Ch7 Digestive System

Gastrointestinal (GI) tract

• The GI tract runs from mouth to anus.
• The lumen is external to body tissues (contents are not “inside” the body until absorbed).

Major segments

• oral cavity (mouth)
• pharynx
• esophagus
• stomach
• small intestine
• large intestine
• anus

Accessory organs

• salivary glands
• liver
• gallbladder
• pancreas

Core functions

Motility

Movement through the tract.

Secretion

Secretion of digestive enzymes into the lumen of the GI tract.

Digestion

Enzymes break down large molecules into smaller ones.

Absorption

Small molecules move from lumen → circulatory system.

Motility and muscle types

• GI movements do not require motor neuron activity to occur (smooth muscle can respond to regulation but does not require motor neurons).

Where skeletal vs smooth muscle is found

• mouth, jaw, top 1/3 esophagus: skeletal muscle, some voluntary
• rest of esophagus, stomach, intestines: smooth involuntary muscle
• remaining: skeletal voluntary

Swallowing

Oral phase (voluntary)

• bolus of food forced into pharynx
• soft palate lodges against pharynx

Pharyngeal phase (involuntary)

• mechanoreceptors initiate reflex in brainstem
• epiglottis closes
• upper esophageal sphincter relaxes

Esophageal phase (involuntary)

• esophageal sphincter contracts
• peristaltic contractions move bolus to stomach

Motility patterns

Peristalsis

• rhythmic wave of smooth muscle contraction that moves bolus through the GI tract

• occurs in:

• esophagus

• stomach
• small intestine
• large intestine

Segmentation

• mixing movement
• pinching of intestine into compartments and subsequent mixing of undigested material
• no net movement

• occurs in:

• small intestine

• large intestine

Saliva

Composition :

• enzymes, mucus, water

Functions:

• moistens food
• amylase and lipase begin digestion of carbohydrates and triglycerides
• lysozyme: antimicrobial

Stomach

Functions:

• holds ingested food (chyme-food)
• passes chyme in small increments to the small intestine (via peristaltic waves)
• kills most microbes
• degrades food physically
• begins digestion of proteins with pepsin
• secretes intrinsic factor for absorption of vitamin B12

Hydrochloric acid (HCl)

HCl does not digest chyme, but:

• kills most microbes (acidity)
• denatures proteins and inactivates enzymes in food (acidity)
• breaks down plant cell walls and connective tissues in meat (acidity)
• activates pepsinogen → pepsin

Secretion of HCl (parietal cell)

Apical (toward lumen):

• \(H^+/K^+\) ATPase (moves \(H^+\) to lumen, \(K^+\) to cell)
• \(K^+\) channel (apical; flows toward lumen)
• \(Cl^-\) channel (apical; flows toward lumen)

Within parietal cell:

• \(CO_2\) and \(H_2O\) convert to \(H_2CO_3\) with carbonic anhydrase
• \(H^+\) flows to lumen
• \(HCO_3^-\) flows to capillary

Basolateral (toward capillary):

• \(HCO_3^- / Cl^-\) exchanger (AE2): \(HCO_3^-\) out of cell, \(Cl^-\) into the cell

**Summary **

• \(Cl^-\) enters from basolateral side → lumen of stomach
• \(HCO_3^-\) goes to the capillary
• \(H^+\) goes to the lumen of stomach

Small intestine

Key points:

• secretions from liver and pancreas enter the small intestine
• most absorption occurs here
• lining has villi and microvilli
• internal surface is the brush border

Digestion and absorption of macronutrients

Carbohydrates

Enzymes by location

• salivary amylase (salivary glands, mouth): carbohydrates → disaccharides
• pancreatic amylase (pancreas): carbohydrates → disaccharides
• intestinal wall (brush border enzymes): disaccharides → monosaccharides

Absorption

Apical side (brush border):

• polysaccharides → (pancreatic amylase) → disaccharides → (brush border enzymes) → monosaccharides
• fructose enters via GLUT5
• glucose and galactose enter with Na\(^+\) via SGLT1

Basolateral side:

• Na/K pump
• monosaccharides exit via GLUT2
• \(K^+\) channel allows \(K^+\) to flow out

Proteins

Major enzymes

• pepsin:

• released from pepsinogen (stomach wall)

• activated by HCl
• breaks proteins → small chain amino acids

• trypsin:

• released from trypsinogen (pancreas)

• activated by enteropeptidase (secreted by enteroendocrine cells in small intestine lining)
• breaks proteins → small chain amino acids
• activates other digestive enzymes

Absorption

• amino acid small chains absorbed via transporters

Fats (triglycerides)

Enzymes:

• salivary lipase (salivary glands): triglycerides → monoglycerides + fatty acids (absorbed by small intestine)
• pancreatic lipase (pancreas): triglycerides → monoglycerides + fatty acids (absorbed by small intestine)

Bile salts:

• released by liver
• emulsify fats from large globules to small globules
• have \(-OH\) regions that prevent small globules from re-forming large ones

Large intestine

Functions:

• stores, concentrates, and excretes undigested material
• some absorption (water and ions)

Features:

• shorter than small intestine
• not covered with villi and microvilli
• contains most microbes in GI tract

Regulation: hormones and neurons

• GI tract is regulated by hormones and neurons.
• enteroendocrine cells line the GI tract and release hormones according to the environment.

Stomach regulation

• stretching of stomach sensed by mechanoreceptive neurons in lining
• neurons stimulate lining cells to secrete more HCl

Small intestine regulation (CCK)

• sensed by enteroendocrine cells → releases CCK (cholecystokinin) into blood

• effects:

• increases pancreatic enzyme secretion

• facilitates liver and pancreas secretions
• decreases stomach contractility
• CCK relaxes the sphincter of Oddi (restricts secretions of liver and pancreas)

GI muscle layers and roles

Circular muscle

• thins tract on contraction
• responsible for segmentation and peristalsis

Longitudinal muscle

• shortens tract on contraction
• responsible for peristalsis

Sphincters

• smooth muscle, involuntary

Muscularis mucosa

• inner layer responsible for local movements

Muscularis externa

• outer layer responsible for large-scale movements