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Chapter 18

Anatomy and Physiology HAP101 Chapter Notes - Chapter 18: Hypophyseal Portal System, Anterior Pituitary, Pituitary Gland


Department
Anatomy and Physiology
Course Code
Anatomy and Physiology HAP101
Professor
Tania Killian
Chapter
18

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HAP101 Week 13/Chapter 18: The Endocrine System
LO 18.1: Compare the control of body functions by the nervous and endocrine systems
The nervous and endocrine system act together to coordinate functions of all body systems
Similar to how the nervous systems releases neurotransmitters, the endocrine system also controls body activities by
releasing mediators, called hormones, but the means of control of the two are very different
Hormone: a mediator molecule that is released in one part of the body but regulates the activity of cells in the body
o Most enter through interstitial fluid and then the bloodstream. The circulating blood delivers hormones to cells
throughout the body. The hormones will then bind to receptors on their “target” cells and the effect will begin.
Responses of the endocrine system are slower than those of the nervous system, as most take several minutes or more to
cause a response. However, effects of the NS are briefer than the endocrine systems, as the influence of the endocrine system
is much broader (it helps regulate virtually all types of body cells)
LO 18.2: Distinguish between exocrine and endocrine glands
The body contains two kinds of glands
o Exocrine glands: these glands secrete their products into ducts that carry secretions into body cavities, the lumen of an
organ, or the outer surface of the body. They include sudoriferous (sweat), sebaceous (oil), mucous and digestive
glands
o Endocrine glands: secrete their products (hormones) into the interstitial fluid surround the secretory cells rather than
into the ducts. From this fluid, hormones diffuse into blood capillaries and blood carries them to target cells throughout
the body. They are some of the most vascular tissues of the body, since they are very dependent on the cardiovascular
system to distribute their products. Circulating levels are low, since most hormones are required in very small amounts
1. Pituitary gland
2. Thyroid gland
3. Parathyroid gland
4. Pineal gland
5. Some organs and tissues are not classified as this gland nut contain cells that secrete hormones: hypothalamus,
thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, adipose tissue, and placenta.
Endocrine system: the science of the structure and function of the endocrine glands
Endocrinology: the diagnosis and treatment of disorders of the endocrine
LO 18.3: Describe how hormones interact with target-cell receptors
A given hormone affects only specific target cells, as they chemically-bind to specific protein receptors. These receptors are
continuously being synthesized and broken down
Down-regulation: if a hormone is present in excess, the number of target-cell receptors may decrease. This process makes a
target cell less sensitive to a hormone.
Up-regulation: if a hormone is deficient, the number of receptors may increase. This process makes a target cell more
sensitive to a hormone
Circulating hormones: hormones that pass from the secretory cells that make them into interstitial fluid and then into the
blood
Local hormones: hormones that act locally on neighboring cells or on the same cell that secreted them without first entering
the bloodstream.
Local hormones that act on neighboring cells are called paracrines and those that act on the same cell that secreted them are
called autocrines
Local hormones are usually are inactivated quickly; circulating hormones may linger in the blood and exert their effects for a
few minutes or occasionally for a few hours. In time, circulating hormones are inactivated by the liver and excreted by the
kidneys. In cases of kidney or liver failure, excessive levels of hormones many build up in blood
LO 18.4: Compare the 2 classes of hormones based on their solubility and mechanisms of action
Hormones are divided into two chemical classes:
o Lipid-soluble hormones: include steroid hormones, thyroid hormones, and nitric oxide
1. Steroid hormones: derived from cholesterol; each is unique due to the presence of different chemical groups
attached at various sites on the four rings at the core of its structure these small differences allow for diversity of
functions
2. Thyroid hormones: T3/T4 are synthesized by attaching iodine to tyrosine. The presence of two benzene rings
with T3/T4 make the molecule very lipid soluble
3. Nitric Oxide: this is a hormone and an NT and its synthesis is catalyzed by the enzyme nitric oxide synthase
o Water-soluble hormones:
1. Amine hormones
2. Peptide hormones and protein hormones
3. Eicosanoid hormones
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Hormone Transport in the Blood
o Most water-soluble hormone molecules circulate in the watery blood plasma in a “free” form (not attached to other
molecules)
o Most lipid-soluble hormone molecules are bound to transport proteins (are synthesized by cells in the liver)
o Transport proteins have three functions
1. Make lipid-soluble hormones temporarily water-soluble, increasing their solubility in blood
2. They retard passage of small hormone molecules through the filtering mechanism in the kidneys, slowing the rate
of hormone loss in the urine
3. They provide a ready reserve of hormone, already present in the bloodstream
Mechanisms of Hormone Action
o A response to a hormone depends on both the hormone itself and the target cell, as various target cells respond
differently to the same hormone
o The response is not always the synthesis of new molecules. Other hormonal effects including changing the
permeability of the plasma membrane, stimulating transport of a substance into or out of the target cells, altering the
state of specific metabolic reactions or causing contraction of smooth muscle or cardiac muscle
Action of LIPID-SOLUBLE HORMONES
o A free lipid-soluble hormone molecule diffuses from the blood, through interstitial fluid, and through the lipid bilayer
of the plasma membrane into a cell
o If the cell is a target cell, the hormone binds to and activates receptors located within the cytosol or nucleus. The
activated receptor-hormone complex then alters gene expression: turns specific genes of the nuclear DNA on or off
o As the DNA is transcribed, new messenger RNA (mRNA) forms, leaves the nucleus, and enters the cytosol. There, it
directs synthesis of a new protein, often an enzyme, on the ribosomes
o The new proteins alter the cell’s activity and cause the responses typical of that hormone
Actions of WATER-SOLUBLE HORMONES
o A water-soluble hormone (the first messenger) diffuses from the blood through interstitial fluid and then binds to its
receptor at the exterior surface of a target cell’s plasma membrane. The hormone-receptor complex activates a
membrane protein called a G protein. The activated G protein in turn activates adenylate cyclase
o Adenylate cyclase converts ATP into cyclic AMP (cAMP). Since the enzyme’s active site in on the inner surface of the
plasma membrane, this reaction occurs in the cytosol of the cell
o Cyclic AMP (the second messenger) activates one or more protein kinases, which may be free in the cytosol or bound
to the plasma membrane. A protein kinase is an enzyme that phosphorylates (adds a phosphate group to) other cellular
proteins (such as enzymes). The donor of the phosphate group is ATP ADP
o Activated protein kinases phosphorylates one or more cellular proteins. Phosphorylation activates some of these
proteins and inactivates others, rather like turning a switch on or off
o Phosphorylated proteins cause reactions that produce physiological responses. Different proteins kinases exist within
different target cells and within different organelles of the same target cells. So, one protein kinase might trigger
glycogen synthesis, a second might cause the breakdown of triglyceride, a third may promote protein synthesis and so
forth.
o After a brief period, an enzyme called phosphodiesterase inactivates cAMP. Thus, the cell’s response is turned off
unless new hormone molecules continue to bind to their receptors in the plasma membrane
Hormone Interactions
o The responsiveness of a target cell to a hormone depends on the hormone’s concentration in the blood, the abundance
of the target cell’s hormone receptors, and influences exerted by other hormones. It responds more when hormone
levels rise or when to up-regulation.
o Actions of some hormones on target cells require a simultaneous or recent exposure to a second hormone, as it has a
permissive effect (increase the number of receptors for the other hormone; promotes the synthesis of an enzyme
required for the expression of the other hormone’s effects)
o Synergistic effect: when the effect of two hormones acting together is greater or more extensive than the effect of each
hormone acting alone, e.g. the development of oocytes in ovaries requires both follicle-stimulating hormones from
anterior pituitary and estrogen from the ovaries
o Antagonistic effect: when one hormone opposes the actions of another hormone, e.g. insulin, which promotes
synthesis of glycogen by liver cells, and glucagon which stimulates breakdown of glycogen in the liver
Control of Hormone Secretion
o Most hormones are released in short bursts, with little or no secretions in between.
o When stimulated an endocrine gland will release its hormone in more frequent bursts, increasing the concentration of
the hormone in the blood. The blood level of the hormone will decrease in the absence of stimulation
o Regulation of secretion prevents overproduction or underproduction of any given hormone to help maintain
homeostasis:
1. Signals from the NS
2. Chemical changes in blood
3. Other hormones
o Most hormone regulation works via negative feedback, while some operate through positive feedback
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LO 18.5: Describe the location, histology, hormones, and functions of the hypothalamus, anterior and posterior
pituitary, thyroid gland, parathyroid glands, adrenal glands and the pancreatic islets
The pituitary gland and the hypothalamus work together to control the endocrine system
o Pituitary gland: is a pea-shaped structure lies in the hypophyseal fossa of the sella turcia of the sphenoid bone;
attaches to the hypothalamus by the infundibulum. It has two anatomically and functionally separate portions: the
anterior pituitary and the posterior pituitary
Anterior pituitary (adenohypophysis) secretes hormones that regulate a wide range of bodily activities, from growth to
reproduction. The release of hormones from here is stimulated by releasing hormones and suppressed by inhibiting
hormones from the hypothalamus. It is mainly composed of epithelial tissue. In total, it secrets seven hormones
o Hypophyseal Portal System:
Blood passes from one capillary network into a portal vein, and then into a second capillary network before
returning to the heart. In this network, blood flows from capillaries in the hypothalamus into portal veins that
carry blood to capillaries of the anterior pituitary
Superior hypophyseal arteries: these are branches of the internal carotid arteries that bring blood into the
hypothalamus
Primary plexus of the hypophyseal portal system is a capillary network that is created by arteries at the
junction of the median eminence of the hypothalamus and the infundibulum. From here blood drains into the
hypophyseal portal veins that pass down the outside of the infundibulum (rd more on 623)
o Types of Anterior Pituitary Cells and Their Hormones: these are the five types of cells that secrete seven hormones
1. Somatotrophs: secrete the human growth hormone that stimulates several tissues to secrete insulin-like
growth factors (IGFs), hormones that stimulate general body growth and regulate aspects of metabolism
2. Thyrotrophs: secrete thyroid-stimulating hormone (TSH), i.e. thyrotropin to control secretions and other
activities of the thyroid gland
3. Gonadotrophs: secrete two gonadotropins: follicle-stimulating hormone (FSH) and luteinizing hormone
(LH). FSH and LH both act on the gonads and stimulate secretion of estrogen and progesterone and the
maturation of oocytes in the ovaries, and they stimulate sperm production and secretion of testosterone in the
testes
4. Lactotrophs: secrete prolactin (PRL), which initiates milk production in the mammary glands
5. Cortioctrophs: secrete adrenocorticotropic hormone (ACTH), also known as corticotrophin, which stimulates
the adrenal cortex to secrete glucocorticoids such as cortisol. Some also secrete melanocyte-stimulating
hormone (MSH)
o Control of Secretion by the Anterior Pituitary
1. Neurosecretory cells in the hypothalamus secrete five releasing hormones, which stimulate secretion of anterior
pituitary hormones and two inhibiting hormones, which suppress secretion of anterior pituitary hormones
2. Negative feedback in the form of hormones released by target glands decrease secretions of three types of anterior
pituitary cells: secretory activity of Thyrotrophs, Gonadotrophs, and Cortioctrophs decrease when blood levels of
their target gland hormones rise.
o Human Growth Hormone and Insulin-like Growth Factors
Somatotrophs are the most numerous cells in the anterior pituitary and hGH is the most plentiful such hormone.
Its main function is to promote synthesis and secretion of small protein hormones celled insulin-like growth
factors. As a result, IGFs are secreted by cells in the liver, skeletal muscles, cartilage, bones and other tissues,
and may enter the bloodstream from the liver or act locally in tissues as autocrines or paracrines
The functions of IGFs include the following
o Cause cells to grow and multiply by increasing uptake of amino acid into cells and accelerating protein
synthesis. Decrease breakdown of proteins and the use of amino acids for ATP production. Thus hGH
increase growth rate of skeleton/muscles during childhood/teenage years. In adults, hGH and IGFs
maintain mass of muscles and bones, promoting healing of injuries and tissue repair
o They enhance lipolysis in adipose tissue, which results in increased use of the released fatty acids for ATP
production by body cells
o hGH and IGFs also influence carbohydrate metabolism by decreasing glucose uptake, which decreases the
use of glucose for ATP production by most body cells. This ensures glucose is available to neurons for
ATP production in times of glucose scarcity. They may also stimulate liver cells to release glucose into the
blood
Somatotrophs release bursts of hGH every few hours, especially during sleep. Their secretory is controlled by
two hypothalamic hormones: (1) growth hormone-releasing hormone (GHRH) promotes secretion of hGH and
(2) growth hormone-inhibting hormone (GHIH) suppresses it. The blood glucose level is a major regulator of
these two hormones (figure 18.7 627)
Posterior Pituitary (neurohypophysis): stores and releases two hormones produced by the neurosecretory cells
o Oxytocin (OT) and antidiuretic hormone (ADH)
o Axons from the neurosecretory cells form the hypothalamohypophyeal tract
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