Human Digestive System

Human Digestive System

Structure of the gut wall

The general cross-section of the gut above is basically the same from the oesophagus to the end of the colon. However, the folding of the mucosa varies from region to region (e.g. less in the oesophagus, much more in the ileum) and the glands vary (e.g. salivary glands in the mouth, the liver and pancreas being connected via ducts to the duodenum).

Tissue layers of the gut wall

The submucosa contains nerves, blood and lymph vessels, collagen and elastic fibres.

The nerves regulate:

  • The gut movements by muscle contractions to force the food along or to mix the food with secretions in a particular region.
  • The digestive secretions into the lumen of the gut.

The two layers of muscle create the waves of contraction of the gut wall known as peristalsis. During peristalsis, the circular muscle contracts behind the food and the longitudinal muscle contracts in front of the food. The constricted gut behind and the shortened gut in front forces the food along.

Mouth and saliva

Even before food enters the mouth the sight, smell and thought of the food stimulates a conditional reflex that results in the release of saliva into the mouth.

When food enters the mouth, the stimulation of the taste buds results in an unconditional reflex whereby impulses are relayed to the brain via sensory neurones and then via motor neurones to the salivary glands. Again, releasing saliva.

1 - 1.5 litres of saliva is released each day.

The saliva contains mucus, which lubricates the food, mineral salts to activate enzymes, lysozyme which kills bacteria entering with the food, and amylase, an enzyme that breaks down starch into shorter polysaccharides and then into maltose.

Chewing mechanically breaks up the food so that there is a larger surface over which the amylase can work. The food and saliva mixture is pushed into a ball called a bolus and swallowed.

Oesophagus

The oesophagus is a muscular tube with a squamous epithelium lining and mucus glands to lubricate the passageway down to the stomach.

Peristalsis moves the food down and when the food reaches the lower portion of the tube, the circular muscle making up the sphincter (a muscular ring controlling the passage of food between consecutive sections of the gut) relaxes and opens.

With no food present, the sphincter remains closed so that no acid can enter and burn the oesophagus.

Stomach

Once food enters the stomach the muscular wall contracts and relaxes to churn and mix the food. To make this really efficient, the stomach lining has an extra layer of oblique (diagonal) muscle on the lumen side of the circular muscle.

The mucosa contains mucus-secreting goblet cells in the columnar epithelium layer. This mucus creates a barrier and prevents autodigestion (digestion of an organisms own tissue by its own enzymes).

In the wall of the stomach are pits (gastric glands) created by infoldings of the epithelium. Some cells lining the pits are called chief cells. These release an inactive enzyme called pepsinogen.

Other cells called parietal cells (or oxyntic cells) release HCl (hydrochloric acid).

The acid has several beneficial effects:

  1. It creates the right pH for the enzymes to work efficiently.
  2. It kills bacteria that have escaped the lysozyme in the mouth.
  3. It removes a portion of the pepsinogen protein so that it becomes active pepsin.

It is important to have the pepsin enzyme since it begins the digestion of proteins into shorter polypeptides.

The pepsinogen and acid together are called the gastric juice. They are released because the presence of food in the mouth, the smell of the food etc. triggers off impulses to the brain which then send impulses to the gastric glands in the stomach to release these substances.

The physical stretching of the stomach when food is in it stimulates the glands themselves to secrete a hormone gastrin. The hormone is released into the blood and its target cells are those of the gastric glands so this too causes the release of the gastric juice.

Gastrin also stimulates the pyloric sphincter (between the stomach and first part of the small intestine) to relax. Acidic, churned, partially digested food known as chyme is slowly released into the duodenum.

Duodenum

Most chemical digestion by enzymes takes place in the duodenum.

The mucosa is folded and the millions of microscopic projections created by this folding of the inner surface of the wall are called villi. In between the villi are intestinal glands (or crypts of Leiberkuhn) which secrete intestinal juice.

The possession of the villi and the folds in the cell surface membranes of the epithelial cells lining the villi (microvilli) massively increases the surface area.

The epithelial cells produce various enzymes that are not released into the lumen but are bound to the cell surface of the microvilli (also called brush border because of its appearance).

Other enzymes, found free in the lumen of the duodenum have been produced and released by the pancreas. As well as these enzymes, sodium bicarbonate solution is secreted into the duodenum to provide the optimal pH for the enzymes found in this region of the gut.

The liver also secretes a substance called bile into the duodenum. (Bile may be stored in the gall bladder before release). Bile consists of bile salts that emulsify fats. The large globules of fat are mechanically dispersed into many smaller globules. This increases the surface are over which lipases can act.

The release of these substances, as in the stomach, is controlled:

  1. When the acidic chyme touches the mucosa of the duodenum impulses are sent to the brain. The brain then sends impulses to the gastric glands to slow down the release of gastric juice (since the food has moved on). The acid also stimulates the release of a hormone called secretin. Secretin affects the pancreas and liver and brings about:
    • the release of sodium bicarbonate solution from the pancreas and,
    • the production of bile from the liver.
  2. The presence of the food in the duodenum stimulates the glands to produce their enzymes. It also stimulates the release of a hormone called either CCK (cholecystokinin) or pancreozymin. It brings about:
    • the release of bile from the liver and,
    • the production and release of pancreatic juice (containing enzymes).

Enzyme:

Released by duodenum:

Released by pancreas:

Action:

Aminopeptidase

Yes

 

Peptides to amino acids

Enterokinase

Yes

 

Inactive trypsinogen into trypsin

Maltase

Yes

 

Maltose to glucose

Sucrase

Yes

 

Sucrose to glucose and fructose

Exopeptidase

Yes

 

Peptides into amino acids

Lactase

Yes

 

Lactose into glucose and galactose

Amylase

Yes

Yes

Starch / glycogen into maltose

Chymotrypsin

 

Yes

Proteins into amino acids

Trypsin

 

Yes

Proteins into amino acids Chymotrypsinogen into chymotrypsin

Lipase

 

Yes

Lipid into fatty acids and glycerol

Carboxypeptidase

 

Yes

Peptides to amino acids

Endopeptidase

 

Yes

Proteins into peptides

 

Ileum

Large molecules have now been chemically digested into their constituent subunits by enzymes. They now need to be absorbed through the intestine wall, along with mucus and the digestive juices.

Absorption takes place by simple diffusion, facilitated diffusion and active transport.

Monosaccharides, amino acids, dipeptides and tripeptides are actively transported into the epithelial cells lining the wall. It is thought that their absorption is coupled with the absorption of sodium. This would mean that the carrier proteins have 2 receptor sites, one for sodium and one for glucose. Only when both are filled would they be actively transported from the lumen side of the epithelium, and into the cells. From here they diffuse into the capillaries.

The fatty acids, glycerol and monoglycerides form complexes with bile salts called micelles. The micelles come into contact with epithelium cells and the fat-soluble fat components diffuse into the epithelium cell, leaving behind the fat insoluble bile salts.

In the epithelium, the fatty acids and glycerol are reformed by the endoplasmic reticulum to make triglycerides. They are packaged into globules with cholesterol and phospholipids and then coated with protein to be passed out from the epithelial cell into the lacteal.

Eventually these packages, called chylomicrons, are emptied into the blood.

Large intestine

The colon, rectum and anus make up the final section of the digestive tract. In humans these are mainly for absorbing water and for forming faeces from undigested food, dead cells and bile pigments. These wastes are then egested.

Sodium is actively transported out of the lumen and water follows by osmosis. This solidifies what is left. There are mutualistic bacteria that live in the colon that produce certain vitamins in exchange for a constant environment in which to live.

Faeces are consolidated and stored in the rectum until elimination from the anus when the rectum is distended.

 

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