If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Putting it all together: Pressure, flow, and resistance

See how pressure, flow, and resistance relate to one another and how they each affect the human body. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

Want to join the conversation?

• This circulatory system equations seem to follow the same rules as an electrical circuit. Is the equation P=Q*R, analogous to Ohm's Law?
• No one answered it for ya- even though it looks like you found the answer already. Yes V=IR is analgous. V = P or some sort change of pressure/charge etc. I = Q or flow. R is resistance. So [P=QR] = [V=IR].
• Why do we actually count the resistance? Does is say something interesting about our health?
• Increased resistance can drastically reduce blood flow, causing problems. For example, Angina Pectoris (symptomatic chest pain) can occur when your coronary arteries are too reduced in size to get proper blood flow; the vessels are just too small and the resistance is too high. You can administer medication to patients with this condition and it will dilate their arteries, decreasing resistance and allowing blood flow to resume.
• Does the standard medical courses of MBBS and MD include the study of all the phenomenon in body through the physical and mathematical point of view or is this just a part of some other course? is physiology really taught based on our understanding and explanation provided ny physics?
• I can't speak with authority on medical school, but as an exercise physiology graduate student, I can say that there is a lot of explanation that kind of goes this way (obviously at a fast pace and without too much time spent on the more basic physics concepts). It is acknowledged that many of the phenomena that we observe in the body can be best explained through principles of physics (electrical conduction, fluid dynamics, pressure/volume relationships, tissue elasticity & plasticity, etc.).

I have found that some of my best teachers have had a pretty good grasp of the physics behind physiology :)
• I'm sort of confused about how to relate the circulatory system with fluid dynamics and resistance. In Bernoulli's Equation (assuming constant height) we found that pressure and velocity where inversely related. In the continuity equation, since volume is constant, we said that area and velocity where also inversely related. Why then, when the arteriole increases in area, does the pressure in the artery decrease?

Intuitively it makes sense, but mathematically I don't get it.
• Hi, I'm still unclear why the equation {delta}P = Q x R is as such. In the video he just applies the equation, but does not explain the basis behind the equation.
• it is a way to see fluid as Ohm's law (Ohm’s law is for electricity but it can be analogous to flow)

it comes from the logical postulate that If you lower the resistance or up the flow of fluid/electrons you get a larger potential difference. (that logic is described in the formula)

Like Ohm’s law in terms of:
V = I x R

Change in pressure for blood formula:

P = Q x R

P = change in pressure along vessel [analogous to V = voltage (difference/change of electric potential is like change in pressure)]
Q = blood flow [analogous to I = current as electron flow]
R = resistance of flow [analogous to R = resistance]

Sources:

• At , why does Q stand for blood flow instead of F or something that would be easier to remember?
(1 vote)
• You can consider Q as Cardiac Output. Cardiac output is the volume of blood pumped by the heart per minute (mL blood/min). Cardiac output is a function of heart rate and stroke volume. The heart rate is simply the number of heart beats per minute. The stroke volume is the volume of blood, in milliliters (mL), pumped out of the heart with each beat.
• How'd he simplify 4900 mL/min to 5 L/min?
• 5000ml/min=5 L/min, and 4900lm/min=4.9L/min. These numbers are very close.
• I now this is kind of stupid but can somebody tell the differences between resistance and pressure?
• Pressure is how hard fluid is peing pushed along the tube or vessel. It's used to overcome the resistance from the vessel wall.

Resistance is how hard the vessel is trying to stop the flow of fluid. It's made up of how small the vessel is, and the friction along the sides.

Flow is the amount of fluid that's passing through the vessel over an amount of time. It's determined by how much the pressure can push against the resistance.