Hormones of the neurohypophysis

Antidiuretic hormone (ADH), also referred to as vasopressin, has two major effects, both of which are reflected by its names: (1) antidiuresis (decrease in urine formation by the kidney); and (2) vasoconstriction of arterioles.

Antidiuretic hormone promotes the reabsorption of water from the tubules of the kidney, or antidiuresis. Specifically, it acts on the collecting ducts and increases the number of water channels, which increases the diffusion coefficient for water. This results in the body's conservation of water and the production of a low volume of concentrated urine. The reabsorbed water affects plasma osmolarity and blood volume. This effect of ADH on the kidney occurs at relatively low concentrations. At higher concentrations, ADH causes constriction of arterioles, which serves to increase blood pressure. Antidiuretic hormone secretion is regulated by several factors:

• Plasma osmolarity

• Blood pressure

The primary factor that influences ADH secretion is a change in plasma osmolarity. Osmoreceptors in the hypothalamus are located in close proximity to the ADH-producing neurosecretory cells. Stimulation of these osmorecep-tors by an increase in plasma osmolarity results in stimulation of the neu-rosecretory cells; an increase in the frequency of action potentials in these cells; and the release of ADH from their axon terminals in the neurohypo-

LONG-LOOP NEGATIVE FEEDBACK

Hypothalamus 4

Thyrotropin-releasing hormone (hypothalamic hormone)

I Hypothalamic-hypophyseal I portal veins

Adenohypophysis

Thyroid-stimulating hormone (Adenohypophyseal hormone)

I Systemic circulation

SHORT-LOOP NEGATIVE FEEDBACK

Thyroid gland (Target endocrine gland)

Thyroid hormone (Endocrine gland hormone)

Systemic circulation

Target tissues

Figure 10.3 Negative-feedback regulation ofhormone release. Hormones released from the adenohypophysis are often part of a three-hormone axis that includes the hypothalamic hormone, the adenohypophyseal hormone, and the target endocrine gland hormone. Long-loop negative feedback occurs when the final hormone in the axis inhibits release of hypothalamic and/or adenohypophyseal hormones. Short-loop negative feedback occurs when the adenohypophyseal hormone inhibits release of the hypothalamic hormone. This figure illustrates the thyrotropin-releasing hormone-thyroid-stimulating hormone-thyroid hormone axis.

physis. The water conserved due to the effect of ADH on the kidney helps to reduce plasma osmolarity or dilute the plasma back to normal.

Hypothalamic osmoreceptors have a threshold of 280 mOsM. Below this value, they are not stimulated and little or no ADH is secreted. Maximal ADH levels occur when plasma osmolarity is about 295 mOsM. Within this range, the regulatory system is very sensitive, with measurable increases in ADH secretion occurring in response to a 1% change in plasma osmolarity. Regulation of ADH secretion is an important mechanism by which a normal plasma osmolarity of 290 mOsM is maintained.

Other factors regulating ADH secretion include blood volume and blood pressure. A decrease in blood volume of 10% or more causes an increase in ADH secretion sufficient to cause vasoconstriction as well as antidiuresis. A decrease in mean arterial blood pressure of 5% or more also causes an increase in ADH secretion. The resulting water conservation and vasoconstriction help increase blood volume and blood pressure back to normal. Furthermore, the effect of blood pressure on ADH secretion may be correlated to the increase in secretion that occurs during sleep, when blood pressure decreases. The result is the production of a low volume of highly concentrated urine that is less likely to elicit the micturition (urination) reflex and interrupt sleep. In contrast, alcohol inhibits the secretion of ADH, thus allowing for loss of water from the kidney. Therefore, the consumption of alcoholic beverages may actually lead to excessive water loss and dehydration instead of volume expansion.

Oxytocin also exerts its major effects on two different target tissues. This hormone stimulates:

• Contraction of uterine smooth muscle

• Contraction of myoepithelial cells

Oxytocin stimulates contraction of the smooth muscle in the wall of the uterus. During labor, this facilitates the delivery of the fetus and, during intercourse, may facilitate the transport of the sperm through the female reproductive tract. Oxytocin also causes contraction of the myoepithelial cells surrounding the alveoli of the mammary glands. This results in "milk letdown" or the expulsion of milk from deep within the gland into the larger ducts from which the milk can be obtained more readily by the suckling infant.

The secretion of oxytocin is regulated by reflexes elicited by cervical stretch and by suckling. Normally, as labor begins, the fetus is positioned head down. This orientation exerts pressure on the cervix and causes it to stretch. Sensory neurons in the cervix are thus activated to transmit signals to the hypothalamus, which will stimulate the release of oxytocin from the neurohypophy-sis. This hormone then enhances uterine contraction which causes further pressure and stretch of the cervix, additional oxytocin release, and so on, until pressure has built up adequately so that delivery can take place. In the lactating breast, suckling activates sensory neurons in the nipple to transmit signals to the hypothalamus to stimulate oxytocin release from the neuro-hypophysis and therefore milk letdown. Interestingly, this reflex may also be triggered through a conditioned response in which the sight or sound of the hungry infant is sufficient to enhance oxytocin secretion. In contrast, the release of oxytocin from the neurohypophysis may be inhibited by pain, fear, or stress.

The function of oxytocin in males is not clearly understood.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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