Chapter VII Energy Metabolism and Temperature
In adults the rates at which molecules are synthesized (anabolism) and broken down (catabolism) are approximately equal, and so the chemical composition of the body does not change appreciably. The energy released during a chemical reaction is either released as heat or is transferred to other molecules. In all cells, energy is transferred from the catabolism of fuel molecules to adenosine triphosphate (ATP). The hydrolysis of ATP to adenosine triphosphate (ADP) and Pi then transfers this energy to cell functions. ATP is formed by oxidative phosphorylation and substrate phosphorylation.
Regulation of Total Body Energy Balance
The energy liberated during a chemical reaction appears either as heat or work. Total energy expenditure = heat produced + external work done + energy stored. Metabolic rate is influenced by the muscular activity, environmental temperature, emotional state, food intake, age, sex, height, weight, body surface area and body temperature. Basal metabolic rate is increased by the thyroid hormones and epinephrine.
Energy storage, as fat, can be positive or negative when metabolic rate is less than or greater than, respectively, the energy content of ingested food. Energy storage is regulated mainly by reflex adjustment of food intake to metabolic rate. In addition, metabolic rate increases or decreases, to some extent, when food intake is chronically increased or decreased, respectively.
Regulation of Body Temperature
Body temperature, measured as rectal temperature, shows a circadian rhythm, being highest during the day and lowest at night. Body temperature is regulated by altering heat production and/or heat loss so as to change total-body heat content. Heat production is altered by increasing muscle tone, shivering, and voluntary activity. Heat-loss mechanisms include radiation, conduction, convection, and evaporation of water from the body surface. Heat loss by radiation, conduction, and convection depends on the difference in temperature between the skin surface and the environment. . 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 the nerves. Behavioral responses such as putting on more clothes also influence heat loss.
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. Increased heat production is essential for temperature regulation at environmental temperatures below the thermoneutral zone, and sweating is essential at temperatures above this zone. The hypothalamus and other brain areas are the integrating centers for temperature-regulating reflexes. Both peripheral and central thermoreceptors participate in these reflexes.
The most common causes of hyperthermia are fever and exercise. 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 pyrogens, probably including IL-I, TNF, and IL-6. The hyperthermia of exercise is not due primarily to a changed set point but to the increased heat produced by the muscles.
