Question

#1: List 3 hormones that are involved in the digestive system and give their site of origin, what they do and what are their targets.

#2: The kidney regulates blood pressure. List 3 hormones that are involved in regulating one’s blood pressure. Give their site of origin, what they do and what are their targets

Answer 1

Question 1

Hormones involved in digestive system are gastrin, cholecystokinin(CCK), secretin, gastric inhibitory peptide, vasoactive intestinal peptide, motilin, somatostatin.

Gastrin : It is secreted by G cells of gastric antrum. It acts on the parietal cells of the stomach to secrete gastric acid (HCl) which is it's it's primary action. It increases gastric motility and emptying.

It also helps in increasing pepsin which helps in protein digestion and intrinsic factor which helps in absorption of vitamin B12 from ileum.

Gastrin production is stimulated by distension of the stomach. Proteins are most potent stimulus for gastrin production. It's production is inhibited by the hormone somatostatin and if the pH of the stomach decreases below 1.5

Cholecystokinin (CCK) : It is produced by I cells of proximal small intestine (duodenum and jejunum). It acts on the gall bladder and causes it's contraction and simultaneous relaxation of spincter of Oddi which promotes the release of bile into the duodenum for fat digestion. It also acts on the pancreas and increases pancreatic enzyme and bicarbonate secretion which eventually goes into the duodenum and helps in digestion and neutralizing the acidity. It decreases gastric acid secretion and gastric motility.

CCK's secretion is stimulated by fatty acids and proteins but not carbohydrates. Since, CCK increases pancreatic enzymes, it is also known as pancreozymin.

Secretin : It is produced by S cells of proximal small intestine ( duodenum and jejunum ). It acts on the pancreas and increases pancreatic bicarbonate secretion which helps in neutralizing the duodenal acidity. It acts on the stomach and decreases gastric acid and pepsin secretion.

The most potent stimulus for secretin production is gastric acid. Products of protein digestion in the small intestine also increases its synthesis.

Vasoactive intestinal peptide (VIP) : It is produced by mucosal cells from stomach to colon but most importantly in the colon. It increases pancreatic and intestinal secretion rich in water and electrolytes and hence its over production can cause watery diarrhoea. Besides it's action on gastrointestinal tract, it also acts on blood vessels causing vasodilatation and thereby hypotension. It also decreases the tone of smooth muscles of gastrointestinal tract including the tone of lower esophageal sphincter.

It's production is stimulated by the distension of small intestine as well as vagal stimulus and inhibited by adrenergic inputs from sympathetic nervous system.

Question 2

Kidneys are important organs in regulating the blood pressure of the body. Hormones involved in regulating the blood pressure are renin, angiotensin II and aldosterone.

These there hormones form the renin-angiotensin-aldosterone axis in the body which regulates the blood pressure.

When the blood pressure is low like after severe blood loss and subsequent hypovolemia, the blood flow to the kidneys are decreased as the brain and heart are prioritised over other organs. This reduced blood flow to the kidneys cause the juxtaglomerular cells (JG cells) of the kidneys to produce a hormone called renin.

Renin which is produced by JG cells of the kidneys causes the conversion of angiotensinogen (produced by liver) to angiotensin I which is converted into angiotensin II by angiotensin converting enzyme (ACE). ACE is synthesized by the endothelial cells of the lungs.

This angiotensin II is a potent vasoconstricter. It causes constriction of the blood vessels causing increase in total peripheral resistance of the systemic circulation thereby increasing the blood pressure and helping in bringing back the blood pressure to optimum and normal range.

Angiotensin II also helps in the synthesis of another hormone called aldosterone from the adrenal cortex. Aldosterone acts on the collecting ducts of nephrons in the kidneys and increases the reabsorption of sodium and water as well as increasing the excretion of potassium to maintain the electrolyte balance. This increased reabsorption of water and sodium increases the blood volume and corrects the hypovolemia. This helps in increasing the blood pressure back to normal range.