Physiology
When blood is lost, the greatest immediate need is to stop further blood loss. The second greatest need is replacing the lost volume. This way remaining red blood cells can still oxygenate body tissue. Normal human blood has a significant excess oxygen transport capability, only used in cases of great physical exertion. Provided blood volume is maintained by volume expanders, a quiescent patient can safely tolerate very low haemoglobin levels, less than 1/3 that of a healthy person.
The body automatically detects the lower haemoglobin level, and compensatory mechanisms start up. The heart pumps more blood with each beat. Since the lost blood was replaced with a suitable fluid, the now diluted blood flows more easily, even in the small vessels. As a result of chemical changes, more oxygen is released to the tissues. These adaptations are so effective that if only half of the red blood cells remain, oxygen delivery may still be about 75 percent of normal. A patient at rest uses only 25 percent of the oxygen available in his blood. In extreme cases, patients have survived with a haemoglobin level of 2 g/dl, about 1/7 the norm, although levels this low are very dangerous.
With enough blood loss, ultimately red blood cell levels drop too low for adequate tissue oxygenation, even if volume expanders maintain circulatory volume. In these situations, the only alternatives are blood transfusions, packed red blood cells, or oxygen therapeutics (if available). However in some circumstances, hyperbaric oxygen therapy can maintain adequate tissue oxygenation even if red blood cell levels are below normal life-sustaining levels.
Read more about this topic: Volume Expander
Famous quotes containing the word physiology:
“A physician’s physiology has much the same relation to his power of healing as a cleric’s divinity has to his power of influencing conduct.”
—Samuel Butler (1835–1902)
“If church prelates, past or present, had even an inkling of physiology they’d realise that what they term this inner ugliness creates and nourishes the hearing ear, the seeing eye, the active mind, and energetic body of man and woman, in the same way that dirt and dung at the roots give the plant its delicate leaves and the full-blown rose.”
—Sean O’Casey (1884–1964)
“Now the twitching stops. Now you are still. We are through with physiology and theology, physics begins.”
—Alfred Döblin (1878–1957)