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Endocrine System

The Endocrine System and Hormone Function Overview

1. The endocrine system (Figure 9.1)is a major controlling system of the body. Through hormones, it stimulates such long-term processes as growth and development, metabolism, reproduction, and body defense.

2. Endocrine organs are small and widely separated in the body. Some are mixed glands (both endocrine and exocrine in function). Others are purely hormone producing.

3. All hormones are fat-souble (steroid)or water-soluble (amino acid-based) hormones.

4. Endocrine organs are activated to release their hormones into the blood by hormonal, humoral, or neural stimuli. Negative feedback is important in regulating hormone levels in the blood.

5. Blood-borne hormones alter the metabolic activities of their target organs. The ability of a target organ to respond to a hormone depends on the presence of receptors in or on its cells to which the hormone binds or attaches.

6. Fat-soluble (steroid) hormones directly influence the target cell's DNA by binding to receptor sites in the nucleus (Figure 9.2). Water-soluble (amino acid-based) hormones act through second messengers (Figure 9.3).

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The Major Endocrine Organs

1. Pituitary gland

   The pituitary gland hangs from the base of the brain by a stalk and is enclosed by bone. It consists of a glandular (anterior) portion and a neural (posterior) portion (Figure 9.4).

   Except for growth hormone and prolactin, hormones of the anterior pituitary are all tropic hormones.

   1. Growth hormone (GH): An anabolic and protein-conserving hormone that promotes total body growth. Its most important effect is on skeletal muscles and bones. Hyposecretion during childhood results in pituitary dwarfism; hypersecretion produces giantism (in childhood) and acromegaly (in adulthood).

   2. Prolactin (PRL): Stimulates production of breast milk.

   3. Adrenocorticotropic hormone (ACTH): Stimulates the adrenal cortex to release its hormones.

   4. Thyroid-stimulating hormone (TSH): Stimulates the thyroid gland to release thyroid hormone.

   5. Gonadotropic hormones

   Follicle-stimulating hormone (FSH): Beginning at puberty, stimulates follicle development and estrogen production by the female ovaries; promotes sperm production in the male.

   Luteinizing hormone (LH): Beginning at puberty, stimulates ovulation, converts the ruptured ovarian follicle to a corpus luteum, and causes the corpus luteum to produce progesterone; stimulates the male's testes to produce testosterone.

   Releasing and inhibiting hormones made by the hypothalamus regulate release of hormones made by the anterior pituitary. The hypothalamus also makes two hormones that are transported to the posterior pituitary for storage and later release.

   The posterior pituitary stores and releases hypothalamic hormones on command.

   1. Oxytocin: Stimulates powerful uterine contractions and causes milk ejection in the nursing woman.

   2. Antidiuretic hormone (ADH): Causes kidney tubule cells to reabsorb and conserve body water and increases blood pressure by constricting blood vessels. Hyposecretion leads to diabetes insipidus.

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2. Thyroid gland

   1. The thyroid gland is located in the anterior throat.

   2. Thyroid hormone (thyroxine [T4] and triiodothyronine [T3]) is released from the thyroid follicles when blood levels of TSH rise (Figure 9.5). Thyroid hormone is the body's metabolic hormone. It increases the rate at which cells oxidize glucose and is necessary for normal growth and development. Lack of iodine leads to goiter. Hyposecretion of thyroxine results in cretinism in children and myxedema in adults. Hypersecretion results from Graves' disease or other forms of hyperthyroidism.

   3. Calcitonin is released by C cells surrounding the thyroid follicles in response to high blood levels of calcium (Figure 9.6). It causes calcium to be deposited in bones.

3. Parathyroid glands

   1. The parathyroid glands are four small glands located on the posterior aspect of the thyroid gland.

   2. Low blood levels of calcium stimulate the parathyroid glands to release parathyroid hormone (PTH). It causes bone calcium to be liberated into the blood. Hyposecretion of PTH results in tetany; hypersecretion leads to extreme bone wasting and fractures.

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4. Adrenal glands

   1. The adrenal glands are paired glands perched on the kidneys. Each gland has two functional endocrine portions, cortex and medulla.

   2. Three groups of steroid hormones are produced by the adrenal cortex.

      1. Mineralocorticoids, primarily aldosterone, regulate sodium ion (Na+) and potassium ion (K+) reabsorption by the kidneys (Figure 9.7). Their release is stimulated primarily by low Na+ and/or high K+ levels in blood.

      2. Glucocorticoids enable the body to resist long-term stress by increasing blood glucose levels and depressing the inflammatory response.

      3. Sex hormones (mainly male sex hormones) are produced in small amounts throughout life.

   3. Generalized hypoactivity of the adrenal cortex results in Addison's disease. Hypersecretion can result in hyperaldosteronism, Cushing's disease, and/or masculinization.

   4. The adrenal medulla produces catecholamines (epinephrine and norepinephrine) in response to sympathetic nervous system stimulation. Its catecholamines enhance and prolong the effects of the fight-or-flight (Sympathetic nervous system) response to short-term stress. Hypersecretion leads to symptoms Typical of sympathetic nervous system overactivity.

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5. Pancreatic islets

   1. Located in the abdomen close to the stomach, the pancreas is both an exocrine and endocrine gland. The endocrine portion (islets) releases insulin and glucagon to blood (Figure 9.8).

   2. Insulin is released when blood levels of glucose are high. It increases the rate of glucose uptake and metabolism by body cells. Hyposecretion of insulin results in diabetes mellitus, which severely disturbs body metabolism. Cardinal signs are polyuria, polydipsia, and polyphagia.

   3. Glucagon is released when blood levels of glucose are low. It stimulates the liver to release glucose to blood by accelerating the conversion of glycogen to glucose, thus increasing blood glucose levels.

6. The pineal gland, located in the third ventricle of the brain, releases melatonin, which affects biological rhythms and reproductive behavior.

7. The thymus gland, located in the upper thorax, functions during youth but atrophies in old age. Its hormone, thymosin, promotes maturation of T lymphocytes, important in body defense.

8. Gonads

   1. The ovaries of the female, located in the pelvic cavity, release two hormones.

      1. Estrogens: Release of estrogens by ovarian follicles begins at puberty under the influence of FSH. Estrogens stimulate maturation of the female reproductive organs and development of secondary sex characteristics of the female. With progesterone, they cause the menstrual cycle.

      2. Progesterone: Progesterone is released from the corpus luteum of the ovary in response to high blood levels of LH. It works with estrogens in establishing the menstrual cycle.

   2. The testes of the male begin to produce testosterone at puberty in response to LH stimulation. Testosterone promotes maturation of the male reproductive organs, male secondary sex characteristics, and production of sperm by the testes.

   3. Hyposecretion of gonadal hormones results in sterility in both females and males.

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Other Hormone-Producing Tissues and Organs

1. The placenta is a temporary organ formed in the uterus of pregnant women. Its primary endocrine role is to produce estrogen and progesterone, which maintain pregnancy and ready breasts for lactation.

2. Several organs that are generally nonendocrine in overall function, such as the stomach, small intestine, kidneys, and heart, have cells that secrete hormones.

3. Certain cancer cells secrete hormones.

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