24 PHYSIOLOGY FOR NURSES 



venous is changed to arterial blood and thence back to 

 the heart to go over the same route as long as life lasts. 

 The heart is, therefore, the main force of the circula- 

 tion. Anatomy has taught us that it is a four-chambered 

 hollow muscle. The two thin-walled chambers at the 

 base are called auricles, the two thick-walled chambers 

 forming the apex, the ventricles. Both the left auricle 

 and ventricle are thicker than the right, but the left 

 ventricle is much thicker than any other part of the or- 

 gan. The right half is concerned with venous blood and 

 the pulmonary circulation. The blood from the upper 

 extremities, head and neck is collected and poured into 

 the right auricle through the superior vena cava; that 

 from the lower part of the body enters the same cavity 

 through the inferior vena cava. From the right auricle 

 the course is into the right ventricle through the auric- 

 iiloventricular opening, thence through the pulmonary 

 artery into the lungs whence it is collected by the four 

 pulmonary veins and carried into the left auricle from 

 which it flows through the left auriculoventricular 

 opening into the left ventricle from which it is pumped 

 through the aorta to all parts of the body. 



The auriculoventricular openings are guarded by 

 valves composed of triangular flaps, the right tricuspid 

 having three, and the left bicuspid, or mitral hav- 

 ing two. The two auricles fill at the same time. As the 

 blood flows in through cavse or pulmonary veins, it passes 

 through the auriculoventricular openings into the cor- 

 responding ventricle until that cavity is nearly full. As 

 the blood rises in the ventricle it floats the valve flaps 

 away from the walls of the ventricles and toward the 

 opening. This action continues until there is but a slit- 

 like aperture between the flaps. Just at this moment 

 the auricle contracts, forces into the ventricle all the 



