Regulation of Body Temperature.
PKGhatak, MD
Humans are warm-blooded and maintain a constant level of body temperature. The core or internal temperature is near constant and environmental factors do not change the core temperature. The shell or skin temperature is much lower and varies with the prevailing weather condition.
The core temperature is defined as the temperature recorded inside the body, recorded by inserting a temperature sensitive detector in the central vasculature. Oral and rectal temperatures also record the same. In normal conditions, the core temperature of humans is 37C (+/- 2C) or 98.7F. The skin temperature is lower and usually around 21C. and different in the exposed skin and covered skin areas.
Cellular functions are mostly derived from enzymes. The enzymes function at their maximum efficiency at core temperature. Certain immunocytes, however, can increase efficiency at a higher temperature. Maintaining a normal core temperature requires a steady state balance between heat generation and dissipation of heat from the body.
Sets of temperature sensors, a center for incoming information, sub-centers for coordination, and a center for outgoing instructions exist in the body just like a well designed environment control center.
Source of body heat:
Involuntary metabolic processes and physical work are the principal sources but warm food and drink also contribute to body heat.
Heat dissipation follows the Laws of Physics- evaporation, radiation, conduction and convection.
The distribution and equilibrium of heat in different compartments of the body are achieved by blood circulation and help maintain a constant core temperature.
Mechanism of heat generation.
Mitochondria are the metabolic factory of the body. As food is undergoing oxidative phosphorylation, the synthetic or secretory functions of cells are paired with it. This coupled action minimizes the loss of energy as heat.
The body has two distinct pathways to generate heat – (a) shivering and (b) uncoupled respiration and thermogenesis by brown fat.
(a) Shivering.
In exposure to cold, stress, fear, and in the face of danger, the body releases Adrenaline and Thyroxine. The effects of these hormones are many. The voluntary muscle mass responds to these inputs by muscle contractions and using up stored energy Glycogen as heat, CO2, and water.
(b) Uncoupled respiration.
The site of uncoupled respiration and heat generation takes place in the Brown fat. In fact, the brown color is due to the presence of large numbers of mitochondria in the fat cells. The oxidative phosphorylation is coupled with ATP (adenosine triple phosphate) generation. ATP is a high energy packet and the mitochondria use them for other functions requiring energy.
A protein, Thermogenine, regulates the rate of Proton (H+) transfer across the mitochondria and cell wall membranes. ATP generation depends on the availability of H+. Thermogenine keeps these two processes linked together. H+(proton) combines O-(- = electron) is the essence of cellular respiration. A H+ pump situated on the inner side of the mitochondria membrane maintains the H+ gradient. In the situation when extra heat is required the Thermogeneine uncouples the two processes and the H+ pump allows H+ to cross to the outside mitochondria membrane. This action stops ATP generation and increases heat production. Catecholamines, stress hormones, and sympathetic nerve impulses accelerate the rate of heat generation by recruiting additional mitochondria.
Minimizing heat loss.
Goosebumps or Piloerection. A tiny bunch of smooth muscle by contracting lifts the skin hair upright. Standing hairs in a close group, trap air in between providing a barrier to heat loss. The sympathetic nerve supplies the motor impulse for muscle contraction and adrenaline is the neurotransmitter of piloerection.
Dissipation of body heat.
In addition to the law of physics of the flow of heat from a higher temperature to a lower temperature, there are additional measures the body can utilize to lower body temperature.
Sweating. In summer we know how sweat cools the body. Sweat glands are adrenergic. And stress, anxiety, and fever also increase sweating.
Skin vasodilation.
In the non-hair region of the skin – the blood vessels of the palms, soles, and lips are supplied by only vasoconstrictor nerve fibers. There are special channels that connect skin arterioles to capillary loops in the top layer of the skin. At a higher temperature, the sympathetic tones of blood vessels are withdrawn thereby increasing the skin blood flow. Cooling takes by radiation.
Temperature sensors.
Skin sensors:
The entire skin is covered by sensors -touch, pain, pressure, position sensation, and cold and hot temperatures. The skin has separate cold and heat sensors. These are discussed elsewhere. The spinal nerves carry temperature sensation via the lateral spinothalamic tract of the spinal cord. The second order neurons of temperature are located in the lateral horn of the spinal cord, from there the information reaches the lateral parabrachial nucleus located in the dorsolateral pons. From there the sensation goes to Median Preoptic Hypothalamic Nuclei. The second order neurons also carry the sensation by another tract to the ventromedial nucleus of the thalamus via the medial lemniscus.
A similar layout is seen in the sensory division of the 5th cranial nerve carrying temperature sensation from the face. And the sensory division of the 9th nerve from the structures of the mouth and throat. They carry the sensations to the ventromedial thalamic nuclei via the medial lemniscus.
Thalamus relays the sensations to (a). Preoptic Hypothalamus nuclei from there to the pituitary gland, (b). sensory cerebral cortex, (c). other midbrain nuclei.
Visceral censors:
The temperature sensors of the internal organs and central blood vessels are carried by the Vagus nerve. And then the sensations are relayed to preoptic nuclei of the thalamus and from the Thalamus- to Hypothalamus – to the sensory cerebral cortex.
Regulating Center of Temperature.
The sensory input comes to the Median Preoptic nuclei of the Hypothalamus, and by another tract to the Ventromedial nuclei of the Thalamus.
The outgoing instructions are generated by the Medial Preoptic nuclei of the hypothalamus. These two sections of the hypothalamus are interconnected.
From the medial preoptic nuclei, the outgoing instruction is carried to the midbrain dorso-median nuclei of the thalamus. From there the outflow impulses are relayed to the Rostral Raphe Nucleus of the Globus Pallidum of the basal ganglia. Then the final instruction travels down the spinal cord to 1. Brown fat tissue. 2. Blood vessels of the skin. 3. The skeletal muscles.
