Heart rate increases while breathing in (inspiration) and decreases while breathing out (expiration). This is to compensate for the decreased left ventricular output while breathing in. During inspiration, more blood gets pooled in the lungs so that left ventricle gets less blood to pump out. By increasing the heart rate with a reflex mechanism, body maintains the cardiac output. Cardiac output is the product of heart rate and the amount of blood pumped out by the heart in a beat (stroke volume). In contrast, left ventricular output is higher in expiration and heart rate lower.