Become one red blood cell and travel from the right atrium through the lungs, the aorta, the capillaries and the calf veins, back to the heart in about a minute — stretching one heartbeat and watching the moment the colour changes.
Right now, a little left of the middle of your chest, a muscle about the size of a fist is squeezing and relaxing. It is about 12 cm long and weighs about 300 g. In a resting adult it beats around 75 times a minute, about 100,000 times a day. On this journey you become one red blood cell in the blood it sends out: a disc about 8 µm across, dimpled in the middle. It has just come back through the great vein, dark red now that it has given up its oxygen. From here, around the body and back again takes about a minute at rest.
Put a finger on your wrist or neck and tap along with your pulse at least four times. The heart starts beating at your pace, and the clocks ahead keep that pace too.
The pulse under your finger: is that the blood arriving? The answer is waiting partway along, in the aorta.
Beating at 75 per minute · about 108,000 a day
