Aneurysm -- A ballooning of the wall of an artery, a vein, or the heart, at a weak spot in the wall; it is filled with fluid or clotted blood, often forming a pulsating tumor. A sacciform or saccular aneurysm is a localized distension involving only part of the vessel wall; a fusiform aneurysm distends the whole circumference of the artery; in a dissecting aneurysm, the artery wall is split longitudinally by blood pumping through a tear in the inner arterial wall.

Aorta -- The great arterial trunk that carries oxygenated blood from the heart to be distributed throughout the body by branch arteries.

Atrial septal defect -- A congenital defect in which the wall (septum) that normally separates the heart's two upper chambers is incompletely formed, leaving an opening that compromises efficient blood circulation.

Atrium -- Either of the two upper chambers of the heart, which receives blood from the veins and forces it into the ventricles (lower chambers).

Coarctation of the aorta -- A congenital deformity in which the aorta's upper portion is narrowed.

Estivation -- The dormant state of decreased metabolism in which certain animal species, e.g., some tropical amphibians, survive a hot, dry summer; summer dormancy.

Hibernation -- The dormant state in which certain animal species pass the winter; it is characterized by a reversible depression of central nervous system functions marked by stupor or insensibility, and sharp reduction in body temperature and metabolic activity.

Homograft -- A graft of tissue between individuals of the same species. Also called allograft.

Hypothermia -- Subnormal temperature of the body.

Infundibular stenosis -- An abnormal narrowing of the area below the pulmonary valve in the right ventricle of the heart.

Mitral commissurotomy -- A surgical operation that re-opens the flaps of the mitral valve of the heart when they have been partially fused by scarring.

Mitral valve stenosis -- A narrowing of the mitral valve opening caused by partial fusing of the valve flaps. The fusing can occur as a congenital defect or as the result of inflammation and scarring (as in rheumatic fever.)

Pulmonary valvular stenosis -- A narrowing of the opening between the pulmonary artery and the right ventricle of the heart.

Thoracic -- Relating to, located within, or involving the thorax (the part of the mammalian body between the neck and the abdomen.)

Torpor -- a state of mental and motor inactivity with partial or total insensibility; dormancy.

Ventricle -- A cavity of a bodily part or organ; in the heart, either of two larger lower chambers that receives blood from a corresponding atrium and from which blood is forced into the arteries. The right ventricle pumps blood back to the lungs to be oxygenated, while the left ventricle pumps oxygenated blood out to the body and to the coronary arteries that supply the heart muscle.

Ventricular fibrillation -- Irregular heart rhythm characterized by fibrillary contractions of the ventricular muscle due to rapid repetitive excitation of the heart muscle fibers without the coordinated contraction of the ventricle needed to pump blood.