Blood consists of formed elements that are suspended and carried in a fluid called plasma.The formed elements—erythrocytes, leukocytes, and platelets—function, respectively, in oxygen transport, immune defense, and blood clotting. Plasma contains different types of proteins and many water-soluble molecules.
The total blood volume in the average-sized adult is about 5 liters, constituting about 8% of the total body weight. Blood leaving the heart is referred to as arterial blood. Arterial blood, with the exception of that going to the lungs, is bright red because of a high concentration of oxyhemoglobin (the combination of oxygen and hemoglobin) in the red blood cells. Venous blood is blood returning to the heart. Except for the venous blood from the lungs, it contains less oxygen, and is therefore a darker red than the oxygen-rich arterial blood.
Blood is composed of a cellular portion, called formed elements, and a fluid portion, called plasma. When a blood sample is centrifuged, the heavier formed elements are packed into the bottom of the tube, leaving plasma at the top (fig. 13.1). The formed elements constitute approximately 45% of the total blood volume (a measurement called the hematocrit), and the plasma accounts for the remaining 55%.
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