Lesson 2 — Antidiuretic Hormone and Oxytocin


Antidiuretic Hormone (ADH)

Read page 441

Antidiuretic hormone (ADH) targets cells in the kidney tubules to reabsorb water into the blood to produce a smaller volume of concentrated urine. ADH is also called a vasopressin because a high concentration of antidiuretic hormone (ADH) can cause the blood vessels to constrict, thereby increasing the blood pressure.


This diagram represents the regulation of water volume in the body by antidiuretic hormone (ADH). When a person loses water by sweating profusely but does not rehydrate with water, the result is a decrease in the plasma (watery part) of the blood so that the solute (dissolved substances) concentration of the blood increases. As a result, both the blood volume and the blood pressure decrease.

Sensors (or receptors) in the hypothalamus called osmoreceptors sense the increased solute concentration, decreased blood volume, and decreased blood pressure. The hypothalamus sends messages to the posterior pituitary gland to increase the release of the hormone ADH.

An increased level of ADH causes the kidneys (kidney tubules) to retain (reabsorb) more water and release less in the urine. Reabsorbed water in the blood increases blood volume, increases blood pressure, and decreases solute concentration. Information on these three factors is fed to the sensors in the hypothalamus and results in decreased secretion of ADH.



Disorder Related to Antidiuretic Hormone (ADH)

Although ADH is regulated by a negative feedback loop, a tiny tumor in the posterior pituitary can prevent the release of ADH and result in a condition known as diabetes insipidus. This could result in the following positive feedback loop.



In the example above, a person is losing water by sweating profusely but does not rehydrate with water. The tiny tumor in the posterior pituitary prevents the release of ADH. The kidney cannot reabsorb more water, so water is released as an increased volume of urine. This information is fed to the sensors in the hypothalamus. Positive feedback reinforces the loss of water in the body, not only from sweating and less drinking, but also from excreting increased amounts of urine.

If ADH cannot be produced or secreted, then copious amounts of urine are produced (up to 24 L in 24 h). The person experiences what is known as diabetes insipidus, which is treated with ADH in pill form. (Diabetes means to pass through and insipidus means without taste.)



Oxytocin

Read pages 523 fig. 15.14 and 526 fig. 15.16


Oxytocin, the other hormone secreted by the hypothalamus and released by the posterior pituitary, begins the contractions of the smooth muscles of the uterus at the end of pregnancy, thus initiating childbirth.

In addition, it causes tiny smooth muscle cells that surround the milk glands in the breasts to contract and squeeze milk into the nipples. Unit B studies oxytocin in more detail.

Note: The nervous system regulates the endocrine system by way of the hypothalamus, which controls the "master gland" or pituitary.



Watch and Listen

To review and further your understanding of the hormones associated with the posterior pituitary,  watch the following segment of Biologix-09 on Oxytocin and ADH (4 minutes):

 















©Alberta Education. The Hypothalamus and Pituitary: The Master Complex (7:20 - 12:18); Series 09. LearnAlberta.ca

 


Self-Check

To further your understanding and application of concepts on posterior pituitary hormones, complete the following. Answer in full sentences where appropriate.

  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

     

Self-Check Answers

  1. A hormone is able to recognize its target cells because the target cells have receptors on their cell membranes that have complementary shapes 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 reduces the urine volume. The increased water in the blood reduces the solute/osmotic concentration, which inhibits the osmoreceptors, and less ADH is released.



  3. Diabetes insipidus usually is caused by a tumor 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 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.

  4. 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


Biology 30 © 2008  Alberta Education & its Collaborative Partners ~ Updated by ADLC 2019