lec 5 physiology

Lec.5 Physiology Dr.Israa H.Mohsen

Endocrine system
The endocrine system consists of a variety of glands and specialized cells throughout the body. These glands and cells secrete chemicals (called hormones) that influence almost every cell and organ in the body.
Endocrine glands are ductless glands: they secrete hormones directly into the bloodstream. (In contrast, exocrine glands secrete hormones into ducts, which, in turn, lead to a surface in some other location, such as the body s surface—as in sweat glands—or the digestive tract—as in digestive enzymes.) Once in the bloodstream, each hormone travels throughout the body. In the process, the cells of many different organs are exposed to a particular hormone; however, only cells having receptors for that hormone (called target cells) will respond. In other words, a hormone acts only on cells with receptors specific to that hormone; this is called specificity.
Hormones can be classified as steroid or nonsteroid.
1.Steroid hormones are synthesized from cholesterol; they include male and female sex hormones as well as aldosterone (secreted by the adrenal cortex).
2.Nonsteroid, or protein-based, hormones are synthesized from amino acids. They can be further divided into protein hormones (such as insulin), peptide hormones (such as antidiuretic hormone), or amino acid derivative hormones (such as epinephrine and norepinephrine).
Hypothalamus
Neurons within the hypothalamus synthesize various hormones. Some, called releasing hormones, stimulate the anterior pituitary to secrete its hormones. Others, called inhibiting hormones, suppress hormone secretion by the anterior pituitary. The neurons of the hypothalamus release their hormones into a system of blood vessels called the hypophyseal portal system. This stimulates the anterior pituitary to release, or to suppress the release of, certain hormones into the general circulation.
•Gonadotropin-releasing hormone: Promotes secretion of FSH and LH
• Thyrotropin-releasing hormone: Promotes secretion of TSH
• Corticotropin-releasing hormone: Promotes secretion of ACTH
• Prolactin-releasing hormone: Promotes secretion of prolactin
• Prolactin-inhibiting hormone: Inhibits secretion of prolactin
• Growth hormone–releasing hormone: Promotes secretion of GH
• Somatostatin: Inhibits secretion of GH and TSH
Anterior Pituitary gland
Most of the hormones produced and secreted by the anterior pituitary are tropic (or trophic) hormones (so called because the hormone names end with the suffix -tropin or -tropic, such as gonadotropin, thyrotropin, etc.). Tropic hormones stimulate other endocrine cells to release their hormones,
1. Thyroid-stimulating hormone (TSH), or thyrotropin, stimulates the thyroid gland to secrete thyroid hormone.
2. Prolactin stimulates milk production in the mammary glands in females. In males, it may make the testes more sensitive to LH.
3. Growth hormone (GH), or somatotropin, acts on the entire body to promote protein synthesis, lipid and carbohydrate metabolism, and bone and skeletal muscle growth.
4. Luteinizing hormone (LH)—a gonadotropin— stimulates ovulation and estrogen and progesterone synthesis in females and the secretion of testosterone by the testes in males.
5. Follicle-stimulating hormone (FSH)—one of the gonadotropins—stimulates the production of eggs in the ovaries of females and sperm in the testes of males.
6. Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to secrete corticosteroids.
Posterior Pituitary gland
Unlike the anterior pituitary, which is composed of glandular tissue, the posterior pituitary is made of neural tissue. Also, instead of synthesizing hormones as the anterior pituitary does, the posterior pituitary simply stores hormones synthesized by the hypothalamus. The hormones stored by the posterior pituitary are antidiuretic hormone (ADH) and oxytocin (OT).
Oxytocin stimulates contraction of the uterus during childbirth. It also triggers the release of milk from the breasts during lactation.
Antidiuretic hormone (ADH) acts on the kidneys to reduce urine volume and prevent dehydration. ADH is also called vasopressin.

Pineal gland
The pineal gland produces melatonin, a hormone that rises at night, when sunlight is absent, and falls during the day. High melatonin levels trigger sleepiness, making it a key factor in the sleep-wake cycle. Although yet to be proven, scientists speculate that the pineal gland may also regulate the timing of puberty.


Thymus gland
The thymus secretes thymosin and thymopoietin, two hormones having a role in the development of the immune system. Although it secretes hormones, making it a member of the endocrine system, the actions of the hormones make the thymus part of the immune system.

Thyroid Gland
The largest endocrine gland, the thyroid, consists of two large lobes connected by a narrow band of tissue called the isthmus. two main thyroid hormones: T3 (triiodothyronine) and T4 (thyroxine). Unlike other glands, the thyroid gland can store the hormones for later use. The overall effect of TH is to increase the body’s metabolic rate, which, in turn, increases heat production. (This explains why TH is released when the body is exposed to cold.) Just a few of the other effects of TH are increased rate and strength of heart contractions, increased respiratory rate, and increased appetite. TH is also crucial for growth and development: it promotes the development of bone; the nervous system; and skin, hair, nails, and teeth.
Cells between the thyroid follicles, called parafollicular cells, secrete another hormone called calcitonin. Secreted in response to rising blood calcium levels, calcitonin triggers the deposition of calcium in bone, thus promoting bone formation. The effects of calcitonin are particularly important in children.

Parathyroid glands
Located on the posterior surface of the thyroid are four parathyroid glands. These glands secrete parathyroid hormone (PTH) in response to low blood levels of calcium .PTH is the main hormone the body uses to maintain normal levels of calcium in the blood. Normal nerve and muscle function, blood clotting, cell membrane permeability, and the function of certain enzymes all depend on adequate levels of calcium. In an effort to achieve calcium homeostasis, PTH exerts its influence on the bones, kidneys, and intestines
• PTH inhibits new bone formation while stimulating the breakdown of old bone, causing calcium (and phosphate) to move out of bone and into the blood.
• PTH encourages the kidneys to reabsorb calcium— blocking its excretion into the urine—while promoting the secretion of phosphate. PTH also prompts the kidneys to activate vitamin D, necessary for intestinal absorption of calcium.
• After its activation by the kidneys, vitamin D allows the intestines to absorb calcium from food; the calcium is transported through intestinal cells and into the blood.
Adrenal glands
The adrenal glands perch on the top of each kidney. Instead of being one gland, though, each adrenal gland is actually two distinct glands. The inner portion—called the adrenal medulla—consists of modified neurons and functions as part of the sympathetic nervous system. The outer portion—called the adrenal cortex—is glandular tissue and secretes steroid hormones called corticosteroids.
Adrenal Medulla
The adrenal medulla contains modified neurons (called chromaffin cells) that act as part of the sympathetic nervous system. These cells secrete catecholamines (specifically, epinephrine and norepinephrine) in response to stimulation. Catecholamines:
• Prepare the body for physical activity by increasing heart rate and blood pressure, stimulating circulation to the muscles, and dilating the bronchioles; to maximize blood flow to the areas needed for physical activity, they also inhibit digestion and urinary production.
• Boost glucose levels (a source of fuel) by breaking down glycogen into glucose (glycogenolysis) and converting fatty acids and amino acids into glucose (gluconeogenesis).
Adrenal Cortex
The adrenal cortex consists of three layers of glandular tissue:
1.Zona glomerulosa (the outermost layer): Secretes mineralocorticoids: The principal mineralocorticoid is aldosterone.
• Aldosterone acts on the kidneys to promote Na+ retention and K+ excretion
• In turn, it also causes water retention.
2.Zona fasciculata (the middle layer): Secretes glucocorticoids The principal glucocorticoid is cortisol.
• Glucocorticoids help the body adapt to stress and repair damaged tissue by stimulating the breakdown of fat and protein, converting fat and protein to glucose, and releasing fatty acids and glucose into the blood.
• They have an anti-inflammatory effect.
• They also suppress the immune system if secreted over a long term.
• Glucocorticoids are essential for maintaining a normal blood pressure.
3.Zona reticularis (the innermost layer): Secretes sex steroids
Sex steroids include a weak form of androgen that is converted to the more potent androgen testosterone.
• The testes produce much more testosterone, making this an unimportant source of testosterone in men.
• Androgens stimulate development of pubic and axillary hair and sustain sex drive (libido) in both sexes.
• The sex steroids also include small amounts of estrogen.
• Because the amount is small, it has little importance during reproductive years. However, it is the only source of estrogen after menopause

Pancreas
The pancreas is somewhat unique in that it contains both endocrine and exocrine tissues. The vast majority of the pancreas acts as an exocrine gland, but a small percentage serves an important endocrine function.
• Alpha cells secrete the hormone glucagon. Between meals, when blood glucose levels fall, glucagon stimulates liver cells to convert glycogen into glucose and also to convert fatty acids and amino acids into glucose (gluconeogenesis). The resulting glucose is released into the bloodstream, causing blood glucose levels to rise.
• Beta cells secrete the hormone insulin. After eating, the levels of glucose and amino acids in the blood rise. Insulin stimulates cells to absorb both of these nutrients, causing blood glucose levels to fall.
• Delta cells secrete somatostatin, a hormone that works within the pancreas to regulate the other endocrine cells. Specifically, it inhibits the release of both glucagon and insulin. It also inhibits the release of growth hormone.

Gonads
Gonads—the testes in males and the ovaries in females—are the primary sex organs. They produce sex hormones, which stimulate the production of sperm (in males) and eggs (in females). They also influence the development of secondary sex characteristics during puberty.
The cells of the ovarian follicle secrete estrogen. Estrogen promotes the development of female characteristics (such as breast development) and also contributes to the development of the reproductive system.
After ovulation, the corpus luteum (the tissue left behind after a rupture of a follicle during ovulation) secretes progesterone. Progesterone, in combination with estrogen, helps maintain the uterine lining during pregnancy.
Specialized cells within the testes secrete testosterone. Testosterone triggers the development of male sexual characteristics; it also sustains sperm production.


Other Endocrine Cells and Chemicals
Not all hormones are secreted from specific, well-defined glands. In fact, organs such as the heart, liver, kidneys, stomach, small intestines, and placenta contain clusters of cells that secret hormones. Just a few examples:
? When stretched by rising blood pressure, heart muscle cells secrete atrial natriuretic peptide (ANP), a hormone that triggers changes to lower blood pressure.
? The placenta secretes estrogen, progesterone, and other hormones that help maintain a woman’s pregnancy.
? The stomach secretes several hormones that help regulate the digestive process.