The meaning of blood pressure

Figures

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  Fig. 1. Effect of age and initial volume on thoracic aortic elastance. The slopes of the lines are elastance. The right upper insert shows the increase in circumferential tension versus increases in aortic circumference in percent for age < 18 to > 80 years [33]. The lower left shows a schematic pressure–volume relationship for the aorta. The boxes represent stroke volumes. The same stroke volume A starting from the same initial volume produces increasing pulse pressures depending upon the shape and position of the start of the stroke volume. The stroke volume B is the same size as in A but starts at a higher initial volume and produces a much larger pulse pressure

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  Fig. 2. Pressure measurements in a vessel with an aneurysmal region. Pressures measured with fluid filled tubes facing the flow [1, 3, 5] measure elastic and kinetic energy, whereas tubes with the opening perpendicular to the flow just measure lateral pressure [2, 4, 6]. An assumption is that the energy loss due to resistance (dashed line) is minimal. In A, the tube facing the flow [1] shows a higher pressure than the tube measuring lateral pressure [2] because it includes kinetic energy. In B the vessel diameter is larger and velocity of flow is slower. The kinetic energy converts into elastic energy and the difference between tubes 3 and 4 is much smaller than between 1 and 2. In C, the tube narrows again so that kinetic energy increases and lateral energy decreases, which again increases the difference between 5 and 6

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  Fig. 3. Gravitation effect on arterial pressures (adapted from [9]). The numbers on the right in mmHg refer to the gravitational potential energy related to the difference between the base of the measuring device relative to the mid-point of the right atrium (dashed line) assuming a 182-cm male. The loss of pressure due to resistance is assumed to be 5 mmHg. When the transducer is set at the level of the right atrium, the mean pressure is 83 mmHg. At the top of the head the pressure would only be 39 mmHg. If the transducer was at the level of the foot, the pressure would be a mean of 171 mmHg

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  Fig. 4. Flow vs pressure for kidney (left) and heart (right) based on data from hemorrhaged dogs [11]. The dotted line indicates baseline state and the solid line indicates maximal vasodilation with nitroprusside. The initial flow vs pressure line for the kidney is steep and is only a little steeper with vasodilation. The heart starts with a much flatter flow vs pressure line but can increase fivefold in the pressure range of 70–80 mmHg. Note that the peak conductance of flow to the heart is only mildly higher than the baseline conductance to the kidney

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  Fig. 5. Pressure versus volume relationship of the left ventricle. The series of lines with increasing slopes indicate the time-varying elastance of the aorta as described by Sagawa and colleagues [22]. Note that aortic valve opening occurs much before peak aortic elastance, peak left ventricular pressure, and accordingly peak aortic pressure