THE SIZE OF THE HEART

The size of the heart varies very little over a whole range of normal physiological activities. Physiologists, in animals and man, measure c hanges in cardiac output and heart volumes during exercise. Cardiac ou tput can increase 5, 6, or 7 times in athletes but the stroke volume n ever more than doubles, the end-diastolic volume increases only by abo ut 50% and the end-systolic (residual) volume decreases by the same am ount; the heart rate increases about two and a half times in the untra ined to 5 times in the physically fit athlete. It certainly appears as though there are some controlling mechanisms. The best way to conside r these potential controlling mechanisms is not to accept the proposit ion that the heart provides most of the force necessary to propel the blood round the body during these various activities; this only occurs when you are flat on your back with your chest and abdomen open-not a very common occurrence. It is easier to regard the heart as having me chanisms available to it which allow the heart to accept all the blood which is pumped back to it during activity by the muscle pumps. The F rank-Starling mechanism allows an increased force of contraction to fo llow an increase in volume of each chamber, but from the evidence prov ided above this is by no means the whole story. It is proposed that ch anges in heart rate form the basis of the mechanism controlling the he art volumes and its size. Evidence is provided to allow me to postulat e that the atrial receptors and the effect on blood volume and the eff ect on heart rate together form a remarkable control system which cont rols the size of the heart-and keeps it small.