PRESSURE OF BLOOD IN THE ARTERIES 59 



(91 millimeters) in the carotid, and 3.44 inches (86 millimeters) in the 

 crural. These experiments show that the pressure is not the same 

 in all parts of the arterial system, that it is greatest in the arteries 

 nearest the heart and that it gradually diminishes toward the capil- 

 laries. The difference is slight, almost inappreciable, except in vessels 

 of small size ; but here the pressure is directly influenced by the dis- 

 charge of blood into the capillaries. The cause of this diminution of 

 pressure in the smallest vessels is the proximity of the great outlet of 

 the arteries, the capillary system ; for, as will be seen farther on, the 

 flow into the capillaries has a constant tendency to diminish the pres- 

 sure in the arteries. 



Influence of Respiration. It is easy to see, in studying the arterial 

 pressure, that there is a marked increase with expiration and a diminu- 

 tion with inspiration. In tranquil respiration the influence on the flow 

 of blood is due simply to the mechanical action of the thorax. With 

 every inspiration the air-cells are enlarged, as well as the bloodvessels 

 of the lungs, the air rushes in through the trachea and the movement 

 of the blood in the veins near the chest is accelerated. At the same 

 time the blood in the arteries is somewhat retarded in its flow from the 

 thorax, or at least does not feel the expulsive influence which follows 

 with the act of expiration. The arterial pressure at that time is ati 

 its minimum. With the expiratory act the air is expelled by com-' 

 pression of the lungs, the flow of blood into the thorax by the veins 

 is retarded to a certain extent, while the flow into the arteries is 

 favored. This is shown in the increased force, with expiration, in the 

 jet from a divided artery. Under these conditions, the arterial pres- 

 sure is at its maximum. In perfectly tranquil respiration, the changes 

 due to inspiration and expiration are slight, presenting a difference of , 

 not more than half an inch or an inch (12.7 or 25.4 millimeters) in the \ 

 cardiometer. When the respiratory movements are exaggerated, the 

 oscillations are more extensive. 



Interruption of respiration is followed immediately by a great in- 

 crease in the arterial pressure. This is due, not to causes within the 

 chest, but to obstruction to the circulation in the capillaries. With an 

 interruption of the respiratory movements, the non-aerated blood passes 

 into the arteries but can not flow readily through the capillaries ; and 

 as a consequence the arteries are abnormally distended and the pres- 

 sure is increased. If respiration is permanently arrested, the arterial 

 pressure becomes, after a time, diminished below the normal standard, 

 and is finally abolished on account of the stoppage of the action of the 

 heart. When respiration is resumed before the action of the heart has 

 become arrested, the pressure soon returns to the normal standard. 



