ABSTRACT
Because not every cell in the human body is near enough to the environment to easily exchange with it’s mass (including nutrients, oxygen, carbon dioxide, and the waste products of metabolism), energy (including heat), and momentum, the physiologic system is endowed with a major highway network — organized to make available thousands of miles of access tubing for the transport to and from a different neighborhood (on the order of 10 µm or less) of any given cell whatever it needs to sustain life. This highway network, called the cardiovascular system, includes a pumping station, the heart; a working fluid, blood; a complex branching configuration of distributing and collecting pipes and channels, blood vessels; and a sophisticated means for both intrinsic (inherent) and extrinsic (autonomic and endocrine) control.
Accounting for about 8± 1% of total body weight, averaging 5200 ml, blood is a complex, heterogeneous suspension of formed elements — the blood cells, or hematocytes — suspended in a continuous, strawcolored fluid called plasma. Nominally, the composite fluid has a mass density of 1.057 ± 0.007 g/cm3, and it is three to six times as viscous as water. The hematocytes (Table 1.1) include three basic types of cells: red blood cells (erythrocytes, totalling nearly 95% of the formed elements), white blood cells (leukocytes, averaging <0.15% of all hematocytes), and platelets (thrombocytes, on the order of 5% of all blood cells). Hematocytes are all derived in the active (“red”) bone marrow (about 1500 g) of adults from undifferentiated stem cells called hemocytoblasts, and all reach ultimate maturity via a process called hematocytopoiesis.
