Blood pressure, blood flow, and resistance

Definitions & Key takeaways

Blood pressure is the force your circulating blood exerts against the walls of your arteries. Blood flow is the movement of blood through your body, and resistance is the pushback that's against the blood flow in the circulatory system. Blood pressure, flow, and resistance are all closely related. Your blood pressure is determined by two things: the amount of blood flowing through your arteries and the diameters (widths) of those vessels. The more blood that flows through the arteries and the narrower those vessels are, the higher your blood pressure will be.

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Introduction0:00–0:51

Pressure is a force over an area, so with blood pressure, we’re measuring the force that the blood exerts on the surface area of the walls of the blood vessels.
Differences in blood pressure throughout the body keep blood flowing from high-pressure areas, like the arteries, to low-pressure areas, like the veins.
When we say “blood flow,” we’re referring to the volume of blood that flows through a vessel or an organ over some period of time.
Now, the amount of blood flow from one end of a blood vessel to another is affected by the blood pressure, and by the resistance, which comes from the vessels themselves.
Vasoconstriction, where the vessels constrict, decreases blood flow, and vasodilation, where the blood vessels expand, increases blood flow.
Now, blood flow is not the same thing as the velocity of blood. Blood flow is the volume of blood that moves by a point over some period of time.

Blood Flow0:51–1:15

So let’s say this chunk of blood has a volume of 83 cm^3, and it took 1 second for this much to flow past the blue circle—this is the blood flow, represented by the variable capital Q.
Now, velocity on the other hand, is the distance traveled in a certain amount of time. So maybe in the same one second, a red blood cell at the very edge here traveled a distance of 27 cm, then it’d be moving 27 cm/s, represented by lowercase v.

Velocity1:15–3:04

Even though these aren’t equal, they are related, and the last piece is area, specifically the cross-sectional area of the blood vessel, which in reality is the same as the blood cross section like this.
So, based on units, since area’s going to be expressed in cm^2, we see that flow rate equals area times velocity! Alright, so for example, let’s say we want to calculate blood velocity, and we have a person’s cardiac output of 5L/min, which is average for an adult, and the diameter of their aorta, which is 2cm.
First off, using the equation for the area of a circle, (D/2)^2 x pi, we get (2 / 2)^2 x pi = 3.14 cm^2. Next, since cardiac output is the same as blood flow, we just need to convert this L/min to cubic cm per second, so there are 1000 cubic cm in a L, and 60 seconds in a minute, so multiplying those out we get 83 cubic cm per second.
Then, rearranging our little formula, velocity equals flow rate divided by area, and we get about 26 cm per second! Which is also about 1 km / hr!

Ohm's Law3:04–4:03

Going back to blood pressure, blood flow, and resistance, that relationship can be written out mathematically as well. So, to start, you have an initial, higher pressure at one end, and a final, lower pressure at the other.
The difference between these, or the initial minus the final pressure, sometimes expressed as delta P, equals blood flow through that vessel multiplied by resistance.
This can be also written as Q equals change in pressure over resistance. So, for example, let’s say the the blood vessel narrows, which increases the resistance, in order to keep the flow of blood to organs the same, the pressure difference has to increase, and this is typically what happens.
This equation might look familiar to a similar equation, where change in voltage V equals current I times resistance R, also known as ohm’s law!
Okay, let’s talk about pressure. Blood pressure is usually highest in a large artery like the brachial artery, which is where blood pressure is usually measured, That’s because at that point the blood still needs to be able to push through the smaller arterioles and capillaries, which means lots of resistance.

Blood Pressure4:03–5:03

In an average adult, blood pressure is about 120/80 mmHg or millimeters of mercury. That might look like a fraction, but it’s not.
The first number, 120, is the systolic pressure, which is the force that the blood exerts on the walls of the arteries during systole, when the heart contracts to pump blood through the body, and the second number, 80, is the diastolic pressure, which is the pressure on the walls of the arteries during diastole, when the heart relaxes and refills with blood between heartbeats.
That’s why the first number is bigger: the arteries are under more pressure when there’s more force from the heart pushing the blood.

Mean Arterial Pressure5:03–6:51

The mean arterial pressure, or MAP, is the average pressure on the arteries during a complete cardiac cycle, including the systolic and diastolic pressures.
It can be calculated a couple ways. The first is the quickest: we assume that about ⅓ of the cardiac cycle is spent in systole and about ⅔ of the cardiac cycle is spent in diastole and say that the MAP = ⅓ (systolic blood pressure) + ⅔ (diastolic blood pressure).
Another measuremnt is the pulse pressure, which is the difference between the systolic pressure and the diastolic pressure, so in our example, it would be 120 - 80 or 40 mm Hg.
So another way to figure it out is to say that MAP = ⅓ (pulse pressure) + diastolic pressure, which is exactly the same thing, but just written differently.
Now, a completely different way to think about MAP is by using the relationship between blood pressure, blood flow, and resistance.
Let’s go back to our big artery, say this time it’s the aorta, the biggest of the arteries coming right from the heart itself.
That artery goes to arterioles and capillaries, representing the whole human body, and then the venous blood returns to the right atrium of the heart.
So in this zoomed out view, the initial pressure here is pressure in the arteries, in other words—the mean arterial pressure, and the final pressure over here is we have the central venous pressure, or CVP, in the vena cavae right before they enter the right atrium.
Looking back at our equation, Pi - Pf = Q x R, blood flow is the total blood flow through all of the blood vessels and organs, which is called the cardiac output and is measured in L/min.

Systemic Vascular Resistance6:51–7:54

Pi is MAP and Pf is CVP, and finally, the combined resistance of all of the blood vessels of systemic circulation would be the systemic vascular resistance, called SVR,.
So the equation is MAP - CVP = CO x SVR. CVP is usually a small number, so it’s usually ignored and the equation is simplified to MAP = CO x SVR.
Based on this relationship, you can see why for example, an increase in resistance, which is mostly caused by arteriole vasoconstriction, will cause the blood pressure to rise.

Review7:54–9:21

All right, as a quick recap: blood pressure refers to the force the blood exerts on the blood vessels, blood flow refers to the volume of blood that flows through the blood vessels, an organ, or the entire body, over a certain amount of time, and resistance refers to the physical and mechanical pushback the blood gets from things like vasoconstriction.