# RIU 324 Lecture Notes - Lecture 41: Hemodynamics, Vasomotor, Cardiac Output

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23 Apr 2020
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I. Dynamic Pressure and Capacitance
A. At the extremities, typical venous pressures are around 15
mmHg
B. When entering the right atrium, mean pressure is typically
between 2 and 8 mmHg
C. With such a low pressure gradient, what drives venous return?
D. The capacitance of the venous system acts as a reservoir to
supply blood volume under higher demand
E. Control of vasomotor tone (changing capacitance) is principal
regulator of mean circulatory pressure
F. Venoconstriction acts as a pump, decreasing the stored blood
volume to increase cardiac output
II. Reference for Hydrostatic Pressure
A. By referencing the hydrostatic pressure to the right atrium of
the heart, the central pressure of a patient does not need to
be corrected by the hydrostatic pressure differences for
different height patients. The result of referencing the RA as
0 mmHg is that the hydrostatic pressure is considered negative
in the head for a standing patient.
B. For quick calculations, assume every inch is equivalent to 2
mmHg
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C.
III. Hydrostatic Pressure
A. Static water pressure= hydrostatic pressure
B. Pressure related to weight of blood pressing on a vessel
measured at a height above or below heart level
C. Units of mmHg, like patients blood pressure
D. BP measurements are erroneous when measured above or
below heart level
E. Standing
1. The reference level is at the heart (right atrium)
2. Below heart level, such as ankle level, measured BP is
higher than actual BP due to additional hydrostatic
pressure from gravity
3. Above heart level, hydrostatic pressure is negative and
measured BP will be lower than true circulatory BP
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4.
5. In a patient with 140 mmHg systolic pressureMeasured
pressure is the sum of circulatory pressure +
hydrostatic pressure
6.
IV. Calf Muscle Pump
A. The calf muscle pump helps overcome the effect of gravity to
aid with venous return for a patient in the standing position.
By muscle contraction, the venous volume is ratcheted back
toward the right heart through a series of valves which open
and close with muscle contraction
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