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Hypothermia is defined as a body core temperature below 35.0 °C (95.0 °F) in humans. [48] Symptoms range from shivering and mental confusion, [49] to hallucinations and cardiac arrest. [48] In addition to cold injuries, breathing cold air can cause dehydration, because the air is warmed to body temperature and humidified from body moisture. [15]
The human body always works to remain in homeostasis. One form of homeostasis is thermoregulation. Body temperature varies in every individual, but the average internal temperature is 37.0 °C (98.6 °F). [1] Sufficient stress from extreme external temperature may cause injury or death if it exceeds the ability of the body to thermoregulate.
Simplified control circuit of human thermoregulation. [8]The core temperature of a human is regulated and stabilized primarily by the hypothalamus, a region of the brain linking the endocrine system to the nervous system, [9] and more specifically by the anterior hypothalamic nucleus and the adjacent preoptic area regions of the hypothalamus.
Hypoxia is the principal stimulus that increases the number of erythrocytes, increasing the hematocrit from 40 up to 60%, with an increase of the hemoglobin concentration in blood from 15 g/dl up to 20–21 g/dl. Also the blood volume increases 20% producing an increase of the corporal hemoglobin up 15% or more. [3]
They show a sustained increase in cerebral blood flow, lower hemoglobin concentration, and less susceptibility to chronic mountain sickness than other populations due to their longer history of high-altitude habitation. [33] [34] With the proper physical preparation, individuals can develop short-term tolerance to high-altitude conditions.
The Armstrong limit or Armstrong's line is a measure of altitude above which atmospheric pressure is sufficiently low that water boils at the normal temperature of the human body. Exposure to pressure below this limit results in a rapid loss of consciousness, followed by a series of changes to cardiovascular and neurological functions, and ...
It varies with the temperature and pressure of the parcel and is often in the range 3.6 to 9.2 °C/km (2 to 5 °F/1000 ft), as obtained from the International Civil Aviation Organization (ICAO). The environmental lapse rate is the decrease in temperature of air with altitude for a specific time and place (see below). It can be highly variable ...
Following the tropopause is the stratosphere. This layer extends from the tropopause to the stratopause, which is located at an altitude of about 50 km (31 mi). Temperatures remain constant with height from the tropopause to an altitude of 20 km (12 mi), after which they start to increase with height.