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Digestive system physiology summary


Department
Biological Sciences
Course Code
BIOC32H3
Professor
Stephen Reid

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Digestive System Physiology
The digestive system consists of the “tube through which food (and digested products pass
(mouth, esophagus, stomach, small intestine, large intestine, rectum and anus) as well as
accessory organs (liver, pancreas, salivary glands, gall bladder) that produce substances (e.g.,
enzymes, bile) that assist in digestion.
There are four main functions (“actions is probably a better way to describe these four things) of
the digestive system.
1) Motility – contraction of smooth muscle lining the esophagus, stomach and intestine helps to
move food stuff through the digestive system from mouth to anus.
2) Digestion – this is the process whereby food is broken down into its constituent molecules
(i.e., proteins broken down into amino acids; carbohydrates broken down into simple sugars;
lipids broken down into free fatty acids and monoglycerides).
3) Secretion – this is the secretion of substances that aid digestion from the digestive organs or
accessory glands into the lumen of the stomach or intestine.
4) Absorption – this refers to the uptake of digestive break down products (i.e., amino acids,
small peptides, simple sugars, free fatty acids, monoglycerides) from the lumen of the digestive
system into the blood. Most absorption takes place in the small intestine. A few substances (i.e.,
aspirin; alcohol) can be taken up across the stomach while the large intestine is predominately
involved in water reabsorption.
Entry of food into the esophagus is regulated by the upper esophageal sphincter while entry of
food into the stomach is regulated by the lower esophageal sphincter (which also prevents
backflow of food from the stomach to the esophagus. Digestion of carbohydrates begins in the
mouth as the salivary glands release salivary amylase into the mouth. This begins the process of
carbohydrate digestion. However, salivary amylase is inactivated in the acidic environment of the
stomach.
The Stomach and Intestine
The stomach is divided into three regions – the fundus (the top portion of the stomach located
above (in a human) the lower esophageal sphincter), the body (the main central region of the
stomach) and the antrum (the region closest to the small intestine).
The small intestine is divided into three regions – the duodenum (the region closest to the
stomach), the jejunum (the middle region) and the ileum (the region closest to the large
intestine).
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The large intestine consists of the cecum/ascending colon, transverse colon, descending colon
and sigmoidal colon. The sigmoidal colon empties into the rectum.
The movement of food stuff from the stomach (antrum) to the small intestine (duodenum) is
regulated by the pyloric sphincter. The movement of digested material from the small intestine
(ileum) to the large intestine (ascending colon) is regulated by the illeocelcal sphincter. The
voiding of feces from the rectum is regulated by the internal and external anal sphincters.
Chemical Processes within the Stomach
The stomach digests foods by mechanical and chemical processes. The mechanical processes
involved the movement of food stuff back and forth as alternating waves of muscle contraction
move the food stuff forward and backward within the stomach. The chemical processes involve
the secretion of acid and an enzyme from specialised cells that line the stomach. The mixing
helps to bring the acid and enzyme in contact with the food stuff to facilitate digestion.
Lining the wall of the stomach are numerous invaginations called gastric pits. These gastric pits
contain three main cell types: 1) Parietal cells that secrete acid (HCl), 2) Chief cells that secrete
pepsinogen (which is the inactive form of pepsin) and 3) Mucus cells that secrete mucous.
HCl Secretion from Parietal Cells
HCl plays three main roles in the stomach. 1) It breaks down connective tissue, 2) It helps
convert pepsinogen into pepsin and 3) It kills microorganisms.
HCl is formed from the reaction of CO2 and H2O which react, within the parietal cell to form H+
(a proton) and HCO3- (bicarbonate ion). The CO2 in the cell is a byproduct metabolism within the
cell. It is always there.
The H+ ion is then moved across the apical membrane of the parietal cell (into the stomach;
gastric pit) in exchange for a K+ ion (H+ - K+ exchange).
The HCO3- ion is moved across the basolateral membrane in exchange for a Cl- ion (HCO3- - Cl-
exchange).
The Cl- ion that moves into the parietal cell then enters the stomach (gastric pit) through a
chloride channel.
The K+ that was taken into the cell moves back into the stomach through a K+ channel (K+ is
therefore recycled).
The net result is the secretion of HCl (H+ and Cl-) into the stomach and the movement of
NaHCO3 into the blood (Na+ and HCO3-)
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