Physiology, Temperature Regulation PDF

NCBI Bookshelf. Aserviceof the National Library of Medicine, National Institutes of Health. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Physiology, Temperature Regulation Authors Eva V. Osilla1; Sandeep Sharma2. Affiliations 1 Uni...

NCBI Bookshelf. Aserviceof the National Library of Medicine, National Institutes of Health. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Physiology, Temperature Regulation Authors Eva V. Osilla1; Sandeep Sharma2. Affiliations 1 University of South Alabama, DeBusk COM 2 Mery Fitzgerald Hospital Last Update: March 16, 2019. (NOTE: Text in brackets that is italicized and in a purple color provides a definition for terms that students may not know. These definitions were inserted by Dr. Fasenko and were not part of the original paper.) NOTE: Some of the information in this paper has been simplified or removed by Dr. Fasenko in order to improve student readability and understanding of the information. Introduction Thermoregulation, by definition, is a mechanism by which mammals maintain body temperature by tightly controlled self-regulation, no matter the temperature of their surroundings. Temperature regulation is a type of homeostasis, which is a process that biological systems use to preserve a stable internal state to survive. Ectotherms are animals that depend on their external environment for their body heat, and endotherms are animals that use thermoregulation to maintain a somewhat consistent internal body temperature to survive, even when their external environment changes. Humans and other mammals and birds are endotherms. Human beings have a normal core, or internal, temperature of around 37 degrees Celsius, which is equivalent to around 98.6 degrees Fahrenheit. Core temperature is most accurately measured via rectal probe thermometer. This is the temperature at which the human body's systems work together at their optimum, which is the reason the body has such tightly regulated mechanisms. Thermoregulation is crucial to human life. Without thermoregulation, the human body would not be able to adequately function and, inevitably, will expire. Issues of Concern When the body's ability to thermoregulate becomes hindered and is left untreated, organ failure is imminent. Blood flow will be reduced, leading to ischemia (this is a condition in which some part of your body is not getting enough blood, so therefore it is not getting enough oxygen), and, ultimately, multiple organ failures. Cellular Viral illness or another infectious disease can cause a person to develop a fever, and the body no longer has that same core temperature of 37 degrees Celsius. This is because when the body experiences an infection from invading pathogens, it tries to fight back by releasing pyrogens such as cytokines, prostaglandins, and thromboxane-all of which increase the body's temperature. By releasing these pyrogens, the foreign pathogens are not able to breed. This allows for antibodies to develop and enzymes to be activated to fight the infection further. https://www.ncbi.nlm.nih.gov/books/NBK507838/?report=printable 1/5  Development The brain, or more specifically the hypothalamus, controls thermoregulation. If the hypothalamus senses external temperatures growing too hot or too cold, it will automatically send signals to the skin, glands, muscles, and organs. For example, when the body is in a very hot external environment, or simply undergoing high activity levels such as exercise, it's temperature will rise, causing the hypothalamus to send signals to the cells of the skin that produce sweat. Sweating is the body's approach to cooling itself down. As the body's temperature rises, sweat is expelled, the muscles relax, and body hair lies flat against the skin. These are all ways to release heat and therefore lower the temperature of the body. In contrast, when the body experiences a cold environment, the skeletal muscles tense up leading to the shivering reflex, and the arrector pili muscles, a type of smooth muscle, raise the bodily hair follicles where they are attached. These processes, in turn, create warmth and trap heat, respectively. Organ Systems Involved Multiple organs and body systems are affected when thermoregulation is not working correctly. During heat illness as a result of improper thermoregulation, the following organs and systems are impaired. Notice that many of these issues cause or are influenced by the other issues. Increased heart rate. Hypovolemia (intravascular dehydration -dehydration just within blood vessels) Lack of oxygen to brain and brain swelling Bleeding in gastrointestinal tract and sepsis (bodies extreme reaction to an infection) Fluid builds up in the lungs, making it difficult to breathe Kidney failure Liver cells are stressed Abnormal blood clotting throughout the body Electrolyte abnormalities Hypoglycemia (low blood sugar), Metabolic acidosis (too much acid in body fluids) Respiratory alkalosis (too little carbon dioxide in the blood, causing the blood's pH to rise) When body temperatures severely decrease in hypothermia, the body's systems are negatively affected. Irregular heart beat Central nervous system activity declines Fluid in lungs Decrease in blood flow and subsequent reduction in filtration of blood by kidney https://www.ncbi.nlm.nih.gov/books/NBK507838/?report=printable 2/5  Function Slight changes in core body temperature occur every day, depending upon variables such as circadian rhythm (the natural physical, mental, and behavioral changes that occur in the body over a 24-hour cycle) and menses; but otherwise, the temperature is tightly controlled. When a person is unable to regulate his or her body temperature, various pathologies may occur. The human body has four different methods for keeping itself at its core temperature: vaporization (evaporation), radiation, convection, and conduction. To keep the body functioning, it must be at its ideal temperature, and for this to happen, physical factors must be sufficient. This includes having enough intravascular volume and cardiovascular function; the body must be able to transport the rising internal heat to its surface for release. The reason that elderly people are at higher risk for disorders of thermoregulation is that they, as a whole, have less intravascular volume and decreased cardiac function. Mechanism Thermoregulation has three mechanisms: afferent sensing, central control, and efferent responses. There are receptors for both heat and cold throughout the human body. Afferent sensing works through these receptors to determine if the body is experiencing either too hot or too cold of a stimulus. Next, the hypothalamus is the central controller of thermoregulation. Lastly, efferent responses (behavioral thermoregulation) are carried out primarily by the body's behavioral reactions to fluctuations in body temperature. For example, if a person is feeling too warm, the normal response is to remove an outer article of clothing. If a person is feeling too cold, they choose to wear more layers of clothing. Efferent responses also consist of automatic responses by the body to protect itself from extreme changes in temperature, such as sweating, vasodilation, vasoconstriction, and shivering.18] Related Testing The thermoregulatory sweat test (TST) is a specific clinical test that is used to diagnose certain conditions that cause abnormal temperature regulation and defects in sweat production in the body. It measures a patient's ability to produce sweat in a controlled, heated and humidified environment and assesses the patient's central and autonomic nervous systems to determine if the thermoregulatory centers are working correctly. To perform the thermoregulatory sweat test, the patient is placed in a chamber that slowly rises in temperature. Before the chamber is heated, the patient is coated with a special kind of indicator powder that will change in color when sweat is produced. This powder, when changing color, will be useful in visualizing which skin is sweating versus not sweating. Results of the patient's sweat pattern will be documented by digital photography, and abnormal TST patterns can indicate if there is dysfunction in the autonomic nervous system. Pathophysiology (the study of the physical and functional changes that occur during a disease process) When external environments are exceedingly warm, or a person is engaging in an excessive amount of physical activity, the heat that is produced inside his or her body is typically transported to the blood. The blood then carries the heat through numerous capillaries that are located directly under the skin. Because the blood is near the surface, it can cool the person down. This cooled blood can then be transported back https://www.ncbi.nlm.nih.gov/books/NBK507838/?report=printable 3/5  through the body to prevent the body temperature from becoming too high. Sweat is also a means by which the body cools itself down; it is created by glands to carry out evaporation at the topmost skin layer, the epidermis, to release heat. This describes vaporization, one of the four mechanisms used to maintain core body temperature. Radiation is when the heat that is released from the body's surface is moved into the surrounding air; convection occurs when cooler air surrounds the body's surface, and conduction comes into play when a person is either immersed in cold water or uses an ice pack-their internally generated heat is transferred to the cold water or the ice pack. This is another reason why it is very important to stay hydrated in the heat or during physical activity-not only to maintain adequate intravascular fluid volume, but also to aid in conduction processes that cool the body down. When cold fluids are ingested, the heat is released into the fluid and excreted out of the body as sweat or urine. When patients are dehydrated, they contain a decreased amount of intravascular volume. This in itself is another risk factor for dysfunctional thermoregulation. If there are not enough fluids in the body, there is not enough blood. When there is not enough blood, the body cannot transport the internally made heat to the body's surface to be cooled. Heat will remain retained, blood will become thicker, and the heart becomes strained. This leads to hyperthermia. In contrast, hypothermia is defined as low internal body temperature, or a temperature less than 35 degrees Celsius (95 degrees Fahrenheit). It is usually caused by too much heat loss from cold weather exposure or cold water immersion. During cold water immersion, there is a reflex known as the diving reflex, which causes vasoconstriction in the visceral muscles. The body does this as a protective mechanism to keep a person's essential organs, like their heart and brain, supplied with blood. The body also does this to prevent the brain from hypoxia (lack of oxygen), since metabolic demand will have decreased. There are two different types of hypothermia: primary and secondary. Primary hypothermia is when the S cold environment is the direct pathology (cause), and secondary hypothermia is when a patient's illness is what causes his hypothermia. Conduction, convection, and radiation also come into play with hypothermia; this is how the rate of heat loss is determined. Hypothermia decelerates all physiologic roles include metabolic rate, mental awareness, nerve conduction, neuromuscular reaction times, and both the cardiovascular and respiratory systems. As previously mentioned, the vasoconstriction caused by hypothermia induces renal dysfunction, and,eventully, cold diuresis due to the decreased levels of ADH (antidiuretic hormone). These decreased levels of antidiuretic hormone mean that the patient's urine will be very dilute. Because of the vasoconstriction, thismay lead to cardiac arrest or abrupt shock as the patient's vasculature is dilated while being rewarmed, a condition referred to as the rewarming collapse. Some diseases that can cause decreased heat production, or hypothermia include endocrinological diseases such as diabetes, hypothyroidism, hypoadrenalism, and hypopituitarism. Those who are most at risk for hypothermia are elderly patients, trauma patients, those who are mentally ill, those who are abusing alcohol, drugs, or on other types of medication, and, lastly, those with low socio- economic status. Ordinarily, people who get hypothermia have an underlying issue-either from a disease or surgery. When a patient with one of these diseases presents with hypothermia, it is imperative to treat the underlying disease to effectively treat the hypothermia. Hypothermia, like hyperthermia, affects all of the human body's systems. When the core temperature drops below 30 degrees Celsius, the heart responds with arrhythmias. The reason that patients with traumatic brain injury are likely to have impaired thermoregulation is that the hypothalamus regulates the core body temperature. When this essential body part is injured, the body is https://www.ncbi.nlm.nih.gov/books/NBK507838/?report=printable 4/5  unable to control how it regulates the body's heat. Other problems in the CNS (Central Nervous System) that affect thermoregulation by the same mechanism can include tumors in the CNS, spinal cord injuries, intracranial hemorrhage, and diseases such as Parkinson, and multiple sclerosis. Patients who are on the extreme spectrums of age (such as infants and elderly persons) are at higher risk for disorders of thermoregulation and exhibit these features more readily when sick. The very young and the very old cannot increase their metabolic rates, and this can provoke them into hypothermia since they do not have the shivering reflux or much muscle mass. Changes that occur as one ages include those affecting vasomotor sweating function, skeletal muscle response, and temperature perception. Elderly persons have lower than normal internal body temperatures and decreased immunity, so when they have an infection, they may not get the normal pyretic (fever) response. Instead, they may present with hypothermia caused by a septic infection. In fact, studies have shown that the core temperatures of elderly patients with sepsis within their first 24 hours of presentation are a huge predictor of their mortality. This is the reason that elderly patients who are septic have a higher mortality rate than younger patients who are septic. References 1. Lim CL, Byrne C, Lee JK. Human thermoregulation and measurement of body temperature in exercise and clinical settings. Ann. 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Anesth Prog. 2010 Spring;57(1):25-32; quiz 33-4. [PMC free article: PMC2844235] (PubMed: 20331336] https://www.ncbi.nlm.nih.gov/books/NBK507838/?report=printable 5/5