Zoology Notes: 018 Chapter 14

Chapter 14. The Endocrine System The endocrine system consists of specialized glands located in the different parts of the body. These glands secrete chemical substances called hormones, which transfer information from one set of cells to another. The endocrine system glands have no ducts connecting to their organs or tissues, hence the endocrine system is often called the ductless glands, and so they liberate hormones directly into the bloodstream. The endocrine system also relies on the circulatory system to deliver hormones to the target tissues. In general, this system is concerned principally with the control of the different metabolic functions of the body such as the rates of chemical reactions in the cell and the transport of substances through cell membranes or other aspects of cellular metabolism like growth and secretion. Glands of the Endocrine System: Hypothalamus. This gland is connected to the pituitary gland by way of the infundibular (funnel shape) stalk, and it secretes several factors that stimulate or inhibit various pituitary secretor cells. Responsible for controlling body Fig. 14.1. Glands of the endocrine system. temperature, hunger, thirst, and other homeostatic systems and involved in sleep and emotional activity. Pituitary Gland. The pituitary gland is a pea-sized body attached to the based of the brain. This gland produces several hormones such as: • Growth Hormone (GH) – promotes muscle and bone growth • Prolactin Hormone – milk production after childbirth • Thyroid-stimulating Hormone (TSH) – stimulates thyroid tissue growth and the production of thyroid hormones. • Adrenocorticotrophic Hormone (ACTH) – influences metabolism in adipose tissues. • Gonadotophics Hormone – stimulate production of male and female sex hormones, spermatogenesis, and ovarian development. • Melanophore-stimulating Hormone – influences skin pigmentation. Thyroid Gland. This composed of microscopic units called follicles. Each follicle consists of a layer of secretory epithelium surrounding a hormone storing colloid. The follicles extract iodine from the blood and concentrate it for incorporation into the hormones thyroxine and triiodothyronine. These hormones play a major role in general metabolism, including calorigenesis, protein turnover, hydromineral balance, and growth and maturation of the individual. Adrenal Gland. The adrenal glands are a pair of ductless glands located above the kidney. It produces adrenaline, a hormone that is released in emergency or emotional situations and stimulates

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the heart and the vascular system. Epinephrine also stimulates various metabolic activities and elevates blood glucose levels. Gonads. The gonads of the reproductive system produce steroid sex hormones that stimulate reproductive process. The testes produce androgens (male) and the ovaries produce estrogen, progesterone and a peptide hormone called relaxin. Diseases of the Endocrine System Diseases of the endocrine system primarily result in excess release (hypersecretion) or insufficient release (hyposecretion) of hormones by one or more glands. Hypersecretion of hormones, usually associated either with tumors or above-normal cell proliferation (hyperplasia), occurs because of overstimulation of endocrine glands by factors such as tropic hormones from the pituitary gland. Hyposecretion results from several causes, including: surgical or radiation destruction of glands; glandular atrophy; congenital defects in the enzymes involved in hormone synthesis; and dietary conditions such as low iodine intake that may result in insufficiency of the thyroid hormones. Hypothalamus and Pituitary Gland The hypothalamus and the pituitary gland are closely related. The hypothalamus modulates the activity of the anterior portion of the pituitary through release or release-inhibitor hormones, which travel through a portal venous system to the anterior pituitary. These neurohormones regulate the release of such hormones as growth hormone (GH), adrenocorticotrophin (ACTH), follicle-stimulating hormone (FSH), prolactin, luteinizing hormone (LH), and thyroid-stimulating hormone (TRH). Decreased secretion in the hypothalamus results in decreased secretion of the pituitary, a state called secondary hypopituitarism. This under secretion is usually caused by tumors, encephalitis, or inflammatory lesions. Hypersecretion Cause Result Growth Hormone (GH) pituitary tumor gigantism or acromegaly Gonadotrophins pituitary tumor hypogonadism, menstrual disturbance in women. Adrenocorticotrophin ACTH tumor Cushing’s disease Prolactin pituitary tumor galactorrhea, amenorrhea, infertility Table 14.1. Cause and results of hypersecretions of the hypothalamus and pituitary glands. Hyposecretion Growth Hormone (GH) Thyroid-SH

Cause pituitary tumor pituitary tumor

Result dwarfism hypothyroidism, intolerance of cold, slow speech, anemia ATCH (cortisol) pituitary tumor weakness, low blood pressure, diminished blood sugar FSH and LH pituitary tumor amenorrhea, atrophy of genitals, absence of libido, infertility Gonadotrophins pituitary tumor hypogonadism, menstrual disturbance in women. Adrenocorticotrophin ACTH tumor Cushing’s disease Prolactin pituitary tumor inhibits lactation ADH pituitary tumor diabetes insipidus, mental retardation Table 14.2. Cause and results of hyposecretions of the hypothalamus and pituitary glands.

Thyroid Gland A deficiency of the hormones thyroxine and triiodothyronine (hypothyroidism) originates either in the thyroid (primary) or in the controlling pituitary of hypothalamus (secondary). Hyposecretion Thyroxine and Triiodothyronine Various Condition

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Cause mentioned iodine def

Result cretinism, dry, wrinkled skin, enlarged tongue, drooling mouth, broad face. dwarfism, mental retardation Goiter

Table 14.3. Cause and results of hyposecretions of the thyroid glands.

Adrenal Gland Hypersecretion Cause Result Aldosterone and Cortisol --Addison’s Disease Table 14.4. Cause and results of hypersecretions of the adrenal glands. Hyposecretion Androgen

Cause tumor

Result early development of male sex char, (female) baldness, acne, deep voice, decreased breast size, inc masculinity Glucocorticoids tumor Cushing’s syndrome Aldosterone tumor high blood pressure, normal to high serum Na, low serum K Table 14.5. Cause and results of hyposecretions of the adrenal glands.

Parathyroid Gland Deficiency of parathyroid hormone (PTH) leads to a tendency for chronic tetany, or increased neuromuscular excitability. Chemically, hypoparathyroidism is characterized by low serum calcium and high serum phosphorus. The disease is caused either by accidental removal or by damage during removal of the thyroid gland, or by a form of autoimmune disease. Hypersecretion PTH

Cause Result tumor, high serum calcium, low levels inorganic phosphate, enlargement, abnormal bone metabolism cancer Table 14.6. Cause and results of hypersecretions of the parathyroid glands.

Gonads Premature development of the ovaries and testes through early but normal hypothalamus-pituitary maturation, without apparent cause, accelerates body growth but ultimately results in stunting due to premature fusing of the growing ends of bones. This condition is called true sexual precocity; reproduction is possible as early as age 6. Brain tumors and encephalitis can also result from premature development of the ovaries and testes. The development and functioning of the male reproductive organs and related structures and characteristics depend on the pituitary gonadotrophins. LH stimulates the cells of the testes to produce androgen. FSH stimulates the sperm-producing cells of the sex glands. Primary male hypogonadism consists of a failure to synthesize androgens (testosterone) due to defective enzymes. Secondary male hypogonadism is caused by a lack either of LH, which inhibits androgen production, or of FSH, which leads to a failure of spermatogenesis. A deficiency of FSH and LH prior to or during puberty delays the development of genitalia and secondary sex characteristics. Dwarfism occurs if GH is also deficient. Primary amenorrhea, the failure of a women to begin menstruation, may be due to pituitary dysfunction (such as tumors) or to other endocrine disorders (such as hypothyroidism). Secondary amenorrhea halts the menstrual cycle. Pancreas Juvenile diabetes mellitus (Type 1) results from both hereditary and environmental factors and is characterized by insulin deficiency and a high blood-sugar (glucose) level. In the absence of insulin treatment, the disease rapidly causes dehydration, ketoacidosis, low blood volume, hypotension, coma, and death. 59