Endocrine System Organs Notes

Pineal Gland

Situation And Structure: Pineal gland is otherwise called epiphysis. It is a small cone-shaped structure. In humans, it is about 10 mm long. Pineal gland is located in diencephalic area of brain above the hypothalamus. In human, pineal gland has two types of cells:

1. Parenchymal cells, which are large epithelial cells
2. Neuroglial cells.

In adults, the pineal gland is calcified. But, the epithelial cells exist and secrete the hormonal substance.

Read And Learn More: Medical Physiology Notes

Functions: Pineal gland has two functions:

1. It controls the sexual activities in animals by regulating seasonal fertility. However, the pineal gland plays little role in regulating the sexual functions in human being
2. The parenchymal cells of pineal gland secrete a hormonal substance called melatonin.

• Melatonin:
  • Source of secretion: Melatonin is secreted by the parenchymal cells of the pineal gland.
  • Chemistry: It is an indole (N-acetyl-5 methoxytryptamine).
  • Actions: Melatonin acts mainly on gonads. Its action differs from species to species. In some animals, it stimulates the gonads while in other animals it inhibits the gonads.
    • In humans, it inhibits the onset of puberty by inhibiting the gonads.
  • Diurnal variation in melatonin secretion: Melatonin secretion is more in darkness than in daylight. In animals the secretion of melatonin varies according to activities in different periods of the day, i.e. circadian rhythm. Hypothalamus is responsible for the circadian fluctuations of melatonin secretion.

Thymus

Situation: It is situated in front of trachea below the thyroid gland. Thymus is small in newborn infants and gradually enlarges till puberty, and then decreases in size.

Functions: Thymus has lymphoid function and endocrine function. It plays an important role in the development of immunity in the body. It has two functions:

1. Processing the T lymphocytes
2. Endocrine function.

 

1. Processing the TLymphocytes: Thymus plays an essential role in the development of immunity by processing the T lymphocytes. The lymphocytes, which are produced in bone marrow, are processed in thymus into T lymphocytes. It occurs during the period between three months before birth and three months after birth. 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: It is also called thymopoietin. It suppresses the neuromuscular activity by inhibiting acetylcholine release. Hyperactivity of thymus causes myasthenia gravis.

Kidneys

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 a small quantity of C-type natriuretic peptide.

1. Erythropoietin:
   • Source of Secretion: Endothelial cells of peritubular capillaries in the kidney secrete erythropoietin. The stimulant for its secretion is hypoxia. Erythropoietin is a glycoprotein with 165 amino acids.
   • Action of Erythropoietin: Erythropoietin stimulates the bone marrow and causes erythropoiesis. More details are given
2. Thrombopoietin: Thrombopoietin is a glycoprotein. It is secreted by the kidneys and liver. It stimulates production of platelets.
3. Renin
   • Source of Secretion: The granular ceils of the juxtaglomerular apparatus of the kidney secrete renin.
   • Actions of Renin
     • When renin released into the blood, it acts on a specific pis-sma protein called alpha2 globulin. It is also called angiotensinogen or renin substrate.
     • Renin converts angiotensinogen into angiotensin 1 which is converted into angiotensin 2 by a converting enzyme. The other details of renin and angiotensin are given.
4. 1,25-Dihydroxycholecalciferol – Calcitriol
   • Formation of 1,25-Dihydroxycholecalciferol
     • 1,25-dihydroxycholecalciferol is otherwise known as calcitriol or activated vitamin D. It is formed from cholecalciferol which is present in skin and intestine.
     • The cholecalciferol (vitamin D₃) from skin or intestine is converted into 25-hydroxycholecalciferol in the liver. This in turn, is activated into 1,25-dihydroxycholecalciferol by parathormone in the kidney.
   • Action of 1,25-Dihydroxycholecalciferol: The activated vitamin D plays an important role in the maintenance of blood calcium levels. It acts on the intestinal epithelium and enhances the absorption of calcium from intestine into the blood. Details are given.
5. Prostaglandins: The prostaglandins secreted from kidney are PGA₂ and PGE₂. These hormones are secreted by juxtaglomerular cells and type 1 interstitial cells present in the medulla of kidney. The prostaglandins decrease the blood pressure by systemic vasodilatation, diuresis and natriuresis. Details of prostaglandins are given.

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

• Atrial natriuretic peptide (ANP) is a polypeptide with 28 amino acids. It is secreted by atrial musculature of the heart. Recently, it is found in the hypothalamus of brain also. However, its action in brain is not known.
• ANP is secreted during overstretching of atrial muscles in conditions like increase in blood volume. ANP in turn increases the excretion of sodium (followed by water excretion) through urine and helps in the maintenance of ECF volume and blood volume. It also lowers blood pressure.
  • Effect of ANP on Sodium Excretion: ANP increases the excretion of sodium ions through urine by
    1. Increasing glomerular filtration rate by relaxing mesangial 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 the 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 the juxtaglomerular apparatus of the kidney
    3. Inhibiting vasoconstrictor effect of angiotensin 2
    4. Inhibiting vasoconstrictor effects of catecholamines.

Brain Natriuretic Peptide

• Brain natriuretic peptide (BNP) is also called B-type natriuretic peptide. It is a polypeptide with 32 amino acids. It is secreted by the cardiac muscle. It is also secreted in some parts of brain. The stimulant for its secretion is not known
• BNP has the same actions of ANP (see above). On the brain, its actions are net known.
  • Clinical importance of BNP: Measurement of prasma level of BNP (BNP test) is becoming an important diagnostic tool for heart diseases.
    • Normally, blood contains very small amount of BNP. However, in conditions like heart failure, BNP level is increased in blood.

C-Type Natriuretic Peptide

• C-type natriuretic peptide (CNP) is the newly discovered peptide hormone. It is a 22 amino acid peptide. Initially, it was identified in brain.
• Now it is known to be secreted by several tissues which include the myocardium, endothelium of blood vessels, gastrointestinal tract and kidneys. The functions of this hormone are not fully studied. It is believed that it has similar action of atrial natriuretic peptide.