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Physiology (BY03) revision

Questions

Books always make topics look far too big, and not in the order in which you get asked questions. This is a bit of a summary of the stuff you actually need to know. You should know it already if you have finished the module, but not in this order. It is not the order in which to learn it, but is the order in which to revise it. Not much is it!

 

SO HERE'S EVERYTHING ABOUT….

…the digestive system's….

…digestion of protein, starch and lipids (walk-through) and control of secretions

Physical digestion begins in the mouth with mastication. This process breaks the food up and increases the surface area for enzymes to work on. Saliva from the salivary gland contains amylase, which begins the digestion of starch to maltose. Stimulation of salivary glands is by the nervous system in response to real or imagined food. Saliva also lubricates the movement of the food, which is then rolled into a bolus by the tongue and travels down the oesophagus to the stomach. Here it is mixed with gastric juice (pH 2), which is produced in gastric pits/glands in the stomach epithelium. Gastric juice is released in response to the presence of food in the buccal cavity. As the stimulation by the nervous system wears off, the presence of food stimulates cells in the stomach lining to produce gastrin. This passes into the bloodstream and stimulates the release of gastric juice for several hours. These consist of:

Gastric juice therefore contains pepsin and pepsinogen, as well as HCl, and chemical digestion begins. Both pepsin and HCl catalyse hydrolysis. Pepsin is an endopeptidase: it is an enzyme which catalyses the hydrolysis of proteins to polypeptides. It does this by hydrolysing the inside of the protein molecule, thus increasing the number of ends for the exopeptidases to work on. The pyloric sphincter opens when the chyme (the mixture of food and gastric juice) is of the correct consistency, and releases the food into the small intestine. Lipid stimulates the production of entergasterone by the stomach mucosa, which has an antagonistic effect on gastrin. Therefore, the more fat the food contains, the longer it remains in the stomach. The first section of this is the duodenum, which runs nearly horizontally across your body below your stomach. When chyme enters the duodenum, its lining releases a number of hormones. One of these is secretin, which stimulates the production of the sodium hydrogencarbonate portion of the pancreatic juice. Cholecystokinin-pancreozymin (CCK-PZ) stimulates the gall bladder to contract, and the release of digestive enzymes from the pancreas. At the beginning of this, ducts running from the gall bladder (bile duct) and the pancreas (pancreatic duct) form the common duct and secrete bile and pancreatic juice into the duodenum. Pancreatic juice

Type of enzyme

Name of enzyme

Substrate

Product(s)

Polysaccharidase   

Amylase

Starch

Maltose/sucrose/lactose

Endopeptidase

Trypsin

Protein

Polypeptides

Endopeptidase

Chymotrypsin

Protein

Polypeptides

Exopeptidase

Carboxypeptidase

Polypeptides

Dipeptides/Dipeptides

Lipase

Lipase

Lipids

Fatty acids and glycerol

Nuclease

Nuclease

Nucleic acids

Nucleotides

Bile

The duodenum contains deep folds between its villi, which have Brunner's glands below them. The latter secrete a viscous fluid containing water, hydrogencarbonate ions and mucoprotein, and serves to protect the wall of the duodenum from the pepsin and chyme. Both extracellular digestion and intracellular digestion occur in the small intestine. The cells of the epithelium contain enzymes in their cytoplasm and embedded within their cell surface membrane. Digestion therefore occurs outside, on the way into, and inside the cytoplasm of, these cells. Resulting from the digestion in the lumen of the small intestine, carbohydrates have been hydrolysed to disaccharides and proteins to dipeptides and tripeptides. Their further digestion takes place in the cell surface membrane of the microvilli of the intestinal mucosa. Disaccharidases break disaccharides down into their constituent monosaccharides. Most of these are released back into the lumen of the gut. A similar process occurs with dipeptides and tripeptides, which are taken up by the microvilli and broken down into their respective amino acids. Some also diffuse into the cytoplasm of the cells of the villi, where they are digested.

…Histology

The gut or alimentary canal can be considered as an extension or inversion of the epithelium or outer skin, as it is physiologically not part of the body. It runs from the mouth from the rectum, and varies in thickness. It has the following functions (comments in brackets for my Yorkshire-born counterparts):

The wall of the alimentary canal consists of:

The buccal cavity (gob) has a tongue (no…!), which rolls food into a bolus so that it passes easily down the oesophagus.

The stomach has cardiac and pyloric sphincters which keep food in and regulate it's flow into the small intestine in small squirts. It is surrounded by muscle which helps to churn and mix the contents.

The small intestine (duodenum, jujenum, ileum) is long thin and folded, and therefore it has a large area over which to digest food. It has villi and microvilli, which further increase the available surface area. Its cells produce mucous, which lubricates the passage of food and reduce damage and friction.

The large intestine (colon) has the job of re-absorbing all the fluids secreted and ingested into the alimentary canal, and compacting the undigested remains for storing in the rectum. It is fairly wide. Bacteria exist here which use some of the undigested remains (fibre) and synthesise vitamins such as K, B12 and riboflavin.

…The kidneys'…

Ultrafiltration and tubular reabsorption control

…the liver's…

role in controlling glucose levels

Copyright © 2000 Simon the A-Level Man. Full reproduction permission is granted provided the URL http://start.at/alevel is included.

If you are reproducing, please bear in mind that every third baby born on Earth is Chinese so maybe you should stop at two (Note: the author has nothing against Chinese)