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Pressure and Gravity

September 19, 2010

Our blood pressure is an anti-gravity tool.  It actually doesn’t make sense for blood pressure to be the primary resuscitation goal for a supine patient.

The only animal with valves in its carotid arteries is the giraffe.  It’s head is 17 feet higher than the heart, so it needs valves to keep the blood flowing up there from falling back down.  And, to provide the oomph to push it up there, the giraffe’s blood pressure is over twice as high as our is.  Our systolic blood pressure is normally about 120 millimeters of mercury (mmHg), and we are considered to have high blood pressure (hypertension) when it is higher than 130 or 140 mmHg, depending on age.  Giraffes have systolic blood pressures well over 200 mmHg, and they have been recorded to be as high as 300 mmHg. 

Also, in order further to prevent the entire circulatory volume from pooling at the bottom of the giraffe’s towering anatomy, the blood vessel walls in its lower extremities (figure at right) are extremely thicker than those in its head (figure at left), preventing the former from stretching under the tremendous gravitational pull.  In addition, they have very thick connective tissues wrapping their thin legs to prevent fluid extravasation that would otherwise occur as a result of the extremely high blood pressure.

The pressure within our circulation’s microscopic capillaries has been determined to be about 25 mmHg, which is the pressure necessary to drive blood across the capillary network and into the venous system, nourishing cells on the way.  However, the mean arterial pressure within the aorta and major blood vessels is about 90 mmHg — a difference of 65 mmHg.  Why should the heart have to pump our pressure into the such a high level of 90 if it only takes 25 to get it through the circulation?  Doesn’t that make the heart work overtime?

It would, of course, if there weren’t any reason for the pressure discrepancy.  But, like most things in biology, there is a very good reason.  You see, we have the same problem as the giraffe, although not to the same degree.  Our head is also above our heart, so the heart needs to  be able to pump blood upwards against gravity.  But we’re not always upright, because we sleep lying down.  For about a third of the day, our heart is at the same level as our head and our legs. 

So our circulation has to adapt for changes in our position.  When we’re supine, our lower extremity vessels open up.  But when we stand, they must constrict.  Giraffes, having a less flexible circulation, have to sleep while standing.

The relationship of our physiology to our life on planet Earth (with its gravity) is most apparent when astronauts return after spending prolonged periods in space.  They have swollen, “puffy” faces and skinny, “bird-leg” legs as a result of having antigravity circulation exposed to zero gravity.

The paradigm shift in medicine will be to use precise, specific measures of circulatory performance to determine the nature and impact of our therapeutic interventions, rather than potentially misleading stand-ins such as blood pressure.

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