Mammalian heart structure

The mammalian heart is a muscular organ that pumps blood. Its muscular wall contracts to force blood out of the heart. A wall called the septum separates the heart into right and left sides, preventing blood from mixing between them.

Chambers and valves

Each side has two chambers. An upper chamber, called an atrium, receives blood entering the heart. A lower chamber, called a ventricle, pumps blood out. Blood passes from the right atrium to the right ventricle, and from the left atrium to the left ventricle.

One-way valves keep blood moving in the correct direction. Valves between the atria and ventricles, and at the exits from the ventricles, prevent blood from flowing backwards when the chambers contract.

Recognising the heart’s structures

In a heart diagram, look for the septum separating the two sides, then identify the atrium above the ventricle on each side. The labels left and right refer to the animal’s own left and right. In a front-view diagram, the heart’s left side therefore often appears on the right of the image.

The coronary arteries run over the surface of the heart and supply its muscle with oxygen and nutrients. This supply allows the heart muscle to keep contracting.

Arteries and veins

Blood travels away from the heart and returns to it through two main types of blood vessel:

• Arteries carry blood away from the heart.
• Veins carry blood back to the heart.

The direction of blood flow relative to the heart determines whether a vessel is an artery or a vein. A vessel carrying blood out of the heart is an artery; one bringing blood back is a vein.

This classification does not depend on whether the blood is oxygenated or deoxygenated. Some arteries carry deoxygenated blood, and some veins carry oxygenated blood.

Monitoring heart activity

The heart can be monitored by detecting its electrical activity, the pressure waves it produces in arteries, or the sounds made by its valves. Each signal gives different information about how the heart is working.

An electrocardiogram (ECG) records the heart’s electrical activity using electrodes placed on the skin. The pattern of electrical signals shows the rate of the heartbeat—how many times the heart beats in a given time—and its rhythm, or whether the beats occur regularly.

Pulse rate measures how often the heart is beating. Each heartbeat normally produces a pressure wave that travels along the arteries. The pulse can be felt where an artery is close to the skin, and the waves can be counted to find the pulse rate, usually in beats per minute.

Heart sounds can be heard using a stethoscope. The main sounds come from different sets of heart valves closing during the cardiac cycle, the sequence of events in one heartbeat. Listening to these sounds makes the heartbeat audible and gives information about valve action. The two main sounds do not represent two separate heartbeats.

Physical activity and heart rate

During physical activity, muscles need more energy and oxygen than they do at rest. The heart responds by beating faster. Heart rate is the number of heartbeats per minute.

A higher heart rate helps move more blood to the working muscles. The blood supplies oxygen and carries carbon dioxide away more quickly.

After exercise stops, the body's demand for energy and oxygen falls. The heart rate then gradually decreases towards its resting level: the heart rate before exercise.

Measuring heart rate around exercise

Heart rate can be investigated by measuring the pulse: the regular beat felt in an artery as the heart pumps blood. Count the pulse beats for one minute to find the heart rate in beats per minute.

1. Measure the pulse before exercise to find the resting heart rate.
2. Measure it during exercise and compare it with the resting rate. It should be higher during physical activity.
3. Measure it after exercise, at intervals during recovery. It should gradually fall towards the resting rate as the body's demand decreases.

Reducing coronary heart disease risk

The coronary arteries supply the heart muscle with blood. In coronary heart disease, fatty deposits build up in these arteries, narrowing them and reducing blood flow to the heart muscle. Diet and physical activity can lower the risk of this narrowing developing.

Diet and coronary heart disease risk

A healthy diet helps maintain a healthy body mass and reduces conditions that put extra strain on the heart and blood vessels. In particular, limiting excessive intake of saturated fat can help reduce the build-up of fatty deposits in coronary arteries.

Exercise and coronary heart disease risk

Regular exercise improves fitness and helps the heart and circulatory system work efficiently. It can also help maintain a healthy body mass and reduce problems associated with inactivity. Together, these effects can lower the risk of coronary heart disease.

These lifestyle choices reduce risk rather than eliminate it: they make coronary heart disease less likely, but they do not guarantee that it will be prevented.

Cardiac cycle

The heart pumps blood by repeatedly contracting and relaxing its chambers. Each repetition is a cardiac cycle. Blood returning to the heart enters the upper chambers, the atria, before passing into the lower chambers, the ventricles. Valves help keep blood moving in one direction.

Contraction and relaxation

As the heart relaxes, blood enters the atria and flows into the ventricles. The atria then contract, pushing more blood into the ventricles through the atrioventricular valves. These valves lie between the atria and ventricles.

Next, the ventricles contract. This closes the atrioventricular valves, preventing blood from flowing back into the atria. The contraction also opens the semilunar valves at the exits of the ventricles, allowing blood to be pumped into the arteries.

When the ventricles relax, the semilunar valves close. This prevents blood in the arteries from flowing back into the ventricles. The chambers fill again, and the cycle repeats.

The two sides of the heart

The right side of the heart pumps deoxygenated blood towards the lungs. The left side pumps oxygenated blood towards the rest of the body. A wall called the septum separates the two sides, preventing oxygenated and deoxygenated blood from mixing.

Heart chamber wall thickness

The heart’s chambers have muscular walls that contract to push blood onwards. The thickness of each wall reflects the pressure needed to move blood to its destination: a thicker muscular wall can produce a stronger contraction and generate greater pressure.

Atrial and ventricular walls

The atria have thinner walls than the ventricles. They only need to push blood into the ventricles, which are immediately below them, so less pressure is needed.

The ventricles have thicker walls because they must pump blood out of the heart. The right ventricle pumps blood to the lungs, while the left ventricle pumps blood around the body.

Left and right ventricular walls

The left ventricle has a thicker muscular wall than the right ventricle. Its stronger contraction generates the higher pressure needed to pump blood around the whole body.

The right ventricle pumps blood to the nearby lungs at lower pressure, so it does not need a wall as thick as the left ventricle’s.