Press two fingers to the side of your neck and you can feel a pump that has been running since a few weeks after you were conceived. At rest it beats about once a second, roughly 100,000 times a day.

From Galen to Harvey

Galen taught that the liver turned digested food into blood, which flowed outward to the body and was used up there, like water soaking into a field. Some blood, he thought, seeped from one side of the heart to the other through invisible pores in the wall between them. (A thirteenth century physician in Cairo, Ibn al-Nafis, had already described blood passing through the lungs.)

The English physician William Harvey overturned Galen in his 1628 book On the Motion of the Heart and Blood. His most famous argument was simple arithmetic. Even if the heart pushes out only a little blood with each beat, multiply by the number of beats in an hour and the total far exceeds the weight of the whole body. The heart pumps far more blood in an hour than the body could possibly make from food, so the same blood must travel in a circle. Harvey also pressed on the veins of a volunteer's arm and showed that their valves let blood flow only toward the heart. What he could not see was how blood got from the smallest arteries to the smallest veins. In 1661 the Italian Marcello Malpighi, using a microscope on a frog's lung, saw them: capillaries linking arteries to veins.

A double pump

The heart has four chambers. Two upper chambers, the atria, receive blood. Two lower chambers, the ventricles, pump it out. The right side sends oxygen-poor blood to the lungs to pick up oxygen and drop off carbon dioxide. The left side sends oxygen-rich blood out through the aorta to everywhere else. Because it must push blood through the entire body, the left ventricle has the thickest, most muscular wall. Valves between the chambers and at the exits act as one-way doors, and the "lub-dub" sound a doctor hears through a stethoscope is the sound of those valves snapping shut.

Each beat begins inside the heart itself. A small cluster of cells in the right atrium, the sinoatrial node, fires on its own about once a second and acts as the heart's natural pacemaker. Nerves and hormones speed it up or slow it down, but they do not start it. That is why a transplanted heart, whose nerves have been cut, still beats on its own.

Arteries, veins and capillaries

Vessels are named by direction, not by oxygen. An artery carries blood away from the heart; a vein carries it back. Arteries have thick, elastic, muscular walls that stretch with each surge of pressure. Veins have thinner walls and low pressure, and in the legs they rely on valves and on the squeezing of nearby muscles to move blood upward. Between them lie the capillaries, with walls only one cell thick, where oxygen, nutrients and wastes pass between blood and tissue. The rule about direction explains a puzzle: the pulmonary artery, leaving the right ventricle, carries oxygen-poor blood, because it carries blood away from the heart.

What blood is

An adult has about five liters of blood. A little over half is plasma, a pale fluid of water, salts, proteins, nutrients and hormones. Most of the rest is red blood cells, packed with the protein hemoglobin, which carries oxygen. White blood cells, the subject of Level 5, defend the body, and platelets, tiny cell fragments, start the clotting that seals a cut.

Pressure and heart disease

Blood pressure is written as two numbers, such as 120/80. The higher number is the pressure while the heart contracts; the lower is the pressure while it relaxes between beats. High blood pressure usually causes no symptoms, yet over years it strains the heart and damages vessels, raising the risk of stroke and heart failure. In atherosclerosis, fatty deposits called plaques build up in artery walls. A heart attack usually happens when a clot forms on a ruptured plaque and blocks a coronary artery, one of the vessels that feed the heart muscle itself. Starved of oxygen, that muscle begins to die within minutes to hours. According to the World Health Organization, this kind of heart disease is the leading cause of death worldwide. Smoking, high blood pressure, high cholesterol, diabetes and inactivity all raise the risk.

Worked example: one red cell's journey

Follow a red blood cell from a vein in your foot. It reaches the right atrium, drops into the right ventricle, and is pushed through the pulmonary artery to a lung capillary, where it unloads carbon dioxide and fills with oxygen. It returns through a pulmonary vein to the left atrium, falls into the left ventricle, and is blasted into the aorta. From there it might travel to a coronary artery and feed the heart muscle that just launched it. At rest the whole loop can take about a minute. Now imagine a plaque in that coronary artery cracks and a clot blocks the way. The muscle downstream gets no oxygen, which is why, in a heart attack, every minute before the artery is reopened costs heart muscle.