Lecture 24: GI Physiology

What organs make up the alimentary canal?

Mouth, esophagus, stomach, small intestine, large intestine, rectum, and the anus

What are the accessory digestive organs?

Salivary glands, liver, gallbladder, pancreas.

The alimentary canal is approximately ______ feet long

30

The GI tract is divided by ______ and ______ that regulate flow.

Sphincters; valves.

T/F: The pyloric valve separates the ileum from the cecum.

False: the ileocecal valve separates the ileum from the cecum; the pyloric valve separates the stomach from the duodenum.

Which three accessory organs are shown connected to the duodenum?

Liver, gallbladder, and pancreas.

Which structure is shown between the stomach and small intestine?

The pyloric sphincter

Which organ is shown inferior to the liver and stores bile?

Gallbladder

How many liters of fluid enter the GI tract daily?

9

How many liters are absorbed by the small intestine?

7.5

Daily ingestion contributes ______ liters of fluid to the GI tract.

2

Daily secretions contribute ______ liters of fluid to the GI tract.

7

T/F: Most GI secretions are reabsorbed.

True: 9L enter and 9L are reabsorbed

Which secretion contributes the largest volume to GI fluid?

Gastric secretions (2.0 L/day).

What is the definition of secretion?

Movement of substances from cells into the lumen or ECF

What is the definition of digestion?

Chemical and mechanical breakdown of food into absorbable units.

What is the definition of absorption?

Movement of substances from lumen to ECF.

Motility is the movement of ______ through the GI tract.

Chyme

T/F: Excretion removes large amounts of metabolic waste.

False: fecal matter contains very little metabolic waste; most is undigested material and bacteria.

Which macromolecule is broken down into amino acids?

Proteins

Which macromolecule is broken down into monosaccharides?

Carbohydrates

Which macromolecule is broken down into nucleotides?

Nucleic acids

What do lipids break down into?

Monoglycerides and free fatty acids

Which enzymes break down polysaccharides?

Carbohydrate‑digesting enzymes.

Which products of fat digestion enter absorption?

Glycerol and fatty acids

Which digestive enzymes are bound to apical membranes?

Brush‑border enzymes

Which nutrient type is absorbed into lacteals?

Fats

What are the four layers of the GI tract wall?

Mucosa, submucosa, muscularis externa, serosa

What are the three components of the mucosa?

Epithelium, lamina propria, and muscularis mucosae.

The submucosa contains the ______ plexus.

submucosal

The muscularis externa contains the ______ plexus.

Myenteric

T/F: The serosa is continuous with the mesentery

True

Which structures increase surface area by 600×?

Villi, crypts, and microvilli

Which immune structure is shown in the mucosa?

Peyer’s Patch

Which vessel type absorbs fats?

Lacteals

Which cells secrete mucus?

Goblet cells

Which cells secrete hormones?

Enteroendocrine cells

What type of epithelium lines the small intestine?

Simple columnar epithelium with microvilli.

What is the function of enteroendocrine cells?

Secrete hormones.

Stem cells in the crypts replace ______ cells.

damaged

The brush border is formed by ______.

Microvilli

T/F: Crypt cells secrete digestive enzymes.

False: they secrete ions and water; enzymes come from enterocytes or pancreas.

Which vessels run inside each villus?

Capillaries and lacteals

Which cells are shown transporting nutrients?

Enterocytes

Which cells divide to replace the epithelium?

Stem cells in crypts

What type of muscle makes up the muscularis externa?

Smooth muscle

Which nervous system provides input to the myenteric plexus?

ANS

The myenteric plexus primarily regulates ______.

Motility

The submucosal plexus primarily regulates ______.

secretion

T/F: The serosa is a thick muscular layer.

False: it is a thin connective tissue layer continuous with the peritoneum.

Which layer of the stomach contains the oblique muscle?

Muscularis externa.

Which layer of the stomach contains the gastric glands?

Mucosa

Which layer of the stomach contains the submucosal blood vessels?

Submucosa

Which stomach layer is the outermost?

Serosa

Which muscle layer in the stomach is responsible for peristalsis?

Circular and longitudinal layers of the muscularis externa.

What are the three modes of GI communication?

Endocrine, paracrine, and neurocrine.

What do endocrine cells in the gut “taste”?

Luminal contents

Histamine is released from ______ cells.

Enterochromaffin‑like (ECL) cells.

Somatostatin is released from ______ cells.

D

T/F: Paracrine signals enter the bloodstream before reaching their target.

False; they act locally in the interstitial fluid.

Where do endocrine hormones travel to reach their targets?

Through the bloodstream

What is the immediate environment where paracrine signals act?

Interstitial fluid

Which structure releases neurocrine transmitters?

Neurons

Which type of cell receives neurocrine signals?

Smooth muscle of gland cells

What are the two major plexuses of the ENS?

Myenteric and submucosal plexus

What type of neurons are found in the ENS?

Sensory, interneurons, and motor neurons.

The ENS can function independently of the ______.

CNS

The ENS increases the ______ of rhythmic contractions.

Intensity.

T/F: The ENS inhibits sphincter contraction.

True

Which plexus lies between the circular and longitudinal muscle layers?

Myenteric plexus

Which neurotransmitter is know to be inhibitory?

Norepinephrine

Which neurotransmitter is known to be excitatory?

Acetylcholine

Which cells detect luminal stimuli

Sensory neurons in the mucosa

What are slow‑wave potentials?

Cyclical depolarizations in GI smooth muscle.

What cells generate slow waves?

Interstitial cells of Cajal

Action potentials occur when slow waves exceed ______.

Threshold

The force of contraction depends on the number of ______.

Action potentials

T/F: Slow waves always cause contractions.

False: only slow waves that reach threshold trigger action potentials and contractions.

What happens when slow waves do not reach threshold?

No action potentials; no contraction.

What increases the amplitude of slow waves?

Parasympathetic stimulation (ACh).

What decreases the amplitude of slow waves?

Sympathetic stimulation (NE).

Which type of contraction lasts minutes to hours?

Tonic contractions

Which type of contraction lasts only seconds?

Phasic contractions.

What is a short reflex?

A reflex contained entirely within the ENS.

What is a long reflex?

A reflex involving the CNS.

The gastrocolic reflex increases ______ after eating.

Colon motility

The enterogastric reflex inhibits ______ activity.

Gastric

T/F: The vagovagal reflex involves both afferent and efferent vagus fibers.

True

Which organ signals the colon to contract after eating?

Stomach

Which phase of digestion begins with sight, smell, or thought of food?

Cephalic phase

Which system mediates the cephalic phase?

Parasympathetic nervous system.

What cells release gastrin?

G cells in the antrum and duodenum

What stimulates gastrin release?

Peptides, amino acids, stomach distention, GRP, ACh

Gastrin is inhibited by ______.

Somatostatin

Gastrin’s primary target cells are ______ and ______.

ECL cells; parietal cells

T/F: Gastrin is less potent than histamine at stimulating acid secretion.

False: gastrin is ~1500× more potent than histamine

Which neurotransmitter stimulates G cells?

GRP (gastrin‑releasing peptide)

Which cell releases histamine in response to gastrin?

ECL cell

Which receptor on parietal cells binds histamine?

H₂ receptor

Which cell releases somatostatin to inhibit gastrin?

D cell