Cardiac Output

Cardiac output (CO) is the volume of blood each ventricle pumps out per minute, and it is simply the product of how fast the heart beats and how much blood it ejects with each beat: CO = HR x SV. Heart rate (HR), or pulse, is the number of beats per minute, and stroke volume (SV) is the volume of blood ejected by one ventricle in a single beat; SV is the arithmetic difference between how full the ventricle was just before contracting and how full it is just after - SV = EDV - ESV. According to the Frank-Starling law of the heart, the critical factor controlling stroke volume is how much the cardiac muscle fibres are stretched immediately before they contract, and the single biggest influence on that stretch is venous return - the amount of blood flowing back to the heart and filling its ventricles; during vigorous exercise, increased venous return can double stroke volume. Because the heart's two sides are really two independent pumps working in series, one side can fail while the other keeps working: left-heart failure causes blood to back up into the lungs (pulmonary congestion), while right-heart failure causes it to back up into the body (peripheral congestion), and the Frank-Starling mechanism itself acts as a safety valve against sudden, abnormal swings in blood volume. Related to cardiac output is blood pressure, the force blood exerts on vessel walls, with two components: systolic pressure, measured while the heart's chambers are contracting, and diastolic pressure, measured while they are relaxed, together measured with a sphygmomanometer and reported as systolic/diastolic (a healthy adult reading is about 120/80 mm Hg). Mean arterial pressure depends on both cardiac output and arteriolar resistance and is kept ste …