Fig. 23—*General scheme of the circulation*, showing places where the
blood takes on and gives off materials. 1. Body in general. 2. Lungs. 3.
Kidneys. 4. Liver. 5. Organs of digestion. 6. Lymph ducts. 7. Pulmonary
artery. 8. Aorta.


*Blood Pressure and Velocity.*—The blood, in obedience to physical laws,
passes continuously through the blood vessels, moving always from a place
of greater to one of less pressure. Through the contraction of the
ventricles, a relatively high pressure is maintained in the arteries
nearest the heart.(21) This pressure diminishes rapidly in the small
arteries, becomes comparatively slight in the capillaries, and falls
practically to nothing in the veins. Near the heart in the superior and
inferior venæ cavæ, the pressure at intervals is said to be _negative_.
This means that the blood from these veins is actually drawn into the
right auricle by the expansion of the chest walls in breathing.(22)

The velocity of the blood is greatest in the arteries, less in the veins,
and _much_ less in the capillaries than in either the arteries or the
veins. The slower flow of the blood through the capillaries is accounted
for by the fact that their united area is many times greater than that of
the arteries which supply, or the veins which relieve, them. This allows
the same quantity of blood, flowing through them in a given time, a wider
channel and causes it to move more slowly. The time required for a
complete circulation is less than one minute.

*Summary of Causes of Circulation.*—The chief factor in the circulation of
the blood is, of course, the heart. The ventricles keep a pressure on the
blood which is sufficient to force it through all the blood tubes and back