The solubility of o2 in water (especially warm salty water) is not enough to provide o2 to active tissues. Many organisms use respiratory pigments that bind o2 and transport it to tissues. Respiratory pigments can be in solution or enclosed in pigments. A hematocrit is a process where you centrifuge whole blood and measure the proportion of solids" (cells). It is a pretty good measure of blood oxygen carrying capacity in vertebrates. In humans, blood picks up oxygen from the lungs and holds 200ml of o2 per litre of blood in chemical combination as opposed to holding 4ml of o2 per litre in solution. This 50-fold increase in oxygen demonstrates the fact that without hemoglobin, the heart would need to pump 50 litres for every single litre it pumps now. A respiratory pigment is a that combines reversibly with oxygen.