THE CIRCULATION OF THE BLOOD. 



the average of the pressures in all the arteries. The pressure in the 

 veins is never more than one tenth of the pressure in the corresponding 

 arteries, and is greatest at the time of auricular systole. There is no 

 periodic variation in venous pressure, as there is in the arterial, except in 

 the great veins. 



Variations of Blood-Pressure. Many circumstances cause con- 

 siderable variations in the amount of the blood-pressure. The following 

 are the chief : (1) Changes in the beat of the Heart; (2) Changes in the 



FIG. 136. Normal arterial tracing obtained with Fick's kymograph in the dog. (Burdon-San- 

 derson.) 



Arteries and Capillaries ; (3) Changes due to Nerve Action; (4) Changes 

 in the Blood ; (5) Respiratory Changes. 



1. Changes in the Beat of the Heart. The systole and diastole of the 

 muscular chambers. The arterial tension increases during systole and 

 diminishes during diastole. The greater the frequency, moreover, of the 

 heart's contractions, the greater is the blood-pressure, cc&teris paribus. 

 As a rule, however, when the heart contracts frequently, the beats lose 

 in strength, and the increase in frequency may be compensated for by 

 the delivery into the arteries at each beat of a comparatively small quan- 

 tity of blood. The greater the quantity of blood expelled from the heart 

 at each contraction the greater is the blood-pressure. 



The quantity and quality of the blood nourishing the heart's sub- 

 stance through the coronary arteries must exercise also a very consider- 

 able influence upon its action, and therefore upon the blood-pressure. 



2. Changes in the Arteries and Capillaries. Variations in the degree 

 of contraction of the smaller arteries modify the blood-pressure by favor- 

 ing or impeding the accumulation of blood in the arterial system which 

 follows every contraction of the heart; the contraction of the arterial 

 walls increasing the blood-pressure, and their relaxation lowering it. 



3. Changes due to Nerve Action. The nervous system has a very 

 important action in regulating the blood-pressure. Its influence is 

 exerted chiefly upon the muscular coat of the arteries and not upon the 

 elastic element, which possesses, as must be obvious, rather physical than 

 vital properties. The muscular tissue in the walls of the vessels increases 

 in amount relatively to the other coats as the arteries grow smaller, so 

 that in the smallest arteries it is developed out of all proportion to the 

 other elements; in fact, in passing from capillary vessels, made up as we 

 have seen of endothelial cells with a ground substance, the first change 



