VALVES AND OPENINGS 45 



Functions of Valves. The valves are arranged at the 

 openings 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 back- 

 ward 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 are 

 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 distance to which this column is carried by the heart force. 

 The column of blood is that amount that is sent by a single 

 contraction 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 expended in raising one gram one meter. Thus the 

 work of the left ventricle at each contraction is 130.5 gram- 

 metres (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. 



Sounds of the Heart. Listening to the heart's action 

 through the thoracic wall we hear two distinct sounds. The 

 first is a slightly elongated sound and comes immediately 

 after the beat of the radial pulse. It is characterized by the 



