Blood Pressure
in the common use of the term, means the pressure of the blood in the large arteries. The heart BOLL WEEVIL propels the blood into these arteries, from which it is in turn forced into the capillaries, thence flowing back to the heart through the veins. It is evident that, if the arteries are elastic, the influx of blood from the heart with each heart-beat will be temporarily taken up by their expansion, to be gradually distributed throughout the body under an even, steady pressure; while, if the arteries are inelastic, it will be necessary for the heart to force the blood through the farthest capillaries with each beat, and the pressure between beats will fall back to zero. Thus the difference between the maximum and the minimum blood pressure becomes the measure of the elasticity of the arteries and of the work which the heart is doing.
High blood pressure alone may be due to the natural force or rapidity of the heart-beat, but a high maximum pressure combined with a low minimum indicates non-elastic arteries. Apart from any trouble with the larger arteries, blood pressure is increased by contraction of the capillaries, making it harder to force the blood through them. This contraction may be due either to disease or to the normal functioning of the vasoconstrictor system, which in health maintains a fairly constant resistance in the capillaries, thus assuring the blood pressure necessary to the proper blood supply of the brain and skeletal muscles .
Pressure is sometimes under normal, temporarily as in fainting or shock, permanently as in certain wasting diseases. Recently published experiments by Orr and Innes seem to indicate that high blood pressure may commonly be lowered by the drinking of large amounts of water. The theory is that the contraction of the capillaries above described is caused by the formation of certain substances in the blood, called pressors, which are in part flushed out by drinking of water.