What is the functional upstream pressure for venous return, which is a primary determinant of cardiac output?
A. pleural pressure
B. right atrial pressure
C. mean systemic pressure
D. mean circulatory filling pressure
?ANS: C
A. Incorrect response: Pleural pressure fluctuates throughout the ventilatory cycle regardless of whether the breathing is spontaneous breathing or positive-pressure breathing. During spontaneous ventilation (SV), pleural pressure remains subatmospheric. However, during positive-pressure ventilation (PPV), pleural pressure becomes supra-atmospheric.
B. Incorrect response: The right atrial pressure represents the end of the downstream end of the pressure gradient responsible for venous return.
C. Correct response: Mean systemic pressure is the weighted average of all pressure from all parts of the systemic circulation. The elastic properties of the blood vessels and the volume of blood they contain determine the magnitude of the mean systemic pressure.
D. Incorrect response: See explanation C.
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