Doctors grade dangerous blood loss using the scoring of a tennis match
Lose under 15 percent of your blood volume, then 15 to 30, then 30 to 40, then more than 40: those four bands of hypovolemic shock echo tennis scores of 15, 30 and 40. Hypovolemia means the fluid outside your cells has run low, and the body fights hard to hide it.
Hypovolemia, also called volume depletion, is an abnormally low level of extracellular fluid, whether from losing salt and water together or from losing blood. It is not the same thing as dehydration. Causes split into those involving the kidneys, such as excess urine driven by high glucose or impaired response to salt-regulating hormones, and those that do not: vomiting, diarrhoea, heavy sweating, burns, fast breathing, fluid pooling in body cavities, and bleeding, whether visible or internal.
The body's defence starts with pressure sensors in the carotid sinuses and aortic arch. When they detect falling volume they trigger a surge of adrenaline and noradrenaline, which narrows peripheral blood vessels to reserve blood for the brain and heart. That explains the classic picture of cold hands and feet, a racing pulse and pale skin. Early on, a person may simply feel headachy, tired, dizzy or thirsty; later come scant urine, bluish skin, chest pain and a clouding mind.
Children are especially deceptive. They can hold their blood pressure steady, even artificially high, for longer than adults despite losing volume, but when compensation fails they crash quickly and badly, so any hint of internal bleeding in a child is treated aggressively. Internal bleeding can also leave telltale bruising, along the flanks in Grey Turner's sign or around the navel in Cullen's sign.
In hospital, the first priority with hypovolemic shock is finding and stopping the bleeding, and oxygen is given at once. Intravenous fluids restore volume but cannot carry oxygen and dilute clotting factors, so current practice often tolerates somewhat low blood pressure rather than pushing it up and dislodging fresh clots. Drugs that constrict vessels, such as noradrenaline, are generally avoided because they can starve tissues further without fixing the loss.
Source: Hypovolemia
