How Blood Circulation Keeps Your Body Supplied

The heart has four chambers. The two upper chambers, called atria, receive blood. The two lower chambers, called ventricles, push blood out. A wall separates the right and left sides, helping keep oxygen-poor blood from mixing with oxygen-rich blood. The atria contract first, moving blood into the ventricles. The ventricles then contract, creating the stronger pressure needed to send blood toward the lungs or through the entire body.

The right side sends blood toward the lungs. The left side sends blood toward the body. Valves open in one direction and close behind the moving blood, which helps prevent it from flowing backward. The familiar heartbeat sound comes largely from valves closing as the chambers contract and relax.

The heart muscle needs its own supply of oxygen too. Small coronary blood vessels run across its surface and feed the working tissue. This is a useful reminder that the pump is also a living organ with the same basic needs as the body it serves.

Blood returning from the body enters the right side of the heart with less oxygen and more carbon dioxide. The right ventricle pumps it through the pulmonary arteries toward the lungs. These arteries are unusual because they carry oxygen-poor blood.

Inside the lungs, blood passes through tiny capillaries beside air sacs called alveoli. Oxygen moves from the air into the blood, while carbon dioxide moves from the blood into the air to be breathed out. The newly oxygen-rich blood returns to the left side of the heart through the pulmonary veins.

The left ventricle then gives the blood a strong push into the aorta, the body's largest artery. From there, arteries branch into smaller vessels until the blood reaches capillaries throughout the tissues. In body capillaries, oxygen and nutrients move toward cells, while carbon dioxide and other wastes enter the blood. Veins then guide the blood back toward the heart, completing the systemic loop.

The route becomes easier to remember when you trace one red blood cell instead of memorising a pile of vessel names:

• •Body tissues to the right atrium.
• •Right ventricle to the lungs.
• •Lungs to the left atrium.
• •Left ventricle to the body.
• •Capillaries exchange gases, nutrients and wastes.

For many centuries, doctors in Europe followed ideas that went back to the Greek physician Galen. In that view, blood was made continuously in the liver from digested food and then used up by the body, rather than travelling round in a loop. In 1628 the English physician William Harvey published a careful argument against this. He estimated how much blood the heart pushes out with each beat and multiplied by the number of beats. The total was far more than a person could possibly make from food in the same time.

The only sensible explanation was that the same blood was being sent round again and again. Harvey supported this with simple experiments, such as tying a band around a person's arm and pressing along the veins. The valves let blood move only towards the heart, never away from it. One gap remained, because Harvey could not see how blood passed from arteries to veins. A few decades later, Marcello Malpighi used a microscope to observe capillaries in a frog's lung, supplying the missing link.

A blood pressure reading such as 120 over 80 contains two measurements. The first, systolic pressure, is the highest pressure in your arteries as the ventricles contract. The second, diastolic pressure, is the lowest pressure while the heart relaxes between beats. To measure them, a cuff around the upper arm is inflated until it squeezes the artery shut, then slowly released. Blood begins to spurt through at the systolic pressure, and flow becomes smooth and silent again at the diastolic pressure. The units are millimetres of mercury, because early instruments used mercury columns.

Pressure is not the same everywhere in the circuit. It is highest in the aorta, much lower in the capillaries, where thin walls allow exchange without bursting, and lowest in the veins. With so little pressure left, blood returning from the legs relies on vein valves and on surrounding muscles squeezing the veins as you move. This muscle pump explains why soldiers standing still on parade for a long time sometimes faint: blood pools in the legs, less returns to the heart and the brain briefly receives too little.

Before birth, the circulation runs differently, because the lungs are filled with fluid and cannot load oxygen. Instead, oxygen-rich blood arrives from the placenta through the umbilical cord. Sending all the blood through the lungs would waste effort, so the developing heart uses two shortcuts. An opening between the two atria, called the foramen ovale, lets much of the blood pass straight from the right side to the left. A short vessel called the ductus arteriosus links the main artery to the lungs with the aorta, diverting most of the rest away from the lungs and out to the body.

Birth switches the system over within minutes. With the first breaths, the lungs expand, resistance to blood flow through them falls sharply and much more blood returns from the lungs to the left atrium. The rising pressure there pushes a flap of tissue against the foramen ovale, closing it like a door, and it usually seals permanently over the following months. The ductus arteriosus tightens and closes within the first few days. In roughly a quarter of adults the foramen ovale never fully seals, which is usually harmless. If the ductus stays open, doctors can often close it with medicine or a small procedure.

Not every animal has a double circulation. A fish heart has two main chambers and pumps blood in a single loop. Blood goes first to the gills, where it picks up oxygen, and then flows straight on to the rest of the body without returning to the heart. Passing through the fine capillaries of the gills uses up much of the pressure, so blood reaches the body tissues relatively slowly. That works for an animal whose body temperature matches the water around it, but it would struggle to meet the high energy demands of a bird or mammal.

Your arrangement solves this by sending blood back to the heart after the lungs for a second, powerful push. The lung loop can run at low pressure, which protects the delicate air sacs, while the body loop runs at high pressure and delivers oxygen quickly. Amphibians and most reptiles sit in between. Their hearts have two atria but a single ventricle, or a partly divided one, so some mixing of oxygen-rich and oxygen-poor blood can occur. This is less wasteful than it sounds, because it lets them divert blood away from the lungs while diving or holding their breath. Crocodiles, unusually, have four chambers.

Circulation is a continuous two-loop journey. The right side of the heart sends blood to the lungs, the left side sends it to the body, and capillaries handle the quiet exchanges that keep cells supplied. Follow the direction of travel, and the whole system becomes much easier to picture.