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Maintaining Balance (Homeostasis) Homeostasis • The process by which our body maintains an internal balance of all body systems. • Usually it is regulated by one of two major systems Endocrine system Nervous system 2 The Endocrine System works with the Nervous System • Two systems coordinate communication throughout the body: the endocrine system and the nervous system. • The endocrine system secretes hormones that coordinate slower but longer-acting responses including reproduction, development, energy, metabolism, growth, and behavior. • The nervous system conveys high-speed electrical signals along specialized cells called neurons; these signals regulate other cells. Coordination of Endocrine and Nervous Systems in Vertebrates • The hypothalamus receives information from the nervous system and initiates responses through the endocrine system. • Attached to the hypothalamus is the pituitary gland, composed of the anterior pituitary and the posterior pituitary. • The anterior pituitary makes and releases hormones under regulation of the hypothalamus. • The posterior pituitary stores and secretes hormones that are made in the hypothalamus. © 2011 Pearson Education, Inc. Feedback Loops 5 Types of Feedback NEGATIVE FEEDBACK POSITIVE FEEDBACK 6 Feedback Regulation • A negative feedback loop inhibits a response by reducing the initial stimulus, thus preventing excessive pathway activity. NEGATIVE= OPPOSITE • Positive feedback reinforces a stimulus to produce an even greater response. POSITIVE= INCREASE OUTPUT • For example, in mammals oxytocin causes the release of milk, causing greater suckling by offspring, which stimulates the release of more oxytocin. The Process of Communication: Signal-Transduction Pathway Three stages of the SignalTransduction Pathway 1. reception 2. transduction 3. response Typical Signal Transduction Pathway Ligand = Chemical Messenger • Three major classes of molecules function as hormones in vertebrates (ligands) – Polypeptides (proteins and peptides) – Amines derived from amino acids – Steroid hormones 10 Cellular Response Pathways • Water- and lipid-soluble hormones differ in their paths through a body • Water-soluble hormones are secreted by exocytosis, travel freely in the bloodstream, and bind to cellsurface receptors • Lipid-soluble hormones diffuse across cell membranes, travel in the bloodstream bound to transport proteins, and diffuse through the membrane of target cells SECRETORY CELL Lipidsoluble hormone Watersoluble hormone VIA BLOOD Signal receptor Transport protein TARGET CELL Signal receptor NUCLEUS (a) (b) SECRETORY CELL Lipidsoluble hormone Watersoluble hormone VIA BLOOD Signal receptor TARGET CELL Cytoplasmic response Transport protein OR Gene regulation Signal receptor Cytoplasmic response NUCLEUS (a) (b) Gene regulation Pathway for Water-Soluble Hormones • Binding of a hormone to its receptor initiates a signal transduction pathway leading to responses in the cytoplasm, enzyme activation, or a change in gene expression Specific Example Notice the presence of the second messenger Click here to view the animation Pathway for Lipid-Soluble Hormones • The response to a lipid-soluble hormone is usually a change in gene expression • Steroids, thyroid hormones, and the hormonal form of vitamin D enter target cells and bind to protein receptors in the cytoplasm or nucleus • Protein-receptor complexes then act as transcription factors in the nucleus, regulating transcription of specific genes Steroid Hormone Example: Testosterone Simple Hormone Pathways • Hormones are released from an endocrine cell, travel through the bloodstream, and interact with specific receptors within a target cell to cause a physiological response Simple Hormone Pathways • For example, the release of acidic contents of the stomach into the duodenum stimulates endocrine cells there to secrete secretin. • This causes target cells in the pancreas, a gland behind the stomach, to raise the pH in the duodenum. • The increased pH results in a decrease of secretin secretion. Simple Hormone Pathways Example Stimulus Low pH in duodenum Negative feedback Pathway Endocrine cell S cells of duodenum secrete the hormone secretin ( ). Hormone Target cells Response Blood vessel Pancreas Bicarbonate release Negative Feedback • Secretin secretion regulation is an example of negative feedback in action. 22 An example of positive feedback 23 Insulin and Glucagon: Control of Blood Glucose • Hormones work in pairs to maintain homeostasis. • Insulin (decreases blood glucose) and glucagon (increases blood glucose) are antagonistic hormones that help maintain glucose homeostasis. • The pancreas has clusters of endocrine cells called pancreatic islets with alpha cells that produce glucagon and beta cells that produce insulin. 25 Figure 45.13 Insulin Body cells take up more glucose. Blood glucose level declines. Beta cells of pancreas release insulin into the blood. Liver takes up glucose and stores it as glycogen. STIMULUS: Blood glucose level rises (for instance, after eating a carbohydrate-rich meal). Homeostasis: Blood glucose level (70–110 mg/100mL) STIMULUS: Blood glucose level falls (for instance, after skipping a meal). Blood glucose level rises. Liver breaks down glycogen and releases glucose into the blood. Alpha cells of pancreas release glucagon into the blood. Glucagon Out of Balance: Diabetes Mellitus • Diabetes mellitus is perhaps the best-known endocrine disorder. • It is caused by a deficiency of insulin or a decreased response to insulin in target tissues. • It is marked by elevated blood glucose levels. Out of Balance: Diabetes Mellitus • Type 1 diabetes mellitus (insulin-dependent) is an autoimmune disorder in which the immune system destroys pancreatic beta cells. • Type 2 diabetes mellitus (non-insulin-dependent) involves insulin deficiency or reduced response of target cells due to change in insulin receptors. Action of Insulin 29 Increases Ca2 uptake in intestines Active vitamin D Stimulates Ca2 uptake in kidneys PTH Stimulates Ca2 release from bones Parathyroid gland (behind thyroid) STIMULUS: Falling blood Ca2 level Blood Ca2 level rises. Homeostasis: Blood Ca2 level (about 10 mg/100 mL) Homeostasis in blood calcium levels • PTH increases the level of blood Ca2+ – It releases Ca2+ from bone and stimulates reabsorption of Ca2+ in the kidneys. – It also has an indirect effect, stimulating the kidneys to activate vitamin D, which promotes intestinal uptake of Ca2+ from food. • Calcitonin decreases the level of blood Ca2+ – It stimulates Ca2+ deposition in bones and secretion by kidneys. Practice • Blood calcium levels rise • Blood calcium level falls • Parathyroid releases PTH • Thyroid releases calcitonin • If calcium rises above set point • If calcium falls below set point 32 Solution 1. Blood calcium levels rise 3. Blood calcium level falls 5. Parathyroid releases PTH 2. Thyroid releases calcitonin 6. If calcium rises above set point 4. If calcium falls below set point 33 Anterior and Posterior Pituitary © 2011 Pearson Education, Inc. Posterior Pituitary Hormones • The two hormones released from the posterior pituitary act directly on nonendocrine tissues. – Oxytocin regulates milk secretion by the mammary glands and stimulates contraction of the uterus. – Antidiuretic hormone (ADH) promotes retention of water by the kidneys. Production and Release of Posterior Pituitary Hormones Neurosecretory cells of the hypothalamus Hypothalamus Neurohormone Axons Posterior pituitary Anterior pituitary HORMONE TARGET ADH Kidney tubules Oxytocin Mammary glands, uterine muscles Anterior Pituitary Hormones • Hormone production in the anterior pituitary is controlled by releasing and inhibiting hormones from the hypothalamus. • For example, prolactin-releasing hormone from the hypothalamus stimulates the anterior pituitary to secrete prolactin (PRL), which has a role in milk production and secretion. • Another example is Thyroid Stimulating Hormone (TSH), which stimulates the thyroid gland to produce thyroxine which stimulates and maintains metabolic processes. Figure 45.16 Tropic effects only: FSH LH TSH ACTH Neurosecretory cells of the hypothalamus Nontropic effects only: Prolactin MSH Nontropic and tropic effects: GH Hypothalamic releasing and inhibiting hormones Portal vessels Endocrine cells of the anterior pituitary Pituitary hormones Posterior pituitary HORMONE FSH and LH TSH ACTH Prolactin MSH GH TARGET Testes or ovaries Thyroid Adrenal cortex Mammary glands Melanocytes Liver, bones, other tissues Table 45.1a Table 45.1b Thyroid Regulation: A Hormone Cascade Pathway • A hormone can stimulate the release of a series of other hormones, the last of which activates a nonendocrine target cell; this is called a hormone cascade pathway. • The release of thyroid hormone results from a hormone cascade pathway involving the hypothalamus, anterior pituitary, and thyroid gland. • Hormone cascade pathways typically involve negative feedback. Figure 45.17 Pathway Stimulus Example Cold Hypothalamus Negative feedback Target cells Response Increased cellular metabolism Figure 45.17 Pathway Stimulus Example Cold Sensory neuron Hypothalamus Neurosecretory cell Hypothalamus secretes thyrotropin-releasing hormone (TRH). Releasing hormone Blood vessel Negative feedback Anterior pituitary Tropic hormone Endocrine cell Hormone Target cells Response Anterior pituitary secretes thyroid-stimulating hormone (TSH, also known as thyrotropin). Thyroid gland secretes thyroid hormone (T3 and T4). Body tissues Increased cellular metabolism