Digestion and Absorption

Note:

absorb means to take into (i.e. to incorporate)

adsorb means that take onto (i.e. to allow to stick to - as in chromatography)

Food entering the intestinal tract undergoes three processes:

• mechanical digestion - which breaks the food down into smaller pieces
• chemical digestion - which further breaks the food down to molecule size
• absorption - where these molecules are taken from the intestinal tract into the body

The main anatomical structures of the intestinal tract (in the order through which food passes) are:

mouth

oesophagus (gullet)

stomach

ileum (small intestine)

colon (large intestine)

rectum

anus

Mouth:

• site of mechanical digestion - food is moved around the mouth by the tongue and sliced and crushed by the teeth
• Food is mixed with saliva (which lubricates the movement of the food down the oesophagus) and formed into a bolus
• Saliva (produced by the salivary gland) contains salivary amylase with begins the process of chemical digestion of starch
• The bolus is moved to the back of the buccal cavity (i.e. mouth) and swallowed - passing into the oesophagus

Oesophagus:

• bolus is moved down the oesophagus by peristalsis

• circular muscles lining the tube in front of the bolus relax
• circular muscles behind the bolus contract
• like moving a ping-pong ball along a hose pipe

Stomach:

• stomach fluid is very acidic
• this stops the action of salivary amylase
• and is harmful to bacteria reducing the chance of infection
• food remains in stomach for 1-4 hours
• constant rhythmic contractions of the muscles in the stomach wall continually churns the food and mixes it with ......
• gastric juice. This is produced by the gastric glands of the stomach wall
• Gastric juice comprises:
• water
• hydrochloric acid
• proteases (which can work in the low pH)
• mucus
• The resulting mixture of partially digested foods in an acidic fluid is called chyme
• Note that the mucus provides a protective layer over the wall of the stomach reducing the amount of damage to the wall as a result of the peptidases and the acid.
• However the cells lining the wall of the stomach have a very high turnover rate

Duodenum and Ileum

• the small intestine is divided into he duodenum and ileum
• these are the site of the remainder of chemical digestion and the beginning of absorption
• The duodenum receives secretions from the liver and the pancreas
• The liver produces bile. This is an alkaline solution (so neutralises the stomach acid) containing bile salts. Bile salts emulsify lipids (break large lipid droplets into many smaller ones) increasing the surface area for digestion.
• Bile is stored in the gall bladder until it is required
• The pancreas produces pancreatic juice. This is an alkaline solution of enzymes: amylase and peptidases and lipases
• Bile and pancreatic juice and both released into the bile duct which leads to the pancreas
• The results of the action of pancreatic juice and bile are:
• fatty acids and glycerol
• amino acids
• disaccharides
• The disaccharides are then converted to monosaccharides by enzymes embedded in the wall of the small intestine
• The fatty acids, amino acids and monosaccharides are then absorbed across the wall

Colon

• The colon is primarily associated with absorption of water from the diet
• Remaining starch is also digested (by intestinal bacteria) and absorbed here

Rectum

• This is where undigested food is stored as faeces

Anus

• A sphincter (ring of circular muscle) controls the passage of faeces from the rectum through the anus

Adaptations of the Ileum for Absorption

The intestinal tract has a common structure along its length from oesophagus to rectum.

In section the wall comprises 4 layers surrounding a central lumen:

• mucosa - glandular epithelium (i.e. epithelial cells containing glands [secretory structures]) and connective tissue
• submucosa - connective tissue
• external muscles (muscularis externa) - circular and longitudinal muscles
• serosa - connective tissue

The secretory structures found inthe stomach the mucosa varying according to site:

• are goblet cells secreting mucus

In the stomach:

• gastric glands secreting gastric juice

In the duodenum and ileum:

• intestinal glands at the base of the Crypts of Lieberkühn secreting intestinal juice

The Ileum has additional refinements to the overall plan to facilitate chemical digestion and absorption

• it is long - food therefore remains in it for a lengthy period and there is a large surface area across which food can be absorbed
• the mucosa is made up of villi - finger-like projections into the lumen

• The villi have a network of capillaries within them ensuring that any molecules absorbed across the epithelial cells are quickly taken into the blood
• in the central region of the villus is a lacteal. This is a part of the lymphatic system and takes in lipids. These molecules thus enter the blood at the lymphatic ducts
• the surface area of the ilium is further increased by the microvilli (foldings of the membrane) of the epithelial cells

hydrolyse glycosidic bonds

salivary amylase (in mouth) - breaks starch down to maltose

pancreatic amylase (in pancreatic juice) - breaks starch down to maltose

maltase (in the membranes of microvilli in the ileum) - breaks maltose down to glucose

lactase (in the membranes of microvilli in the ileum) - breaks lactose down to glucose and galactose

sucrase (in the membranes of microvilli in the ileum) - breaks sucrose down to glucose and fructose

For these last three the position of the enzyme means that, as soon as the enzyme has worked, the products can be absorbed

hydrolyse peptide bonds to release amino acids

endopeptidases - break bonds within the protein molecule - the result is more ends on which exopeptidases can work

exopptidases - break bonds near the end of the protein molecule - product is dipeptides and free amino acids

hydrolyse the bonds in lipids to produce free glycerol and fatty acids

the products are absorbed across the microvilli but, surprisingly, lipids are resynthesised in the epithelial cells of the microvilli and then absorbed into the lacteal

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