Heart

a hollow muscular orgaf, the function of which is to maintain the circulation of the blood. The organs of circulation are the heart, the arteries, the veins, and the capillary ve els, The blood is either arterial or venous, according to the relative proportions of oxygen and carbonic acid gas it contains. The venous blood which has returned to the right side of the heart from the body, is propelled along the pulmonary artery by the contraction of the heart into the lungs, where by the absorption of oxygen and the giving off of carbonic acid gas it becomes arterial blood, which then is conveyed along the pulmonary veins to the left side of the heart, and is thence distributed throughout the body to nourish it and repair its lo es.