11th Class Biology Chemical Coordination and Integration Gastro-Intestinal Mucosa, Placenta, Skin, Kidney And Heart

(1) Gastro-intestinal mucosa : Inner most layer of the wall of the alimentary canal is called mucosa. Certain cell of the mucosa of the stomach and intestine secrete important hormones. Gastro-intestinal mucosa is endodermal in origin.

(i) Stomach : The mucosa of the pyloric stomach near the duodenum secretes a hormone called gastrin. Presence of food in the stomach provides a stimulus for gastrin secretion. Gastrin stimulates the gastric glands to produce the gastric juice. It also stimulates the stomach movements.

(ii) Intestine : The intestinal mucosa secretes six hormones : secretin, cholecystokinin, enterogastrone, enterocrinin, duocrinin and villikinin. Entry of acidic food from the stomach into the duodenum serves as a stimulus for the release of these hormones.

(a) Secretin : It is produced by the small intestinal mucosa. It causes the release of sodium bicarbonate solution from the pancreas for pancreatic juice and from the liver for bile. It also inhibits the secretion and movements of stomach.

(b) Cholecystokinin-pancreozymin (CCK-PZ) : This hormone is secreted by the mucosa of entire small intestine. The actions of cholecystokinin and pancreozymin were discovered independently. But it has been discovered that both hormones have similar effects and hence it is considered one hormone. As the name suggest \[CCK-PZ\] has two main functions. The word cholecystokinin is derived from three roots : Chol meaning bile, Cyst meaning bladder, and kinin meaning to remove. The word pancreozymin is derived from pancreas and Zymin, which means enzyme producer. This hormone stimulates the gall bladder to release the bile and also stimulates the pancreas to release its enzymes.

(c) Enterogastrone : It is secreted by the duodenal mucosa. It shows gastric contractions and stops the secretion of gastric juice.

(d) Enterocrinin : It is secreted by duodenal mucosa. It stimulates crypts of Lieberkuhn to secrete the enzymes in the intestinal juice.

(e) Duocrinin : It is secreted by the duodenal mucosa. It stimulates the release of viscous mucus from Brunner’s glands into the intestinal juice.

(f) Villikinin : It is secreted by the mucosa of the entire small intestine. It accelerates the movements of villi to quicken absorption of food.

(2) Placenta : When the early embryo reaches into the uterus from fallopian tube, it becomes implanted with uterine wall by a placenta for support and nutrition. The cells of placenta secrete two steroid hormones (estradiol and progesterone) and two protein hormones (human chorionic gonadotropin-hcG and human chorionic somato mammotrophin-HCS). Early placenta secretes so much of chorionic gonadotropin that the latter starts being exerted in mother’s urine just after about two weeks of pregnancy. Its presence in urine is used for pregnancy test. It serves to maintain the corpus luteum, and to stimulate it for secretion. Due to its effect, the corpus luteum continues secreting estrogens, progesterone and relaxin. It also serves to maintain pregnancy by preventing contraction of uterine wall. After about three months of pregnancy, secretion of progesterone by the placenta increases. Hence, importance of corpus luteum decreases, and it starts degenerating. If therefore, ovaries are surgically removed at this stage, pregnancy remains unaffected, i.e. there is no abortion and the fetus grows and develops normally.

The chorionic somatomammotropin was formerly known as placental lactogen. Reaching into mother’s body, its serves as a mid growth hormone and promote growth of milk glands.

Relaxin hormone : This hormone has been obtained from corpus luteum of ovaries and from the placenta. It is a polypeptide. During pregnancy it causes relaxation of the ligaments of pubic symphysis, and towards the termination of pregnancy, softens and widens the opening (cervix) of uterus for easy child birth (parturition). A temporary structure with endocrine function is placenta.

(3) Skin : Vitamins of D group are synthesized in skin cells under the effect of ultraviolet (UV) rays of sunlight from cholesterol-derived compounds. Cholecalciferol \[({{D}_{3}})\] is the main D vitamin. It circulates in blood. Liver cells convert it into hydroxycholecalciferol (calcidiol) by hydroxylation and release back into blood. Certain cells of proximal convoluted tubules of nephrons in the kidneys convert calcidiol into dihydroxycalciferol (calcitriol) by further hydroxylation and release back into blood. Calcitriol is an important regulator of \[C{{a}^{2+}}\] homeostasis. It promotes absorption of \[C{{a}^{2+}}\] and phosphorus in intestine and bone-formation. It is therefore, required for growth of body and bone healing. Its deficiency in childhood causes thin, weak and curved bones, a condition called rickets. Its deficiency after growth period, causes weak, porous and fragile bones. This called osteomalacia.

(4) Kidney : Whenever the rate of ultrafiltration in kidneys decreases due to low blood pressure (BP), the cells of juxtamedullary complexes secrete into blood a compound named renin. The latter is a proteolytic enzyme. It acts upon a large plasma-protein formed in liver and called angiotensinogen, separating a small protein from it called angiotensin-I. Besides their function of excretion, the kidneys secrete three hormones, viz calcitriol, renin and erythropoetin. Calcitriol is the active form of vitamin \[{{D}_{3}}\] as already described. While the blood flows in blood capillaries of liver, an angiotensin-converting enzyme (ACE) converts angiotensin-I into angiotensin-II which acts as a hormone. This hormone accelerates heartbeat and constricts arterioles increasing blood pressure. Consequently, the rate of ultrafiltration increases. Simultaneously, it stimulates adrenal cortex to secrete aldosterone, and enhances water and sodium reabsorption from nephrons. These factors also increase the volume of ECF, elevating blood pressure.

Erythropoetin (EPO) controls formation of erythrocytes (red blood corpuscles-RSCs) in red bone marrow. That is why, its secretion increases on decrease in blood volume, or RBC count, or haemoglobin deficiency (anaemia). Hence EPO is also called renal erythropoitic factor. Contrarily, its secretion decreases when RBC count tends to increase due to blood transfusion or other reasons.

(5) Heart : When volume of ECF and blood pressure (BP) increase due to retention of more \[NaCl\] in the body, certain cardiac muscle cells of the atria of heart secrete an atrial natriuretic peptide (ANP) which acts as a hormone. The effect of ANP is to promote copious urination (diuresis) and excretion of \[NaCl\] (natriuresis) to normalise ECF volume and BP. It also inhibits the effect of vasoconstrictor hormones and secretion of renin, aldosterone and vasopressin hormones.

List of hormones, their chemical nature and functions

Disease caused by hormonal irregularities