There are five digestive juices, viz., saliva, gastric juice, pancreatic juice, succus entericus (intestinal juice) and bile, secreted from salivary, gastric, pancreatic, intestinal and hepatic gland respectively, which are poured in the alimentary canal at its different levels successively from oral to aboral side.
The term, mechanism of secretion is meant by- (a) How the glands respond to secrete by the stimuli and how its flow and composition are maintained, (b) How the glands modify their secretion as regards their flow and composition by various types of stimuli and site of stimulation.
Types # 1. Salivary Secretion:
Purely a reflex process; two types of reflexes present; conditioned and unconditioned. Conditioned reflex is proved by the fact that even sight and smell of food stimulate secretion. Other conditioned stimuli can be established.
The stimulus for unconditional reflex arises chiefly in the mouth. But it also arises from the oesophagus (oesphago-salivary reflex), from the stomach (gastro-salivary reflex), as well as from other viscera such as gravid uterus.
Types # 2. Gastric Secretion:
Experiments:
i. Sham feeding.
ii. Pavlov’s pouch.
Phases:
Three phases:
(a) Cephalic (or nervous),
(b) Gastric, and
(c) Intestinal.
(a) Cephalic Phase:
Starts immediately after taking food. It is a reflex process involving both conditioned and unconditioned reflexes. The juice secreted is called appetite juice. It is constant in composition and does not vary with the nature of food. In man it is small in amount but is important.
(b) Gastric Phase:
Starts half an hour after the entry of food in the stomach. The stimulus is chemical. The chemical substance is manufactured by the pyloric mucous membrane from some products of protein digestion and is known as gastrin. Gastrin enters blood stream carried to the gastric glands and stimulates their secretion independent of all nerves.
Largest quantity of gastric juice is secreted during this phase. This part of gastric juice varies in quality and quantity with the nature of food. Proteins increase both the amount and the HCl content. Fats inhibit both. Bread stimulates a secretion having the greatest digestive power. Water, coffee, spices stimulate.
(c) Intestinal Phase:
Starts when food enters the duodenum. It is small in amount and is independent of nerves; consequently, the stimulus is chemical but its exact nature is not known. Presence of fat in the duodenum inhibits gastric secretion. This, according to Ivy, is due to the liberation of an inhibitory hormone from the intestine called enterogastrone.
The three phases are closely interrelated. Cephalic phase initiates appetite juice, which digests the proteins partly. Form these products of digestion gastrin are manufactured which initiates the second phase. After this when gastric digestion has proceeded to the required stage, stomach empties into duodenum and thereby intestinal phase starts. Thus each phase initiates the next.
Even in fasting condition stomach secrets at the rate of 10-60 ml per hour. Its cause is not known. It acts as an antiseptic against pathogenic bacteria that may be swallowed with saliva in empty stomach.
Types # 3. Pancreatic Secretion:
Two phases:
(a) Nervous.
(b) Chemical.
(a) Nervous Phase:
Starts 1-2 minutes after taking food. The reflex is purely unconditioned. Unlike gastric juice there is no conditioned reflex here. The stimulus for this secretion arises both in the mouth (during chewing) as well as from the stomach after the food is swallowed.
(b) Chemical Phase:
Starts when stomach empties into duodenum. This is due to hormone-like substances called secretin and pancreoxymin. Secretin thus enters blood stream, goes to pancreas and stimulates the secretion. The secretin fraction stimulates the secretion of water, alkali and other salts of pancreatic juice; whereas, pancreozymin increases the enzyme content.
On the whole, juice stimulated by secretin is rich in alkali but poor in enzyme. Whereas secretion produced by the stimulation of the vagus is poor in alkali but rich in enzyme. Pancreatic secretion varies with the type of food. Meat stimulates a ‘secretin’ type of juice, fat stimulates the ‘vagus’ type; whereas, bread elicits a mixed type of secretion.
Types # 4. Bile Secretion:
Experiments:
(1) Altercursive intubation.
Mechanism:
Bile secretion by liver is active and continuous, total amount 800 -1,000 (15 ml per Kg body weight) per day. The rate of secretion increases one hour after taking food remains high for 2 – 5 hours and then falls. At the end of intestinal digestion it comes to the fasting level. The mechanism involves only chemical stimuli, nerves are of no practical importance.
The chemical stimuli are the following:
i. Bile Salts:
Chief cholagogues.
ii. Foodstuffs:
Fats and proteins stimulate. It has been noted that bile secretion increases about one hour after meal, remains high for about 2-5 hours and then declines.
iii. Humoral:
Cholecystokinin which stimulates bile secretion by contraction of gall-bladder and hepatocrinin which stimulates the liver cells to secret bile.
5. Succus Entericus:
Experiment:
Thiry-Vella fistula.
Mechanism:
During fasting, no secretion. Flow starts one hour after food, maximum in the third hour, then gradually declines.
Mechanism involves two factors:
(i) Mechanical, and
(ii) Chemical.
(i) Mechanical Factor:
When the mucosa is mechanically irritated secretion takes place. It is independent of the vagus and the sympathetic (splanchnic) but depends upon local nerve plexuses. Presence of food in the intestine acts as a normal stimulus.
(ii) Chemical Stimulus:
(a) Some products of digestion, particularly of proteins,
(b) Enterocrinin, and
(c) Duocrinin which acts on duodenum only.
All three independent of nerves, act as important stimuli in the normal process of secretion of intestinal juice. Enterocrinin is a hormone manufactured by intestinal mucosa which is believed to act as important stimulus in the normal process of secretion of intestinal juice. It is obvious that all these factors only operate so long as intestinal digestion and absorption continue. Consequently, after this period— (normally 2-5 hours)—the flow will cease.