Ch 22 metabolism and energy balance

Ch 22 metabolism and energy balance - CH 22: METABOLISM AND...

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CH 22: METABOLISM AND ENERGY BALANCE HOMEOSTATIC CONTROL OF METABOLISM The endocrine system has primary responsibility for metabolic regulation, although the nervous system does have some influence, particularly in terms of governing food intake - Hour-to-hour regulation depends primarily on the ratio of insulin to glucagon, two hormones secreted by endocrine cells of the pancreas o Both hormones have short half-lives and must be continuously secreted if they are to have a sustained effect THE PANCREAS SECRETES INSULIN AND GLUCAGON The endocrine cells of the pancreas make up less than 2% of the organ’s total mass; most pancreatic tissue is devoted to the production and exocrine secretion of digestive enzymes and bicarbonate - Small clusters of cells, now known as islets of Langerhans scattered throughout the body of the pancreas contain four distinct cell types, each associated with secretion of one or more peptide hormones Nearly ¾ of the islet cells are beta cells , which produce insulin and a peptide called amylin - Another 20% are alpha cells, which secrete glucagon - Most of the remaining cells are somatostatin- secreting D cells - A few rare cells called PP cells (or F cells ) produce pancreatic polypeptide Like all endocrine glands, the islets are closely associated with capillaries into which the hormones are released - Both sympathetic and parasympathetic neurons terminate on the islets, providing a means by which the nervous system can influence metabolism THE INSULIN-TO-GLUCAGON RATIO REGULATES METABOLISM Insulin and glucagon act in antagonistic fashion to keep plasma glucose concentrations within an acceptable range - Both hormones are present in the blood most of the time - It is the ratio of the two that determines which hormone dominates In the fed state, when the body is absorbing nutrients, insulin dominates, and the body undergoes net anablosim - Ingested glucose is used for energy production; excess glucose is stored as glycogen or fat
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CH 22: METABOLISM AND ENERGY BALANCE - Proteins primarily go to protein synthesis In the fasted state, metabolic regulation prevents low plasma glucose concentrations ( hypoglycemia ) - When glucagon predominates, the liver uses glycogen and nonglucose intermediates to synthesize glucose for release into the blood In a normal person, fasting plasma glucose is maintained around 90 mg/dL of plasma - After absorption of nutrients from a meal, plasma glucose rises - The increase in glucose stimulates insulin release, which in turn promotes glucose transfer into cells - Plasma glucose concentrations thus fall back toward the fasting level shortly after each meal During an overnight fast, plasma glucose concentrations fall to their lowest values, and insulin secretion also decreases - Glucagon secretion remains relatively steady during the 24-hour period, supporting the theory that it is the ratio of insulin to glucagon that determines the direction of metabolism INSULIN IS THE DOMINANT HORMONE OF THE FED STATE Insulin is a typical peptide hormone
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This note was uploaded on 04/07/2008 for the course BIOL 425 taught by Professor Tondi during the Spring '08 term at George Mason.

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Ch 22 metabolism and energy balance - CH 22: METABOLISM AND...

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