Blood vessels of the heart

Four main vessels connect the heart to the body and the lungs. To identify a vessel, follow its connection and determine whether blood enters or leaves the heart through it.

Vessels entering the heart

• The vena cava brings blood from the body into the right atrium.
• The pulmonary vein brings blood from the lungs into the left atrium.

Vessels leaving the heart

• The aorta carries blood from the left ventricle to the body.
• The pulmonary artery carries blood from the right ventricle to the lungs.

Identifying the vessels on a heart diagram

Trace each vessel to the chamber it connects to: the vena cava and pulmonary vein enter atria, while the aorta and pulmonary artery leave ventricles. The heart’s right and left are the person’s right and left; on a diagram viewed from the front, the right side of the heart usually appears on the left of the page.

A diagram may show more than one vena cava or pulmonary vein. Follow each vessel’s connection to the heart and its route to or from the body or lungs.

Double circulation

In a double circulatory system, blood passes through the heart twice during one complete circuit. It travels from the heart to the lungs and back, then from the heart to the rest of the body and back.

Pulmonary circuit: heart and lungs

The right ventricle pumps deoxygenated blood through the pulmonary artery to the lungs. There, the blood gains oxygen and loses carbon dioxide. The oxygenated blood returns to the left atrium through the pulmonary vein. This journey between the heart and lungs is the pulmonary circuit.

Systemic circuit: heart and body

Blood passes from the left atrium to the left ventricle, which pumps it through the aorta to the rest of the body. After delivering oxygen, the deoxygenated blood returns to the right atrium through the vena cava. It then passes to the right ventricle, ready to travel to the lungs again. The journey between the heart and the rest of the body is the systemic circuit.

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.