USMLE Step 1 Physiology Review 54 14 Hemodynamics (1 of 2)

# USMLE Step 1 Physiology Review 54 14 Hemodynamics (1 of 2)

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### Begin 54 14 Hemodynamics (1 of 2) Transcription

Okay, now we are going to talk about hemodynamics and pressure as they relate to the heart.

What factors does the study of hemodynamics concern?

• The factors that determine blood flow and blood pressure.

In what terms is pressure usually expressed?

•  Pressure is expressed in terms of the height of a column of water that a given pressure will support.

What are the units of pressure?

• Millimeters of mercury or centimeters of water.

How many centimeters of water is one millimeter of mercury equivalent to?

• One millimeter of mercury equals 1.36 centimeters of water.

What is the term for the difference in pressure between two locations within a system?

• Pressure gradient or delta P.

How does fluid flow on a pressure gradient?

•  Downhill, or from a point of high pressure to a point of lower pressure.

The total energy of any point in a system is the sum of what two components?

• Potential energy, which is analogous to pressure and kinetic energy.

Expressed in the language of hemodynamics, what is kinetic energy?

• It is the momentum that blood gains because of its mass and velocity. The greater the velocity of the blood flow, the greater the kinetic energy.

Student Doctor, please pause the tape and summarize the information on hemodynamics and pressure discussed thus far.

• The study of hemodynamics concern the factors that determine blood flow and blood pressure. Pressure is expressed in terms of height of a column of water that a given pressure will support. The units of pressure are milliliters of mercury or centimeters of water. One milliliter of mercury is equal to 1.36 centimeters of water. The difference in pressure between two locations within a system is referred to as a pressure gradient, or Delta P. In a pressure gradient fluid flows downhill or from a point of high pressure to a point of lower pressure. The total energy at any point in a system is the sum of potential energy, which is analogous to pressure and kinetic energy. Expressed in the language of hemodynamics, kinetic energy is the momentum that blood gains because of its mass and velocity. The greater the velocity of the blood flow the greater the kinetic energy.

Okay, now let’s talk about the components of potential energy.

Potential energy includes what two components?

• Hydrostatic pressure and lateral pressure.

What causes hydrostatic pressure?

• The effects of gravity on a fluid filled system.

Expressed algebraically, what does hydrostatic pressure equal? Please pause the tape.

• Hydrostatic pressure is equal to the density of the fluid multiplied by the height of the fluid column above or below a reference level, multiplied by gravitational constant.

In the human body what is this reference level?

• The heart

What is another term for lateral pressure?

• Static pressure.

What does lateral pressure represent?  Please pause the tape.

• It represents the pressure in the cardiovascular system after eliminating hydrostatic pressure effect. So it is as if every point in the system is at reference level.

Consider the following eight components of the cardiovascular system and list them again in order of descending lateral pressure: veins, capillaries, arterioles, vena cava, venules, left ventricle, arteries, and aorta.

• First, the left ventricle and aorta are first and can be listed together since there is minimal drop in lateral pressure from ventricle to aorta. Second, arteries.  Third, arterioles.  Fourth, capillaries.  Fifth, venules.  Sixth, veins.  Seventh, vena cava. Blood flows from the inferior and superior vena cava into the right atrium and then into the right ventricle.

What happens to the lateral pressure of the blood in the right ventricle?

• It jumps up again to a level roughly equal to the lateral pressure in the venules.

Student Doctor, please pause the tape and summarize the information on hemodynamics discussed since the last summary. The first question in this section was about the components of potential energy.

• Potential energy includes hydrostatic pressure and lateral pressure. The effects of gravity on a fluid filled system cause hydrostatic pressure. Hydrostatic pressure is equal to the density of the fluid multiplied by the height of the fluid column above or below a reverence level multiplied by a gravitational constant. In the human body the heart is the reference level. Another term of lateral pressure is static pressure. Lateral pressure presents the pressure in the cardiovascular system after eliminating for the hydrostatic pressure effect. The following eight components of the cardiovascular system are listed in order of descending lateral pressure: the left ventricle and aorta are first and can be listed together since there is minimal drop in lateral pressure from ventricle to aorta. Next comes the arteries, arterioles, capillaries, venules, veins, and finally the vena cava, Blood flows from the inferior and superior vena cava into the right atrium and then into the right ventricle. The lateral pressure of the blood in the right ventricle jumps up again to a level roughly equal to the lateral pressure in the venules.

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