This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
The phenomena of the circulation in the heart and vessels are very much the same as in the schema. When the heart stands still (as when the vagus is strongly galvanised), the blood flows from the arteries into the veins until the arteries are nearly empty and the pressure within them falls to zero. If the heart now begin to beat, it forces blood into the elastic aorta and arteries at each systole, and distends them, just like the elastic bag of the schema; while at the same time it takes blood from the veins, and they become empty in proportion as the arteries become full. During every diastole of the heart, the distended aorta and other arteries, in virtue of their elasticity, contract on the blood they contain, and keep it flowing on through the capillaries till another systole occurs; the heart, meanwhile, being completely shut off from the aorta by the sigmoid valves (just as the ball of the schema was shut off from the elastic bag). In general, the diastole is longer than the systole; so that for the greater part the circulation through the capillaries is carried on by the elasticity of the arteries, and not directly by the heart. The arteries, which we have supposed to be at first empty, gradually become distended by the heart, just as the elastic bag was by the ball, and exert more and more pressure on the blood in them (so that it would spout higher and higher if one of them were cut), till they are able during the diastole to press the same amount of blood through the capillaries into the veins as had been pumped into them during the systole. The more tensely they are stretched, the greater is the pressure they exert on the blood they contain; and the amount of this is termed the arterial tension or blood-pressure. These two terms mean the same thing, and we use one or other just as the fancy strikes us. At each systole, the fresh supply of blood pumped in by the heart stretches them more; that is, the arterial tension rises. During each diastole, the blood escapes into the wide and dilatable veins, and the arteries become less stretched; that is, the arterial tension falls. This alternation of rise and fall constitutes the pulse.
Besides the oscillations which take place in the blood-pressure at each beat of the heart, a rise and fall in the form of a long wave occurs at each respiration. The wave begins to rise just after inspiration has begun, reaches its maximum just after the beginning of expiration, and then begins to fall again till a new wave succeeds it. The heart-beats are generally quicker during inspiration, and slower during expiration.
The blood - pressure thus oscillates up and down at each heart-beat and rises and falls with each respiration, and the average between the highest and lowest points is called the mean arterial tension or mean blood-pressure.
Besides the oscillations in blood-pressure due to the pulse and to the respiration, there are slowly rising and falling waves to which the name of Traube's curves is given. These are due to alternate contraction and relaxation of the arterioles and capillaries. Rhythmical contraction of the arterioles has been observed in almost all parts of the body of rabbits, and probably occurs both in the lower animals and in man.
The blood-pressure is not equal throughout the whole arterial system. It is greater in the large and less in the smaller arteries, in which it becomes diminished by the friction between the blood and the arterial walls. It is also modified by gravity, so that the position of a limb may alter the pressure in its arteries.