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Chapter 20 Endocrine System 20.1 Endocrine Glands The endocrine consists of glands and tissues that secrete hormones. Hormones* Endocrine glands* Hormones can categorized as either peptides or steroids. Hormones and Homeostasis Unlike exocrine glands (salivary glands), endocrine glands are ductless; they release hormones directly into the bloodstream for distribution throughout the body. Table 20.1 lists endocrine glands and hormones. The glands and their locations in the body are shown in figure 20.3. The endocrine system is involved in homeostasis. There are 2 mechanisms that control the effect of endocrine system. The most common mechanism is a negative feedback mechanism. The gland is sensitive to the level of hormone in the blood or the condition it is regulating. Once the level of hormone increases or the desired effect is achieved, the release of the hormone is then dampened. The presence of an antagonistic hormone is a way the effect of a hormone is controlled. The thyroid and parathyroid glands work opposite to each other in regulating the level of calcium in the blood. 20.2 Hypothalamus and Pituitary Gland The hypothalamus gland regulates the internal environment trough the autonomic system by aiding in control of heartbeat, temperature and water balance. The hypothalamus also controls the hormonal secretions of the pituitary gland. The pituitary gland and the hypothalamus are directly linked by a stalklike structure. The pituitary has 2 parts: the anterior and posterior pituitary. Posterior Pituitary Neurosecretory cells in the hypothalamus release oxytocin and antidiuretic hormone through axons where they are stored in the posterior pituitary in axons endings. ADH promotes the reabsorption of water from collecting ducts of nephrons. The neurons in the hypothalamus are sensitive to osmolarity of the blood. Negative feedback is the mechanism that stimulates this response. The effect of the hormones stops the release of the hormone. Diabetes insipidus results from an inability to produce ADH. Oxytocin causes the uterus to contract more forcefully during labor. This is a result of positive feedback; one uterine contraction brings another more forceful contraction. Oxytocin * Anterior Pituitary A portal system* There is a portal system connecting the hypothalamus and the pituitary gland. It produces hypothalamic-releasing and hypothalamic-inhibiting hormones. These hormones exhibit stimulatory or inhibitory control of the anterior pituitary gland. Three of the six hormones released by the anterior pituitary have an effect on other glands. The three hormones are: thyroid stimulating hormone (TSH)- thyroid hormones adrenocorticotropic hormone (ACTH)- cortisol gonadotropic hormone (FSH,LH)- gametes and sex hormones The other three hormones act directly, they do not affect other glands: Prolactin (PRL)- stimulates milk production Melanocyte (MSH)-stimulating hormone- skin color changes Growth hormone (GH)- promotes skeletal and muscular growth Effect of Growth Hormone Produced during childhood and adulthood. Too little GH – * Too much GH- * If an excess of GH is produced in adulthood, then the result is acromegaly. The bones in the hand, feet, and face grow. 20.3 Thyroid and Parathyroid Gland These glands are attached to the trachea just below the larynx. The parathyroid is attached to the posterior of the thyroids. Thyroid Gland It is composed of a number of follicles; each made of thyroid cells filled with thyroxine (T4), and triiodothyronine (T3). Effects of Thyroid Hormones The thyroid actively acquires iodine to produce its hormones. If it does have access to iodine, the thyroid will become enlarged due to constant stimulation by the pituitary gland to release its hormones. *Goiter Thyroid hormones increase the metabolic rate. They stimulate all organs to increase the metabolic rate. More glucose is produced and more energy is used. Hypothyroidism: Cretinism occurs when the thyroid fails to develop properly* In adults, hypothyroidism results in the condition of myxedema. Symptoms include weight gain, loss of hair, slower pulse rate, lowered body temp, and thickness and puffiness of the skin. Hyperthyroidism: Graves’ disease. The thyroid is enlarged. Eyes protrude because of edema in socket tissues and swelling of eye muscles. Called exophthalmic goiter. Symptoms include irritability, hyperactive, nervous, and insomnia. A tumor on the thyroid can result in this condition as well. Surgery or radiation to destroy the thyroid is used as a treatment. Radioactive iodine is used. The prognosis is excellent. Calcitonin The thyroid gland secretes calcitonin, which regulates the amount of calcium ions in the blood. The primary effect is the deposit of calcium in the bones. This is accomplished by reducing the number and activity of the osteoclasts. When the blood calcium levels return to normal, the hormone is no longer secreted. Parathyroid Glands Produce parathyroid hormone- causes blood phosphate levels to decrease and blood calcium level to increase. Low levels of calcium in the blood stimulates the parathyroid to release hormone that causes bones to release calcium into the blood by promoting the work of osteoclasts Insufficient PTH brings about tetany- muscles shake from continuous contractions due to a drop in calcium level. Figure 20.9 for nice diagram of feedback mechanism control of hormones. 20.4 Adrenal Glands The adrenal glands are on top of the kidneys. The inner adrenal is called the adrenal medulla, the outer layer is known as the adrenal cortex. No connection between the 2 layers The hypothalamus controls the activity of the adrenals. It controls the activity of the medulla by initiating nerve impulses that travel to the medulla and cause it to release its hormone. When the hypothalamus releases ACTH releasing hormone, which stimulates the anterior pituitary to release ACTH to stimulate the cortex. Stress causes the hypothalamus to stimulate the adrenal glands. Epinephrine(adrenaline) and norepinephrine (noradrenaline) are produced in the medulla and they bring about physiological changes needed for an emergency response. The hormones produced by the cortex provide a sustained response to stress. These hormones are mineralocorticoids (salt and water balance) and glucocorticoids (carbohydrate, fat and protein metabolism). The adrenal cortex secretes a small amount of male and female sex hormones in both sexes. Glucocorticoids Cortisol is the most important of the glucocorticoids. It promotes the hydrolysis of proteins to amino acids. The liver converts the amino acids to glucose. Cortisol also favors the metabolism of fattty acids rather than carbohydrates. Works opposite to insulin and increases blood glucose levels. Also counteracts the inflammatory response of bursitis or arhtritis. Very high levels can suppress the body’s defense system. Mineralocorticoids Aldosterone is the most important mineralocorticoid. Promotes renal absorption of Na+ and the renal excretion of K+. Renin is an enzyme in the kidneys that causes a cascade of events that promotes the release of aldosterone. It is called the renin-angiotensin-aldosterone system. The contrary hormone is atrial natriuretic hormone (ANH) and it inhibits the release of aldosterone from the adrenals. Causes Na+ to be excreted along with water and blood pressure returns to normal. Malfunction of the Adrenal Cortex Hyposecretion- Addison’s disease. The presence of ACTH causes a bronzing of the skin. The lack of cortisol results n the ability to replenish blood glucose during stressful situations. The lack of aldosterone causes a loss of water and Na+ and hypotension results. Severe dehydration is possible. Potentially fatal condition. Hypersecretion causes Cushing’s syndrome. The excess of cortisol causes diabetes mellitus as muscle protein is metabolized. Excess aldosterone causes edema due to reabsorption of water. This leads to a basic pH and hypertension. Masculinization of females may occur. 20.5 Pancreas The pancreas lies in the abdomen between the kidneys near the duodenum of the small intestine. It is composed of two types of tissues: Exocrine tissue- produces and secretes digestive juices that are sent to the small intestine by way of ducts. Endocrine tissue- called pancreatic islets that produce and secrete insulin and glucagon. Insulin is secreted when the blood glucose level is high. It stimulates the uptake of glucose by liver, muscle, and adipose cells. In the liver and muscles, glucose is converted to glycogen. Adipose cells metabolize glucose to supply glycerol for the formation of fat. Glucagon is secreted when glucose levels are low. Stimulates the liver to break down glycogen to glucose and to use fat and and protein as energy sources instead of glucose. Adipose cells break down fat to glycerol and fatty acids so that the liver will use them to produce glucose. All these mechanisms are ways of increasing blood glucose levels. Diabetes Mellitus This is a disease where body cells do not take up and/or metabolize glucose. As blood glucose levels rise, water and glucose is excreted in the urine. The person experiences thirst. Protein and fat are metabolized for energy. The metabolism of fat causes excessive presence in the blood and acidosis that can lead to coma and death. Two types of diabetes mellitus: Type I- insulin dependent- The pancreas is not producing insulin. This is thought to be caused by an agent that causes cytotoxic T cells to destroy the islets that produce insulin. *transplant? Type II- non-insulin dependent: Occurs in obese and inactive people. Muscles and the liver do not respond to insulin, as they should. Exercise and diet can control this type of diabetes. The alternative is prescription medication that stimulates the pancreas to produce more insulin and enhance the metabolism of glucose by the liver and muscle cells. 20.6 Other Endocrine Glands Testes and Ovaries Testes are male gonads and are located in the scrotum; ovaries are female gonads and are located in the pelvic cavity. The testes produce androgens and the ovaries produce estrogens and progesterones. The hypothalamus and the pituitary glands control the secretion of these organs. Testosterone is essential for development and functioning of sex organs in males. It is also necessary for maturation of sperm Increased release of testosterone causes the primary and secondary sex characteristics to appear in the male. This is marked by the "growth spurt". It is responsible for muscular strength of males and this why some athletes use anabolic steroids. Testosterone also stimulates oil and sweat glands of the body. Increased chance of acne as a result. Another effect of testosterone is baldness. Estrogen and progesterone are the female sex hormones. Estrogens are responsible for appearance of primary and secondary sex characteristics. It is responsible for fat distribution and the reason why females have a wider pelvis. Estrogens and progesterones are needed for breast development and regulation of the uterine cycle. Thymus Gland This gland is at its largest size and most active stage during childhood. Lymphocytes that originate in the marrow and pass through the thymus become T lymphocytes. Thymoxin* Pineal Gland It is located in the brain and produces melatonin, primarily at night. This hormone is involved in our circadian rhythms. This can cause problems for people who do shift work or fly across time zones. It appears that melatonin also regulates sexual development. In children whose pineal gland has been destroyed, they experience early puberty. Hormones from Other Tissues Some organs secrete hormones. The heart, stomach, small intestine are a few. Leptin This is a protein hormone produced by adipose cells that acts on the hypothalamus to signal satiation. Growth Factors A number of differebt types of organs and cells produce peptide growth factors: Granulocyte and macrophage colony stimulating factor:* Platelet-derived growth factor* Epidermal growth factor and nerve growth factor Tumor angiogenesis Prostaglandins: 20.7 Chemical Signals A Chemical signal* They can be characterized in 3 ways: Environmental signals that act at a distance between organisms- pheromones. Environmental signals that act at a distance between body parts-include hormones Environmental signals that act locally between adjacent cells- neurotransmitters and local hormones, i.e. histamine. The Action of Hormones Hormones fall into 2 basic categories: nonsteroid hormone: amino acid, peptide or a protein Steroids hormone- same complex of 4 rings with different side chains. Work at low concentration- effect is amplified by cellular mechanisms. Organs have specific receptors for hormones; the hormone doesn’t select only certain organs. Steroids are lipids and can move through the cell membrane. When they are inside the nucleus, they bind to receptor proteins. The hormone-receptor complex binds to DNA and activates certain genes. This leads to mass production of a cellular enzyme. Nonsteroids cannot pass through the membrane, as they are not lipids. They bind to receptor proteins on the surface of the cellular membrane. This starts a cascade effect through the cell. The hormone is the first messenger, whatever is formed by this binding is the messenger and the second messenger sets in motion an "enzyme cascade". It is a cascade because 1 enzyme will activate another. Enzymes work repeatedly and every step in a cascade leads to more reactions. The effect of the hormone is amplified.
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