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Total peripheral resistance (TPR) is the amount of resistance to blood flow present in the vascular system of the body. It can be thought of as the amount of force working against the heart as it ejects blood into the vascular system. Although total peripheral resistance plays an integral role in determining blood pressure, it is an exclusively defined measure of the cardiovascular system and should not be confused with the pressure against arterial walls, which is a measure of blood pressure.
The vascular system, which is responsible for the flow of blood both to and from the heart, can be divided into two components: systemic and pulmonary. The pulmonary system delivers blood to and from the lungs, where it becomes oxygenated, and the systemic vasculature is responsible for the transporting this blood to the cells of the body through the arteries and returning the blood to the heart after perfusion. TPR affects the flow of this system and can, in turn, greatly affect the perfusion to organs.
Total peripheral resistance is calculated by using a specific equation. This equation is TPR = change in pressure/cardiac output. Change in pressure is the difference in mean arterial pressure and venous pressure. Mean arterial pressure is equal to diastolic blood pressure plus one-third of the difference between the systolic and diastolic pressures. Venous blood pressure can be measured using an invasive instrumental technique that physically measures the pressure inside a vein. Cardiac output is the amount of blood pumped through the heart in a one-minute increment.
There are a number of factors that can significantly change the components of the TPR equation, thus changing total peripheral resistance. These factors include vascular vessel diameter and blood property dynamics. The diameter of a blood vessel is inversely proportional to blood pressure, so a smaller vessel would increase resistance, hence increasing TPR. Contrarily, a larger blood vessel equates to a less concentrated volume of blood particles pushing against vessel walls, which translates into lower pressure.
The fluid dynamics of the blood also can heavily contribute to an increase or decrease in TPR. The mechanism behind this is a change in clotting factors and blood components that might change the viscosity of blood. As would be predicted, a more viscous fluid causes more resistance to flow. A less viscous fluid would move more easily throughout the pipeline of the body, causing less resistance. Analogous to this would be the force needed to move water versus molasses.
I read a book a while ago which went through experiments people had done to prove or disprove old wives tales.
One of the things they had was a story about some scientists who shipped a lot of water from hot pools, famous for their healing powers, to their laboratory and heated it up and sat in it, then sat in normal hot water to see if the water made any difference.
What they found out was sitting in water decreases the TPR, making it easier for blood to get around the body. It also did a bunch of other things that are beneficial, although the person had to be submerged up to their neck.
The two different kinds of water worked in exactly the same way.
@indigomoth - Not only do you decrease the distance the blood has to travel, if you are able to make sure you don't have too much fatty tissue between your organs, you're giving your heart and lungs more room to work.
Also, depending on what kind of exercise you do, you're making them stronger, so the level of resistance doesn't matter as much.
Just bear in mind, though that TPR isn't really the distance that the heart has to pump. It's more related to how much force is needed to pump into the arteries.
So, obesity may increase TPR by creating fatty deposits in the arteries, thus blocking part of the blood flow to the body, but the distance the blood has to go doesn't make a difference to the total peripheral resistance equation.
I imagine that the size of a person would also increase the total peripheral resistance.
I remember seeing on one of the many doctor shows they once had, a doctor talking to an obese patient and explaining to her that the heart is about the same size in all of us, and that she was giving hers about three times as much work as a normal heart.
He was trying to impress upon her that she would wear her heart out before its time.
The patient wasn't very impressed, but it made an impression on me.
I struggle with my weight sometimes, but I think that the health benefits of keeping it within a certain range are immense. For one thing, you give your poor heart a break by lowering the resistance against it.