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Module 2 The Endocrine System

Lesson 1.2.2

1.2.2 page 2

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The Posterior Pituitary

 

You have learned that the hypothalamus lies just above the tiny, pea-sized pituitary gland in the middle of the head. The two structures are connected by a stalk. Anatomically, the pituitary gland is made up of two very different kinds of cells. The posterior pituitary is made up of modified nerve cells. It is connected to the hypothalamus by neurosecretory cells in the stalk, which secrete two hormones – antidiuretic hormone (ADH, also called vasopressin) and oxytocin. Releasing hormones (releasing factors) from the hypothalamus signal the release of the stored ADH and oxytocin from the posterior pituitary into the blood stream.

 

You have already learned that ADH targets cells in the kidney to reabsorb more water into the blood producing a smaller volume of concentrated urine. You have also practiced drawing the negative feedback loop that regulates ADH production in Lesson 1. If ADH cannot be produced or secreted, then copious amounts of urine are produced (up to 24 L/24 h). The person experiences what is known as diabetes insipidus (diabetes meaning to pass through and insipidus meaning without taste) which is treated with ADH in pill form.

 

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Oxytocin, the other hormone secreted by the hypothalamus and released by the posterior pituitary begins the contractions of the uterine muscles at the end of pregnancy, thus initiating childbirth. It also causes tiny cells that surround the milk glands in the breasts to contract and squeeze milk into the nipples. You will study oxytocin in more detail in Unit B.

 

hypothalamus: region of the brain located below the cerebral hemispheres and thalamus and just above the pituitary gland; functions in maintaining homeostasis, especially in coordinating the endocrine and nervous systems; secretes hormones of the posterior pituitary as well as releasing hormones which regulate the anterior pituitary

 

anterior pituitary: an endocrine gland which consists of secretory cells which synthesize and secrete several hormones directly into the blood

 

posterior pituitary: an extension of the hypothalamus composed of nervous tissue that secretes hormones produced in the hypothalamus into the blood; consists of a temporary storage site for hormones produced in the hypothalamus

 

neurosecretory cells: specialized nerve cells in the hypothalamus that extend into the posterior pituitary and secrete ADH and oxytocin into the posterior pituitary and subsequently into the blood stream

 

diabetes insipidus: a condition caused by a lack of ADH, which results in excessive production of very dilute urine; ADH may be produced in insufficient quantities by the hypothalamus or the posterior pituitary may fail to release it into the bloodstream when a tumor develops

 

vasopressin: also called antidiuretic hormone

Watch and Listen

 

To review and further your understanding of the hormones associated with the posterior pituitary you can watch the following video segments:

 

The Pituitary

 

Oxytocin

 

Oxytocin and child birth

 

Antidiuretic Hormone

 

 

Try This

 

TR 1.

 

To further your understanding and application of concepts on posterior pituitary hormones, complete the following questions. Answer in full sentences where appropriate. After checking your work, file it in your course folder.

  1. In general, how is a hormone able to recognize and stimulate its target cells?
  2. Describe how the secretion of ADH is regulated by negative feedback. 
  3. What causes diabetes insipidus? Describe the symptoms of this condition.
  4. Complete the following table:

Hormone

Produced By

Released By

Function(s)

Oxytocin

      

ADH

      

 

Check your work.
Self-Check Answers
  1. A hormone is able to recognize its target cells because the target cells have receptors on their cell membrane that have a complementary shape to the shape of the hormone molecules. When the hormone fits into the receptors, the cells are stimulated by the hormone.
  2. When the solute/osmotic concentration of the blood increases, tiny receptors called osmoreceptors in the hypothalamus are stimulated. In turn, neurosecretory cells in the hypothalamus are stimulated to secrete ADH, which moves along the axons of the neurosecretory cells into the posterior pituitary and is released into the blood stream. ADH moves in the blood to the kidneys where it fits into receptors on the kidney tubules. This causes the tubules to become more permeable and reabsorb more water into the blood, which in turn reduces the urine volume. The increased water in the blood reduces the solute/osmotic concentration, which inhibits the osmoreceptors, and less ADH is released. Students may include a diagram of a feedback loop as shown below.

 

 

  1. Diabetes insipidus is usually caused by a tumor which prevents the hypothalamus from producing ADH, or prevents the posterior pituitary from releasing the ADH. Consequently, the kidney tubules cannot reabsorb adequate amounts of water, which in turn increases the urine output. The main symptom of diabetes insipidus is a very large output of urine with the other components of urine remaining stable.
  2. The completed table should resemble the following:

 

Hormone

Produced By

Released By

Function(s)

Oxytocin

Hypothalamus

Posterior pituitary

Initiates uterine contractions at the end of pregnancy thus starting parturition

 

Initiates release of milk from the breast

ADH

Hypothalamus

Posterior pituitary

Stimulates kidney tubules to reabsorb water into blood, which increases blood volume

 

Stimulates constriction of blood vessels, thereby raising blood pressure