Chapter 37 Integration and Control: Endocrine Systems

I. “The Endocrine System”
A. In the early 1900s, Bayliss and Starling first demonstrated that a hormone (later named secretin)
released into the blood triggers secretion of pancreatic juices.
B. Starling coined the word hormone for internal secretions released into the bloodstream that influence
the activities of other tissues and organs.
C. The sources of hormones may be collectively called the “endocrine system,” which shows intimate
connections with the nervous system.

II. Hormones and Other Signaling Molecules
A. Signaling molecules are hormones and secretions that can bind to target cells and elicit in them a response. B. There are four main types of signaling molecules:
1 Hormones are secreted from endocrine sources and some neurons, and are then transported by the
blood to remote targets.
2 Transmitter substances are secreted from neurons and act on immediately adjacent target cells for a
short time.
3 Local signaling molecules are secreted from cells of many different tissues; they act locally and are
swiftly degraded.
4 Pheromones, which are secreted by exocrine glands, have targets outside the body; they integrate
social activities between animals.

III. The Hypothalamus-Pituitary Connection
A. The hypothalamus and pituitary gland work jointly as the neuroendocrine control center.
1 The hypothalamus is a portion of the brain that monitors internal conditions and emotional states. 2 The pituitary is connected to the hypothalamus by a stalk.
a. The posterior lobe of the pituitary consists of nervous tissue and releases two neurohormones
made in the hypothalamus.
b. The anterior lobe consists of glandular tissue and secretes six hormones and controls the release
of others.
c. An intermediate lobe (not in humans) produces secretions that induce color changes in fur color
of vertebrates.
B. Posterior Lobe Secretions
1 Neuron cell bodies in the hypothalamus produce antidiuretic hormone (ADH) and oxytocin, which are
released from axon endings in the capillary bed of the posterior lobe.
2 ADH acts on the walls of kidney tubules to control the body’s water and solute levels.
3 Oxytocin triggers uterine muscle contractions to expel the fetus and acts on mammary glands to
release milk.
C. Anterior Lobe Secretions
1 Role of the Hypothalamus
a. The hypothalamus produces releasing and inhibiting hormones that target the anterior pituitary.
b. They travel through two capillary beds before leaving the blood and binding to receptors on target
cells in the anterior lobe of the pituitary.
2 Anterior Pituitary Hormones
a. In response, the anterior lobe releases four hormones that stimulate (“tropic”) other endocrine glands:
1) Corticotropin (ACTH) stimulates the adrenal cortex.
2) Thyrotropin (TSH) stimulates the thyroid gland.
3) Follicle-stimulating hormone (FSH) causes ovarian follicles to develop and produce an egg.
4) Luteinizing hormone (LH) also acts on the ovary to release an egg.
5) Prolactin acts on the mammary glands.
6) Somatotropin (STH), or growth hormone (GH), acts on body cells in general to promote growth.
b. Prolactin promotes milk production by the mammary glands but only when the tissues of those
glands have been primed by other hormones.
c. Somatotropin stimulates protein synthesis and cell division in many cells of the body and thus
influences overall growth.
1) In childhood, too little STH can cause pituitary dwarfism, while too much causes gigantism.
2) Oversecretion in adulthood causes a thickening of skin and bones called acromegaly.
IV. Selected Examples of Hormonal Action
A. When considering the actions of hormones, three points are relevant:
1 The response of target cells depends on the number of their receptors and the concentration of the
hormone.
2 Many hormones are linked to the neuroendocrine control center by homeostatic feedback loops. 3 Hormones interact to produce some effect on body functions.
a. In antagonistic interaction, the effect of one hormone may oppose that of another.
b. Synergistic interaction is the result of two or more hormones acting together to produce an overall
action.
c. In permissive interaction, one hormone exerts its effect only when the target cell has become
“primed” to respond in an enhanced way to that hormone.
B. Adrenal Glands
1 Adrenal Cortex
a. One adrenal gland is located on top of each kidney.
b. Among the secretions of the outer portion are the glucocorticoids such as cortisol, which helps control
blood glucose levels.
c. When the body is stressed, as in painful injury, the nervous system provides an override
mechanism in which the levels of cortisol remain high to promote healing.
2 Adrenal Medulla
a. This inner portion secretes epinephrine and norepinephrine under direction from sympathetic
nerves from the hypothalamus.
b. Its secretions mobilize the body during times of excitement or stress (“fight-or-flight” response).
C. Thyroid Gland
1 Its hormones, thyroxine and triiodothyronine, influence metabolic rates, growth, and development. a. These two hormones contain critical amounts of iodine.
b. If the blood levels of iodine are too low, the pituitary responds with too much TSH causing the
thyroid gland to enlarge abnormally in what we call a goiter.
2 Hypothyroidism in adults results in lethargy and weight gain; in infants, retardation and dwarfism can
result (cretinism).
3 Hyperthyroidism increases heart rate and blood pressure and causes weight loss (Grave’s disease).
D. Parathyroid Glands
1 These glands are embedded in the thyroid gland and respond to the changing levels of calcium in
the blood.
a. A drop in calcium level causes parathyroid hormone (PTH) levels to rise, resulting in removal of
calcium from bone and an activation of vitamin D (to help in calcium absorption from the gut).
b. When calcium levels rise, the PTH levels are reduced.
2 Calcitonin from the thyroid gland acts antagonistically to PTH and promotes deposition of calcium in
bones.
E. Gonads
1 The ovaries and testes—gonads—produce gametes and hormones governing reproduction.
2 The names and actions of these will be discussed in Chapter 43.
F. Pancreatic Islets
1 Certain cells within the pancreas have an endocrine function:
a. Alpha cells secrete glucagon, which causes glycogen stored in the liver to be converted to
glucose, raising its levels in the blood.
b. Beta cells secrete insulin, which stimulates the uptake of glucose by liver, muscle, and adipose to
reduce glucose levels in the blood, especially after a meal.
c. Delta cells secrete somatostatin, which can inhibit the secretion of glucagon and insulin.
2 Diabetes mellitus is a disease resulting from imbalances of insulin; its effects include weight loss,
ketone production, water-solute problems, and possible death.
a. In type 1 diabetes, insulin is no longer produced because the beta cells have been destroyed by
an autoimmune response; treatment is by insulin injection.
b. In type 2 diabetes, the insulin levels are near normal but the target cells cannot respond to the
hormone; controlling diet is an effective treatment.
G. Thymus Gland
1 This gland is located behind the breastbone and between the lungs.
2 Certain lymphocytes multiply and mature in this gland; it also secretes groups of hormones called
thymosins, which affect the functioning of lymphocytes that defend the body against disease.
H. Pineal Gland
1 Located in the brain, this gland is a modification of a primitive “third eye” and is sensitive to light and
seasonal influences.
2 In the absence of light, melatonin is secreted; thus in winter, high levels of the hormone are
instrumental in suppressing reproductive activity in hibernating animals.
3 Decreased melatonin secretion in humans might help trigger the onset of puberty.

V. Local Signaling Molecules
A. Prostaglandins
1 More than sixteen prostaglandins have been identified in tissues throughout the body.
a. When stimulated by epinephrine and norepinephrine, prostaglandins cause smooth muscles in
blood vessels to constrict or dilate.
b. Allergic responses to dust and pollen may be aggravated by prostaglandin effects on airways in
the lungs.
2 Prostaglandins have major effects on menstruation and corpus luteum function.
B. Growth Factors
1 Epidermal growth factor influences the growth of many cell types.
2 Nerve growth factor promotes growth and survival of neurons in the developing embryo.

VI. Signaling Mechanisms
A. A target cell’s response to a hormone is dependent on two factors:
1 Different hormones activate different cellular response mechanisms.
2 Not all cells have receptors for all hormones; the cells that respond are selected by means of the type
of receptor they possess.
B. There are two main kinds of hormones:
1 Steroid hormones, assembled from cholesterol, are lipid-soluble and therefore cross plasma
membranes readily.
2 Nonsteroid hormones, assembled from amino acids, include protein and peptide hormones plus the
catecholamines; all are water-soluble and cannot cross the lipid bilayer without assistance.
C. Steroid Hormone Action
1 Steroids stimulate or inhibit protein (especially enzyme) synthesis by switching certain genes on or off.
2 They easily diffuse through the lipid bilayer of the plasma membrane, bind to chromosomal proteins
in the nucleus, and then activate transcription.
3 Testosterone is the male hormone with receptors throughout the body; however, in testicular
feminization syndrome, none of the target cells respond correctly, so the XY individual develops
female characteristics.
D. Nonsteroid Hormone Action
1 Protein hormones and other water-soluble signaling molecules cannot cross the plasma membrane
of target cells so must first bind to a receptor on the plasma membrane.
2 The hormone-receptor complex enters the cell to stimulate the production of cyclic AMP, a “second
messenger” which amplifies the signal by activating numerous enzymes.