Misconception 13. Pressure in arteries is always high (i.e. above 50 mmHg); higher than that in any vein.
When an individual is lying down, neither the arterial pressure nor the venous pressure is affected by gravity. Venous pressure at the feet may be 5 mmHg, and the same at the head. Blood is flowing from the head to the heart and from the feet to the heart. Some loss of pressure occurs in the venous system because of a slight vascular resistance.
The pressure at the heart on the arterial side, approximately 100 mmHg, is mainly produced by the cardiac contractions. When the individual stands up, that is, undergoes orthostasis, the pressures are altered everywhere except at the heart (Figure 1.04), (Table 1.06).
At the arterial side in the feet, for example, the pressure has increased to 183 mmHg and on the venous side to 93 mmHg. The increase is due to gravity. A gravitational component acting through the hydrostatic columns from the heart down to the feet is added to both the venous and arterial pressures. The increased transmural pressures in both vessels has the capability of stretching the blood vessels, and thus causing increased blood flow. Perfusion pressure on the other hand is unchanged, i.e. still 90 mmHg, the same as when the individual was lying down.
The same thing has occurred at the level of the head, however in an opposite manner. Pressure is now subtracted from the pressure generated by the heart. For example, on the arterial side the pressure is now only 51 mmHg because 44 mmHg was subtracted from 100. On the venous side 44 mmHg is also subtracted, from 5, yielding -39 mmHg. Therefore the venous pressure at the head level is actually negative. This has the potential for the collapse of flexible vessels, especially those such as the jugular vein, which are not protected by the bony cranium (skull).
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