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Cecie Starr Christine Evers Lisa Starr www.cengage.com/biology/starr Chapter 31 Endocrine Control (Sections 31.6 - 31.11) Albia Dugger • Miami Dade College 31.6 Thyroid and Parathyroid Glands • The thyroid gland regulates metabolic rate, and the adjacent parathyroids regulate calcium levels • thyroid gland • Endocrine gland at the base of the neck • Produces thyroid hormone, which increases metabolism • parathyroid glands • Four small endocrine glands whose hormone product increases the level of calcium in blood Thyroid Function • The thyroid secretes thyroid hormone (two iodine-containing molecules: triiodothyronine and thyroxine) which increases metabolic activity of tissues throughout the body • The thyroid gland also secretes calcitonin, a hormone that causes deposition of calcium in bones of growing children • The anterior pituitary gland and hypothalamus regulate thyroid hormone secretion by a negative feedback loop Feedback Control of Thyroid Function 1. A low level of thyroid hormone causes the hypothalamus to secrete thyroid-releasing hormone (TRH) 2. TRH causes the anterior pituitary to secrete thyroidstimulating hormone (TSH) 3. TSH stimulates the secretion of thyroid hormone 4. When the blood level of thyroid hormone rises, secretion of TRH and TSH declines Negative Feedback Loop Negative Feedback Loop STIMULUS Blood level of thyroid hormone falls below a set point. RESPONSE Hypothalamus 1 TRH 4 Anterior Pituitary 2 TSH Rise of thyroid hormone level in blood inhibits the secretion of TRH and TSH. Thyroid Gland 3 Thyroid hormone is secreted. Fig. 31.7, p. 511 RESPONSE STIMULUS Blood level of thyroid hormone falls below a set point. + Hypothalamus TRH Anterior Pituitary TSH Rise of thyroid hormone level in blood inhibits the secretion of TRH and TSH. Thyroid Gland Negative Feedback Loop Thyroid hormone is secreted. Stepped Art Fig. 31.7, p. 511 Hypothyroidism • A diet deficient in iodine can cause thyroid hormone deficiency (hypothyroidism) • Hypothyroidism can also arise when the body’s immune system mistakenly attacks the thyroid • In either case, ongoing stimulation of the thyroid can lead to thyroid enlargement, or goiter Goiter • Enlarged thyroid (goiter) caused by a dietary iodine deficiency Parathyroid Glands and Calcium Levels • Four parathyroid glands located at the rear of the thyroid gland are the main regulators of calcium levels in the blood • When blood calcium level declines, the glands release parathyroid hormone (PTH), which increases breakdown of bone – blood calcium level rises • PTH also encourages calcium reabsorption by kidneys and activation of vitamin D, which helps the intestine take up calcium from food Rickets • Rickets caused by a lack of vitamin D • Parathyroid hormone softens the bones, causing bowed legs ANIMATION: Parathyroid hormone action To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE 31.7 The Adrenal Glands • There are two adrenal glands, one above each kidney • An adrenal gland has two functional zones, controlled by different mechanisms: • The outer cortex secretes steroid hormones • The inner medulla releases molecules that function as neurotransmitters Key Terms • adrenal gland • Endocrine gland that is located atop the kidney • adrenal cortex • Outer portion of adrenal gland • Secretes aldosterone and cortisol • adrenal medulla • Inner portion of adrenal gland • Secretes epinephrine and norepinephrine The Adrenal Cortex • The adrenal cortex secretes two steroid hormones: • Aldosterone controls sodium and water reabsorption in the kidneys • Cortisol helps maintain blood glucose available to the brain by inducing the liver to break down glycogen, adipose cells to degrade fats, and skeletal muscles to degrade proteins Negative Feedback and Cortisol • Cortisol secretion is governed by a negative feedback loop to the anterior pituitary gland and hypothalamus • In times of stress, the central nervous system overrides the feedback controls so that cortisol levels rise • Over the long term, excess cortisol has negative impacts on health Negative Feedback Mechanism 1. A decrease in cortisol triggers secretion of CRH (corticotropinreleasing hormone) by the hypothalamus 2. CRH stimulates secretion of ACTH by the anterior pituitary 3. ACTH causes release of cortisol from the adrenal cortex 4. Cortisol level increases, causing hypothalamus and anterior pituitary to secrete less CRH and ACTH, and cortisol secretion slows Negative Feedback Mechanism Negative Feedback Mechanism Blood level of STIMULUS RESPONSE Hypothalamus cortisol declines. 1 CRH 4 Anterior Pituitary adrenal cortex 2 ACTH adrenal medulla Rise of cortisol level in the blood inhibits the secretion of CRH and ACTH. Adrenal Cortex 3 Cortisol secretion increases and has the following effects: Cellular uptake of glucose from blood slows in many tissues, especially muscles (but not in the brain). Protein breakdown accelerates, especially in muscles. Some of the amino acids freed by this process get converted to glucose. kidney Fats in adipose tissue are degraded to fatty acids and enter blood as an alternative energy source, indirectly adrenal cortex adrenal medulla kidney conserving glucose for the brain.Fig. 31.9, p. 512 STIMULUS A Blood level of cortisol falls below a set point. adrenal cortex adrenal medulla + RESPONSE Hypothalamus Negative Feedback Mechanism B CRH Anterior Pituitary ACTH Adrenal Cortex D Hypothalamus and pituitary detect rise in blood level of cortisol and slow its secretion. C Cortisol is secreted and has the following effects: Cellular uptake of glucose from blood slows in many tissues, especially muscles (but not in the brain). Protein breakdown accelerates, especially in muscles. Some of the amino acids freed by this process get converted to glucose. Fats in adipose tissue are degraded to fatty acids and enter blood as an alternative energy source, indirectly conserving glucose for the brain. Stepped Art Fig. 31.9, p. 512 The Adrenal Medulla • The adrenal medulla contains specialized neurons of the sympathetic division that release norepinephrine and epinephrine that enter the blood and function as hormones • Like sympathetic stimulation, norepinephrine and epinephrine cause a fight-flight response: they dilate the pupils, increase breathing, and make the heart beat faster Stress, Elevated Cortisol, and Health • When an animal is frightened or under physical stress, commands from the nervous system trigger increased secretion of cortisol, epinephrine, and norepinephrine • Physiological responses to chronic stress interfere with growth, the immune system, sexual function, and cardiovascular function • Chronically high cortisol levels harm cells in the hippocampus, a brain region central to memory and learning Elevated Cortisol: Cushing Syndrome • Before and after removal of a adrenal gland tumor Hypocortisolism: Addison’s Disease • Tuberculosis and other infectious diseases can damage adrenal glands, resulting in adrenal insufficiency • President John F. Kennedy had an autoimmune form of Addison’s Disease 31.8 Pancreatic Hormones • The pancreas has both exocrine and endocrine functions: • Exocrine cells secrete digestive enzymes into a duct to the small intestine • Endocrine cells are grouped in pancreatic islets; each islet contains three types of hormone-secreting cells • pancreas • Organ that secretes digestive enzymes into the small intestine and hormones into the blood The Pancreas and Digestion • Delta cells in pancreatic islets secrete somatostatin • Somatostatin helps control digestion and nutrient absorption The Pancreas and Digestion Fig. 31.12a, p. 514 The Pancreas and Digestion stomach pancreas small intestine Fig. 31.12a, p. 514 The Pancreas and Blood Sugar • Two pancreatic hormones with opposing effects work together to regulate the level of sugar in the blood • Insulin (secreted by beta cells) stimulates glucose uptake by muscle and liver cells and thus lowers the blood glucose • Glucagon (secreted by alpha cells) stimulates the release of glucose, which increases blood levels Regulating High Blood Sugar 1. 2. 3. 4. 5. Blood glucose rises Glucagon is blocked Insulin is secreted Glucose is taken up Blood glucose level decreases Regulating High Blood Sugar 1 Stimulus Increase in blood glucose PANCREAS 2 glucagon LIVER 3 insulin MUSCLE FAT CELLS 4 Body cells, especially in muscle and adipose tissue, take up and use more glucose. Cells in skeletal muscle and liver store glucose in the form of glycogen. 5 Response Decrease in blood glucose Fig. 31.12b, p. 514 Regulating Low Blood Sugar 6. 7. 8. 9. Low blood glucose Glucagon is secreted Insulin is blocked Liver breaks down glycogen into glucose 10.Blood glucose increases Regulating Low Blood Sugar 6 Stimulus Decrease in blood glucose 7 8 glucagon insulin 9 Cells in liver break down glycogen faster. The released glucose monomers enter blood. 10 Response Increase in blood glucose Fig. 31.12c, p. 514 ANIMATION: Hormones and glucose metabolism To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE 31.9 Diabetes • Diabetes mellitus is a disorder in which the body does not make or does not respond to insulin • When cells do not take up and store glucose as they should, high blood sugar (hyperglycemia) disrupts normal metabolism • Cells have to use proteins and fats for energy – breakdown of these substances yields harmful waste products Some Complications of Diabetes Type 1 Diabetes • Type 1diabetes develops after white blood cells wrongly identify insulin-secreting beta cells as foreign (nonself) and destroy them (autoimmune response) • All affected individuals require injections of insulin, and must monitor their blood sugar level carefully • When fats and proteins are used as energy sources, ketones accumulate in the blood and urine (ketosis) Insulin Pump • An insulin pump helps smooth out fluctuations in blood sugar, lowering risk of complications Type 2 Diabetes • In type 2 diabetes insulin levels are normal or even high • However, target cells do not respond to the hormone as they should, and blood sugar levels remain elevated • Western diets and sedentary life-styles are contributing factors in type 2 diabetes – diet, exercise, and oral medications can control most cases 31.10 Gonads, Pineal Gland, and Thymus • Outputs from gonads, pineal gland, and thymus all change as an individual enters puberty • gonads • Primary reproductive organs (ovaries or testes) that produce gametes and sex hormones • pineal gland • Endocrine gland deep inside the brain that secretes melatonin when the retina is not stimulated by light • thymus • Endocrine gland beneath the breastbone; secretes hormones that encourage maturation of T lymphocytes The Gonads • Gonads are primary reproductive organs, which produce gametes (eggs or sperm) • Gonads produce steroid sex hormones with roles in reproduction and development of secondary sexual traits • Male gonads (testes) secrete mainly testosterone • Female gonads (ovaries) secrete mainly estrogens and progesterone Location of Human Gonads Location of Human Gonads testis (where sperm originate) Fig. 31.14a, p. 516 Location of Human Gonads ovary (where eggs develop) Fig. 31.14b, p. 516 Control of Sex Hormone Secretion • The hypothalamus and anterior pituitary control secretion of sex hormones The Pineal Gland • The pineal gland lies deep within the vertebrate brain • Melatonin secreted by the pineal gland affects the daily sleep/wake cycle and the onset of puberty • Melatonin also protects against some cancers • Melatonin secretion declines when the retina detects light and sends action potentials along the optic nerve to the brain The Thymus • The thymus lies beneath the breastbone • It secretes thymosins that encourage maturation of infectionfighting white blood cells (T lymphocytes, or T cells) • At puberty, the surge of sex hormones causes the thymus to shrink, and its secretions decline – this can be a problem for people with HIV infection, because AIDS kills T cells Key Concepts • Other Hormone Sources • Endocrine glands throughout the body respond to signals from the hypothalamus and the pituitary • Others secrete hormones in response to internal changes such as a shift in blood glucose level • Poor diet, immune problems, and genetic factors can cause hormone disorders 31.11 Invertebrate Hormones • Vertebrate hormone receptor proteins often resemble similar receptor proteins in invertebrates and probably evolved from them • Invertebrates also have hormones with no vertebrate counterpart, such as the steroid hormone ecdysone, which regulates molting in arthropods such as crabs and insects Evolution of Receptor Diversity • Genetic analysis has revealed the invertebrate ancestry of some vertebrate hormone receptors • Sea anemones have receptors that are structurally similar to vertebrate receptors for TSH, LH, FSH, and other signaling molecules • Genes that encode these receptors have similar nucleotide sequences in vertebrates and invertebrates, and have the same number and type of introns in similar regions Control of Molting • In arthropods, hormones regulate the periodic molting of the cuticle that allows the animal to grow • Details vary, but in all cases, molting is largely controlled by ecdysone, a steroid hormone unique to invertebrates • A molting gland produces and stores ecdysone and releases it at molting time Molting in Crustaceans Molting in Crustaceans Absence of suitable stimuli Presence of suitable stimuli X organ releases molt-inhibiting hormone (MIH) Signals from brain inhibit release of MIH MIH prevents Y organ from making ecdysone Y organ makes and releases ecdysone No molting Molting Fig. 31.16, p. 517 Molting in Crustaceans Fig. 31.16c, p. 517 Key Concepts • Invertebrate Hormones • Hormones control molting and other events in invertebrate life cycles • Vertebrate hormones and receptors for them first evolved in ancestral lineages of invertebrates Hormones in the Balance (revisited) • The photo shows breast development in a girl less than two years old • Exposure to high levels of synthetic chemicals (phthalates) may cause such premature breast enlargement