BIOLOGY Volume 2 - A Guide to General Biology - 2004

19. HOMEOSTASIS

19.5. Endothermic animals

19.5.2. Sources of heat (heat production)

In endothermic animals, the primary source of heat is the exothermic (heat-releasing) biochemical reactions occurring within living Cells. Thermal energy is generated mainly through The breakdown of CARBOHYDRATES and fats obtained from food. The majority of metabolic energy released as heat originates from active Tissues such as The Liver and skeletal Muscles. The amount of heat produced by a fasting Organism at rest is referred to as the basal metabolic rate. This parameter serves as a baseline when comparing energy expenditures during various activities across different organisms (see Section 9.5.8). For a man of average height and weight, the daily energy requirement to maintain basal METABOLISM is approximately 8000 kJ. This parameter varies widely depending on an individual's body mass, age, and sex, and is generally slightly higher in males than in females.

Heat production is regulated by a variety of factors, notably ambient Temperature and Hormones. For instance, thyroxine, secreted by The Thyroid Gland, increases metabolic rate and consequently heat production. This effect is long-term, whereas the effects of adrenaline are short-lived. Other sources of heat are triggered by nerve impulses. Repetitive stimulation of skeletal muscles by motor Neurons induces shivering, which can increase heat production nearly five-fold compared to the basal metabolic rate. During shivering, different groups of Muscle fibers within the same muscle contract and relax asynchronously, with the net result being uncoordinated movements. The thermal effect can be enhanced through Other types of muscular activity, such as rubbing hands together, stamping feet, and light Physical Exercise. In many mammals, areas rich in brown adipose tissue are located around the Blood Vessels of the chest cavity. The cells of this tissue contain numerous fat droplets that are rapidly metabolized upon stimulation by sympathetic nerves. For animals that undergo hibernation, heat production by the Cell/35.html">Mitochondria of these cells is of particular importance, as it enables a rapid increase in body temperature during arousal from hibernation. Most of the thermogenic mechanisms described above are regulated by the Hypothalamus, whose role will be discussed in more detail in Section 19.5.4.