Blood has long been recognized as a vital body fluid. Prehistoric humans must have observed the spillage of blood that followed aggressive exchanges or accidents and quickly learned that loss of blood resulted in death.

It is not surprising, then, that early physicians, including Hippocrates and Galen, placed a great deal of importance on defining blood as an essential bodily humor and that the examination of the pulse was described as the most important component of a correct medical diagnosis in The Yellow Emperor’s Classic of Internal Medicine, which dates back to medical traditions in China around 2700 B.C.

Although modern medicine recognizes these early belief systems as being overly simplistic and often mystical in nature, the importance of the circulation of blood in sustaining life among virtually all vertebrate and invertebrate animals is a known scientific fact.

Our modern understanding of the human circulatory system is generally credited to William Harvey (1628/1941). Influenced by empirical study of the dissection of animals, he demonstrated that the circulation of blood worked like a hydraulic water-pumping system.

In this regard, circulation of blood was conceptualized as a closed system containing blood that traverses a complex set of blood vessels to transport oxygen and nourishment to every type of cell in the body as well as to remove cellular waste products.

The heart worked like the pump in the hydraulic water-pumping system, orchestrating the rate of blood flow throughout the entire circulatory system.

As in the hydraulic pumping system, pressure could be increased within the system to force the fluid (blood in this case) to flow in any direction, even against the force of gravity.

What is called water pressure in a hydraulic water-pumping system is referred to as blood pressure in the circulatory system. Blood pressure clearly differs at various locations in that system.

For example, blood pressure is much higher in the vessels through which it flows immediately after leaving the heart (arterial pressure) than in the vessels through which it flows as it reenters the heart (venous pressure).

This difference in blood pressure is clearly evident when injuries result from severed arteries or veins. Most of the minor injuries individuals sustain throughout life involve severed veins close to the surface of the skin.

In these types of injuries, blood oozes out, and blood flow can generally be stopped with gentle external pressure at the site of the wound, although it may take a few minutes.

Arterial damage, in contrast, is a more dangerous situation; in this type of injury, blood ejects from the wound, pulsing with the beating of the heart.

Failure to respond adequately to arterial injuries will quickly result in bleeding to death. The greater blood pressure within arteries versus veins is responsible for the rapid loss of huge amounts of blood with arterial injuries.

An organism can generate additional blood cells to release into circulation (increasing the density of circulating blood cells) or alter the resistance to the flow of blood by constricting or dilating blood vessels.

Therefore, in contrast to the relatively stable water pressure that can be maintained in a hydraulic water-pumping system, blood pressure is constantly changing as the body creates and releases new blood cells and alters blood flow resistance.

Additionally, because the heart does not pump blood continuously as a water pump does, blood pressure differs while the heart is pumping and while the heart is at rest.

The higher arterial pressure observed during heart action (ejection of blood from the heart) is referred to as systolic blood pressure (SBP).

And the lower arterial pressures that occur during rest just prior to the next heart beat (while blood refills the heart) is referred to as diastolic blood pressure (DBP). Obviously, the circulation of blood is much more complicated than early physicians thought!