88 



ELEMENTS OF PHYSIOLOGY 



pulmonary 

 veins 



pulmonary 

 veins 



below it, and the left auricle communicates with the left 



ventricle. 



145. Circulation of the Blood. From all parts of the 



body (except the lungs) the blood arrives at the right 



auricle, dark red in color, 

 and charged with carbon 

 dioxid, a gas that is unfit 

 for supporting life. The 

 blood has come through 

 two large veins, the supe- 

 rior vena cava, from the 

 head, arms, etc. (Fig. 85), 

 and the inferior vena cava, 

 from the lower parts of 

 the body. The first cham- 

 ber of the heart that it 

 enters is the right auricle. 

 The auricle contracts and 

 presses the blood into the 

 right ventricle (Fig. 86). 



FIG. 85.- Diagram tTthow the Course of the Jt be S ins tO Squeeze tO- 



Biood through the Heart. gether just around the 



The vessels containing impure blood are drawn openings of the VCinS, SO 

 darker than the others. 



that it closes their open- 

 ings. The blood, owing to this, cannot go back into the 

 veins, but is forced into the ventricle. The right ventricle 

 thus filled with blood at once begins to contract. The first 

 effect of the pressure thus produced is to force blood behind 

 the flaps of the tricuspid valve, the valve between the auri- 

 cle and ventricle, consisting of three flaps made of white 

 fibrous tissue. The blood behind the flaps brings the flaps 

 together and so blocks the way to the auricle (Fig. 87). 

 The contraction of the ventricle goes on, and soon the 

 blood presses hard enough upon the semilunar valve to 

 open it and go on into the pulmonary artery (Fig. 85); 



