Physiology 9 GI

GI system functions

Digestion, secretion, absorption, motility

Digestion

Breakdown of macromolecules into absorbable molecules

Secretion

Release of substances such as enzymes, acid, bile, mucus, and bicarbonate

Absorption

Movement of nutrients into blood or lymph

Motility

Mixing and movement of GI contents

Accessory organs

Salivary glands, liver, gallbladder, exocrine pancreas

GI tract organs

Mouth, pharynx, esophagus, stomach, small intestine, large intestine

Four GI wall layers

Mucosa, submucosa, muscularis externa, serosa

Function of mucosa

Protection, secretion, absorption

Function of submucosa

Contains blood vessels, lymphatics, and nerves

Function of muscularis externa

Motility and mixing

Function of serosa

Outer protective covering

Three adaptations that increase surface area

Circular folds, villi, microvilli

Goblet cells

Secrete mucus

Brush border

Microvilli containing digestive enzymes

Lacteals

Absorb dietary fats into lymph

Salivary amylase function

Begins carbohydrate digestion

Esophagus function

Peristalsis and mucus secretion

Stomach secretions

HCl, pepsinogen, intrinsic factor, mucus

Chyme

Semi-liquid mixture leaving the stomach

Small intestine function

Major site of digestion and absorption

Large intestine function

Absorbs water and salts; stores feces

Salivary amylase location

Mouth

Pancreatic amylase location

Small intestine

Fiber digestion

Fiber is not digested by human enzymes

Pepsin function

Begins protein digestion in stomach

Trypsin location

Small intestine

Chymotrypsin function

Protein digestion in small intestine

Carboxypeptidase function

Removes amino acids from protein ends

Lipase function

Digests triglycerides

Emulsification

Breaks fat into smaller droplets

Bile salts function

Prevent fat droplets from re-forming

Micelles

Carry lipids to intestinal cells

Chylomicrons

Transport absorbed fats into lymph

Fat-soluble vitamins

A, D, E, K

Vitamin B12 absorption

Requires intrinsic factor

Stimuli regulating digestion

Distension, acidity, chyme, solute concentration

Enteric nervous system

Local nervous system controlling GI tract

Short reflexes

Occur entirely within ENS

Long reflexes

Involve CNS

Parasympathetic effect on digestion

Stimulates digestion

Sympathetic effect on digestion

Inhibits digestion

Gastrin stimulus

Amino acids, peptides, parasympathetic stimulation

Gastrin actions

Increases acid secretion and stomach motility

CCK stimulus

Fatty acids and amino acids

CCK actions

Stimulates pancreatic enzymes, contracts gallbladder, decreases stomach motility

Secretin stimulus

Acid entering duodenum

Secretin actions

Stimulates bicarbonate secretion and inhibits acid secretion

GIP stimulus

Glucose and fat

GIP action

Stimulates insulin secretion

Cephalic phase

Sight, smell, taste, emotions

Gastric phase

Distension and gastrin release

Intestinal phase

Chyme triggers CCK, secretin, and GIP

Parietal cells

Produce HCl and intrinsic factor

Chief cells

Produce pepsinogen

Mucous cells

Produce protective mucus

ECL cells

Produce histamine

D cells

Produce somatostatin

Intrinsic factor function

Required for vitamin B12 absorption

Pepsinogen activation

Converted to pepsin by acidic pH

Receptive relaxation

Allows stomach expansion after eating

Retropulsion

Backward mixing movement in stomach

Pancreatic bicarbonate

Neutralizes acidic chyme

Pancreatic enzymes

Trypsin, chymotrypsin, elastase, carboxypeptidase, lipase, amylase, nucleases

Primary stimulus for bicarbonate secretion

Secretin

Primary stimulus for pancreatic enzymes

CCK

Components of bile

Bile salts, lecithin, bicarbonate, cholesterol, bilirubin, trace metals

Site of bile salt recycling

Ileum

Migrating myoelectric complex

Peristaltic waves between meals

Colon sections

Ascending, transverse, descending

Functions of gut bacteria

Ferment fiber, produce vitamins, produce gas

External anal sphincter

Skeletal muscle

Heartburn cause

Weak lower esophageal sphincter

Ulcer cause

Helicobacter pylori

Cystic fibrosis GI effect

Thick pancreatic secretions due to CFTR mutation

Gallstones

Crystallized cholesterol in gallbladder

Jaundice

Bilirubin accumulation in blood

Diarrhea

Excessive water loss

Constipation

Commonly improved by increased fiber intake

Fat malabsorption deficiency

Vitamins A, D, E, and K