Question

Physiology, Temperature Regulation

Osilla EV, Marsidi JL, Sharma S.

Publication Details

Introduction

Thermoregulation is a mechanism by which mammals maintain body temperature with tightly controlled self-regulation independent of external temperatures. Temperature regulation is a type of homeostasis and a means of preserving a stable internal temperature in order to survive. Ectotherms are animals that depend on their external environment for body heat, while endotherms are animals that use thermoregulation to maintain a somewhat consistent internal body temperature even when their external environment changes. Humans and other mammals and birds are endotherms. Human beings have a normal core internal temperature of around 37 degrees Celsius (98.6 degrees Fahrenheit) measured most accurately via a rectal probe thermometer. This is the optimal temperature at which the human body’s systems function. Thermoregulation is crucial to human life; without thermoregulation, the human body would cease to function. Thermoregulation also plays an adaptive role in the body's response to infectious pathogens. [1][2]

Issues of Concern

The body's core internal temperature has a narrow range and typically ranges 97-99 F with tight regulation. When the body’s ability to thermoregulate becomes disrupted it can result in overheating (hyperthermia) or being too cool (hypothermia). Either state can have deleterious effects on the various body systems, most significantly reduced blood flow leading to ischemia and multiple organ failure.

Cellular

Viral illness or another infectious disease can cause a person to develop a fever, raising the core temperature above 37 degrees Celsius. Fever is a result of the body releasing pyrogens such as cytokines, prostaglandins, and thromboxane. These pyrogens induce cyclooxygenase 2 (COX2) to convert arachidonic acid to prostaglandin E2 (PGE2). PGE2 binds to receptors in the hypothalamus, increasing the thermogenic set point. This elevated temperature set point results in the body working to achieve a higher internal temperature. [1][3]

Development

The brain, more specifically the hypothalamus, controls thermoregulation. If the hypothalamus senses internal temperatures growing too hot or too cold, it will automatically send signals to the skin, glands, muscles, and organs. For example, if the body is generating heat during high-level exercise or if the external ambient temperature is elevated enough to cause a rise in the core temperature, afferent signals to the hypothalamus result in efferent signals to the cells of the skin to produce sweat. Sweating is one mechanism the body can use to cool itself as heat is lost through the process of sweat evaporation. In contrast, when the body experiences a cold environment, a shivering reflex results in skeletal muscles contracting and generating heat; additionally, the arrector pili muscles (a type of smooth muscle) raise the bodily hair follicles to trap the heat generated.[2]

Organ Systems Involved

Multiple organs and body systems are affected when thermoregulation is impaired. During a heat-related illness, insufficient thermoregulation can result in multiple organ and system impairment. (Notice that many of these issues are interconnected.)

The heart experiences increased work as it increases both heart rate and cardiac output.

The circulatory system can experience intravascular volume depletion.

The brain can experience ischemia and/or edema.

The gastrointestinal tract is vulnerable to hemorrhage and infection as the intestinal mucosa becomes increasingly permeable.

The lungs become impaired if sustained hyperventilation, hyperpnea, and pulmonary vasodilation lead to ARDS. 

Acute renal failure is an effect of intravascular volume depletion and impaired circulation.

Liver cells suffer because of the fever, ischemia, and cytokine increase in the intestinal tract.

Various organs can become ischemic from microthrombi or DIC. 

Electrolyte abnormalities are likely as well as hypoglycemia, metabolic acidosis, and respiratory alkalosis. 

When body temperatures are severely decreased in hypothermia, the body’s systems are also adversely affected. The cardiovascular system is susceptible to dysrhythmias such as ventricular fibrillation. The central nervous system's (CNS) electrical activity is noticeably diminished. Noncardiogenic pulmonary edema can occur as well as cold diuresis. Additionally, hypothermia causes preglomerular vasoconstriction which leads to decreased glomerular filtration rate (GFR) and decreased renal blood flow (RBF). [3]​

Answer 1

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