Page 17, History of Cardiovascular Physiology, Dr. D. Penney

Misconceptions (Continued...):

A number of graduate nursing students had told me that the reason one becomes dizzy during hyperventilation is because the brain is receiving too much oxygen. Of course the reason is because the brain is receiving too little oxygen!

Students have told me that the sounds heard through the stethoscope when taking blood pressure are made by the heart valves opening and closing, rather than by the turbulence of blood flow. I have also been told in laboratory that vascular resistance can be determined by assessing the change in arterial blood pressure alone, whereas resistance is a function of both pressure and blood flow.

Many people think that flow velocity in veins is always sluggish. In some veins this is true, but in the vena cavae, the largest veins in the body, flow velocity approaches that in the aorta.

The time it takes one milliliter of blood to flow through a single capillary is approximately 18 months. This may be surprising, but it is because erythrocytes usually traverse capillaries in single file at a velocity of about 0.5 mm/sec, and because there are about 5,000,000,000 erythrocytes in 1 ml of blood.

Finally, the Valsalva maneuver is not a military exercise, but something each of us does several times each day. Do you know how to do it?

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