358 TEXT-BOOK OF PHYSIOLOGY 



in the arteriole resistance, it is evident that a rise in arterial pressure alone would 

 increase the capillary pressure. If both arterial and venous pressures rise, 

 the capillary pressure increases; if both fall, it decreases. 



C. In Venous Pressure. Independent of any change in the venous 

 pressure in a given area from local or temporarily acting causes e.g., aspira- 

 tion' of the thorax or heart, muscle contractions, change of position, etc. the 

 general venous pressure will be increased by a decrease in the value of those 

 factors which produce the difference of pressure between the arteries and 

 veins. An increase in the value of these factors would necessarily decrease 

 the pressure. 



Variations in the Relation of the Arterial and Venous Pressures. 

 So long as the heart maintains a given rate and force and the resistance at 

 the periphery of the arterial system (due to the contraction of the arteriole 

 muscle) a given value, will the usual physiologic difference between the pres- 

 sure in the arteries and veins remain unchanged. If, however, either 

 factor changes in one direction or another, there will arise a change in the 

 relative degree of pressure in the different divisions of the vascular apparatus. 

 Thus if the heart force increases and a larger volume of blood is discharged 

 into the arteries in a unit of time, the amount of blood in the venous system 

 diminishes, and the result is a rise of the arterial and a fall of the venous 

 pressures. If, on the contrary, the heart force decreases or the mitral valve 

 permits of a regurgitation, a smaller volume of blood is ejected into the 

 arteries in a unit of time, the amount of blood in the venous system increases, 

 and the result is a fall of the arterial and a rise of the venous pressure. 



Again if the arteriole muscle relaxes and a larger volume of blood flows 

 from the arteries into the veins in a unit of time, the result will be a fall of 

 arterial and a rise of venous pressure. If, on the contrary, the arterial muscle 

 contracts and a smaller volume of blood flows into the veins, the reverse 

 change of pressure obtains. 



The Determination of the Arterial Blood-pressure in Man. Inas- 

 much as the blood-pressure undergoes considerable variation in both physio- 

 logic and pathologic conditions as well as in response to the action of drugs, 

 it seemed desirable to possess some means by which an accurate knowledge 

 of the pressure under a variety of conditions could be obtained both for 

 diagnostic and therapeutic purposes. The foregoing method of obtaining 

 the blood-pressure not being of general application to human beings for 

 obvious reasons, special instruments have been devised by which the pres- 

 sures may be determined at least approximately without resorting to any 

 surgical procedure. 



By reason of the fact that both the systolic and diastolic pressures are 

 regarded as important factors in clinical conditions, and their determination 

 of value for diagnostic and therapeutic purposes in diseases of the circulatory 

 apparatus, it is desirable to have clear ideas of what is meant by these terms. 

 If the changes of pressure are registered by a sphygmograph, a curve re- 

 sembling that shown in Fig. 162, will be recorded, which shows more or 

 less accurately the qualitative, if not the quantitative variations, of the 

 pressure in the arteries during a cardiac cycle (see page 371). 



Systolic Pressure. Systolic pressure may be defined as the highest 

 pressure developed in the artery during the systole of the heart and occurs 

 in the first half of the systole and is therefore of very short duration; after 



