Chapter 18 Summary

Section A Summary

Events of the Absorptive and Postabsorptive States
1. During absorption, energy is provided primarily by absorbed carbohydrate, and net synthesis of glycogen, triacylglycerol, and protein occurs.
   1. Some absorbed carbohydrate not used for energy is converted to glycogen, mainly in the liver and skeletal muscle, but most is converted, in liver and adipocytes, to a-glycerol phosphate and fatty acids, which then combine to form triacylglycerol. The liver releases its triacylglycerols in very low density lipoproteins, the fatty acids of which are picked up by adipocytes.
   2. The fatty acids of some absorbed triacylglycerol are used for energy, but most are rebuilt into fat in adipose tissue.
   3. Some absorbed amino acids are converted to proteins, but excess amino acids are converted to carbohydrate and fat.
   4. There is a net upake of glucose by the liver.
2. In the postabsorptive state, blood glucose level is maintained by a combination of glucose production by the liver and a switch from glucose utilization to fatty acid and ketone utilization by most tissues.
   1. Synthesis of glycogen, fat, and protein is curtailed, and net breakdown of these molecules occurs.
   2. The liver forms glucose by glycogenolysis of its own glycogen and by gluconeogenesis from lactate and pyruvate (from breakdown of muscle glycogen), glycerol (from adipose-tissue lipolysis), and amino acids (from protein catabolism).
   3. Glycolysis is decreased, and most of the body’s energy supply comes from the oxidation of fatty acids released by adipose-tissue lipolysis and of ketones produced from fatty acids by the liver.
   4. The brain continues to use glucose but also starts using ketones as they build up in the blood.

Endocrine and Neural Control of the Absorptive and Postabsorptive States
1. The major hormones secreted by the pancreatic islets of Langerhans are insulin by the beta cells and glucagon by the alpha cells.
2. Insulin is the most important hormone controlling metabolism.
   1. In muscle, it stimulates glucose uptake, glycolysis, and net synthesis of glycogen and protein; in adipose tissue, it stimulates glucose uptake and net synthesis of triacylglycerol; in liver, it inhibits gluconeogenesis and glucose release and stimulates the net synthesis of glycogen and triacylglycerols.
   2. The major stimulus for insulin secretion is an increased plasma glucose concentration, but secretion is also influenced by many other factors, which are summarized in Figure 18 – 8.
3. Glucagon, epinephrine, cortisol, and growth hormone all exert effects on carbohydrate and lipid metabolism that are opposite, in one way or another, to those of insulin. They raise plasma concentrations of glucose, glycerol, and fatty acids.
   1. Glucagon’s physiological actions are all on the liver, where it stimulates glycogenolysis, gluconeogenesis, and ketone synthesis.
   2. The major stimulus for glucagon secretion is hypoglycemia, but secretion is also stimulated by other inputs, including the sympathetic nerves to the islets.
   3. Epinephrine released from the adrenal medulla in response to hypoglycemia stimulates glycogenolysis in the liver and muscle, gluconeogenesis in liver, and lipolysis in adipocytes. The sympathetic nerves to liver and adipose tissue exert effects similar to those of epinephrine.
   4. Cortisol is permissive for gluconeogenesis and lipolysis; in higher concentrations, it stimulates gluconeogenesis and blocks glucose uptake. These last two effects are also exerted by growth hormone.

Fuel Homeostasis in Exercise and Stress
1. During exercise, the muscles use as their energy sources plasma glucose, plasma fatty acids, and their own glycogen.
   1. Glucose is provided by the liver, and fatty acids are provided by adipose-tissue lipolysis.
   2. The changes in plasma insulin, glucagon, and epinephrine are similar to those that occur during the postabsorptive period and are mediated mainly by the sympathetic nervous system.
2. Stress causes hormonal changes similar to those caused by exercise.

Diabetes Mellitus
1. Insulin-dependent diabetes is due to absolute insulin deficiency and can lead to diabetic ketoacidosis.
2. Noninsulin-dependent diabetes is usually associated with obesity and is caused by a combination of insulin resistance and a defect in beta-cell responsiveness to elevated plasma glucose concentration. Plasma insulin concentration is usually normal or elevated.

Regulation of Plasma Cholesterol
1. Plasma cholesterol is a precursor for the synthesis of plasma membranes, bile salts, and steroid hormones.
2. Cholesterol synthesis by the liver is controlled so as to homeostatically regulate plasma cholesterol concentration; it varies inversely with ingested cholesterol.
3. The liver also secretes cholesterol into the bile and converts it to bile salts.
4. Plasma cholesterol is carried mainly by low-density lipoproteins, which deliver it to cells; high-density lipoproteins carry cholesterol from cells to the liver and steroid-producing cells. The LDL/HDL ratio correlates with the incidence of coronary heart disease.

Bone Growth
1. A bone lengthens as osteoblasts at the shaft edge of the epiphyseal growth plates convert cartilage to bone while new cartilage is being laid down in the plates.
2. Growth ceases when the plates are completely converted to bone.

Environmental Factors Influencing Growth
1. The major environmental factors influencing growth are nutrition and disease.
2. Malnutrition during in utero life may produce irreversible stunting and mental deficiency.

Hormonal Influences on Growth
1. Growth hormone is the major stimulus of postnatal growth.
   1. It stimulates the release of IGF-I from the liver and many other cells, and IGF-I then acts locally (and perhaps also as a hormone) to stimulate cell division.
   2. Growth hormone also acts directly on cells to stimulate protein synthesis.
   3. Growth hormone secretion is highest during adolescence.
2. Because thyroid hormones are required for growth hormone synthesis and the growth-promoting effects of this hormone, they are essential for normal growth during childhood and adolescence. They are also permissive for brain development during infancy.
3. Insulin stimulates growth mainly during in utero life.
4. Mainly by stimulating growth hormone secretion, testosterone and estrogen promote bone growth during adolescence, but these hormones also cause epiphyseal closure. Testosterone also stimulates protein synthesis.
5. Cortisol in a high concentration inhibits growth and stimulates protein catabolism.

Basic Concepts of Energy Expenditure
1. The energy liberated during a chemical reaction appears either as heat or work.
2. Total energy expenditure = heat produced + external work done + energy stored.
3. Metabolic rate is influenced by the many factors summarized in Table 18 – 7.
4. Metabolic rate is increased by the thyroid hormones and epinephrine. The other functions of the thyroid hormones are summarized in Table 18 – 8.

Regulation of Total-Body Energy Stores
1. Energy storage as fat can be positive or negative when the metabolic rate is less than or greater than, respectively, the energy content of ingested food.
   1. Energy storage is regulated mainly by reflex adjustment of food intake.
   2. In addition, the metabolic rate increases or decreases to some extent when food intake is chronically increased or decreased, respectively.
2. Food intake is controlled by leptin, secreted by adipose-tissue cells, and a variety of satiety factors, as summarized in Figure 18 – 17.
3. Being overweight or obese, the result of an imbalance between food intake and metabolic rate, increases the risk of many diseases.

Regulation of Body Temperature
1. Core body temperature shows a circadian rhythm, being highest during the day and lowest at night.
2. The body exchanges heat with the external environment by radiation, conduction, convection, and evaporation of water from the body surface.
3. The hypothalamus and other brain areas contain the integrating centers for temperature-regulating reflexes, and both peripheral and central thermoreceptors participate in these reflexes.
4. Body temperature is regulated by altering heat production and/or heat loss so as to change total body heat content.
   1. Heat production is altered by increasing muscle tone, shivering, and voluntary activity.
   2. Heat loss by radiation, conduction, and convection depends on the difference between the skin surface and the environment.
   3. In response to cold, skin temperature is decreased by decreasing skin blood flow through reflex stimulation of the sympathetic nerves to the skin. In response to heat, skin temperature is increased by inhibiting these nerves.
   4. Behavioral responses such as putting on more clothes also influence heat loss.
   5. Evaporation of water occurs all the time as insensible loss from the skin and respiratory lining. Additional water for evaporation is supplied by sweat, stimulated by the sympathetic nerves to the sweat glands.
   6. Increased heat production is essential for temperature regulation at environmental temperatures below the thermoneutral zone, and sweating is essential at temperatures above this zone.
5. Temperature acclimatization to heat is achieved by an earlier onset of sweating, an increased volume of sweat, and a decreased sodium concentration of the sweat.
6. Fever is due to a resetting of the temperature set point so that heat production is increased and heat loss is decreased in order to raise body temperature to the new set point and keep it there. The stimulus is endogenous pyrogen, which is interleukin 1 and other peptides as well.
7. The hyperthermia of exercise is due to the increased heat produced by the muscles.