RESISTANCE TO FLOW

Resistance is a measure of the ease with which a fluid will flow through a tube: the easier it is, the less the resistance to flow, and vice versa. In the circulatory system the resistance is usually described as the vascular resistance; as it mainly originates in the peripheral blood vessels, it is also known simply as the peripheral resistance.

Resistance is essentially a measure of the friction between the molecules of the fluid, and between the tube wall and the fluid. The resistance depends on the viscosity of the fluid and the radius and length of the tube.

RADIUS OF THE TUBE (BLOOD VESSEL)

The smaller the radius of a vessel, the greater is the resistance to the movement of particles; this increased resistance results from a greater probability of the particles of the fluid colliding with the vessel wall. When a particle collides with the wall, some of the particle's kinetic energy (energy of movement) is lost on impact, resulting in the slowing of the particle. Thus, in a smaller diameter vessel, there will be a greater number of collisions and a reduction in the energy content and speed of the particles moving through the vessel. This results in a decrease in the hydrostatic pressure.

Small alterations in the size of the radius of the blood vessels, particularly of the more peripheral vessels, can greatly influence the flow of blood. Atheromatous changes in the walls of large and medium-sized arteries cause narrowing of the lumen of the vessels and result in an increased vascular resistance.

LENGTH OF THE TUBE

The longer the tube, the greater the resistance to the flow of liquid through it. A longer vessel will require a greater pressure to force a given volume of liquid through it than will a shorter vessel. However, the length of the blood vessels in the body is not altered significantly and the overall length is kept to a minimum because of the parallel circuits in the systemic circulation.

VISCOSITY OF THE FLUID

Viscosity is a measure of the intermolecular or internal friction within a fluid or, in other words, of the tendency of a liquid to resist flow. The rate of flow varies inversely with the viscosity: the greater the viscosity of a fluid, the greater is the force required to move that liquid.

Thus, changes in blood viscosity affect flow. Normally the viscosity of blood remains fairly constant, but in polycythaemia, in which there is an increased red cell content, the viscosity of the blood can be considerably increased and the blood flow reduced.

Severe dehydration, where there is a loss of plasma, can also lead to increased viscosity.

Cooling of the blood similarly increases its viscosity.

The nature of the lining of the tube or vessel also influences the way fluids flow.

If the lining of the blood vessel is smooth, the fluid will flow evenly; this is known as streamline or laminar flow. However, if the lining is rough or uneven or the fluid flows irregularly, turbulent flow is set up. Laminar flow is characteristic of most parts of the vascular system and is silent, whereas turbulent flow can be heard, e.g. during blood pressure measurements with a sphygmomanometer.

It is sometimes necessary to measure blood flow in patients and it is usual simply to measure the quantity of blood that passes a given point in the circulation over a given period of time. One method used in the clinical situation is by means of an ultrasonic flowmeter applied to the surface of the skin over a blood vessel. This makes use of the Doppler effect (a shift in the frequency of the ultrasonic waves when they are reflected off the moving blood cells). It is a useful and non-invasive method of assessing the condition of the peripheral arteries, in peripheral vascular disease or after vascular surgery for example.