Digestive System Physiology

Basic functions

1. ingestion

2. motility

propulsive movements

mixing movements

3. mechanical processes

4. secretion

5. digestion: chemical processes

6. absorption

7. elimination (defecation)

Digestion: the chemical processes

hydrolysis

carbohydrates ¹ and sugars ²

types

monosaccharides

glucose

galactose

fructose

disaccharides

sucrose

lactose

maltose

isomaltose

polysaccharides ⁸

α– vs. β–glycosidic linkages

glycogen

α-1,4-glycosidic bonds and α-1,6-glycosidic bonds

starch

amylose

amylopectin

cellulose

phycopolysaccharides

agar

carrageenans

alginate

fucoidan

enzymes used

salivary amylase

pancreatic amylase

brush border disaccharidases

sucrase, etc.

proteins and amino acids ³

enzymes used

pepsin in the stomach

enzymes acting in the small intestine

trypsin

chymotrypsin

carboxypolypeptidase

brush border enzymes

aminopeptidases

carboxypeptidases

dipeptidases

fats ⁴

types

triglycerides

cholesterol

chemicals & enzymes used

bile salts

pancreatic lipases

nucleic acids

pancreatic ribonucleases

deoxyribonuclease in the small intestines

Absorption

carbohydrates

cotransport with Na⁺

facilitated diffusion

enter the capillary bed in the villi

transported to the liver via the hepatic portal vein

proteins

similar to carbohydrates

fatty acids and monoglycerides

diffuse into intestinal cells

combine with proteins and extrude chylomicrons

enter lacteals

transported to systemic circulation via lymph

glycerol and short chain fatty acids

into the capillary blood in villi

transported via the hepatic portal vein

nucleic acids

active transport via membrane carriers

absorbed in villi

transported to liver via hepatic portal vein

electrolytes

most ions are actively absorbed along the length of small intestine

Na⁺ is coupled with absorption of glucose and amino acids

ionic iron is transported into mucosal cells where it binds to ferritin

anions passively follow the electrical potential established by Na⁺

K⁺ diffuses across the intestinal mucosa in response to osmotic gradients

Ca²⁺

related to blood levels of ionic calcium

regulated by Vitamin D and parathyroid hormone (PTH)

water

95% absorbed in the small intestines by osmosis

moves in both directions across intestinal mucosa

net osmosis occurs whenever a concentration gradient is established by active transport of solutes into the mucosal cells

water uptake is coupled with solute uptake

as water moves into mucosal cells, substances follow along their concentration gradients

Regulation of digestive function

autonomous smooth-muscle function

slow-wave potentials

regulation of digestion involves:

receptor activation

chemoreceptors

mechanoreceptors

osmoreceptors

mechanical and chemical stimuli

presence of substrate in the lumen

distention

osmolarity

reflexes affect glands and movement

intrinsic nerve plexuses ⁶

nerve plexuses near the GI tract initiate short reflexes

short reflexes are mediated by local enteric plexuses

myenteric plexus (Auerbach’s plexus)

submucosal plexus (Meissner’s plexus)

extrinsic nerves

long reflexes arising within or outside the GI tract

involve CNS centers and extrinsic autonomic nerves

vagus n

gastrointestinal hormones ⁷

gastrin

stimulus: protein-rich chyme in stomach

gastro-colic reflex

secretin

stimulus: acidic chyme in duodenum

cholecystokinin

stimulus: fatty and protein-rich chyme in duodenum

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Questions for thought
1.		Diagram and label the layers of the gut tube wall. Give a brief description of the function of each layer.
2.		List the enzymes involved in the digestion of complex carbohydrates to simple sugars; for each, note where it is secreted and what its particular action is.
3.		What is meant by the term hydrolysis? Explain, using real examples, how this process breaks macromolecules into their constituent monomers.
4.		Describe the enteric nervous system and how is it regulated by the autonomic.
5.		Explain the processes, in sequence, necessary for the digestion, absorption, and transport of fats.
6.		Briefly explain the functions of the liver and pancreas (exocrine and endocrine).
7.		Describe how hormones produced by the duodenal and gastric mucosa (naming them as appropriate) are used to regulate emptying of the stomach and secretion of digestive juices by the liver and pancreas.

Go to beginning

Links

Referenced sites
- ¹ Digestion of dietary carbohydrates
- ² Fructose metabolism
- ³ Amino acid metabolism
- ⁴ Various aspects of fat metabolism and ketogenesis
- ⁵ Microscopic anatomy of the various organs is portrayed elegantly in The Virtual Labguide by Christopher Brandon, Ph.D., and Douglas S. Mehr, M.D.
- ⁶ Michael D. Gershon’s article on the enteric nervous system
- ⁷ Extensive documentation of the motilides as motilin agonists
- ⁸ An overview of polysaccharide structure
Other sites
- An interesting analysis of what our anatomy tells us about what we should eat
- An historical review of gastric secretion