I recently learned that the velocity of blood moving in veins is faster than that in capillaries, but the blood pressure in veins is much lower than that of any other blood vessel. Since veins have a relatively high blood velocity (at least compared to capillaries), shouldn’t they also have a higher blood pressure? Why don’t velocity and pressure in fluids go hand in hand?

A California students wants to know why velocity and pressure in fluids don’t go hand in hand. That is to say, why veins do not have high velocity = high pressure or low pressure= low velocity. 

 A layman’s (non-professional) answer could be that veins experience x amount of pressure and the body relieves the excess of x amount of pressure by increasing flow (velocity) of blood.    An exchange, if you will, in order to create a system of balance in your body.

Let’s discuss some real-world examples.

Liquid: You have to pee really bad, that feeling that causes you to squirm, your face to turn red, and your chest to hurt is due to the high pressure exerted on the bladder wall from the encased urine (the liquid which is experiencing low/no velocity). As soon as your bladder (the thing that holds urine) begins to evacuate (the urine or fluid moves at high velocity), you can feel the pressure begin to decrease (the bladder wall begins to feel a low/no pressure) to the point you feel relief.  Simply put, this happened because the urine vacated or left the body with velocity; thereby, lowering the pressure which in turn relieved the discomfort the person was feeling.  

Gas: you breathe in very deeply; you feel your lungs fill with air. You feel like your lungs are full and you know this is true because you cannot breathe any more air (the gas which is experiencing low/no velocity) into your lungs.  That feeling of fullness is the high pressure being felt on the lung walls. You then exhale, as you exhale (the air or fluid moves at a high velocity) you notice that there is less fullness or low/no pressure in your lungs. Simply put, this relief or lowering of pressure happened because the air vacated the body with velocity.    Try it and experiment with intaking air slowly and quickly; thereby, changing the fluid’s velocity, pay attention to how your lungs feel or perceive pressure as you breathe in and out.

To this end, have you noticed that when you exercise, that your blood pressure increases then it will level out even though your body is at work?  That is because your blood will flow at an increased rate (velocity) to lower the pressure on the vein walls and will do so just enough to keep the system going at the pace of work, it is undertaking.

It operates as a self-regulating system of balance and it is only achieved with a low to high or high to low exchange.  In fluid physics, most systems try to find a natural balance between the object such as the bladder, lungs hose, balloon, container, tube, etc. (non-fluid) and the fluid such as urine, air, oil, water, helium, milk, etc. (liquid or gas).

Two other posts you may enjoy:

1.      Why do veins allow for a higher velocity as compared to capillaries? Are we saying, if it has velocity, it is moving fast?

2.      What is Bernoulli’s Principle and Pascal’s Law in relation to blood pressure and velocity?

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