Veins and Venous Pump

Types of veins

1. Postcapillary venules

· Smallest of veins and immediately follow capillaries

· Even more porous than capillaries and allow for fluid exchange with tissues

· Most leukocytes emigrate from the blood stream through the walls of these venules

2. Muscular venules

· Receive blood from postcapillary venules

· Slightly larger than postcapillary venules

3. Medium veins

· Many medium veins, especially in the limbs, exhibit infoldings of the tunica interna that meet in the middle of the lumen, forming venous valves directed towards the heart.

· The venous pressure alone is not enough to pump the blood towards the heart against gravity, the functional venous pump is partly responsible for it.

4. Venous sinuses

· Veins with very thin walls, large lumens and no smooth muscle. Examples include the coronary sinuses of the heart and dural sinuses of the brain.

5. Large veins

· Examples include venae cavae, pulmonary veins, internal jugular and renal veins.

Venous Return Mechanism – Valves and ‘pumps’

MUSCLE PUMP

Everytime you contract the muscles in your legs you compress the veins in or adjacent to the muscles which forces the blood to flow proximally in the veins, because of the unidirectional valves.

This is known as the venous or muscle pump and is efficient enough to reduce the venous pressure in the feet to about 20mmHg when walking, as opposed to the 90mmHg if there were no pump or valves. If a person stands perfectly still, therefore not making use of the venous pump, the venous pressure in their feet will rise to 90mmHg with time because of the gravitational force fighting against the return of the blood superiorly to the heart. There would also be a pressure increase in the capillaries inferiorly which would cause between 10 to 20 percent of the blood volume to leak into tissue spaces within 15 to 30 minutes of standing still. (possible reference to pcl case)

Saladin pp.760-761

Guyton pp176-