Hormones of Posterior Pituitary : Endocrine physiology USMLE Step 1

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Hormones of Posterior Pituitary : Endocrine physiology USMLE Step 1

The posterior pituitary gland is composed of nervous tissue, rather than endocrine tissue, and is an extension of the hypothalamus. The hypothalamus produces and releases two hormones, oxytocin and antidiuretic hormone (ADH), which are transported down axons to the posterior pituitary gland for storage and release into the bloodstream.

Oxytocin:
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Oxytocin is produced by the paraventricular nucleus of the hypothalamus.
Its primary function is to stimulate uterine contractions during labor and delivery.
Oxytocin also stimulates milk letdown during breastfeeding by causing the contraction of myoepithelial cells surrounding the alveoli in the mammary glands.
In addition to its role in reproductive physiology, oxytocin is involved in social bonding, trust, and empathy. It is sometimes referred to as the "cuddle hormone" because of its effects on human behavior.

Antidiuretic Hormone (ADH):
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ADH is produced by the supraoptic nucleus of the hypothalamus.
Its primary function is to regulate the body's water balance by reducing urine output and increasing water reabsorption in the kidneys. This is achieved by promoting the insertion of aquaporin-2 channels in the renal collecting ducts, which allows for water to be reabsorbed back into the bloodstream.
ADH also has vasoconstrictive effects, which increase blood pressure by constricting blood vessels.
In response to dehydration or increased blood osmolality, ADH release is stimulated to increase water reabsorption and decrease urine output.
Conversely, in response to overhydration or decreased blood osmolality, ADH release is inhibited to decrease water reabsorption and increase urine output.

Clinical Conditions:
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Diabetes insipidus (DI) is a condition characterized by excessive urine output and thirst due to a deficiency in ADH. There are two types of DI: central DI, which is caused by a deficiency in ADH production, and nephrogenic DI, which is caused by the kidneys' inability to respond to ADH. Central DI can be treated with exogenous ADH, while nephrogenic DI is treated with diuretics or other medications to decrease urine output.
Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a condition characterized by excessive ADH secretion, leading to water retention, hyponatremia, and concentrated urine output. SIADH can be caused by various conditions, such as lung cancer, head trauma, or medication use, and is treated by addressing the underlying cause and restricting fluid intake.

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