Neural and Hormonal Control of Digestive System
The neural control is effected by an intrinsic and extrinsic network of nerves and nerve endings. The intrinsic network is in the form of plexus in the wall of the alimentary canal with two tiers, submucosal and myenteric. The extrinsic network of nerves belongs to the autonomic system. The sympathetic nerves come from post-ganglionic fibres and the parasympathetic from the pre-ganglionic fibres of the vagus nerve.
Hormonal control of digestion is by hormones.
Gastrin: Gastrin stimulates the parietal cells to secrete hydrochloric acid and the chief cells to secrete enzymes. Gastrin secreted by the G-cells of the gastric, duodenal and jejunal mucosa. It controls the motility of the stomach wall.
Secretin: is secreted by the mucosal epithelium of the duodenum in response to the acidity inside the stomach. It causes the release of insulin and bile, stimulates the secretion of pepsinogen.
Cholecystokinin: stimulates the gall bladder to pour in the bile and the pancreas to pour in the pancreatic juice.
Gastric inhibitor: inhibits the secretion of hydrochloric acid and induces the pancreas to secrete insulin.
Role of Liver in Metabolism
The liver is a reddish brown gland situated in the upper part of the abdomen on the right side. It is the largest gland in the body. It secretes bile juice that is stored in a sac called the gall bladder. Its numerous functions can be grouped under categories:
Blood Related Functions
Produces red blood cells in the embryo. (In adults, RBCs are produced in bone marrow).
Produces prothrombin and fibrinogen required for blood clotting.
Produces heparin which prevents unnecessary coagulation of blood.
Destroys dead and worn out red blood cells.
Removes toxic and metallic poisons from the blood (protective function).
Storage of iron and some other metallic ions.
Storage of vitamins A, D and B12.
Converts extra blood glucose into glycogen and stores it
Regulation of blood sugar level by retaining excess glucose received as product of carbohydrate digestion from the intestines, and storing it as insoluble glycogen to release it again as soluble glucose when the blood sugar level falls.
Breaking down of excess amino acids. Amino acids are the end products of protein digestion. Liver breaks down excess amino acids into urea and sugar. Urea is excreted out in urine and sugar is stored for use.
Synthesizes fatty acids from carbohydrates, which can be used or stored as fat.