestine because it comes after)

Jejunum: site for most chemical digestion and nutrient absorption

Ileum: ends at the ileocecal valve; continues absorption, connects to large intestine

What movement types occur in the small intestine

Segmentation: mixing chyme with secretions

Peristalsis: propelling food forward

What are the functions of the liver

• regeneration

• filters blood and detoxifies

• metabolism

• forms clotting factors

• stores vitamins (A, E, K, B12) and minerals (iron, copper)

What are the basic functional units of liver called?

Hepatic lobules

What are functions of the pancreas

• Exocrine (digestive): produces enzymes —- amylase (carbs), lipase (fats), proteases (proteins), nucleases (nucleic acids)

• Endocrine (hormonal): releases insulin and glucagon to regulate blood sugar

What do cholecystokinin (CCK) and secretin do?

• CCK: Released in response to fatty chyme → stimulates gallbladder contraction, pancreatic enzyme release, and relaxes hepatopancreatic ampulla.

• Secretin: Released in response to acidic chyme → stimulates bicarbonate secretion from liver/pancreas and inhibits gastric motility.

What are the main functions of the large intestine

• absorbs water, electrolytes, and vitamins (B and K)

• Houses normal bacterial flora (microbiota)

• Forms and stores feces for elimination

What do the intestinal glands (crypts) secrete

Mucin, which lubricates undigested material and protects the lining

What vitamins are produced by bacteria in the large intestine

B vitamins and vitamin K

What are common digestive disorders 

• constipation: slow motility; dry, compacted feces

• diarrhea: disrupted water absorption; excess fluid in feces

What enzymes break down carbohydrates in the small intestine

• pancreatic amylase

• brush border enzymes: dextrinase, glucoamylase, maltase, lactase

What enzyme digests protein in the stomach

Pepsin (activated by HCl from parietal cells)

What enzymes digest lipids

Pancreatic lipase (requires bile salts for emulsification)

What enzymes digest nucleic acids

Deoxyribonuclease and ribonuclease (from pancreas) plus brush border enzymes phosphatase and nucleosidase

Bile Canaliculi

small channels between hepatocytes that collect bile, flow into bile ducts, gallbladder, duodenum

think: canaliculi=canals for bile

Indigenous Microbiota

In the large intestine produce vitamins B and K, and ferment undigested materials

remember B & K made by bacteria

Parietal cells

Secrete hydrochloric acid (HCl) + Intrinsic factor (for B12 absorption)

remember parietal = acid + absorption helper