The Human Circulatory System: How It Works
All the functions of the circulatory system occur in the capillary beds. The rest of the system consists of two pumps (in the heart) and associated plumbing:
- arteries
- their terminal branches, the arterioles
- veins
- and their tributaries, the venules.
Blood Pressure
Blood moves through the arteries, arterioles, and capillaries because of the force created by the contraction of the ventricles.Blood pressure in the arteries.
The surge of blood that occurs at each contraction is transmitted through the elastic walls of the entire arterial system where it can be detected as the pulse. Even during the brief interval when the heart is relaxed — called diastole — there is still pressure in the arteries. When the heart contracts — called systole — the pressure increases.Blood pressure is expressed as two numbers, e.g., 120/80.
The first is the pressure during systole. The unit of measure is the torr, in this example, the pressure equivalent to that produced by a column of mercury 120 mm high. The second number is the pressure at diastole.
Although blood pressure can vary greatly in an individual, continual high pressure — especially diastolic pressure — may be the symptom or cause of a variety of ailments. The medical term for high blood pressure is hypertension.
