ENDOCRINE SYSTEM Flashcards
ENDOCRINE SYSTEM AFFECTS
- Reproduction
- Development
- Conception, pregnancy, birth, lactation
- Metabolism
- Growth, development & puberty
- Sleeping/waking (biological clock) • Stress Response
- Ageing
HORMONE
Molecule released by endocrine cells into interstitial fluid which then enters blood, and acts as a chemical messenger to regulate specific body functions
Hormones have the ability to alter the function of other cells in the body but…
• Whilst a hormone reaches all parts of the body, it will only act upon cells with a specific receptor – “target cells”
HORMONE TYPES
Lipid-soluble hormones
e.g. Steroid Hormones – Aldosterone, Testosterone, And Thyroid hormones – T3 and T4
Water-soluble hormones
e.g. Amine hormones - Epinephrine, Norepinephrine Peptide hormones – Antidiuretic hormone (ADH), Oxytocin Protein hormones – Insulin, Glucagon
WATER SOLUBLE HORMONE
All protein and amino acid-based hormones
• Travel “free” form in blood plasma
• Can’t enter the target cell…
• Act on plasma membrane receptors (First Messenger) to activate G protein
• cAMP acts as second messenger
• A chain reaction resulting in a
physiological response
LIPID SOLUBLE HORMONE
Steroid & thyroid hormones
• Travel in blood bound to transport proteins
• Can diffuse through the plasma membrane…
• Act on intracellular receptors to activate genes resulting in new proteins that alter cell activity
Humoral Stimuli
Hormones released in direct response to changing levels of substances in blood
• Low Ca2+ → Parathyroid Hormone release…
• High blood glucose → Insulin release
Neural Stimuli
Nerve fibres stimulate release of hormones
• Acute stress causes the sympathetic nervous system to stimulate the release of epinephrine & norepinephrine (adrenaline & noradrenaline) from the adrenal glands
• Hypothalamus can stimulate posterior pituitary to release hormones - antidiuretic hormone (ADH) & oxytocin
Hormonal Stimuli
Many endocrine glands release hormones in response to hormones produced by other organs
• For example, the hypothalamic-pituitary- target endocrine organ feedback loop
Hypothalamus-Posterior Pituitary
- Nerve fibres originating in the hypothalamus travel through the infundibulum to terminate in the posterior pituitary
- These are specialised ‘neurosecretory cells” that can release hormones from the axon terminals
- Nerve impulses from the hypothalamus stimulate secretion of these hormones (e.g. ADH, oxytocin) into the blood from the posterior pituitary
Hypothalamus-Anterior Pituitary
- Releasing/inhibiting hormones are secreted by the hypothalamus, diffuse into blood and travel by a portal venous system to target anterior pituitary cells
- Hypothalamic hormones stimulate or inhibit the release of anterior pituitary hormones (e.g. growth hormone, thyroid stimulating hormone…)
CALCITONIN
If calcium levels are too high, the thyroid gland releases calcitonin.
The hormone produced by the parafollicular cells of the thyroid gland (see figure 18.10b) is calcitonin (CT). CT can decrease the level of calcium in the blood by inhibiting the action of osteoclasts, the cells that break down bone extracellular matrix. The secretion of CT is controlled by a negative feedback system (see figure 18.14).
When its blood level is high, calcitonin lowers the amount of blood calcium and phosphates by inhibiting bone resorption (breakdown of bone extracellular matrix) by osteoclasts and by accelerating uptake of calcium and phosphates into bone extracellular matrix. Miacalcin, a calcitonin extract derived from salmon that is 10 times more potent than human calcitonin, is prescribed to treat osteoporosis.
PARATHYROID HORMONE (PTH)
If calcium levels are too low, the parathyroid gland releases PTH
Parathyroid hormone is the major regulator of the levels of calcium (Ca2+), magnesium (Mg2+), and phosphate (HPO42−) ions in the blood. The specific action of PTH is to increase the number and activity of osteoclasts. The result is elevated bone resorption, which releases ionic calcium (Ca2+) and phosphates (HPO42−) into the blood. PTH also acts on the kidneys. First, it slows the rate at which Ca2+ and Mg2+ are lost from blood into the urine. Second, it increases loss of HPO42− from blood into the urine. Because more HPO42− is lost in the urine than is gained from the bones, PTH decreases blood HPO42− level and increases blood Ca2+ and Mg2+ levels. A third effect of PTH on the kidneys is to promote formation of the hormone calcitriol (kal′-si-TRĪ-ol), the active form of vitamin D. Calcitriol, also known as 1,25-dihydroxyvitamin D3, increases the rate of Ca2+, HPO42−, and Mg2+ absorption from the gastrointestinal tract into the blood.
ADRENAL CORTEX
The adrenal cortex is subdivided into three zones, each of which secretes different hormones (figure 18.15d). The outer zone, just deep to the connective tissue capsule, is the zona glomerulosa (glo-mer′-ū-LŌ-sa; zona = belt; glomerul- = little ball). Its cells, which are closely packed and arranged in spherical clusters and arched columns, secrete hormones called mineralocorticoids (min′-er-al-ō-KOR-ti-koyds) because they affect mineral homeostasis. The middle zone, or zona fasciculata (fa-sik′-ū-LA-ta; fascicul- = little bundle), is the widest of the three zones and consists of cells arranged in long, straight columns. The cells of the zona fasciculata secrete mainly glucocorticoids (gloo′-kō-KOR-ti-koyds), primarily cortisol, so named because they affect glucose homeostasis. The cells of the inner zone, the zona reticularis (re-tik′-ū-LAR-is; reticul- = network), are arranged in branching cords. They synthesise small amounts of weak androgens (andro- = a man), steroid hormones that have masculinising effects.
ALDOSTERONE
Aldosterone (al-DOS-ter-ōn) is the major mineralocorticoid. It regulates homeostasis of two mineral ions— namely, sodium ions (Na+) and potassium ions (K+)— and helps adjust blood pressure and blood volume. Aldosterone also promotes excretion of H+ in the urine; this removal of acids from the body can help prevent acidosis
GLUCAGON
The principal action of glucagon is to increase blood glucose level when it falls below normal. Insulin, on the other hand, helps lower blood glucose level when it is too high. The level of blood glucose controls secretion of glucagon and insulin via negative feedback (figure 18.19).
1 Low blood glucose level (hypoglycaemia) stimulates secretion of glucagon from alpha cells of the pancreatic islets.
2 Glucagon acts on hepatocytes (liver cells) to accelerate the conversion of glycogen into glucose (glycogenolysis) and to promote formation of glucose from lactic acid and certain amino acids (gluconeogenesis).
3 As a result, hepatocytes release glucose into the blood more rapidly, and blood glucose level rises.
4 If blood glucose continues to rise, high blood glucose level (hyperglycaemia) inhibits release of glucagon (negative feedback).
5 High blood glucose (hyperglycaemia) stimulates secretion of insulin by beta cells of the pancreatic islets.
6 Insulin acts on various cells in the body to accelerate facilitated diffusion of glucose into cells; to speed conversion of glucose into glycogen (glycogenesis); to increase uptake of amino acids by cells and to increase protein synthesis; to speed synthesis of fatty acids (lipogenesis); to slow the conversion of glycogen to glucose (glycogenolysis); and to slow the formation of glucose from lactic acid and amino acids (gluconeogenesis).
7 As a result, blood glucose level falls.
8 If blood glucose level drops below normal, low blood glucose inhibits release of insulin (negative feedback) and stimulates release of glucagon.
FIGHT OR FLIGHT RESPONSE
- Stress stimulates the hypothalamus.
- The sympathetic nervous system is activated and nerve impulses are conducted throughout the body.
- Acute stress response mobilizes the body’s resources to ensure immediate response to stressor. And, non-essential activities are inhibited.
- Sympathetic nerves stimulate the adrenal medulla to release epinephrine & norepinephrine.
- Sympathetic responses are enhanced further.
The fight-or-flight response, initiated by nerve impulses from the hypothalamus to the sympathetic division of the autonomic nervous system (ANS), including the adrenal medulla, quickly mobilises the body’s resources for immediate physical activity (figure 18.20a). It brings huge amounts of glucose and oxygen to the organs that are most active in warding off danger: the brain, which must become highly alert; the skeletal muscles, which may have to fight off an attacker or flee; and the heart, which must work vigorously to pump enough blood to the brain and muscles. During the fight-or-flight response, nonessential body functions such as digestive, urinary, and reproductive activities are inhibited. Reduction of blood flow to the kidneys promotes release of renin, which sets into motion the renin–angiotensin–aldosterone pathway (see figure 18.16). Aldosterone causes the kidneys to retain Na+, which leads to water retention and elevated blood pressure. Water retention also helps preserve body fluid volume in the case of severe bleeding.
INSULIN
is a peptide hormone produced by beta cells of the pancreatic islets; it is considered to be the main anabolic hormone of the body.[7] It regulates the metabolism of carbohydrates, fats and protein by promoting the absorption of glucose from the blood into liver, fat and skeletal muscle cells.[8] In these tissues the absorbed glucose is converted into either glycogen via glycogenesis or fats (triglycerides) via lipogenesis, or, in the case of the liver, into both.[8] Glucose production and secretion by the liver is strongly inhibited by high concentrations of insulin in the blood.[9] Circulating insulin also affects the synthesis of proteins in a wide variety of tissues. It is therefore an anabolic hormone, promoting the conversion of small molecules in the blood into large molecules inside the cells. Low insulin levels in the blood have the opposite effect by promoting widespread catabolism, especially of reserve body fat.
THYMUS
• Thymic hormones – promote maturation of T lymphocytes (white blood cells that are part of our immune response)
The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, thymus cell lymphocytes or T cells mature. T cells are critical to the adaptive immune system, where the body adapts specifically to foreign invaders. The thymus is located in the upper front part of the chest, in the anterior superior mediastinum, behind the sternum, and in front of the heart. It is made up of two lobes, each consisting of a central medulla and an outer cortex, surrounded by a capsule.
The thymus is made up of immature T cells called thymocytes, as well as lining cells called epithelial cells which help the thymocytes develop. T cells that successfully develop react appropriately with MHC immune receptors of the body (called positive selection) and not against proteins of the body, (called negative selection). The thymus is largest and most active during the neonatal and pre-adolescent periods. By the early teens, the thymus begins to decrease in size and activity and the tissue of the thymus is gradually replaced by fatty tissue. Nevertheless, some T cell development continues throughout adult life.
The thymus facilitates the maturation of T cells, an important part of the immune system providing cell-mediated immunity.[10] T cells begin as hematopoietic precursors from the bone-marrow, and migrate to the thymus, where they are referred to as thymocytes. In the thymus they undergo a process of maturation, which involves ensuring the cells react against antigens (“positive selection”), but that they do not react against antigens found on body tissue (“negative selection”).[10] Once mature, T cells emigrate from the thymus to provide vital functions in the immune system.
