The hormones produced by the posterior pituitary gland include vasopressin and oxytocin. Vasopressin is also referred to as antidiuretic hormone (ADH) and acts on the kidney to conserve water. It is also important for maintaining fluid and electrolyte balance in the body. There are specialized osmoreceptors in the hypothalamus that assist with water homeostasis. Osmoreceptors are nerve cells that have the inherent ability to detect the amount of solute, such as sodium or potassium, in the blood. When the body loses water (i.e., through sweating during exercise, vomiting, diarrhea), dehydration may occur and the plasma solute concentration increases. The hypothalamic osmoreceptors then initiate the release of ADH from the posterior pituitary gland. ADH primarily acts on the kidneys to increase the amount of water reabsorbed from the kidney filtrate back into the blood. The amount of urine produced thereby decreases and, consequently, the urine filtrate becomes more concentrated and darker in color. Increased reabsorption of water helps counter the increased solute concentration of the blood.
A decrease in blood volume or blood pressure, such as during hypovolemic shock, can also prompt ADH release. By promoting greater water reabsorption in the kidney, blood volume increases, thereby maintaining blood pressure in times of volume loss. ADH can also function as a vasopressor, or an agent that constricts the blood vessels, further increasing and normalizing blood pressure. This action of ADH is especially prevalent in the peripheral, small arteries.
Reduced secretion of ADH by the posterior pituitary can lead to diabetes insipidus (DI), a condition associated with polyuria, or frequent urination. This is because those with DI cannot concentrate their urine and large volumes of urine are subsequently produced daily, potentially leading to severe dehydration.
The other hormone released by the posterior pituitary gland is oxytocin. Oxytocin is mainly produced in the hypothalamus, where it is either sent to the pituitary gland for storage and subsequent release into the blood or is sent to other parts of the brain and spinal cord, where it binds to oxytocin receptors to influence behavior and physiology. Oxytocin is important in childbirth since it stimulates the contraction of the smooth muscles of the uterus. Because of this effect, synthetic oxytocin (pitocin) is sometimes used to induce labor or increase the force of contractions. Oxytocin also contracts the smooth muscle of the breast to allow the milk to flow during breastfeeding, referred to as the milk letdown reflex. More recently, oxytocin has been found to have roles in facilitating orgasm, social recognition, bonding, and maternal behaviors. Oxytocin has also been shown to play a role in sperm movement in those assigned male at birth. It also appears to affect testosterone levels in the testes.
Individuals assigned male at birth who have high levels of oxytocin sometimes develop benign prostatic hyperplasia, or the enlarging of the prostate gland. A lack of oxytocin in those assigned female at birth can prevent the milk letdown reflex and make breastfeeding difficult.