One drop of blood, the whole trip
Ride one drop of blood from the heart to a toe and back, then out to the lungs.
Leaving the heart
Follow one drop of blood (red). It has just picked up oxygen in the lungs and is being squeezed out of the strongest room of the heart, the left ventricle. It shoots up into the aorta, the biggest pipe in your body, as thick as a garden hose. The aorta bends over like a walking-stick handle, then runs straight down the back of your chest and belly.
Oxygen-rich blood leaves the left ventricle through the aortic valve into the ascending aorta (about 2.5 to 3 cm across in adults). The aortic arch gives off the three arteries to the head and arms, then the descending aorta runs down in front of the spine as the thoracic and then the abdominal aorta, sending branches to the organs on the way. Each heartbeat pushes about 70 ml into the aorta; at a resting 70 beats a minute that is roughly 5 litres a minute, about the whole blood volume of an adult (4.7 to 5.7 litres). Red here means oxygen-rich; real blood is bright red in the arteries and dark red, never blue, in the veins.
Down the leg
At the bottom of the belly the aorta splits in two, one pipe for each leg. Our drop takes the right one. The pipe runs down the front of the thigh, ducks behind the knee, comes down the shin and ends on the top of the foot, where a doctor can feel your pulse. Watch each pipe light up as the drop passes.
The abdominal aorta divides at the fourth lumbar vertebra into the common iliac arteries. The external iliac artery passes under the inguinal ligament and becomes the femoral artery, which runs down the front and inside of the thigh, then through a gap in the adductor muscles to become the popliteal artery behind the knee. Below the knee it divides; the anterior tibial artery runs down the front of the shin and continues onto the top of the foot as the dorsalis pedis artery, whose pulse can be felt there. Arteries have thick, muscular, elastic walls to take the pressure of each beat.
Turning blue
The pipes get smaller and smaller until, in the toe, they are thinner than a hair. The walls are so thin that oxygen slips out of the blood into the toe, and the waste gas the toe has made slips in. Our drop has given away its oxygen: from here on we draw it blue. It turns round and starts the long journey home in a vein.
Arteries branch into arterioles and then capillaries, tubes about as wide as a single red blood cell, with walls one cell thick. Gas exchange happens there: oxygen diffuses out to the tissue, carbon dioxide diffuses in. The body’s muscles alone hold an estimated 9,000 to 19,000 km of capillaries. They are far too small to model, so the dots simply change colour at the toe. Capillaries drain into venules and then veins: here the dorsal digital veins of the foot, which feed the great saphenous vein in front of the ankle bone.
Back up to the heart
Veins carry the blood back. They run up the inside of the leg, and the blood has to climb all the way from your toe to your chest, against gravity. Veins have little one-way doors inside them, and every time your leg muscles squeeze, they push the blood up a bit and the doors shut behind it. The leg veins join into one huge vein that runs up through the belly and empties into the right side of the heart.
The great saphenous vein, the longest vein in the body, runs up the inside of the leg from the ankle to the groin, where it joins the femoral vein. That becomes the external iliac vein, joins the internal iliac to form the common iliac vein, and the two common iliac veins unite into the inferior vena cava, which climbs beside the aorta, through the diaphragm and into the right atrium. Venous pressure is low, so return from the legs depends on the valves in the veins and the calf muscle pump; when the valves fail, blood pools and the veins become varicose. Blue here means oxygen-poor.
Refuel in the lungs
The drop lands in the right atrium, drops into the right ventricle, and gets pumped out to the lungs. There it swaps its waste gas for fresh oxygen and turns red again. It comes back to the left side of the heart, into the left ventricle, right where we started. The whole trip took about a minute. Your blood does this all day, every day, for your whole life.
From the right atrium the blood passes through the tricuspid valve into the right ventricle, which pumps it through the pulmonary valve and pulmonary trunk into the right and left pulmonary arteries, the only arteries carrying oxygen-poor blood. In the lung capillaries it releases carbon dioxide and takes up oxygen, then returns through the four pulmonary veins to the left atrium and, through the mitral valve, to the left ventricle. That is the pulmonary circuit; the loop through the body is the systemic circuit. At rest the heart pumps about 5 litres a minute, roughly the whole blood volume, so an average drop completes the full round trip in about a minute; during exercise it is much faster.
The whole trip in one go
Now watch the whole thing at once: red down from the heart to the toe, blue all the way back up and out to the lungs, red again into the heart. Round and round, about once a minute when you are resting, and several times a minute when you run.
The two circuits in series: left ventricle → aorta → leg arteries → toe capillaries → leg veins → vena cava → right atrium → right ventricle → pulmonary arteries → lung capillaries → pulmonary veins → left atrium → left ventricle. The 30 s loop shown here is slowed down about twice compared with the resting body, and the drop takes the longest road; a drop that goes to the heart muscle itself or to the brain is back within seconds.