Blood flows in veins because of what?

Question

Blood flows in veins because of what?

Answer 1

Blood flows in veins because there is a difference in the pressure of the blood between the end of the capillaries and the superior vena cava.  That is, the pressure is higher at the capillaries than it is in the large veins that return the blood to the heart.

There are two sources for this pressure difference.  One is the pressure difference between the aorta and the right atrium created by the heart itself in its pumping.  The other is pressure put on the capillaries and veins due to peripheral muscles.  Small valves in the veins keep the blood flowing towards the heart.

This is one of the most important discoveries in physiology made by Sir William Harvey.  He showed that blood circulates, and that there are valves in the veins.

Answer 2

When blood is pumped into the arteries by the heart, it is pushed forwards under high pressure. Only a little of this pressure is left after blood has filtered through the capillaries, to push it through the veins, and the action of the muscles provides a pumping action that helps to push the blood up through the veins. This ‘muscle pump’ is particularly important in the legs, because when we are standing blood has to travel a long way ‘uphill’ to get back to the heart.

Valves in the veins make the blood flow towards the heart.Some of the veins in your legs contain valves at various points that prevent blood from flowing in the wrong direction as there is quite a large distance between your legs and your heart, the valves in the veins need to be working properly in order to maintain a strong, upward blood flow.

Answer 3

valves

Blood pressure accounts for the flow of blood in the arteries, but because blood pressure drops off after the capillaries, it cannot cause blood flow in the veins. Skeletal muscle contraction pushes blood past a venous valve, which then shuts, preventing backward flow.

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edited to add:

Blood wouldn't "flow" through the veins without the valves.  The pressure from the surrounding muscles would cause the blood to surge forward and then (when the muscles relax) the blood would backslide... it would not travel anywhere (one step forward, one step back).  The valves hold the blood in its new position until the next muscle contraction when the blood is pushed forward again.

Answer 4

Contraction of body muscles, particularly in the limbs, compresses the thin-walled veins. The valves in the veins prevent the blood flowing backwards when the vessels are compressed in this way.This assists the return of venous blood to the heart.

Answer 5

Blood flows in the vein by the pressure created by skeletal muscles.