Post. Pituitary gland

Hormones of the Posterior Pituitary are :

(1) Antidiuretic Hormone (ADH) . (2) Oxytocin Hormone

These two hormones are synthesized in and stored in

synthesized in hypothalamus and stored in posterior pituitary gland

posterior pituitary hormones:

synthetized in cell bodies of supraoptic and paraventricular nuclei in hypothalamus.

transported in combination with carrier protein called neurophysins down to the nerve ending in the posterior pituitary gland

stimulation of hypothalamic nuclei causes release of hormone by exocytosis into the Blood

Functions of ADH:

(1) On the kidneys:

increases reabsorption of water that leads to ⬆ conc. of urine,⬇ its volume.

acts on distal portions of nephrons(collecting ducts and tubules) by increasing their permeability to water.

causes opening of many protein-water channels

2) On the Blood vessels :

potent vasoconstrictor

Under normal condition

this action does not significantly alter ABP because it is counteracted by the baroreceptor reflexes.

in hypotensive conditions, such as hemorrhage

The vasoconstrictor effect of ADH is important in maintenance of ABP ⬆ to normal

Control of ADH:

(1) Osmotic stimuli

(2) Volume depletion

(1) Osmotic stimuli:

When osmolality of plasma is increased (as dehydration or high salt level), Secretion of ADH increases leading to water retention by its action on kidney.

normal osmality 290

The change in osmolality is sensed by

by osmoreceptors which are present in hypothalamus

(2) Volume depletion:

Decrease in circulatory volume and mean arterial pressure will increase ADH

Volume depletion are sensed by

sensed by low pressure receptors (present in the vascular system as the atria, pulmonary vessels, great veins).

- Antidiuretic hormone deficiency: (Diabetes Insipidus)

(1) Polyuria

(2) Polydepsia

Polyuria:

Excretion of large amount of diluted urine

up to 18 liters.

Polydepsia:

rapid loss of fluid in urine creates a constant thirst.

Functions of oxytocin:

(1) Milk ejection

(2) Contraction of smooth muscle of uterus

(3) effect on behaviors

(4) In male

Milk ejection

causes contraction of myoepithelial cells that line alveoli and ducts of mammary gland, thus squeezing milk outwards through nipples

Contraction of smooth muscle of uterus:

-it is used clinically for induction of labour, its secretion increases during labour

- it acts on the nonpregnant uterus to facilitate sperm transport by contraction of uterus

Control of Oxytocin Secretion

(1) Suckling reflex:

(2) Stretching of the uterine cervix as during labour:

(3) Genital stimulation:

Suckling reflex:

suckling of breast stimulates touch receptors in the nipple, sensory nerves transmit impulse upward to hypothalamus to cause release of oxytocin.

Stretching of the uterine cervix as during labour:

labour can cause a neurogenic reflex that causes posterior pituitary to increase secretion of oxytocin to enhance labor .