Download
1 / 24
E N D
Thyrocalcitonin A hypocalcaemic factor, originally thought to be secreted by the parathyroid gland and named calcitonin, is now thought to be produced mainly if not entirely by the thyroid gland and for this reason its name has been changed to thyrocalcitonin. It has been suggested that it is produced by cells lying alongside the thyroid vesicles known as parafollicularcells.
This hormone lowers the blood calcium when the level is too high. • The mode of action is probably to suppress the mobilization of calcium from bone. • Its importance in the regulation of the plasma calcium level has not yet been evaluated. • In addition to the parathyroid and thyroid glands, calcium intake and vitamin D, the growth and development of bone is influenced by the pituitary gland.
The Adrenal Glands The two adrenal or suprarenal glands are situated immediately above and in front of the upper pole of each kidney. The adrenal gland is a double gland with a central part or medulla and an outer part or cortex. Each part behaves as a separate and independent endocrine gland.
Adrenal Medulla The adrenal medulla releases hormones which are amines of catechol (dihydroxybenzene) and which are known collectively as catecholamines. The important hormones are noradrenaline (norepinephrine) and adrenaline (epinephrine). Noradrenaline is the chemical transmitter of the sympathetic nervous system.
The adrenal medulla is innervated by preganglionic sympathetic fibers and its activity in many respects augments that of the sympathetic nervous system throughout the body.
The action of these hormones is: • Dilate the pupils of the eyes. • Contract the arrectorespilorum muscles which cause the hair to stand on end. • Relax the bronchioles thereby increasing the size of the airways • Inhibit digestion. • Contract the sphincters of the digestive tract • Inhibit the bladder musculature.
With regard to the cardiovascular system, these hormones: • Increase the blood pressure; they constrict the arterioles and veins by a direct action (with the exception of the coronary vessels and muscle vessels which are dilated by adrenaline). • The hormones increase the activity of the heart by their direct action.
However, since an injection of these hormones increases the blood pressure and increases the baroreceptor activity, any tendency to vasoconstriction and cardiac acceleration will be opposed by the baroreceptor reflexes.
As a result, under certain conditions, there may be a reflex cardiacslowing and/or a reflex vasodilatation, with increasedblood pressure due to an increased cardiac output resulting from a more efficient emptying of the ventricles.
The vasoconstrictor action of noradrenaline and adrenaline is utilized when they are added to a, local anaestheticto localize its action by constricting the surrounding blood vessels. • Adrenalinemobilizes the liver glycogen and converts it to blood glucose.
The adrenal medulla is active under a wide variety of stress conditions such as: • Anger. • Fear. • Cold. • Low blood sugar. • Low blood pressure. • Cerebral anoxia and asphyxia. • It has recently been shown that angiotensinstimulates the release of the adrenal medulla hormones.
Adrenal Cortex Unlike the medulla the adrenal cortex has no nerve supply. It consists of columns of cells which are divided into three zones from the capsules of the gland inwards: Zonaglomerulosa. Zonafasciculata. zonareticularis.
The adrenal cortex is rich in vitamin C and cholesterol. Cholesterol is probably the precursor of the hormones produced which are termed corticoids. They may be divided into three groups:
Group I. Mineralocorticoids-Aldosterone Aldosterone is the importantmineralocorticoid produced by the adrenal cortex, that is, a corticoid that regulates the mineral salts in the body. Aldosterone acts mainly on the sodium ion (Na+) and by so doing maintains the level of sodium chloride in the body.
It maintains the level of sodium chloride by: Stimulating the reabsorption of sodium ions in the kidney tubules. Decreasing the sodium ion content of sweat. At the same time the excretion of potassium by the kidneys is increased.
Any fall in the level of circulating aldosterone will result in a fall of sodium chloride in the extracellular fluid of the body and a corresponding loss of water so that the osmotic pressure is unchanged.
Aldosterone is produced by the zonaglomerulosa. It is not controlled by the adreno-corticotrophic hormone (ACTH) from the pituitary gland, but its release is stimulated by circulating angiotensin.
Aldosterone and Blood Volume Regulation This interrelationship gives a possible mechanism for the regulation of the blood and fluid volumes in the body. Should the blood volume be low, the reduced cardiac output will reduce the blood pressure and pulse pressure in the afferent arterioles of the glomeruli of the kidneys.
Surrounding these arterioles are cells which releaserenin into the circulation under these conditions. They form what is termed the juxta-glomerular apparatus (JGA).
Renin is an enzyme which acts on the α2-globulin fraction of the plasma proteins (angiotensinogen), causing the formation of the peptide angiotensin. The angiotensin will act on the adrenal cortex causing an increase in the release of aldosterone.
Aldosterone will increase the reabsorption of sodium by the kidney tubules and thus raise the sodium level in the blood and extracellular fluid. The sodium retention will increase the osmotic pressure of the blood, and this will stimulate the osmoreceptors in the region of the hypothalamus.
The osmoreceptor stimulation will increase the release of ADH from the posterior pituitary gland which willincrease the reabsorption of water by the kidney tubules. Water is thus retained. This retention of sodium and water will restore the blood volume to normal.
