USMLE Step 1 Physiology Review 52 04 Flow

USMLE Step 1 Physiology Review 52 04 Flow

 

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Play USMLE Audio MP3 52 04 Flow Below

Begin 52 04 Flow Transcription

Now for some questions about how fluids flow.

How will a fluid behave in a system where there is a difference of pressure, that is, there is a pressure gradient?

  • The fluid will flow down the gradient.

How is rate of flow measured?

  • It is measured as volume displaced per unit of time.

What are the variables that the velocity of flow depends on?

  • The rate of flow and the diameter of the conduit.

Okay, imagine if you will the following situation. Blood is flowing with a constant rate through two different blood vessels, one with a small diameter and one with a larger diameter.  Given this constant rate of flow, will the velocity of flow be greater in the smaller blood vessel or the larger blood vessel?

  • The velocity of flow will be greater in the smaller diameter blood vessel.

A homogenous fluid flowing through a tube flows in monomolecular layers.  What are these layers called?

  • Laminae.

Do all laminae flowing through a tube flow at the same velocity?

  • No. Given a pressure gradient, the laminae of the fluid slide against one another against friction creating viscosity.

Student Doctor, please pause the tape and summarize the information about flow discussed this far.

  • In a system where there is a pressure gradient, a fluid will flow down the gradient.
  • The rate of flow is measured as volume displaced per unit of time.
  • The velocity of flow varies according to the rate of flow and the diameter of the conduit.  Given a constant rate of flow and blood vessels of two different diameters, the velocity of flow will be greater in the smaller diameter blood vessel.  A homogenous fluid flowing through a tube flows in monomolecular layers that are called laminae.  The different laminae of a liquid flow at different velocities.  Given a pressure gradient, the laminae of a fluid slide against one another against friction creating viscosity.

A certain amount of resistance must be overcome for any fluid to flow through a tube.  How is this resistance expressed?

  • It is expressed in terms of a ratio of the force that is imposed on the fluid to the rate of flow that results. 

How is the imposed force expressed?

  • It is the difference in pressure between two points, or delta P. 

If the rate of flow is Q, what is the equation for resistance?

  • R equals delta P over Q.  That is, delta P divided by Q.

What is the velocity of flow of the laminae in the center of the tube relative to the velocity of flow of laminae close to the wall of the tube; is it greater than, less than or the same?

  • The velocity of flow of the laminae in the center of the tube is greater than the velocity of flow of laminae near the wall of the tube.

At critical velocities, what happens to laminar flow?

  • The laminae begin to break up and the flow becomes turbulent. 

As laminae break up and the flow becomes turbulent, does resistance to flow increase or decrease?

  • It increases.

In which two major systems of the body does turbulent flow occur?

  • In the circulatory system and in the respiratory system.

Student Doctor, please pause the tape and summarize the information about resistance and laminar flow.

A certain amount of resistance must be overcome for any fluid to flow through a tube.  This resistance is expressed in terms of a ratio of the imposed force to the resulting rate of flow.  The imposed force is the difference in pressure between two points, or delta P. If the rate of flow is Q, then the equation for resistance is R equals delta P over Q.  The velocity of flow of the laminae in the center of the tube is greater than the velocity of flow of laminae near the wall of the tube.  At critical velocities, the laminae begin to break up and the flow becomes turbulent.  As laminae break up and the flow becomes turbulent, resistance to flow increases.  Turbulent flow occurs in the circulatory system and in the respiratory system.

 

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