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 (lower muscular chamber of the heart) gets less blood to pump out. By increasing the heart rate with a reflex mechanism, body maintains the cardiac output. Cardiac output (volume of blood pumped out by the heart in a minute) 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. Corresponding to this the volume of pulse will be higher in expiration and lower in inspiration. Same change can be noted in a blood pressure tracing using a cannula (small tube) inserted into the radial artery (blood vessel carrying oxygenated blood to the fore arm).