Oxytocin stimulates uterine contractions and milk ejection from the breast. During childbirth, stretching of the uterine cervix by the baby's head causes the release of oxytocin from the posterior pituitary via positive feedback. Oxytocin then causes stronger contractions to further dilate the cervix and force the baby out, after which oxytocin levels drop. Suckling also triggers oxytocin release to contract the breast and eject milk via positive feedback. Antidiuretic hormone reduces urine production by increasing water reabsorption in the kidneys. Its release from the posterior pituitary is stimulated by increased osmotic pressure in the blood, for example during dehydration. This helps the body retain

1. Anatomy-POSTERIOR
PITUITARY HORMONES

2. OXYTOCIN
Oxytocin stimulates two target tissues
during and after parturition (childbirth):
uterine smooth muscle
and the muscle cells of the lactating breast.

3. During parturition increasing amounts of
oxytocin are released by the posterior
pituitary into the bloodstream
in response to increasing distension of
sensory stretch receptors in the uterine cervix
by the baby's head.

4. Sensory impulses are generated and travel to
the control Centre in the hypothalamus,
stimulating the posterior pituitary to release
more oxytocin.

5. In turn this stimulates more forceful uterine
contractions
and greater stretching of the uterine cervix
as the baby's head is forced further
downwards.

6. This is an example of a positive feedback
mechanism which stops soon after the baby
is delivered
when distension of the uterine cervix is
greatly reduced.

7. The process of milk ejection also involves a
positive feedback mechanism.

8. Suckling generates sensory impulses that
are transmitted from the breast to the
hypothalamus.

9. The impulses trigger the release of oxytocin
from the posterior pituitary

10. and oxytocin stimulates contraction of the
myo epithelial cells around the glandular cells
and ducts of the lactating breast to contract,
ejecting milk.

11. Suckling also inhibits the release of prolactin
inhibiting hormone (PIH),
prolonging prolactin secretion and lactation.

12. The role of this hormone in males and non-
lactating females remains unclear.

13. ANTIDIURETIC HORMONE (ADH) OR
VASOPRESSIN

14. The main effect of antidiuretic hormone is to
reduce urine output
(diuresis is the production of a large volume
of urine).

15. ADH increases the permeability to water of
the distal convoluted and collecting tubules
of the nephrons of the kidneys.

16. As a result the reabsorption of water from the
glomerular filtrate is increased.

17. The amount of ADH secreted is influenced by
the osmotic pressure of the blood circulating
to the osmo receptors in the hypothalamus.

18. As the osmotic pressure rises,
the secretion of ADH increases as in,
for example, dehydration and following
haemorrhage.

19. More water is therefore reabsorbed
and the urine output is reduced.

20. This means that the body retains more water
and the rise in osmotic pressure is reversed.

21. Conversely,
when the osmotic pressure of the blood is
low,
for example after a large fluid intake,
secretion of ADH is reduced,
less water is reabsorbed and more urine is
produced.

22. At high concentrations,
for example after severe blood loss,

23. ADH causes smooth muscle contraction,
especially vasoconstriction in the blood
vessels of the skin and abdominal organs.

24. This has a pressor effect,
raising systemic blood pressure;
the alternative name of this hormone,
vasopressin, reflects this effect.

25. THANKING YOU