1. Chapter 23 The Endocrine System
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Introduction
The nervous and endocrine systems coordinate functions of all body systems.
The endocrine system releases regulating molecules called hormones into interstitial fluid and then the bloodstream.
The endocrine system not only helps regulate the activity
of smooth muscle, cardiac muscle, and some glands; it affects virtually all
other tissues as well.
Hormones alter metabolism, regulate growth and development,
and influence reproductive processes.
Nerve impulses most often produce an effect within a few milliseconds; some hormones can act within seconds, but others can take several hours or more to cause a response.
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3. Location of Many Endocrine Glands (Fig. 23.1)
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Hormones
Hormones have powerful effects, in very low concentrations.
The body’s has 50 or so hormones and only a few types of cells.
Hormone affects only certain target cells.
Hormones bind to specific protein receptors.
Only the target cells for a given hormone
have receptors that bind and recognize that hormone.
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5. Hypothalamus and Pituitary Gland (Fig. 23.2)
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The Pituitary Gland
The pituitary gland lies in the hypophyseal fossa of the sphenoid bone and attaches to the hypothalamus by a stalk, the infundibulum
The pituitary gland has two anatomically and functionally separate portions:
The anterior pituitary
The posterior pituitary
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Posterior pituitary
The posterior pituitary contains axons and axon whose cell bodies are located in the supraoptic and paraventricular nuclei of the hypothalamus.
The axon terminals in the posterior pituitary
gland are associated with specialized neuroglia called pituicytes.
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Anterior Pituitary
The anterior pituitary, or adenohypophysis secretes hormones that regulate a wide range of bodily activities, from growth to reproduction.
Release of anterior pituitary hormones is stimulated by releasing hormones and suppressed by inhibiting hormones from the hypothalamus.
Hypothalamic hormones reach the anterior pituitary through a portal system.
In the hypophyseal portal system, blood flows from capillaries in the hypothalamus into portal veins that carry blood to capillaries of the anterior pituitary.
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9. Major hormones anterior pituitary
1. Human growth hormone (hGH), or somatotropin, is secreted
by cells called somatotrophs. Human growth hormone stimulates tissues to secrete insulin like growth factors, that stimulate general body growth and regulate of metabolism.
2. Thyroid-stimulating hormone (TSH), or thyrotropin, controls the secretions of the thyroid gland.
3. Follicle-stimulating hormone (FSH) and luteinizing
hormone (LH) secreted by gonadotrophs. FSH and LH both act on the gonads: stimulate secretion of estrogens and progesterone . Stimulate
secretion of testosterone and sperm production in the testes.
4. Prolactin (PRL), which initiates milk production in the mammary
glands, is released by lactotrophs .
5. Adrenocorticotropic hormone (ACTH), or corticotropin stimulates the adrenal cortex to secrete glucocorticoids.
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Posterior Pituitary
Although the posterior pituitary, or neurohypophysis, does not synthesize hormones, it does store and release two hormones.
Different neurosecretory cells produce two hormones: oxytocin and anti-di uretic hormone (ADH), also called vasopressin
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Thyroid gland
The wall of each follicle consists primarily of cells called follicular cells
The follicular cells produce two thyroid hormones: thyroxine also called tetraiodothyronine, or T4, because it contains four atoms of iodine, and triiodothyronine or T3, which contains three atoms of iodine.
The thyroid hormones regulate oxygen use and basal metabolic rate, cellular metabolism, and growth and development.
Parafollicular cells, or C cells, embedded within a follicle produce calcitonin , which regulate calcium homeostasis.
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Thyroid Gland (Fig. 23.4)
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Parathyroid Glands
The parathyroid glands contain two kinds of epithelial cells.
Chief cells produce parathyroid hormone (PTH) and oxyphil cell function is unknown.
PTH increases the number and activity of osteoclasts. The result is elevated bone resorption, the releases ionic calcium and phosphates into the blood.
PTH also produces two changes in the kidneys:
(1) IIcreases the rate at which the kidneys remove calcium and magnesium from urine and returns them to the blood,
(2) it inhibits the reabsorption of (HPO4 ) filtered by the kidneys, so that more of it is excreted in urine.
A third effect of PTH on the kidneys is to promote formation of the hormone calcitriol, the active form of vitamin D.
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14. Location and Blood Supply of Parathyroid Glands (Fig. 23.5)
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Adrenal Glands
The paired adrenal glands, or suprarenal glands, one of which lies superior to each kidney in the retroperitoneal space
The adrenal cortex produces hormones that are essential for life.
Complete loss of adrenocortical hormones leads to death in a few days to a
week due to dehydration and electrolyte imbalances, unless hormone
replacement therapy begins promptly.
The adrenal medulla produces two hormones: norepinephrine and epinephrine.
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Adrenal Cortex
The adrenal cortex is subdivided into three zones, each of which secretes different hormones.
The outer zone called the zona glomerulosa secrete hormones called mineralocorticoids affect metabolism of the minerals sodium and potassium.
The middle zone, or zona fasciculata, secrete mainly glucocorticoids, which affect glucose metabolism.
The cells of the inner zone, the zona reticularis synthesize small amounts
of weak androgens, hormones that have masculinizing effects.
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Adrenal Medulla
The inner region of the adrenal gland, the adrenal medulla, is a modified sympathetic ganglion of the autonomic
nervous system (ANS) releases neurotransmitter.
The hormone producing cells, called chromaffin cells produce two principal hormones epinephrine and norepinephrine (NE), also called adrenaline and noradrenaline.
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Adrenal Glands (Fig. 23.6)
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Pancreas
The pancreas is both an endocrine gland and an exocrine gland.
The pancreatic cells are arranged in clusters called acini ; these cells produce digestive enzymes, which flow into the gastrointestinal tract through a network of ducts.
Scattered among the exocrine acini are the pancreatic islets or
islets of Langerhans .
Each pancreatic islet contains four types of hormone-secreting cells:
1. Alpha or A cells constitute about 15% of pancreatic islet cells
and secrete glucagon (GLOO-ka-gon) (see also Table 23.7).
2. Beta or B cells constitute about 80% of pancreatic islet cells
and secrete insulin (IN-soo-lin) (see also Table 23.7).
3. Delta or D cells constitute about 5% of pancreatic islet cells
and secrete somatostatin
4. F cells constitute the remainder of pancreatic islet cells and
secrete pancreatic polypeptide .
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Pancreas (Fig. 23.7)
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Ovaries and Testes
The female gonads, called the ovaries, produce female sex hormones called estrogens and progesterone.
Along with the gonadotropic hormones of the pituitary gland, the sex hormones regulate the female reproductive cycle, maintain pregnancy, and
prepare the mammary glands for lactation.
The ovaries also produce inhibin, a hormone that inhibits secretion of follicle-stimulating hormone (FSH) from the anterior pituitary.
During pregnancy, the ovaries and placenta produce a hormone called relaxin, which increases the flexibility of the pubic symphysis during pregnancy and helps dilate the uterine cervix during labor and delivery.
The male has two oval gonads, called t e s t e s, that produce testosterone, the primary androgen. Te s t o s t e rone stimulates descent of the testes before birth, regulates production of sperm, and stimulates the development and maintenance of male secondary sexual characteristics such as beard growth.
The testes also produce inhibin, which inhibits secretion of FSH.
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Pineal Gland
It is part of the epithalamus, positioned between the two superior colliculi,
The gland, which is covered by a capsule formed by the
pia mater, consists of masses of neuroglia and secretory cells called pinealocytes.
Sympathetic postganglionic fibers from the superior cervical ganglion terminate in the pineal gland.
One hormone secreted by the pineal gland is melatonin. Melatonin
contributes to the setting of the body’s biological clock, which is
controlled from the suprachiasmatic nucleus of the hypothalamus.
During sleep, levels of melatonin in the bloodstream increase
tenfold and then decline to a low level again before awakening.
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