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Blood Flow Velocity - Review

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This video will present: Velocity of the blood flow in aorta and smaller arteries. We will also look at the graphs that you might see in the USMLE tests. Formula to calculate the velocity.




The assumptions made in this lecture to understand the velocity of blood flow are as follows:

• 1 meter = 39.37 inches

• 1 millilitre = 1 cubic centimetre

• A cubic centimetre is diagrammed as a small cube which has all its lengths equal to 1 centimetre. Cardiac Output at rest: 5000mL

Let’s assume that the blood vessels are like cylinders and their cross section appear to be circular. Hence, the cross sectional area of a blood vessel is equal to the area of the circle. This can be calculated by the formula A=πr^2. Unit is cm2.

Velocity vs. Flow

Velocity is defined as the speed of blood in unit time. 

Flow is the amount of blood moving per unit time.

Velocity and Flow can be integrated in the same equation as per the following formula: 


Where:  V = velocity

              Q = Flow

              A = Cross sectional area of the vessel.

This shows that the velocity is inversely proportional to the cross sectional area and directly proportional to the flow of blood in the vessel. This principle is analogous to water pouring out of a water hose. Squeezing the outlet of the hose and making it narrower (decreasing the cross-sectional area) will cause the water to eject with higher than normal speed. 

The cross sectional area, of any part of the vasculature is taken as the sum of all the vessels at that level and not of a single vessel individually. Hence, the aorta which is a singlevessel, has the smallest cross sectional area of 2.5cm^2. On the other hand, the sum of cross-sectional areas of all the capillaries is calculated to be 3000cm^2.

The calibre of the blood vessels changes as the aorta divides into arteries, arterioles and capillaries during the process of transporting blood to the tissues. The change in vessel calibre is met with a subsequent change in the blood velocity. The aorta, with a cross sectional area of 2.5cm^2, has blood travelling at a velocity of 20m/min. By the time the blood reaches the capillaries, the velocity of blood drops to 1.6cm/min. This is because the cross sectional area of all the capillaries when summated becomes equal to 3000cm^2, a value which is 1000 folds greater than the cross sectional area of aorta. Following calculations can be used to calculate velocity of blood flowing through the aorta and the capillaries respectively: 

Velocity of the blood flow through Aorta:

➢ ​ 

= {ml and cm^3 can be substituted interchangeably}


= 2000 cm/min

= 20 m/min

= 33 cm/s


Velocity of the blood flow through the Capillaries:

➢ ​ 

=  {ml and cm^3 can be substituted interchangeably}

= 1.6 cm/min

= 0.016 m/min

= 0.027 cm/s

To summarize, the aorta acts as a conducting vessel as it conducts blood at high velocity to the rest of the body. The capillaries on the other hand, need to contain the blood with minimal velocity which allows for efficient exchange of gases and transport of nutrients and waste products.


In this video we will learn about :

1. Significance of velocity of blood. 

2. Blood flow and blood velocity. 

3. Formula of calculation of blood velocity 

4. Blood velocity in large and small vessels.

5. Factors affecting velocity difference.

6. Graphic representation of velocity in vessels. 

Following answers are created by ChatGPT. Occasionally the answer may be harmful, incorrect, false, misleading, incomplete, or limited in knowledge of world. Please contact your doctor for all healthcare decisions. Also, double check the answer provided by the AI below.


In addition to the presenter, following authors may have helped with the content writing, review, or approval:

  • Dr. Mobeen Syed

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The DrBeen Corp designates this enduring material for a maximum of 0.25 AMA PRA Category 1 Credits™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

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Faculty Disclosures
Dr. Mobeen Syed Author declares no conflict of interest.

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Dr. Mobeen Syed

Dr. Mobeen Syed

MD., MSc., MSc., BSc

Mobeen Syed is the CEO of DrBeen Corp, a modern online medical education marketplace. Mobeen is a medical doctor and a software engineer. He graduated from the prestigious King Edward Medical University Lahore. He has been teaching medicine since 1994. Mobeen is also a software engineer and engineering leader. In this role, Mobeen has run teams consisting of hundreds of engineers and millions of dollars of budgets. Mobeen loves music, teaching, and doing business. He lives in Cupertino CA.

Cardiovascular System

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