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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. 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. [Human
Biology Contents Page] [My Home Page] This page authored by John Ross. Please e-mail any comments or queries to johnross@cwcom.net or leave a message in the guest book. This page last updated on Wednesday, 30 June 1999 15:11 +0100 |
