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Total Peripheral Resistance & Blood Flow Regulation

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  • 0:38 Blood Flow
  • 2:35 Poiseuille's Law
  • 4:21 Blood Flow Resistance
  • 5:14 Total Peripheral Resistance
  • 6:30 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Blood flows continually through your heart and blood vessels. As the needs of the body change, blood flow needs to increase or decrease. Learn how vasodilation, vasoconstriction and other factors change the amount of resistance within your blood vessels to change blood flow through your body.

Circulatory System

We previously learned that an increase in the amount of resistance in the arteries leaving the heart, called an increased afterload, can make it harder for the ventricles to pump blood out of the heart. However, this difficulty pushing against pressure is not only seen at the heart; it's a pattern we see throughout the circulatory system. Any time there is high resistance in a blood vessel, it will be difficult for the blood to flow through that vessel. In this lesson, you will learn about factors that regulate blood flow through the circulatory system.

Blood Flow

Blood flow is the continuous movement of blood through the circulatory system. As blood moves through your body, it encounters resistance to its flow. There are a number of factors that influence blood flow. These factors include the thickness, or viscosity, of your blood and the length of a blood vessel. In other words, if the blood is thicker - more like molasses than it would be like water - there's more resistance to flow. Also, if a blood vessel is longer, it will be harder for blood to pass through that vessel, and there will be more resistance to blood flow. Another factor is the difference in pressure at the beginning and end of the vessel. This pressure difference is important, because later, we will see that blood flows from high to low pressure. However, one of the most important factors influencing blood flow is the size or radius of the blood vessel the blood is passing through.

The artery constricts during vasoconstriction, decreasing blood flow.
Vasoconstriction Diagram

Blood vessels - and in particular, the more muscular arteries - are often the source of resistance. One way an artery can actively resist blood flow is by contracting the smooth muscle in its wall, causing the artery to constrict. When an artery constricts, we call it vasoconstriction. This is an easy term to recall if you remember that 'vaso' refers to 'blood vessel.' During vasoconstriction, the artery gets smaller and offers increased resistance. This causes a decreased blood flow through its lumen, or hollow center. If, on the other hand, the smooth muscle in the wall of the artery relaxes, the blood vessel moves into a state of vasodilation, and its lumen dilates, or gets bigger. This offers decreased resistance and causes an increased blood flow. This is much like a nozzle at the end of a hose. If the nozzle is almost closed, it will allow less water to get through. If the nozzle is opened wide, it will allow more water to flow through.

Poiseuille's Law

We mentioned earlier that one of the most important factors influencing blood flow is the size, or radius, of the blood vessel. You recall that radius is a math term that measures the distance from the center of a circle out to its edge. We explain the size-flow relationship through a law, and that law states that blood flow is proportional to the fourth power of the vessel's internal radius. This is known as Poiseuille's Law, which can be pronounced like the words 'pause' and 'wee' put together. It's an equation that is used to determine the flow of any fluid through any tube, so you might want to use its pronunciation to help you remember what the law talks about. For example, if the blood vessel has a small radius, the blood flow will be small, or we might want to think of it as it will 'pause' from its normal flow. If the blood vessel's radius is big, then blood flow will be big, and we say 'wee' when we open the flood gates.

In simple terms, what Poiseuille's Law tells us is that if the radius of a blood vessel doubles, like it can with vasodilation, then the flow will increase 16 fold, because 2^4 = 16. The same would be true of a hose nozzle. If you take a hose nozzle and open it twice as big, you could expect the water flow to increase 16 fold. The reverse is also true, if the radius of the vessel is reduced in half by vasoconstriction, then the blood flow would reduce by 16 fold. The takeaway learning point here is that it only takes a small change in the size of the artery to give a big change in blood flow.

Blood Flow Resistance

The vessels leading to organs and muscles constrict or expand to control blood flow.
Blood Flow Resistance Organs

If we look closely at the circulatory system, we see that blood flow through the body is considered in different ways. One way we can look at blood flow is how much is needed by an individual organ. For example, if blood flow to an organ is too low, then the blood vessels leading to the organ will dilate and allow more blood to reach the organ. Conversely, if blood flow to an organ is too high, then the blood vessels leading to the organ will constrict and reduce the flow to an appropriate level. This is a piece of cardiac physiology that is important for events such as exercise. For example, if you are doing bicep curls, blood must increase to your bicep muscles so we see the arteries feeding these muscles dilating. But during intense exercise, you do not need to digest your food, so arteries to your digestive system will constrict.

Total Peripheral Resistance

The other way to look at blood flow is to consider the entire circulatory system as a whole. Total peripheral resistance is defined as the total resistance to flow of blood in the systemic circulation. So, with total peripheral resistance, we are looking at the resistance of all of the blood vessels in the body. Of all of the blood vessels in the body, the most important ones to consider when we talk about resistance to blood flow are the arterioles.

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