VALVES AND OPENINGS 45 



what like the tricuspid except that it has only two flaps instead 

 of three. 



Functions of Valves. The valves are arranged at the open- 

 ings of the different chambers of the heart so the blood 

 will be forced in a constant direction. When the auricles 

 are at systole the auriculo-ventricular valves are open thus 

 letting the flow of blood go from auricles to ventricles; but 

 as soon as auricular diastole and ventricular systole begin 

 these valves shut and the blood is kept from flowing backward 

 into the auricles. Then the semi-lunar valves are open and the 

 blood is forced into the aorta and pulmonary artery. When 

 ventricular diastole begins these semi-lunar valves closed and 

 thus blood is prevented from running back into the heart from 

 the arteries. 



Work of the Heart. The work done by the heart is equal to 

 the weight of a column of blood multiplied by the height or dis- 

 tance to which this column is carried by the heart force. The 

 column of blood is that amount that is sent by a single contrac- 

 tion of the heart and the height to which it is carried is equal to 

 the pressure in the aorta and pulmonary arteries. 



The amount of blood thrown into the aorta at each con- 

 traction of the ventricles weighs about 87 grams (about 3 oz.) 

 and the height to which it is forced is about 1.5 meters or 5 feet 

 in man. 



In estimating the work of a machine the English express the 

 result in foot pounds. The French in grammetres. A foot 

 pound is -the energy expended in raising a unit weight (i Ib.) 

 through a unit distance (i ft.). A grammetre is the force ex- 

 pended in raising one gram one meter. Thus the work of the 

 left ventricle at each contraction is 130.5 grammetres (or 15 foot 

 pounds). Add 45 grammetres as the work done by the right 

 ventricle in contracting. If the heart beats 72 times per minute 

 it will, in twenty-four hours, do 18,000 kilogramme-metres of 

 work. 



