Created by CK-12 Foundation/Adapted by Christine Miller
Milk on Demand
This adorable nursing infant (Figure 9.4.1) is part of a positive feedback loop. When he suckles on the nipple, it sends nerve impulses to his mother’s hypothalamus. Those nerve impulses “tell” her pituitary gland to release the hormone prolactin into her bloodstream. Prolactin travels to the mammary glands in the breasts and stimulates milk production, which motivates the infant to keep suckling.
What Is the Pituitary Gland?
The pituitary gland is the master gland of the endocrine system, which is the system of glands that secrete hormones into the bloodstream. Endocrine hormones control virtually all physiological processes. They control growth, sexual maturation, reproduction, body temperature, blood pressure, and metabolism. The pituitary gland is considered the master gland of the endocrine system, because it controls the rest of the endocrine system. Many pituitary hormones either promote or inhibit hormone secretion by other endocrine glands.
Structure and Function of the Pituitary Gland
The pituitary gland is about the size of a pea. It protrudes from the bottom of the hypothalamus at the base of the inner brain (see Figure 9.4.2). The pituitary is connected to the hypothalamus by a thin stalk (called the infundibulum). Blood vessels and nerves in the stalk allow direct connections between the hypothalamus and pituitary gland.
Anterior Lobe
The anterior pituitary is the lobe is at the front of the pituitary gland. It synthesizes and releases hormones into the blood. Table 9.4.1 shows some of the endocrine hormones released by the anterior pituitary, including their targets and effects.
Table 9.4.1
Endocrine Hormones Released by the Anterior Pituitary, and Their Targets and Effects.
| Anterior Pituitary Hormone | Target | Effect |
| Adrenocorticotropic hormone (ACTH) | Adrenal glands | Stimulates the cortex of each adrenal gland to secrete its hormones. |
| Thyroid-stimulating hormone (TSH) | Thyroid gland | Stimulates the thyroid gland to secrete thyroid hormone. |
| Growth hormone (GH) | Body cells | Stimulates body cells to synthesize proteins and grow. |
| Follicle-stimulating hormone (FSH) | Ovaries, testes | Stimulates the ovaries to develop mature eggs. stimulates the testes to produce sperm. |
| Luteinizing hormone (LH) | Ovaries, testes | Stimulates the ovaries and testes to secrete sex hormones; stimulates the ovaries to release eggs. |
| Prolactin (PRL) | Mammary glands | Stimulates the mammary glands to produce milk. |
The anterior pituitary gland is regulated mainly by hormones from the hypothalamus. The hypothalamus secretes hormones (called releasing hormones and inhibiting hormones) that travel through capillaries directly to the anterior lobe of the pituitary gland. The hormones stimulate the anterior pituitary to either release or stop releasing particular pituitary hormones. Several of these hypothalamic hormones and their effects on the anterior pituitary are shown in the table below.
Table 9.4.2
Hypothalamic Hormones and Their Effects on the Anterior Pituitary
| Hypothalamic Hormone | Effect on Anterior Pituitary |
| Thyrotropin releasing hormone (TRH) | Release of thyroid stimulating hormone (TSH) |
| Corticotropin releasing hormone (CRH) | Release of adrenocorticotropic hormone (ACTH) |
| Gonadotropin releasing hormone (GnRH) | Release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) |
| Growth hormone releasing hormone (GHRH) | Release of growth hormone (GH) |
| Growth hormone inhibiting hormone (GHIH) (Somatostatin) | Stopping of growth hormone release |
| Prolactin releasing hormone (PRH) | Release of prolactin |
| Prolactin inhibiting hormone (PIH) (Dopamine) | Stopping of prolactin release |
Posterior Lobe
The posterior pituitary is the lobe is at the back of the pituitary gland. This lobe does not synthesize any hormones. Instead, the posterior lobe stores hormones that come from the hypothalamus along the axons of nerves connecting the two structures (also shown in Figure 9.4.2). The posterior pituitary then secretes the hormones into the bloodstream as needed. Hypothalamic hormones secreted by the posterior pituitary include vasopressin and oxytocin.
- Vasopressin (also called antidiuretic hormone, or ADH) helps maintain homeostasis in body water. It stimulates the kidneys to conserve water by producing more concentrated urine. Specifically, vasopressin targets ducts in the kidneys and makes them more permeable to water. This allows more water to be resorbed by the body, rather than excreted in urine.
- Oxytocin (OXY) targets cells in the uterus to stimulate uterine contractions, as in childbirth. It also targets cells in the breasts of a nursing mother to stimulate the letdown of milk.
9.4 Summary
- The pituitary gland is the master gland of the endocrine system, because most of its hormones control other endocrine glands.
- The pituitary gland is at the base of the brain, where it is connected to the hypothalamus by nerves and capillaries. It has an anterior (front) lobe that synthesizes and secretes pituitary hormones and a posterior (back) lobe that stores and secretes hormones from the hypothalamus.
- Hormones synthesized and secreted by the anterior pituitary include growth hormone, which stimulates cell growth throughout the body, and thyroid stimulating hormone (TSH), which stimulates the thyroid gland to secrete its hormones.
- Hypothalamic hormones stored and secreted by the posterior pituitary gland include vasopressin, which helps maintain homeostasis in body water, and oxytocin, which stimulates uterine contractions during birth, as well as the letdown of milk during lactation.
9.4 Review Questions
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- Explain why the pituitary gland is called the master gland of the endocrine system.
- Compare and contrast the two lobes of the pituitary gland and their general functions.
- Identify two hormones released by the anterior pituitary, their targets, and their effects.
- Explain how the hypothalamus influences the output of hormones by the anterior lobe of the pituitary gland.
- Name and give the function of two hypothalamic hormones released by the posterior pituitary gland.
- Answer the following questions about prolactin releasing hormone (PRH) and prolactin inhibiting hormone (PIH).
- Where are these hormones produced?
- Where are their target cells located?
- What are their effects on their target cells?
- What are their ultimate effects on milk production? Explain your answer.
- When a baby nurses, which of these hormones is most likely released in the mother? Explain your answer.
- For each of the following hormones, state whether it is synthesized in the pituitary or the hypothalamus.
- gonadotropin releasing hormone (GnRH)
- growth hormone (GH)
- oxytocin
- adrenocorticotropic hormone (ACTH)
9.4 Explore More
Common Pituitary Diseases, Swedish, 2012.
Diagnosing and Treating Pituitary Tumors – California Center for Pituitary Disorders at UCSF, UCSF Neurosurgery, 2015.
Attributions
Figure 9.4.1
Breastfeeding by Petr Kratochvil on Wikimedia Commons is used under a CC0 1.0 Universal
Public Domain Dedication (https://creativecommons.org/publicdomain/zero/1.0/deed.en) license.
Figure 9.4.2
The_Hypothalamus-Pituitary_Complex by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
References
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 17.7 Hypothalamus–pituitary complex [digital image]. In Anatomy and Physiology (Section 17.3). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/17-3-the-pituitary-gland-and-hypothalamus
Swedish. (2012, April 19). Common pituitary diseases. YouTube. https://www.youtube.com/watch?v=jUKQFkmBuww&feature=youtu.be
UCSF Neurosurgery. (2015, May 13). Diagnosing and treating pituitary tumors – California Center for Pituitary Disorders at UCSF. YouTube. https://www.youtube.com/watch?v=v41AJGP-XmI&feature=youtu.be
A control mechanism that serves to intensify a response until an endpoint is reached.
A raised, coloured region of tissue on the surface of the breast from which, in females, milk leaves the breast through the lactiferous ducts to feed an infant.
A part of the brain that secretes hormones and connects the brain with the endocrine system.
The master gland of the endocrine system that secretes many hormones, the majority of which regulate other endocrine glands.
The hormone that tells the body to make breast milk when a person is pregnant or breast-feeding. Production of prolactin takes place in the pituitary gland. For those who are not pregnant or breast-feeding, there are only low levels of prolactin in the body.
An exocrine gland in humans and other mammals that produces milk to feed young offspring. Mammals get their name from the Latin word mamma, "breast".
The body system which acts as a chemical messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In humans, the major endocrine glands are the thyroid gland and the adrenal glands.
A hormone is a signaling molecule produced by glands in multicellular organisms that target distant organs to regulate physiology and behavior.
The front lobe of the pituitary gland that synthesizes and secretes pituitary hormones.
The front lobe of the pituitary gland that synthesizes and secretes pituitary hormones.
The back lobe of the pituitary gland that stores and secretes hypothalamic hormones.
A nonapeptide synthesized in the hypothalamus, also referred to as antidiuretic hormone (ADH) or arginine vasopressin (AVP). Science has known it to play essential roles in the control of the body's osmotic balance, blood pressure regulation, sodium homeostasis, and kidney functioning.
An endocrine hormone secreted by the pituitary gland that controls a variety of functions, including during childbirth to stimulate uterine contractions and during lactation to trigger milk letdown.
The back lobe of the pituitary gland that stores and secretes hypothalamic hormones.
A hormone secreted by the anterior pituitary gland that stimulates growth in cells all over the body.
A pituitary hormone that stimulates the thyroid gland to produce thyroxine, and then triiodothyronine which stimulates the metabolism of almost every tissue in the body.
