Physiology of the Circulatory System

It is likely that your blood pressure is different right now from what it was when you started reading this chapter. The difference may not be substantial, but due to the complexity of factors that affect blood pressure, it constantly changes in response to a number of physiological and environmental stimuli.

For example, drinking a caffeinated or alcoholic beverage could influence your blood pressure while you read this page. Smoking a cigarette would have the same result. Your blood pressure will also differ if you are watching television while reading this page or if you are interacting with another person.

Even the simple act of reading affects your blood pressure. In fact, given the constant adjustments in blood pressure that occur, we really should not refer to an individual’s blood pressure as a stable medical parameter.

As depicted in Figure 1, blood pressure is jointly determined by the amount of blood ejected into circulation (cardiac output) and the forces of the circulatory system that impede blood flow (total peripheral resistance).

Increases in either cardiac output or total peripheral resistance will result in increased blood pressure. Cardiac output, in turn, is determined by heart rate and stroke volume (amount of blood ejected from the heart with each stroke).

Again, increases in either heart rate or stroke volume will increase cardiac output, and thus blood pressure. Total peripheral resistance is comprised of the degree of vasodilation and vasoconstriction that occurs in the various blood vessels that compose the entire peripheral circulation.

All of these hemodynamic parameters (heart rate, stroke volume, cardiac output, total peripheral resistance) rarely operate in the same direction. Increased heart rate, for example, is often accompanied by a reduction in stroke volume, potentially resulting in no change in cardiac output or blood pressure at all.

However, all physiologic or psychological states that affect blood pressure will do so by altering cardiac output, total peripheral resistance, or some combination of the two.

Several systems of the body directly influence blood flow through the body and the magnitude of blood pressure, including:

  1. the metabolic demands of the local tissue and associated blood vessels,
  2. the autonomic nervous system,
  3. the neuroendocrine system,
  4. the excretion of fluid by the kidney, and
  5. an extensive feedback system that involves central nervous system activity.

To illustrate these various interrelated systems, let’s consider what happens to blood flow when a person engages in a bout of moderate exercise, like jogging on a treadmill, riding a bicycle, or taking a vigorous walk.

Obviously, blood flow will need to increase to support the metabolic demands of the leg muscles, delivering more oxygen and nutrients while removing the waste by-products from the muscle cells of the legs.

Unusually, although heart rate increases significantly during moderate exercise, very little change in diastolic blood pressure is typically observed. Therefore, during exercise, the body must engage in a variety of regulatory processes to maintain blood pressure in light of the increased cardiac activity.