Dynamics of Blood Flow
The cardiovascular system faces many challenges. It has to maintain blood flow to all tissue, taking care to continue perfusion of vital organs such as the brain and heart, no matter what the situation. When activ-ity increases in one tissue, blood flow has to increase in this region without compromising flow to the brain and heart. For efficiency, blood flow has to be reduced to inactive tissue. Many of the biophysical principles come into play when this system meets these challenges.
Some of the important biophysical considerations that help us understand the circulation of blood are those that apply to any liquid:
· blood (like any liquid) flows from an area of high pressure to one of low pressure
· if the difference in pressure is high, flow is in-creased
· blood flow is decreased if the resistance is more (resistance is any force that opposes movement); in the cardiovascular system, it is referred to as the peripheral resistance.
Arterial blood pressure must be high enough to over-come peripheral resistance and maintain blood flow through capillaries. The force of contraction of the ventricle raises the pressure to about 120 mm Hg (systolic pressure), and the elastic recoil of the ar-teries maintains the pressure at about 90 mm Hg dur-ing ventricular diastole (diastolic pressure). This is enough pressure to keep the blood flowing continu-ously to all parts of the body. However, if there is in-creased resistance to flow, more pressure is needed.
If pressure is high, the work of the heart is in-creased and, if the pressure is too high, the heart may fail. If the pressure is too low, even if the heart is not taxed, it may not be sufficient to perfuse the brain. Hence, the blood pressure has to be constantly mon-itored and maintained.
Peripheral resistance is the resistance offered in the tissue; it has a relationship to blood flow and blood pressure.
If resistance increases, the blood pressure has to be increased to maintain flow.
The major factors that contribute to the peripheral resistance are friction between the blood and vesselwalls, which largely depends on the length of vessels;the caliber (diameter) of blood vessels and the viscos-ity (consistency) of blood; and type of flow (i.e.,whether the flow is turbulent or smooth).
The peripheral resistance increases as the total length of blood vessels increases. As a result, the blood pressure has to be increased to maintain blood flow to the tissue. In individuals who are obese, there is an increase in total blood vessel length to supply the adipose tissue. This may be one cause of hyper-tension observed in obese individuals.
The peripheral resistance in the body is mainly a result of the alteration in the caliber of blood vessels. Of the blood vessels, the arterioles offer the most re-sistance. The high content of smooth muscle in the arteriole walls enables adjustment of resistance ac-cording to regional needs. Sympathetic stimulation creates contraction of smooth muscle.
Blood viscosity is largely determined by the per-centage of the volume of blood occupied by red blood cells. If the red blood cell content increases, viscosity increases, and the heart has to work harder to push the blood forward.