USMLE Step 1 Physiology Review 54 15 Hemodynamics (2 of 2)

# USMLE Step 1 Physiology Review 54 15 Hemodynamics (2 of 2)

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### Begin 54 15 Hemodynamics (2 of 2) Transcription

Now let’s talk about flow and hemodynamics.

Flow, expressed as Q, is the mass movement of a volume of fluid per unit time. According to the continuity principle, in any system arranged in series, the flow through each vascular component must have what relationship to the flow in every other vascular component?

• The flow through each vascular component must be equal to the flow through every other vascular component.

Different vascular segments in the body have different cross-sectional areas.  Therefore, to keep the rate of flow equal, the velocity of flow must vary in what way with cross-sectional area of a vascular segment?

• The velocity of flow must vary inversely with the cross-sectional area of a given vascular segment.

What are the two key variables that determine the flow of blood in the cardiovascular system?

The variables of delta P, resistance expressed as R, and flow expressed as Q, are related to one another in a way that is analogous to Ohms Law. What is the formula for flow?

• Q equals Delta P divided by R.

What are the two factors that affect resistance?

• Radius of the vessels and viscosity of the blood.

What is the major factor that changes the viscosity of the blood?

•  The variation in hematocrit is the major factor that changes the viscosity of blood.

What is the hematocrit?

• It is the percentage of blood that is occupied by red blood cells.

What is the normal value for a hematocrit for men?

• Forty to forty-five.

What is the normal value of a hematocrit for pre-menopausal women?

• Thirty-five to forty.

As the hematocrit increases, what happens to viscosity?

•  It increases as well.

Student Doctor, please pause the tape and summarize the information discussed on flow.

• Flow, expressed as Q, is the mass movement of a volume of fluid per unit time. According to the continuity principle, in any system arranged in series, the flow through each vascular component must be equal to the flow in every other vascular component. Different vascular segments in the body have different cross-sectional areas.  Therefore, to keep the rate of flow equal the velocity of flow must vary inversely with the cross-sectional area of a given vascular segment. Pressure gradient and resistance determine the flow of blood in the cardiovascular system. The variables of delta P, resistance, expressed as R, and flow expressed as Q, are related to one another in a way that is analogous to Ohms Law. The formula for flow is: Q equals Delta P divided by R. The two factors that affect resistance are radius of the vessels and viscosity of the blood. The variation in the hematocrit is the major factor that changes the viscosity of blood. The hematocrit is the percentage of blood that is occupied by red blood cells. The normal value of a hematocrit for men is forty to forty-five and for pre-menopausal women, it is thirty-five to forty. As the hematocrit increases so does viscosity.

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