What is the Composition of Blood?

In the average adult, the total blood volume is almost 5 liters, composing 8 percent of the total body weight. Blood leaving the heart is referred to as arterial blood. For the reason of the high concentration of oxyhemoglobin (composed by the combination of oxygen and hemoglobin) in the red blood cells, arterial blood is bright red with the exception of that going to the lungs. Blood returning to the heart is referred to as venous blood. It is oxygen-poor, and hence a darker red than the oxygen-rich arterial blood except for the venous blood returning from the lungs.

In addition, A ”unit” (half a liter) of blood is drained when you donate blood. This represents approximately one-tenth of your total blood volume.

A cellular portion which is named formed elements, and a fluid portion which is named blood plasma form the blood. The heavier formed elements become packed into the bottom of the tube, leaving blood plasma at the top when a blood sample is centrifuged. Approximately 45 percent of the total blood volume, which is a percentage known as the hematocrit (HCT), is formed by the formed elements. The blood plasma accounts for the remaining 55 percent. The percentage of red blood cells per given volume of blood is closely approximated by the hematocrit, which is an important indicator of the oxygen-carrying capacity of blood.



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