The word thermogenesis means the process of heat production in organisms. When adults are cold, they shiver. Shivering results to increased muscle activity, thus, producing heat. In newborns these mechanisms of heat production do not occur. Newborns rarely shiver except at very low temperatures. Shivering is not an effective way of producing heat in the youngest population.
Nonshivering thermogenesis is the primary method of heat production in infants. It is NOT shivering that produces heat but the metabolism of brown fat increases body temperature when the thermal receptors in the skin detect a skin temperature of 35 to 36 degree Celsius (95 to 96.8 degree Fahrenheit).
Brown fat, also called adipose tissue or brown adipose tissue (BAT), is a special kind of highly vascular fat found in newborns. It contains an ample supply of blood vessels which cause the brown color. Brown fats are located primarily in the following areas:
- Back of the neck
- In the axillae
- Around the kidneys
- Between the scapulae
- Along the abdominal aorta
Some infants have insufficient brown fat stores. Preterm infants may be born before the stores of brown fat have accumulated. Aside from the said case, intrauterine growth restriction also deplete brown fat stores before birth occurs. Newborns that are exposed to prolonged cold stress may have insufficient brown fat stores as large amount brown fat is consumed for heat production in this situation. Thus, these infants will not be able to raise their body temperature if they are subjected to further episodes of cold stress. Without brown fat to be metabolized, no heat production will counteract the cold stress. Hence, the infant is at risk to serious complications.
Processes Involved in Nonshivering Thermogenesis
- Nonshivering thermogenesis begins when the thermal receptors in the skin detect a skin temperature of 35 to 36 degrees Celsius (95 to 96.8 degrees Fahrenheit).
- The thermal receptors stimulation is then transmitted to the hypothalamus thermal center.
- In response to the hypothalamic stimulation, norepinephrine is released in brown fat.
- Presence of norepinephrine in the brown fat initiates its metabolism.
- As brown fat is metabolized, it generates more heat than other fats.
- Thus, blood passing through the brown fats is warmed and carries heat to the systemic circulation or to the rest of the body.
image courtesy of thehealthblog.net