Exam 4 Review:  Chapter 24:  Thermogenesis

behavioral thermoregulation - Activities which are responses in changes in internal or external temperatures, such as making changes in posture, or exercising, or moving about in the environment, or drinking hot or cold beverages, or adding or removing layers of clothing, or huddling with other individuals, or seeking shelter, or using fire or other environmental thermoregulatory technologies, to control body temperature;

thermogenesis - The generation or production of heat, especially by physiological and metabolic processes such as increased basal metabolic rate (BMR); these processes are regulated by thyroid hormones (T₃ and T₄), and epinephrine, and by shivering which is regulated by the CNS.

hypothalamic thermostat - A center in the anterior portion of the hypothalamus of the brain, where there are neurons sensitive to temperature changes reported by sensory thermoreceptors located in the skin, mucous membranes, circulatory system and the brain itself, which regulates body temperature; it works in conjunction with other hypothalamic, autonomic and higher nervous thermoregulatory centers to keep the core temperature constant; some of these thermoregulatory responses are involuntary, mediated by the autonomic nervous system, some are neurohormonal and others are semi-voluntary or voluntary behavioral responses.

thyroid hormones - (1) amines:  thyroxine = tetraiodothyronine = T₄ and triiodothyronine = T₃ which are produced by follicular cells in response to TSH and target most cells in the body to increase their cellular metabolism, a thermogenic effect; and (2) protein:  (thyro)calcitonin produced by the autoregulated parafollicular = C cells released in response to increases in plasma calcium ion levels which targets osteoblasts to increase formation of bone matrix causing a corresponding decrease in plasma calcium and phosphate ion levels.

vasoconstriction - Any decrease in the diameter of blood vessels due to the contraction of the smooth muscle in the vessel's walls; usually regulated by the autonomic NS and certain hormones.

chemical thermogenesis - The generation or production of heat by physiological and metabolic activities which increase the basal metabolic rate (BMR); this is regulated by the hypothalamus which stimulates the release of (1) TSH from the anterior pituitary in order to increase output of T₃ and T₄ from the thyroid gland and the release of (2) epinephrine from the adrenal medulla.

shivering thermogenesis - The generation or production of heat by stimulating involuntary uncoordinated contractions of the skeletal muscles which is regulated by a shivering center in the hypothalamus which then activates motor centers in the brain stem; these contractions require catabolism of nutrients for energy production which provide most of the heat production.

cutaneous vasodilation - A increase in  the diameter of the superficial blood vessels supplying the capillary beds of the dermis of the skin due to the contraction of the smooth muscle in the vessels' walls; usually regulated by the autonomic NS and certain hormones; it increases blood flow in the skin and typically increases heat transfer to a cool environment.

cutaneous vasoconstriction - A decrease in the diameter of the superficial blood vessels supplying the capillary beds of the dermis of the skin due to the contraction of the smooth muscle in the vessels' walls; usually regulated by the autonomic NS and certain hormones; it reduces blood flow and heat transfer to the environment; heat loss from a human is reduced when arms and legs cool to several degrees below the temperature of the body core, where most vital organs are located.

radiation - The propagation of energy through space; the emission of energy in the form of particles or electromagnetic rays or waves; in the case of thermoregulation, the conversion of body heat into infrared rays which are transmitted away from the body.

evaporation - The process by which any substance is converted from a liquid state into, and carried off in, a vapor or gas state; this process requires an input of energy (heat) and the surface (solid or liquid)  from which the vapor escapes is therefore cooled by the process

conduction - The transmission of something through a medium or passage, especially the transmission of electric charge or heat through a conducting medium without perceptible motion of the medium itself; the direct transfer of thermal motion (heat) between molecules in direct contact with each other; heat is always conducted from an object of higher temperature to one of lower temperature; however, the rate and amount of heat transfer varies with different materials; water is 50 to 100 times more effective than air in conducting heat.

convection - Heat transfer in a gas or liquid by the circulation of currents from one region to another; the transfer of heat by the movement of air or liquid within the medium or past a surface; it is caused by the molecular motion; it occurs when a breeze contributes to heat loss from the surface of an animal with dry skin; it also occurs when circulating blood moves heat from an animal’s warm body core to the cooler extremities such as legs.

fever - A rise in the temperature of the body above the normal range; frequently a symptom of infection; stimulated by various chemicals, some of which are present in microorganisms (exogenous pyrogens) while others are internal local hormones released by immune system cells (endogenous pyrogens).

pyrogen - A substance that produces fever; various chemicals with this property are known, some of which are present in microorganisms (exogenous pyrogens) while others are internal local hormones released by immune system cells (endogenous pyrogens); the major endogenous pyrogen in humans is probably interleukin-1.

chill - A sensation of coldness, often accompanied by shivering and pallor of the skin, a pinched face, and blue lips, caused by undue cooling of the body or by nervous excitement, or forming the precursor of some constitutional disturbance, e.g., a fever; often an early symptom of infectious disease due to the invasion of the body by toxins; to be seized with cold.

hypothermia - An abnormally low body temperature, due to exposure to cold weather, or immersion in cold water, or induced by medical means to decrease the metabolism of tissues and, thereby, the need for oxygen, during certain surgical procedures, especially on the heart.

heat stroke - A severe condition caused by impairment of the body's thermoregulatory abilities, resulting from prolonged exposure to excessive heat and characterized by cessation of sweating, severe headache, high fever, hot dry skin, and in serious cases collapse and coma.

hyperthermia - An abnormally high body temperature, due to high fever, over exposure to the sun and heat stroke, burns, severe viral infections, bacterial septicemia and certain toxins, thyrotoxic crisis, as an undesirable side effect of certain drugs used in general anesthesia (anesthetics and muscle relaxants), etc., or induced for therapeutic purposes.

          infrared radiation from body surfaces to the environment
          conduction and convection to air or water around the body
          water evaporation from sweating and from ventilating wet respiratory membranes
          vasodilation of cutaneous (dermal) capillary beds
          decreased thyroid hormone activity lowering basal metabolic rate (BMR)
          changes in behavior:  stop exercising; move into the shade; remove clothes; turn on air conditioning