THE CIRCULATION. 307 



One-half of the transfused blood is transuded into the tissues at the 

 end of the first day. This fluid in the tissue is carried off by the 

 glands, especially the kidneys, and by the intestinal mucous membrane. 

 Where the vessels are underfilled with blood, as by blood-letting, the 

 blood-vessels contract, the secretions diminish and the water of the 

 lymph goes back into the blood-vessels. 



Extremes of Pressure. The highest pressure is registered in the 

 aorta. While traversing the arteries the fall in pressure is very 

 gradual. Immediately upon its passing from the arterioles into the 

 capillaries and there meeting great resistance, the pressure fall is 

 very marked. 



The blood-pressure continues to fall in the capillaries and veins 

 until the cardiac portion of venaB cava3 are reached, when the lowest 

 pressure is registered. As stated elsewhere, this last pressure may 

 be negative. 



The causes of alteration in blood-pressure of arteries, according 

 to Brunton, are as follows : 



It may be raised : 



1. By the heart beating more quickly. 



2. By the heart beating more vigorously and sending more blood 

 ito the aorta at each beat. 



3. By contraction of the arterioles retaining the blood in the 

 irterial system. 



It may be depressed : 



1. By the heart beating more slowly. 



2. By the heart beating less vigorously and completely and send- 

 ing less blood into the aorta at each beat. 



3. By dilatation of the arterioles, allowing the blood to flow more 

 juickly into the veins. 



4. By deficient supply of blood to the left ventricle, as from con- 

 traction of the pulmonary vessels or obstruction to the passage of 

 blood through them, or from stagnation of the blood in the large 

 veins, as in shock. 



The blood-pressure in the pulmonary artery is about one-third to 

 one-seventh that of the aorta. 



Pathological Action. Blood-pressure in disease generally in- 

 creases instead of decreases. Sleep and muscular rest lower blood- 

 pressure; mental and muscular exercise, meals, excitement and cold 

 increase it. We have apoplexies mainly in winter because of the 

 cold, which also is aided by increase of venous tension, caused by any 

 sudden strain, as in defecation. 



