88 PHYSIOLOGY FOR BEGINNERS C:HAI>. 



Let us consider the right side first. The right auricle 

 receives its blood from the superior vena cava and the inferior 

 vena cava. When it becomes full the right auricle contracts 

 and drives the blood into the right ventricle. The auricle 

 begins to contract just round the openings of the veins, so that 

 it squeezes together their flaccid or easily collapsed openings, 

 and then the contraction runs over the whole auricle towards 

 the opening into the ventricle. The blood, owing to this, 

 cannot get back into the veins but is forced into the ventricle. 

 The right ventricle thus filled with blood at once begins to 

 contract. One of the first effects of the pressure thus set up 

 is to force blood behind the flaps of the tricuspid valve ; this 

 brings the flaps together and so closes the way back to the 

 auricle. When the walls of the ventricles contract, the papillary 

 muscles forming part of the walls contract also, and thus 

 tighten the chordae tendineai attached to them ; these pull on 

 the flaps of the tricuspid valve, and prevent them from being 

 forced back into the auricle. The contraction of the ventricle 

 very soon exerts enough pressure to force open the semilunar 

 valves of the pulmonary artery, and then it drives the blood 

 into that vessel. It cannot open the semilunar valves at once, 

 because the pulmonary artery is already full and distended 

 with blood. Soon the ventricle gets up enough pressure to open 

 them, and keeps strongly contracting till it has forced its blood 

 into the artery. This it does partly by forcing the blood in 

 the pulmonary artery farther along into the lungs, and partly 

 by distending the artery still more, so as to make room for the 

 fresh quantity of blood. When the ventricle has emptied, or 

 nearly emptied itself, it begins to relax. The pockets of the 

 semilunar valve are full of blood, and now the great pressure 

 of the blood in the pulmonary artery presses them together 

 and closes the way so that no blood flows back into the 

 ventricle. While the ventricle is contracting the auricle 

 relaxes and the blood flows into it again from the veins. This 

 goes on till the auricle is full, then it contracts and drives 

 the blood into the ventricle, which by this time is relaxed, 

 and the same events occur again. More and more blood 

 is thus driven by the right ventricle into the pulmonary 

 artery, so that the blood which is already in the artery must be 

 forced on through the branches of the artery and through the 



