The pituitary gland regulates numerous bodily functions by releasing chemical messengers into the bloodstream. This small organ, located at the base of the brain, is divided into two distinct parts: the anterior lobe and the posterior lobe. The posterior section connects the nervous system and the endocrine system, governing fundamental processes. This article focuses on the two hormones stored and released by the posterior pituitary.
The Neurosecretory Release Mechanism
The two hormones released by the posterior pituitary are Oxytocin and Arginine Vasopressin (AVP), also known as Antidiuretic Hormone (ADH). Unlike the anterior lobe, the posterior pituitary does not synthesize these hormones but serves as a storage and release center. Production occurs in specialized nerve cells within the hypothalamus, specifically the supraoptic and paraventricular nuclei.
These large nerve cells, called magnocellular neurons, synthesize the hormones as part of larger precursor molecules. The hormones are packaged into secretory granules and transported down the long axons into the posterior pituitary gland. They are stored in the axon terminals. When the hypothalamus receives a neural signal, an electrical impulse triggers the release of the stored hormones directly into the capillary network via exocytosis, allowing them to enter the systemic circulation.
Functions of Oxytocin
Oxytocin, a nine-amino-acid peptide, is known for its actions related to reproduction and childbirth. During labor, stretching of the cervix signals the hypothalamus to release oxytocin. This hormone acts on the smooth muscle of the uterine wall, stimulating the contractions required for delivery.
This process is a positive feedback loop, where uterine contraction stimulates further oxytocin release, intensifying the process until birth. After delivery, oxytocin is released in response to suckling, causing myoepithelial cells surrounding the milk glands to contract. This action facilitates the “milk letdown” or ejection reflex.
Beyond these mechanical functions, oxytocin plays a role in human social behavior and is sometimes called the “bonding hormone.” It fosters attachment and trust, particularly in parent-infant bonds. The hormone influences social recognition, promotes feelings of well-being, and is implicated in sexual arousal and male ejaculation.
Functions of Vasopressin
Vasopressin is primarily responsible for regulating the body’s water balance and maintaining the concentration of solutes in the blood. Its alternative name, Antidiuretic Hormone (ADH), describes its main function: reducing urine production. Specialized osmoreceptors in the hypothalamus monitor the osmolality, or salt concentration, of the blood.
When blood osmolality rises, indicating dehydration, the hypothalamus signals the release of ADH. ADH travels to the kidneys, binding to receptors on the collecting ducts and distal tubules. This triggers the insertion of water channels (aquaporins) into the kidney cell membranes, increasing water reabsorption back into the bloodstream.
By recovering water, ADH restores fluid volume and lowers osmolality. Vasopressin also causes vasoconstriction, or the narrowing of blood vessels, when released in high concentrations. This action helps raise blood pressure, which is important during severe fluid loss. A lack of proper ADH regulation can lead to Diabetes Insipidus, characterized by the excretion of large volumes of dilute urine.