Chapter 14
Endocrine System
14.1Endocrine Glands
Endocrine glands secrete their chemical signals called hormones directly into the bloodstream. There are similarities as well as differences between regulation by the nervous system and the endocrine system. The secretion of a hormone is often controlled by negative feedback, and the effect of a hormone is often opposed by an antagonistic hormone. The end result is homeostasis.
Hormones areChemical Signals
A chemical signal affects the metabolism of cells (such as hormones or prostaglandins) or the behavior of the individual (such as pheromones).
The Action of Hormones
Peptide hormones are usually received by a hormone receptor located in the plasma membrane. Most often their reception leads to activation of an enzyme that changes ATP to cyclic AMP (cAMP). cAMP as a second messenger activates an enzyme cascade. Steroid hormones enter the nucleus and combine with a receptor hormone, and the complex attaches to and activates DNA. Transcription and translation lead to protein synthesis.
14.2 Hypothalamus and Pituitary Gland
The hypothalamus uses the autonomic nervous system to help regulate the body’s internal environment. It also controls the secretions of the pituitary gland which has anterior and posterior portions.
Posterior Pituitary
The posterior pituitary secretes antidiuretic hormone (ADH) and oxytocin which are produced by the hypothalamus. Antidiuretic hormone helps to regulate the water-salt balance of the blood. Oxytocin causes uterine contractions during labor and childbirth, and milk let-down during nursing. Oxytocin release is controlled by positive feedback. The hypothalamus triggers release of posterior pituitary hormones through nervous impulse.
Anterior Pituitary
The anterior pituitary produces at least six types of hormones. Thyroid-stimulating hormone (TSH) stimulates the thyroid; ACTH stimulates the adrenal cortex; the gonadotropic hormones FSH and LH stimulate the gonads; prolactin causes mammary glands to produce milk; and growth hormone promotes bone growth. Melanocyte-stimulating hormone causes skin color changes in fishes, amphibians, and reptiles, although humans produce little of this hormone. The hypothalamus triggers release of anterior pituitary hormones through secretion of releasing- or inhibiting-hormones.
Effects of Growth Hormone
Pituitary dwarfism results from too little and giantism results from too much growth hormone during childhood. If growth hormone is over-produced in an adult, acromegaly occurs.
14.3 Thyroid and Parathyroid Glands
Thyroid Gland
The thyroid gland requires iodine to produce thyroxine and triiodothyronine, which increase the metabolic rate.
Effects of Thyroid Hormones
If iodine is available in limited quantities, a simple goiter develops; if the thyroid is overactive, an exophthalmic goiter develops. Thyroid hormones generally increase metabolic rate. If the thyroid fails to develop properly, cretinism results. Hypothyroidism in adults causes myxedema.
Calcitonin
The thyroid gland also produces calcitonin, which helps lower the blood calcium level.
Parathyroid Glands
The parathyroid glands secrete parathyroid hormone which raises the blood calcium and decreases the blood phosphate level. Calcitonin and parathyroid hormone operate together to regulate calcium levels in the blood.
14.4 Adrenal Glands
The adrenal glands are located atop each kidney and consist of an inner medulla and outer cortex. Both portions of the adrenal glands are controlled by the actions of the hypothalamus. The adrenal medulla responds to stress: immediately, the adrenal medulla secretes epinephrine and norepinephrine, which bring about responses we associate with emergency situations. On a long-term basis, the adrenal cortex produces the glucocorticoids (e.g., cortisol) and the mineralocorticoids (e.g., aldosterone).
Glucocorticoids
Cortisol stimulates hydrolysis of proteins to amino acids that are converted to glucose; in this way, it raises the blood glucose level. High levels of glucocorticoids suppress the body’s immune system.
Mineralocorticoids
Aldosterone causes the kidneys to reabsorb sodium ions (Na+) and excrete potassium ions (K+). The effect of the renin-angiotensin-aldosterone system is to raise blood pressure. The hormone atrial natriuretic hormone produced by the heart opposes the action of aldosterone.
Malfunction of the Adrenal Cortex
Addison disease develops when the adrenal cortex is under active, and Cushing syndrome develops when the adrenal cortex is overactive.
14.5 Pancreas
The pancreatic islets secrete insulin, which lowers the blood glucose level, and glucagon, which has the opposite effect. Together these two hormones maintain steady levels of glucose in the blood.
Diabetes Mellitus
The most common illness due to hormonal imbalance is diabetes mellitus. A glucose tolerance test is used to help diagnose this condition.
Types of Diabetes
There are two types of diabetes. The first is due to the failure of the pancreas to produce insulin (type I or insulin-dependent diabetes) and the second is caused by the failure of the cells to take up glucose (type II or noninsulin-dependent diabetes).
14.6 Other Endocrine Glands
Testes and Ovaries
The testes and ovaries produce the sex hormones which maintain the sex organs and secondary sexual characteristics. Male testes produce androgens (such as testosterone) that are similar to anabolic steroids taken to increase athletic performance. Female ovaries produce estrogen and progesterone.
Thymus Gland
The thymus gland produces thymosins which stimulate T lymphocyte production and maturation.
Pineal Gland
The pineal gland produces melatonin which may be involved in circadian rhythms and sexual development.
Hormones from Other Tissues
A number of other types of body tissues produce hormones.
Leptin
Adipose tissue produces leptin which acts on the hypothalamus where it signals satiety—that the individual has had enough to eat.
Growth Factors
A number of different types of organs and cells produce peptide growth factors, which stimulate cell division and growth.
Prostaglandins
Prostaglandins, which have varied effects, are produced and act locally.
14.7 Homeostasis
The nervous and endocrine systems exert control over the other systems and respond to external changes, internal changes and control the reproductive system. A Human Systems Work Together box shows how the endocrine system functions with other systems of the body to bring about homeostasis.
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