Fevers are caused by the body's immune response to a pathogen. The body raises its own internal temperature to a level that should not cause permanent damage to its own cells but that may kill the disease causing agent (such as a virus or bacteria).
To understand why having a fever may cause someone to become flushed, we need to first understand how the circulatory system is involved in thermoregulation, the process by which the body maintains a stable internal temperature. As blood flows throughout the body, carrying oxygen and nutrients to various tissues, it can also carry heat. The center of the body, in the chest and abdomen, is the warmest and most stable in terms of temperature, with the extremities and areas close to the skin being more prone to fluctuations in temperature. When blood flows through warmer parts of the body it is heated, and when it flows closer to the external environment it loses heat.
When the internal temperature is too low, the body tries to minimize the heat lost from the blood by directing flow away from the skin and extremities. It does this by constricting blood vessels in these areas (known as vasoconstriction). This explains why when you're very cold, your fingers and toes may look pale and feel numb. When the body becomes too warm, the opposite happens- blood is directed away from the warm, internal areas into the extremities and skin. This increase in blood to the thin capillaries of the face causes the capillaries to swell or dilate (this is known as vasodilation) as blood passes through and is cooled by proximity to the external air. This swelling of the capillaries with blood makes the skin appear red, causing what we call "flushing."
So, why does a fever cause a person to flush?
Blood rushes to the skin as part of the body's reaction to an increase in internal temperature, an attempt to maintain a stable internal environment.
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