e. Gravity. Gravity helps to move blood to the trunk and lower members.
However, it is a hindrance in moving blood to the head and neck.
10-34. VENOUS BLOOD FLOW
There is usually a low level of pressure in the veins. There are valves in the
veins that ensure that blood flows continuously toward the heart. Therefore, as pres-
sure is applied to a vein, there will be a pump effect.
a. Pressure from Arteries. The muscular compartments of the upper and low-
er limbs tend to be full in healthy persons. Therefore, as blood enters the arteries within
these compartments, a volume of blood must leave through the veins.
b. Pressure from Muscular Contractions. During muscular activity,
additional forces press against the veins and produce a "milking action." Again, blood
moves through the veins back toward the heart.
c. Gravity. In the head and neck, gravity helps to move the blood down
through the veins. In the trunk and lower limbs, the valves help to prevent a backward
flow of blood in the veins.
Section VI. CAPILLARIES
The capillary beds make up the greatest cross-sectional area of the
cardiovascular system. In the capillary beds, the actual exchange of materials takes
place between the blood and the cells of the body.
10-36. FILTRATION PHENOMENON
The wall of the capillary consists of a single layer of flat cells. The minute
spaces surrounding the capillaries and the individual cells of the body make up the
tissue space (interstitial/ extracellular space). Fluid passes from the capillary into the
tissue space and carries with it various substances. Some of this fluid returns to the
capillary on the venous side.
10-37. CAPILLARY SPHINCTERS
The capillary beds are provided with precapillary sphincters that can reduce or
completely stop the flow of blood into the capillaries. At the other end of the capillary
bed are postcapillary sphincters; when these close, there is a backpressure and more
fluid flows into the tissue space.