<P> The body automatically detects the lower hemoglobin 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 . </P> <P> 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 . </P> <P> There are two main types of volume expanders: crystalloids and colloids . Crystalloids are aqueous solutions of mineral salts or other water - soluble molecules . Colloids contain larger insoluble molecules, such as gelatin; blood itself is a colloid . There is no evidence that colloids are better than crystalloids in those who have had trauma, burns or surgery and as they are more expensive their use is not recommended . </P> <P> The most commonly used crystalloid fluid is normal saline, a solution of sodium chloride at 0.9% concentration, which is close to the concentration in the blood (isotonic). Ringer's lactate or Ringer's acetate is another isotonic solution often used for large - volume fluid replacement . The choice of fluids may also depend on the chemical properties of the medications being given . </P>

What is the difference between a crystalloid and a colloid solution
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