Before discussing the structure and function of blood vessels in detail, it is necessary to consider briefly some of the properties of fluids and the principles that govern the flow of fluids through vessels.

All fluids (when in a confined space) exert a pressure.

The term hydrostatic pressure refers to the force that a liquid exerts against the walls of its container.

The pressure that blood exerts in the vascular system is known as blood pressure.

Pressure varies with the height of the liquid column and this can be observed in the veins of a person standing up.

The venous pressures in the feet are considerably greater than in the head (this is, of course, related to the effect of gravity).

The effect of density on hydrostatic pressure is shown by the fact that 1 mm of mercury (mm Hg) exerts the same pressure as 13 mm of water (mm H2O) because mercury is more than 13 times as heavy as water for an equal volume.

If pressure is exerted on a confined fluid, the pressure will be transmitted equally in all directions - this is known as Pascal's principle. If there is a weak point in the container's wall and the pressure exerted is great enough, the container wall may burst.

This is what happens when an aneurysm bursts.

When an individual is hypertensive, the blood vessels harden or undergo sclerotic changes (arteriosclerosis) to prevent the vessels bursting with the elevated blood pressure.

The distensibility of the container also influences the hydrostatic pressure that develops: if the container is distensible, the pressure in the fluid is less than in a rigid container.

FLOW OF FLUIDS

The flow of a fluid through a vessel is determined by the pressure difference between the two ends of the vessel and also the resistance to flow.

PRESSURE DIFFERENCE

For any fluid to flow along a vessel there must be a pressure difference otherwise the fluid will not move. In the cardiovascular system the 'pressure head' or force is generated by the pumping of the heart and there is a continuous drop in pressure from the left ventricle of the heart to the tissues and also from the tissues back to the right atrium of the heart. Without this drop in blood pressure, no blood would flow around the circulatory system.