The digestive system consists essentially of a long tube about 7. 32 to 10.98 m. (24 to 36 feet) long, winding through the body from the mouth to the anus.
Into this tube various glands pour chemical substances necessary for digestion (Fig.).
The segments of the alimentary canal are called :
(i) Mouth and throat,
(ii) Esophagus or gullet,
(iv) Small intestine (duodenum, jejunum and ileum),
(v) large intestine (caecum, colon and rectum).
Colon and rectum contain a veritable microbe zoo-more than 50 varieties of microbes with a total population in billions.
The vermiform appendix is attached to the alimentary canal are the salivary glands in the mouth, the liver and gall bladder, and the pancreas.
In addition some 30 million small glands in the walls of the stomach and duodenum manufacture gastric juice.
Bile from the liver and gall bladder enters the duodenum through a common duct.
Digestion id the process by which the digestive system breaks down and prepares food substances
taken by mouth into simpler chemical substances that can be absorbed into the blood stream and assimilated by cells and tissues.
The process is partly mechanical but chiefly chemical.
Food is chewed in the mouth and swallowed into the oesophagus by action of muscles in the throat and neck.
However, the food tube is essentially walled by smooth muscle tissue also capable of contracting.
The rhythmic contraction of these muscle, called peristalsis squeezes the food, in various stage of digestion, bit by bit, along the food tube.
It takes place along the entire alimentary canal, but is most evident in the intestine.
It is involuntary, but can be stimulated by eating, by smell of food, by drugs, and by irritating foods in the digestive tract.
The peristaltic action is usually transmitted along the entire length of the alimentary canal.
That is why the urge to have a bowel movement frequently occurs shortly after eating.
Chemistry of Digestion
Chemical digestion is accomplished largely through enzymes contained in the gastric juice after being secreted by the glands along the digestive tract.
The gastric juice work specifically on the food components taken indiscriminately into the body, that is carbohydrates ('energy foods' like starches
and sugars), proteins ('meat' - the 'building blocks', milk products, eggs, fish, beans and pulses), fats ( plant oils, nuts, animal fat on meat, dairy
products like ghee), vitamins (fat-soluble-notably vitamins A, D, E< and K ;water soluble-notably vitamin B complex and ascorbic acid or vitamin C), minerals and water.
The end products of chemical digestion are chiefly amino acids, simple sugars, and tiny globules of natural fat.
The simple sugars are the end products of carbohydrate digestion. They are carried to the liver, where they are transformed into glycogen and redistributed to provide energy, or fuel for the body cells.
Fat digestion ends in fatty acids and glycerol, which are quickly recombined into neutral fat.
Some fat is immediately burned for body fuel ; the rest is stored in fatty tissue.
Any part of the digestive system may be subject to disease, hence a brief discussion on the normal working of teeth, throat, liver and pancreas follows.
Composed mainly of dentine (chiefly calcium phosphate), teeth are covered with enamel. In the center is the pulp cavity, into which blood-vessels and nerves enter through a canal opening at the apex of the roots. Between 6th to 9th month of life and the 24th month, 20 milk teeth are formed in a child, which later fall out, to be replaced by the 32 permanent teeth which are used for biting (incisors), tearing (canines) and grinding the food (premolars and molars).
This cavity in the back of the mouth has pharynx where the respiratory and digestive systems meet ; the mouth and nose enter pharynx from above ; the larynx and the gullet depart from below (Fig.4).
The Eustachian tubes from the ears open into it, one on each side. Pair of tonsils flank it and adenoids may be included.
In front of it hangs the soft palate (uvula). the pharynx is a busy place and it bears the brunt of the common cold and other upper-respiratory infections.
The upper end of the larynx is covered with a flap of cartilage called the epiglottis.
During the act of swallowing the larynx rises and contracts so that food and fluid cannot enter.
When one attempts to talk while eating, this automatic mechanism may be crossed up and food is said to go down wrong way with a chocking and coughing accompaniment.
The larynx proper is a cylinder, about 5cm. high, through which air passes on its way to and from the windpipe (trachea) immediately below.
The larynx is in a vulnerable position in the front of the neck ; but it is protected by a series of five cartilages, notably the thyroid cartilage or 'Adam's apple'.
Inside the larynx are the vocal cords, more exactly vocal folds. They are stretched across the voice box in the shape of a V, with the point toward the front.
The vibration of these cords as the air streams exiting from the lungs passes across them in the beginning of speech and song.
The action of many other parts of the mouth is necessary, however for intelligible speech.
Protected by ribs, filling the upper right part of abdomen, the liver is the largest gland in the body, weighting 1.35 Kg.
and producing over 1000 different enzymes to handle chemical conversions for more than 500 jobs.
This complex and vital organ is intimately concerned with metabolism of carbohydrates, proteins and fats with the regulation of
blood sugar and the storage of glycogen and factors essential for the formation of red blood cells.
It synthesizes prothrombrin, heparin (which retards clotting of blood), and the proteins of the body, renders many toxic
substances ineffective by chemical treatment. Liver secrets bile, important in the digestion of fats.
Obstruction o the flow of bile into the gut can pile up pigments, bilirubin and biliverdin and spill them
over into the bloodstream to produce jaundice. As much as 80 percent of the liver tissue can be cut away as in cancer surgery,
and still the liver functions normally, it can rebuild in a few months, back to the normal size.
Called 'the sweetbread' in animals pancreas is a large, long organ or gland, located behind the lower part of the stomach.
It is an important part of both the digestive system and the endocrine system. Throughout the pancreas are found scattered
islet cells 'islands or langerhans' which secrete insulin, the hormone necessary to proper carbohydrate metabolism.
Failure of the pancreas to produce adequate supplies of insulin results in diabetes mellitus.
The normal fasting blood sugar is 80-120 mg, and postprandial) 2 hours after lunch blood sugar should be up to 150 mg. per 100cc. of blood.