Chapter 45 (4)

Among the vertebrates, thyroid hormone, secreted by the __,r egulates both homeostasis and development. In humans and other mammals, thyroid hormone regulates bioenergetics; helps maintain normal blood pressure, heart rate, and muscle tone, and regualtes digestive and reproductive functions. In these animals, the __ consists of two lobes on the ventral surface of the trachea. In many other ertebrate, the two halves of the gland are separately located on the two sides of the pharynx.

The term __ actually refers to a pair of very similar hormones derived fromm the amino acid tyrosine. __ contains three iodine atoms, wheras __, or __ contains four iodine atoms.

In mammals, the same receptor binds both hormones. The thyrod secretes mainly __, but target cells convert most of it to __ by removing one iodine atom. Because iodine in the body is dedicated to the production of thyroid hormone, radioactive forms of iodine are often used to form images of the thyroid gland.

Too much or too little thyroid hormone in the blood can result in serious metabolic disorders. In humans, excessive secretion of thyroid hormon, called __, can lead to high body temp, profuse sweating, weight los, irritability, and high blood pressure. The most common form of __ is __.
o In this autoimmmune disorder, the immune system produces antibodies that bidn to the receptor for TSH and activate sustained thyroid hormone production. Protruding eyes, caused by fuild accumulation behind the eyes, are a typica sympton. __, a condition of too little thyroid function, can produce symptoms such as weight gain, lethargy and intolerace to cold in adults.

Proper thyroid function requires dietary iodine. W/o sufficient iodine, the thyroid gland cannot synthesize adequate amts of __ and __, and the resulting low blood levels of __ and __ cannot exert the usual negative feedback on the hypothalamus and anterior pituitary. As a consequence, the pituitary continues to secrete TSH. Elevated TSH levels cause an enlargement of the thyroid that results in __, a characteristic swelling of the neck.

True or False:
Among hte vertebrates, thyroid hormones have a variety of roles in development and maturation.
- All vertebrates require thyroid hormones for the normal functioning of bone-forming cells and the branching of nerve cells udring embryoinc development of the brain.
True False

In humans, __, an inherited condition of thyroid deficiecy, results in markedly retarded skeletal growth and poor mental development. These defects can often be prevented, at elast partially, if treatment with thyroid hormones begins early in life. Iodine deficiency in childhood causes the same defects, but it is fully prevetable if iodized salt is used in food preparation.

Because calcium ions are essential to the normal functioning of all cells, homeostatic control of blood calcium level is critical. If the blood Ca²⁺ level falls substantially, __ begin to contract convulsively, a potentially fatal condition called __.

True or False:
If the blood Ca²⁺ level rises substantially precipitates of calcium phospahte can form in body tissues, leading to widespread organ damage.
True False

In mammals, the __, a set of four small structures embedded in the posterior surface of the thyroid, play a major role in blood Ca²⁺ regulation. When blood Ca²⁺ falls below a set point of about 10 mg/ 100 mL, these glands release __.

__ raises the level of blood Ca²⁺ by direct and indrect effects.

o In bone, __ causes hte mineralized matrix to decompose and release Ca²⁺ into teh blood.

o In the kidneys, __ directly stimulates reabsorption of Ca²⁺ through the renal tubules.

__ also has an indirect effect on the kidneyts, promoting the conversion of Vit D to an active hormone. An inactive form of vit D, a steroid-derived moleucle, is obtained from food or synthesized in the skin when exposed to sunlight. Vit D activation begins in the liver and is completed in the kidneys, the process stimulated by __. The active form of Vit D acts directly on the intestines, stimulating the uptake of Ca²⁺ from food and thus augmenting the effect of PTH. As blood Ca²⁺ rises, a - feedback loop inhibits further release of __ from the __.

The thyoid gland can also contribute to calcium homeostasis. If blood Ca²⁺ rises above the set point, the thyroid gland releases __, a hormone that inhibits bone resorption and enhances Ca²⁺ release by the kidney. In fishes, rodents, and some other animals, __ is required for Ca²⁺ hoemostasis. In humans, however, it is aparently needed only during the extensive bone growth of childhood.

The __ of vertebrates are in each case associated with the kidneys. In mammals, each adrenal gland is actually made up of two glands with different cell types, functions, and embryonic origins: the __, the outer portion, and the __, the central portion. The __ consists of true endocrine cells, wherase the secretory cells of the __ derive from neural tissue during embryonic development. Thus, like the pituitary gland, each __ is a fused endocrine and neuroendocrine gland.

The fight-or-flight response:
This coordinated set of physiological changes is triggered by two hormones of the adrenal medulla, __ and __.
Both are __, a class of amine hormones synthesized from the amino acid tyrosine.

The adrenal medulla secretes __ and __ in resposnse to stress-whether extreme pleasure or life threatenig danger. A major avitiviy of these hormones is to increase hte amont of chem energy available for immediate use.

Both __ and __ increase the rate of glycogen breakdown in the liver and skeletal muscles, promote glucose release by lvier cells, and stimulate the release of fatty acids from fat cells. The released glucose and fatty acids circulate in the blood and can be used by body cells as fuel.

In addition to increasing hte availibitiy of energy sources, __ and __ exert profound effects on the cardiovascular and respiratory systems.
o increase both heart rate and stroke volume; dilate bronchioles in the lungs; actions that raise the rate of oxygen delivery to body cells.

The __ also alter blood flow, causing constriction of some blood vessels and dilation of others. THe overalle ffect is to shunt blood away from the skin, digestive organs and kidneys, while increasing the blood supply to the heart, brain and skeletal muscles.

__ generally has a stronger effect on heart and metabolic rates, while the primary role of __ is in modulating BP.

Nerve signals arried from the brain via involuntary (__) neurons regulate secretion by the adrenal medulla. In response to a stressful stimulus, nerve impulses travel to the __, where they trigger the reelase of __. Acting on target tissues, __ and __ each functionin a simple neurohormone pathway.

Hormone sfrom teh __ also function in the body's response to stress. But in contrast to the __, which reacts to nervous input, the __ responds to endocrine signals. Stressful stimuli cause the hypothalamus to secrete a releasing hormone that stimulates the anterior pituitary to release the tropic hormone __.

When __ reaches the adrenal cortex via the bloodstream, it stimmulates the endocrine cells to synthesize and secrete a family of steroids called __. The two main types are __ and __.

__ have a primary effect on gluose metabolism. Augmenting the fuel-mobilizing effects of glucagon from the pancreas, __ promote glucose synthesis from noncarbohydrate sources, such as proteins, making more glucose available as fuel.

__, like cortisol, act on skeletal muscle, causing breakdown of muscle proteins. The resulting amino acids are transported to the liver and kidneys, where they are converted to glucose and released into the blood. The synthesis of glucose from muscle proteins provides circulating fuel when the body requires more glucose than the liver can mobilize from its glycogeon stores.

When __ are introduced into the body at levels above those normally present, they suppress certain components of the body's immune system. Because of htis anti-inflammatory effect, __ are sometiems used to treat inflammatory diseases like arthritis. However, long term use can have serious side effects, reflecting the potent activity of __ on metabolism. or these reasons, __, like aspirin or ibuprofen, generally are preferred for treating chronic inflammatory conditions.

__, named for their effects on mineral metabolism, act principalyl in maintaing salt and water balance. For example, __ funcitons in ion and water homeostasis of hte blood. Low blood volume or pressure leads to production of __, which stimulates secreion of __.

__ in turn stimulates cells in teh kidneys to reabsorb sodium ions and water from filtrate raising BP and volume. __ also functions in the body's response to severe stress. In these circumstances, a rise in blood __ levels increases the rate at which the adrenal cortex secretes __ as wella s __.

THe __ products of the adrenal crtex include small amts of steroid hormone shtat functiona s sex hormones. All steroid hormones are synthesized from __, and their structures differ only in minor ways.

The sex horones produced by the __ are mainly male hormones (__), with small amts of female hormones (__ and __)

Whereas the __ secrete small quantities of these hormones, hte testes of males and ovaries of females are their principle sources.
o The gonads produe and secrete three major categories of steroid hormones: __, __ and __. All three types are found in both males and females but in significantly different proprtions.

The testes primarily syntehsize __, the main one being __.

__ have a major role again at human puberty, when they are responsible for the development of human male secondary sex characteristics.

The muscle building, or __, actin of testosterone and related streroids has enticed some athletes to take them as supplements,d espite prohibitions against their use in nearly all sports.

__, of which the msot important is __, are responsible for the maintenance of the femal reproductive system and teh develpment of female secondary sex characteristics.

In mammals, __ which include __ are primarly involved in preparing and maintaing tissues of the uterus requried to support the growth and development of an embryo.

__, __, and __ are components of hormone cascade pathways. Synthesis of these hormones is controlled by __ ( FSH and LH) from the anterior pituitary gland. FSH and LH secretion is in turn controlled by a releasing hormone from the hypothalamus, GnRH (gonadotropin-releasing hormone).

a small mass of tissue near hte center of the mammalian brain

The __ synthesizes and secretes the hormone __, a modified amino acid. Depending on the species, the __ contains light-sensitive cells or has nervouse connections from the eyes that control its secretory activity.

__ regulates functions related to light and to seasons marked by changes in day length. Although __ affects skin pigmentation in many vertebrates, its primary funcitons relate to biological rhythms associated with reproduction.

__ is secreted at night, and the amt released depends on the length of the night.

The main target of __ is a group of neurons in the hypothalamus called the __, which functions as a biological clock. __ seems to decrease hte activity of the __, and this effect may be related to its role in mediating rhythms.