Digestive System.

Digestion is a two prong attack. One is to pulverize the food to obtain a bigger surface area. The other phase is to break down insoluble bigger molecules to smaller soluble forms. The chemicals which change the complex food do so without themselves changing. Their function is catalytic and they are called enzymes. This is a chart of enzymes and their actions.

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This process starts with the entering of saliva into the buccal cavity. The diagram shows the position of the salivary glands. Just the smell of food or the sight may be sufficient to activate the flow.

Saliva softens and lubricates the dry and hard food. It has the enzyme amylase (ptyalin) which can convert cooked starch to maltose. It also has a germicide, lysozyme. An adult usually secretes about 1 ½ litres of saliva a day.

The tongue while rolling the food inside the buccal cavity recognizes the different tastes. Although it was said that the different parts recognize the different tastes some doubt has arisen about it. It has also come to light that there are more tastes than the five earlier accepted.

A lump of pulverized food, well lubricated by saliva is called a bolus. Food is tasted, and if palatable pushed by the tongue towards the pharynx. (Throat). Then by a reflex (spontaneous) action, the bolus goes into the esophagus.(gullet). The epiglottis guards the trachea (Wind pipe). If an attempt is made to speak while you swallow, epiglottis cannot function.

Esophagus.

The process by which the bolus is pushed along the gullet (Esophagus) is peristalsis. The circular muscles just above the food contract while the similar muscles on the other side of the food relax. The esophagus is only 25cm long and it is well lubricated.

Stomach.

Fig.3 A section through the stomach wall.

The stomach is the dilated part of the alimentary tract having the capacity of 4 to 5 litres. The stomach wall which churns the contents thoroughly is made up of 3 layers of muscles. While these rhythmic movements, every 20 seconds, go on, the contents are contained by the tightening of the cardiac sphincter and the pyloric sphincter muscles.

Numerous tube like glands in the epithelium produce gastric juice. There are three types of cells that produce hydrochloric acid, pepsinogen and mucus. When pepsinogen and acid mix, it is activated to form the enzyme pepsin. These break down proteins to peptides. Only water, salts, alcohol and a little glucose are absorbed here.

After the mixing has been done for about 2 hours, the sphincter muscles in the pyloric end relax allowing the food, now called chyme to squirt into the duodenum. The periodicity will alter according to the nature of the food and the emotional conditions of the person. Excitement may accelerate while fear may retard.

Duodenum

                                                                     Fig.4 The duodenum

This is the first part of the ileum (small intestine) which is only 25 cm long. This is an important part as two major glands pour its secretions into the food. Pancreas makes about 1.5 litres of juice per day. It contains 3 enzymes and HCO₃- ions.

Bile is a greenish liquid from the liver which is stored in the gall bladder. Rate of bile production is also about a litre a day. The gall bladder has a capacity for about 30cm3. This is usually in a concentrated form. When the food is on line this contracts releasing the contents. The green colour is from the degenerating hemoglobin. The alkaline salts in the bile plays and important role in emulsifying the lipids. The speed of food in this region is about 2cm/s.

Self digestion?

It may be noteworthy to consider why the juices that act on meat do not digest the walls of the alimentary tract. This is mainly due to the presence of the mucus cover which is available all along.

The Ileum (The small Intestine).

This is the region which is mainly responsible for the absorption; so it is extremely long extending to about 3 meters when fully stretched in a dead person. The area of absorption is increased by a thousand fold due to tiny velvety like projections called villi. The epithelial cells in these even possess tiny projections called micro villi, which are visible only with an electron microscope the crypt consists of younger cells which would replace the mature ones as they are discarded after a few days.

Absorption

Glucose and amino acids straight away pass on to the blood capillaries in the villi. These capillaries form the hepatic portal vein which takes the food into the liver. The liver may store or alter any of the digestive products. Some of the fatty acids and glycerol may enter the blood capillaries, but a large proportion enters the epithelium to be converted back to lipids.  These products then pass on to lacteals. The fluid in these joins the lymphatic system which will join the blood stream later.

Absorption of digestion products is not always passive. Often they have to be pushed against diffusion gradients. This has been termed active absorption...

The Colon. (large Intestine )

The main function of this is to absorb the water. Along with that some salts may also be absorbed here. Usually there are beneficial bacteria which form vitamin B and K.