Now we're going to discuss venous pressures and how a vascular function curves are
regulated. Venous pressure is a little bit differently regulated than arterial blood pressure.
Venous or veins have smooth muscles so they can constrict and dilate. You can also change
the luminal diameter of any vein. These, however, are less thought of to be regulatory factors
because they don't directly affect blood pressure. They only indirectly affect blood pressure
because when they constrict you return more blood to the heart, therefore you increase
preload and cardiac output. If you dilate a vein, you have lower preload and lower cardiac
output. This is an example of a dilated vein. So now you have an increase in luminal diameter.
The increase in luminal diameter can do a couple of things for you. One is it slows the time fluid
travels through it and it also has higher capacitance meaning that it can hold more fluid. You
can also constrict these veins by pulling them together decreasing their luminal diameter and
this is also a smooth muscle contraction. This speeds up the transit time through the venous
system and it also decreases the volume within the veins and therefore that will pass that
volume up to the heart. A great example of how this movement of vein fluid works is by the
skeletal muscle pump. The skeletal muscle pump is a way by which the muscles around the veins
squeeze together to move the blood against gravity. This can be seen in this diagram where
the muscles on both sides of the vein squeeze, they start to push the blood up towards the
heart and then when they relax the veins have valves in them which allow the veins to collapse
and prevent the back flow of the venous fluid. Then as the muscles constrict again, they'll push
more fluid up and this is a nice pump-like movement to keep returning blood back to the heart.