Pineal Gland .
Structure & Location of Pineal Gland .
[1]. Pineal gland or epiphysis is located in the diencephalic area of brain above the hypothalamus.
[2]. It is a small cone shaped structure with a length of about 10 mm.
[3]. Pineal gland has two types of cells:
1. Large epithelial cells called parenchymal cells .
2. Neuroglial cells. In adults, the pineal gland is calcified.
But, the epithelial cells exist and secrete the hormonal substance.
Functions of Pineal Gland .
Pineal gland has two functions:
1. It controls the sexual activities in animals by regulating the seasonal fertility. However, the pineal gland plays little role in regulating the sexual functions in human being
2. It secretes the hormonal substance called melatonin.
Melatonin .
Source of secretion of Melatonin .
Melatonin is secreted by the parenchymal cells of pineal gland.
Chemistry of Melatonin .
Melatonin is an indole (N-acetyl-5-methoxytryptamine).
Actions of melatonin .
[1]. Melatonin acts mainly on gonads. Its action differs from species to species.
[2]. In some animals, it stimulates the gonads while in other animals, it inhibits the gonads.
[3]. In humans, it inhibits the onset of puberty by inhibiting the gonads.
Diurnal variation in melatonin secretion .
[1]. Melatonin secretion is more in darkness than in daylight.
[2]. In animals, the secretion of melatonin varies according to activities in different periods of the day, i.e. circadian rhythm .
[3]. Hypothalamus is responsible for the circadian fluctuations of melatonin secretion.
Thymus .
Location of Thymus .
[1]. Thymus is situated in front of trachea, below the thyroid gland.
[2]. Thymus is small in newborn infants and gradually enlarges till puberty and then decreases in size.
Functions of Thymus .
[1]. Thymus has lymphoid function and endocrine function.
[2]. It plays an important role in development of immunity in the body.
[3]. Thymus has two functions:
1. Processing the T lymphocytes .
2. Endocrine function.
1. Processing the T Lymphocytes .
[1]. Thymus plays an essential role in the development of immunity by processing the T lymphocytes .
[2]. The lymphocytes which are produced in bone marrow are processed in thymus into T lymphocytes.
[3]. It occurs during the period between 3 months before birth and 3 months after birth.
[4]. So, the removal of thymus 3 months after birth, will not affect the cell-mediated immunity.
2. Endocrine Function of Thymus .
Thymus secretes two hormones:
1. Thymosin .
2. Thymin.
1. Thymosin .
Thymosin is a peptide. It accelerates lymphopoiesis and proliferation of T lymphocytes.
2. Thymin .
[1]. Thymin is also called thymopoietin. It suppresses the neuromuscular activity by inhibiting acetylcholine release.
[2]. Hyperactivity of thymus causes myasthenia gravis.
Kidneys .
Kidneys secrete five hormonal substances:
1. Erythropoietin .
2. Thrombopoietin .
3. Renin .
4. 1,25-dihydroxycholecalciferol (calcitriol) .
5. Prostaglandins.
Recently, it is discovered that kidney secretes small quantity of C-type natriuretic peptide .
1. Erythropoietin .
Source of Secretion of Erythropoietin .
[1]. Endothelial cells of peritubular capillaries in the kidney secrete erythropoietin. The stimulant for its secretion is hypoxia.
[2]. Erythropoietin is a glycoprotein with 165 amino acids.
Action of Erythropoietin .
Erythropoietin stimulates the bone marrow and causes erythropoiesis.
2. Thrombopoietin .
Source of Secretion of Thrombopoietin .
Thrombopoietin is a glycoprotein. It is secreted by kidneys and liver.
Action of Thrombopoietin .
Thrombopoietin stimulates the production of platelets.
3. Renin .
Source of Secretion of Renin .
The granular cells of juxtaglomerular apparatus of the kidney secrete renin.
Actions of Renin .
[1]. When renin is released into the blood, it acts on a specific plasma protein called alpha-2 globulin.
[2]. It is also called angiotensinogen or renin substrate.
[3]. Renin converts angiotensinogen into angiotensin I, which is converted into angiotensin II by a converting enzyme.
4. 1,25-Dihydroxycholecaliferol – Calcitriol .
Formation of 1,25-dihydroxycholecalciferol .
[1]. 1,25-dihydroxycholecalciferol is otherwise known as calcitriol or activated vitamin D.
[2]. It is formed from cholecalciferol, which is present in skin and intestine.
[3]. The cholecalciferol (vitamin D3) from skin or intestine is converted into 25-hydroxycholecalciferol in liver.
[4]. This in turn, is activated into 1,25-dihydroxycholecalciferol by parathormone in kidney .
Actions of 1,25-Dihydroxycholecalciferol .
[1]. The activated vitamin D plays an important role in the maintenance of blood calcium level.
[2]. It acts on the intestinal epithelium and enhances absorption of calcium from intestine into the blood.
5. Prostaglandins .
Source of Secretion of Prostaglandins .
[1]. Prostaglandins secreted from kidney are PGA2 and PGE2 .
[2]. These hormones are secreted by juxtaglomerular cells and type I interstitial cells present in medulla of kidney.
Action of Prostaglandins .
Prostaglandins decrease the blood pressure by systemic vasodilatation, diuresis and natriuresis.
Heart .
Heart secretes the hormones atrial natriuretic peptide and brain natriuretic peptide. Recently, another peptide called C-type natriuretic peptide is found in heart.
Atrial natriuretic peptide .
[1]. Atrial natriuretic peptide (ANP) is a polypeptide with 28 amino acids. It is secreted by atrial musculature of the heart.
[2]. Recently, it is found in hypothalamus of brain also. However, its action in brain is not known.
[3]. ANP is secreted during overstretching of atrial muscles in conditions like increase in blood volume.
[4]. ANP, in turn increases excretion of sodium (followed by water excretion) through urine and helps in the maintenance of extracellular fluid (ECF) volume and blood volume. It also lowers blood pressure.
Effect of ANP on Sodium Excretion .
Atrial natriuretic peptide increases excretion of sodium ions through urine by:
1. Increasing glomerular filtration rate by relaxing mesangeal cells and dilating afferent arterioles .
2. Inhibiting sodium reabsorption from distal convoluted tubules and collecting ducts in kidneys .
3. Increasing the secretion of sodium into the renal tubules.
Escape phenomenon .
Thus, ANP is responsible for escape phenomenon and prevention of edema in primary hyperaldosteronism, in spite of increased ECF volume .
Effect of ANP on Blood Pressure .
ANP decreases the blood pressure by:
1. Vasodilatation by relaxing the smooth muscle fibers, mainly in arterioles and venules .
2. Inhibiting renin secretion from juxtaglomerular apparatus of kidney .
3. Inhibiting vasoconstrictor effect of angiotensin II .
4. Inhibiting vasoconstrictor effects of catecholamines.
Brain natriuretic peptide .
[1]. Brain natriuretic peptide (BNP) is also called B-type natriuretic peptide.
[2]. It is a polypeptide with 32 amino acids. It is secreted by the cardiac muscle.
[3]. It is also secreted in some parts of the brain. The stimulant for its secretion is not known.
[4]. BNP has same actions of ANP (see above). On brain, its actions are not known.
Clinical Importance of BNP .
[1]. Measurement of plasma level of BNP (BNP test) is becoming an important diagnostic tool for heart diseases.
[2]. Normally, blood contains very small amount of BNP. However, in conditions like heart failure, BNP level is increased in blood.
C-type natriuretic peptide .
[1]. C-type natriuretic peptide (CNP) is the newly discovered peptide hormone.
[2]. It is a 22 amino acid peptide. Initially, it was identified in brain.
[3]. Now, it is known to be secreted by several tissues which include myocardium, endothelium of blood vessels, gastrointestinal tract and kidneys.
[4]. The functions of this hormone are not fully studied. It is believed that it has similar action of atrial natriuretic peptide.
Thanks for Visiting us .
