Cardiovascular control of blood pressure

 

Arteries have far more smooth muscle than veins, but all have connective tissue. Arterioles also have elastic tissue, composed of elastin which is a protein than enables elastic arteries to expand by approximately 10% during systole. So when the heart beats, the large arteries expand in order to be able to accommodate the blood. Elastin can also remove the pulse and act as a pressure reservoir (absorbs blood and averages pressure out) throughout the cardiovascular system. This means that blood flow through the capillaries is more of an even flow because the large arteries have taken up the difference of the pulse.

During the heart beat, there’s a pulse pressure which changes between systolic and diastolic, but as you go through the arterial tree, through to the arteries and into the arterioles, pressure diminishes in the capillaries and there’s no pulse, just a continuous flow of the blood. Velocity of blood is high in the arterioles and falls in the capillaries which doesn’t increase much even when it goes through the vena cava. The relative resistance is highest in the arterioles, which is an important control area of blood flow to other tissues.

How do the large arteries take away this pulse pressure?

Large arteries contract, and push the blood along. During the systole, ventricles contract and push blood into the arteries which have a large amount of elastin and so expand which causes the volume in the ventricles to decrease and the volume in the large arteries to increase in order to accommodate this extra blood. When the heart relaxes, this pressure in the ventricles decreases, the aortic valve closes, but the large arteries are still expanded in order to push the blood along. This increase in the volume of blood in the arteries is the stroke volume (amount of blood ejected by the heart in one beat). If the volume increases, so does the pressure. As people get older, large arteries become stiffer and so pulse pressure will tend to increase. Haemodynamics can be used to calculate how blood flows through a tube, in terms of pressure and volume in arteries, veins, etc. The lower the resistance of the tube, the greater the flow will be. This concept serves to explain why blood clots in parts of the circulation tree due to high cholesterol or plaques, will increase the total resistance but will also decrease the flow going through an artery, which is the basis of regulating blood flow to different organs around the body. In an obstructed artery, due to an atherosclerotic plaque for example, the radius of the artery would decrease. If you squeeze a hose pipe for example, it would be logical to think the pressure at the point at which you squeeze the pipe would be the highest. However the pressure on either side would actually be greater. This can be explained by Bernoulli’s theory, in which the flow between points A and B in the steady state is proportional to the difference in mechanical energy of the fluid at points A and B. The kinetic energy is greater in the narrow portion and because of the law of energy conservation; the pressure must decrease, because the kinetic energy is so high. When the vessel opens up again, the velocity slows down, and the pressure increases again. At a blocked region in the blood vessels, the blood goes through at higher velocity, meaning the pressure is quite low, which is going to exacerbate the blockage, so there’s not enough pressure trying to force the vessels apart. If some of the blockage can be removed, (perhaps by dissolving it away), the velocity will slow down, which will cause a pressure increase, helping to keep the vessel open, and cause an alleviation of the symptoms.

 

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