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The tears are unique among body fluids in that they are exposed to the environment. Much like other body fluids, tear fluid is kept in a tight pH range using the bicarbonate buffer system. [15] The pH of tears shift throughout a waking day, rising "about 0.013 pH units/hour" until a prolonged closed-eye period causes the pH to fall again. [15]
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.
Other circumstances also affect the body's temperature. The core body temperature of an individual tends to have the lowest value in the second half of the sleep cycle; the lowest point, called the nadir, is one of the primary markers for circadian rhythms. The body temperature also changes when a person is hungry, sleepy, sick, or cold.
Homeothermy, homothermy, or homoiothermy [1] (from Ancient Greek ὅμοιος (hómoios) ' similar ' and θέρμη (thérmē) ' heat ') is thermoregulation that maintains a stable internal body temperature regardless of external influence. This internal body temperature is often, though not necessarily, higher than the immediate environment. [2]
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature as its own body temperature, thus avoiding the need for internal thermoregulation.
In horses, the lower critical temperature is 5 °C while the upper critical temperature depends on the definition used. [11] Their thermoneutral zone is roughly 5–30 °C (41–86 °F). [12] In mice, the lower critical temperature and upper critical temperature can be the same, creating a thermoneutral point instead of a thermoneutral zone.
Homeostatic capacity refers to the capability of systems to self-stabilize in response to external forces or stressors, or more simply the capability of systems to maintain homeostasis. [1] [2] For living organisms, it is life's foundational trait, consisting of a hierarchy and network of traits endowed by nature and shaped by natural selection.
Thermoreceptors of the skin sense the temperature of water. A thermoreceptor is a non-specialised sense receptor, or more accurately the receptive portion of a sensory neuron, that codes absolute and relative changes in temperature, primarily within the innocuous range.