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Module 2

1. Module 2

1.13. Page 2

Lesson 2

Module 2—The Endocrine System

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The Posterior Pituitary

 

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 that 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 bloodstream.

 

In Biology 20 you learned that ADH targets cells in the kidney to reabsorb more water into the blood, producing smaller volumes of concentrated urine. In Lesson 1 you practised drawing the negative feedback loop that regulates ADH production. If ADH cannot be produced or secreted, copious amounts of urine are produced (up to 24 L in 24 h). The person experiences what is known as diabetes insipidus (from diabetes, which means "to pass through," and insipidus, which means "without taste"), which is treated with ADH in pill form.

 

hypothalamus: the region of the brain located below the cerebral hemispheres and thalamus and just above the pituitary gland; functions in maintaining homeostasis, and is especially important in co-ordinating the endocrine and nervous systems; secretes hormones of the posterior pituitary as well as releases hormones that regulate the anterior pituitary
 
posterior pituitary: an extension of the hypothalamus composed of nervous tissue that secretes hormones produced in the hypothalamus into the bloodstream; 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 bloodstream

 

vasopressin: also called antidiuretic hormone (ADH)

 

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 tumour develops.


 

The photo shows healthcare professionals talking to a woman who is near the end of her pregnancy

© webphotographeer/iStockphoto

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.

 

Read

 

To understand how ADH and oxytocin are part of the endocrine system, and to prepare for the other hormones you will study in this module, read from the top of page 444 to the heading “The Thyroid Gland: A Metabolic Thermostat” on page 446 in your textbook. You may wish to start a concept map similar to the chart on page 445 to facilitate your study of human hormones.

 

Watch and Listen

 

To review and further your understanding of the hormones associated with the posterior pituitary, watch the following segments of “The Hypothalamus and Pituitary Gland: The Master Complex.” You may be required to enter a username and password to access these videos. Contact your teacher for this information.

  • “Oxytocin”
  • “Antidiuretic Hormone (ADH)”
  • “Action of ADH”
  • “Bio Fact: Altered Levels of ADH”
Self-Check

 

To further your understanding and application of concepts related to posterior pituitary hormones, complete the following questions. Answer in full sentences where appropriate. Save your work in your course folder.

 

SC 1. In general, how is a hormone able to recognize and stimulate its target cells?

 

SC 2. Describe how the secretion of ADH is regulated by negative feedback. Include a feedback loop in your answer.

 

SC 3. What causes diabetes insipidus? Describe the symptoms of this condition.

 

SC 4. Copy and complete the following table.

 

Hormone

Produced By

Released By

Function(s)

Oxytocin

      

ADH

      

 

Check your work.
Self-Check Answers

 

SC 1. A hormone is able to recognize its target cells because the target cells have receptors on their cell membranes 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.


SC 2. When the solute concentration or 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 bloodstream. 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 concentration or osmotic concentration, which inhibits the osmoreceptors, and less ADH is released. Your diagram of a feedback loop should be similar to the one shown below.

 

The feedback loop illustrates how low fluid intake, low blood volume, and high solute concentration stimulate osmoreceptors in the hypothalamus, causing neurosecretory cells to increase the secretion of ADH. The posterior pituitary then releases the ADH, which targets the kidney, resulting in reabsorption of water and constriction of blood vessels.

 

SC 3. Diabetes insipidus is usually caused by a tumour that 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.

 

SC 4. Your completed table should resemble the following sample response.

 

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