A large number of readers have requested articles on blood pressure. The questions which are most frequently asked are, “What is normal blood pressure?” “What is dangerous blood pressure?” “What is the meaning of low blood pressure?” and “What is the meaning of high blood pressure?” These cannot be answered for the benefit of the layman without some knowledge of the general principles involved. We will attempt briefly to outline these. If we appear to do so dogmatically, it should be remembered that there is a considerable difference of opinion on the subject among doctors.
Blood pressure means simply, that in order to keep the blood distributed evenly in the tissues of the entire body, it has to be kept in the blood vessels under a certain amount of pressure. Now, in order for life to be maintained, the blood does have to be distributed evenly to all tissues. Therefore, no matter what changes occur in the body, whether these be weakness of the heart muscle, or loss of elasticity of the blood vessels, the pressure must be maintained if life is to go on. When, therefore, in the process of time the arteries lose their elasticity, the heart is compelled to pump harder in order to keep this pressure up. A high blood pressure under such circumstances is a necessary compensation, and is beneficial rather than detrimental.
On the contrary, in a thin young person who has extremely elastic blood vessels, and does not exert the muscles violently, there is not much tissue demand, the blood pressure can be maintained at very low level without affecting health or function.
Thus, neither high blood pressure nor low blood pressure is a disease in itself, but simply the balance of a set of forces.
The heart and the blood vessels must be thought of as a pump and a set of elastic tubes. The tubes divide up into an extremely complicated network. At any point in this network the pressure must be such that if the tube is cut, the fluid which is being pumped through the system will flow out in a steady, even stream. This stream is maintained by two forces; one is the strength of the heart beat; the other is the pressure exerted on the fluid by the elastic walls of the tubes.
In order for the heart to maintain an even pressure in one of the little tubes far away, all of the tubing must constantly exert a gentle pressure on the contained fluid. If, due to the processes of time, the elasticity of the tubing is lost, the heart will necessarily have to beat with greater strength.
Blood pressure is measured by enclosing the vessel of an extremity with an elastic bag and compressing it until the force of the heart is just able to open it. This pressure is communicated from the bag to a column of mercury or a dial, and constitutes the amount of pressure which is necessary in that particular body to keep the circulation maintained.