What Is Blood Pressure
( Originally Published 1916 )
Though the apparatus for measuring blood pressure was invented only a few years ago, most persons know in a more or less indefinite way that high blood pressure is a serious thing. The insurance companies will no longer issue a policy without first taking the applicant's blood pressure. It may be interesting to know how this examination is made.
A cuff with a rubber air-bag is placed on the arm above the elbow. The air-bag is connected by rubber tubing with a column of mercury and a pump. Air is then forced into the bag and glass tube containing the mercury until sufficient pressure is used to stop the pulse in the wrist, the amount required being read from a scale on the mercury tube. Sufficient force to just close off the pulse gives the blood pressure.
Blood pressure results from the force of the heart beat and the resistance of the arteries. One of the causes of high blood pressure is over-eating, particularly of protein foods. I have seen it lowered in stout persons during weight reduction.
If the engineer has a large fire under a boiler when there is little demand for steam, the pressure in the boiler rises to a dangerous degree. To prevent explosions, a boiler is fitted with a safety-valve. The body is, in a measure, comparable to the boiler and engine. Constantly feeding large amounts of food, particularly of the protein class especially to those who are inactive increases blood pressure. Nature may, to some extent, be able to prevent the evil consequences of such over-feeding, but to permit it is tempting Providence, and there is a point beyond which Nature refuses to help those who disobey her laws. It is often said that Providence protects drunken men and children. Though blood pressure may be due to auto-intoxication and to child-like ignorance of what should be eaten, there is, to my knowledge, no Providential dispensation to protect such persons against the evil consequences of their own dietetic sins.
The reason over-eating increases blood pressure may be easily understood. When an individual is living on a diet that just meets requirements, the food is used up as nearly as is possible. The proteins, being employed largely for repair work, such of their products as reach the excretory organs for disposal will be few indeed.
In the chemistry of the body, poisonous by-products are reduced to a minimum. There are no clinkers in the furnace. The proteins are perhaps more injurious when taken in excess than are fats and carbohydrates. They are not adapted for use as fuel, but are mainly suitable for replacing the body structure as it is worn out from time to time. When eaten in amounts greater than is necessary for reparative purposes, the excess is used as fuel. I have said they are not adapted for this purpose. It is impossible for them to be completely oxidized. This throws more work on the kidneys than they would be called upon to perform if the body fuel were in the form of carbohydrates and fats alone. Still, excessive eating of any class of foods is not without its injurious effects. When more is taken into the system than is required, it is certain to interfere with body chemistry and produce poisoning.
While the body has nothing similar to the safety valve of the boiler from which the individual may take warning, there are several signs which may be read by the skilled physician and which give a definite idea of possible dangers ahead. The blood pressure apparatus reveals one of these. Starting with a normal blood pressure of 120 in the young adult, with variations for age and some other conditions, the physician knows that when blood pressure begins to rise to undue heights it is time to reduce the fire in the furnace or increase the demand for steam. Insurance companies hesitate before accepting one with a reading of 150.
If such warnings are neglected, the heart, which is compelled to pump blood through the vessels against an increasing resistance, may enlarge through dilatation and thereby be weakened. If the heart proves equal to its increased task, one of the arteries may enlarge, causing aneurism, or it may break, causing hemorrhage. The latter taking place, as it most often does, in the brain, is a common cause of paralysis. Arterio-sclerosis is similar in its causes, and is frequently associated with high blood pressure.
This little explanation of the significance of blood pressure shows the value of just one more scientific discovery, and just one more medical invention.