Heart output and heart rate

Each contraction of a ventricle forces about 70 ml of blood (about half of its contents) into the circulation. This blood is ejected with such force from the left ventricle that it sends a high-pressure ‘shock wave’ through the circulatory system. You can feel this pressure wave at various points in the body, usually in an artery passing over seomething hard such as bone, as in the wrist. We call the pressure wave our ‘pulse’. If your pulse rate is 70 beats a minute, you can calculate that about 5 litres of blood are ejected into your circulation in one minute. This volume of blood is the ‘cardiac output’. If the demands of the body increase, for example during exercise, the heart responds and increases its output by increasing the number of beats per minute, and also by increasing the strength of each contraction so that more blood is ejected with each beat. The cardiac output can thus increase tenfold or even more if required by strenous physical exercise.

A very slowe pulse, known medically as bradycardia, is a common symptom during convalescence after an infection such as diphteria or influenza. It may also accompany jaundice or any other condition in which the blood is carrying poisonous substances. These types of bradycardia pass away, and the pulse returns to normal, as the patient gets better. Another, permanent, type of bradycardia accompanies a heart block. Because the heart, arteries and veins form a closed system, the volume of blood returned to the heart is the same as that pumped out from it.