Which are the hormones secreted by neurohypohysis what is their role in metabolism?
The hormones of the neurohypophysis (Posterior lobe of the Pituitaiy) are actually synthesised in the hypothalamus by the neurosecretary cells hence the name neurohypophyseal hormones. After secretion, the hormones reach the posterior lobe of the pituitary via the neural pathways. These hormones are stored in the neurohypophysis before their release into circulation.
The neurohypophysis secrets two hormones viz vasopressin and oxytocin.
This is also known as antidiuretic hormone (ADH) and causes the reabsorption of water into the blood from the collecting ducts of the kidney. Chemically ADH is a protein chain made up of 8 amino acids. The following are the functions of ADH
1. It plays an important role in the water balance (osmoregulation) of the body. When the fluid contents of the body become highly concentrated, ADH is secreted. This reaches the kidney via blood capillaries where it increases the permiability of the tubules thereby most of the water is reabsorbed and the urine gets concentrated with reduction in volume.
2. This hormone maintains the proper osmotic concentration of the blood plasma.
3. The hormone is destroyed as soon as its function is over and is excreted out along with the urine.
4. It has the role to play in the maintenance of the blood pressure. The secretion of this hormone is all the more increased during haemoirnage.
5. Alcohol inhibits the secretion of ADH thereby causing diuresis.
6. Insufficient secretion of ADH causes diuresis or diabetes insipidus. Such individuals consume large quantity of water to compensate for the excessive loss of water in urination.
The term oxytocin in Greek means “quick birth”. Chemically it is a peptide made up of 8 aminoacids. It is extremely short lived in human blood having a half life of about 1 to 4 minutes. After its function is over it is either destroyed by the tissues or excreted by the kidneys.
This hormone is released by the posterior lobe of the pituitary by a neural mechanism bringing about contraction of the smooth muscles of the uterus and breast. The secretion of this hormone is stimulated by suckling and other kinds of stimuli. It is believed that suckling stimulates the neural pathways as a result of which oxytocin is released causing release of milk from the breast of the mother.
This hormone has a powerful stimulating action on the pregnant uterus, particularly during parturition. It has been seen that a normal level of oxytocin in the blood is essential for normal delivery. This hormone also has an influence on the neutrality of the uterus which promotes fertilization during coitus.