Biology » Biology 105: Animal Systems » Circulatory Systems In Animals

The Heart and Associated Blood Vessels

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The Heart and Associated Blood Vessels

External Structure of the Heart

The heart is a large muscle, about the size of your clenched fist, that pumps blood through repeated rhythmic contractions. The heart is situated in your thorax, just behind your breastbone, in a space called the pericardial cavity. The heart is enclosed by a double protective membrane, called the pericardium. The region between the two pericardium layers is filled with pericardial fluid which protects the heart from shock and enables the heart to contract without friction.

The heart is a muscle (myocardium) and consists of four chambers. The upper two chambers of the heart are called atria (singular= atrium). The two atria are separated by the inter-atrial septum. The lower two chambers of the heart are known as ventricles and are separated from each other by the interventricular septum. The ventricles have more muscular walls than the atria, and the walls of the right ventricle, which supplies blood to the lungs is less muscular than the walls of the left ventricle, which must pump blood to the whole body.


Clench your fist – the size of your fist is more or less the size of your heart.

In addition, there are a number of large blood vessels that carry blood towards and away from the heart. The terms `artery’ and `vein’ are not determined by what the vessel transports (oxygenated blood or deoxygenated) but by whether the vessel flows to or from the heart. Arteries take blood away from the heart and generally carry oxygenated blood, with the exception of the pulmonary artery. Veins transport blood towards the heart and generally carry deoxygenated blood, except the pulmonary vein.

On the right side of the heart, the superior vena cava transports deoxygenated blood from the head and arms and the inferior vena cava transports deoxygenated blood from the lower part of the body back to the heart, where it enters the right atrium. The pulmonary artery carries deoxygenated blood away from the right ventricle of the heart towards the lungs to be oxygenated. On the left side of the heart, the pulmonary vein brings oxygenated blood from the lungs towards the left atrium of the heart and the oxygenated blood exits the left ventricle via the aorta and is transported to all parts of the body.

Since the heart is a muscle, and therefore requires oxygen and nutrients itself to keep beating, it receives blood from the coronary arteries, and returns deoxygenated blood via the coronary veins.

Did You Know?

In humans, the left lung is smaller than the right lung to make room in the chest cavity for the heart.

The Heart and Associated Blood Vessels

The external structure of the heart: the major part of the heart consists of muscles and is known as the myocardium. The region in which the heart is found is known as the pericardial cavity, which is enclosed by the pericardium.

Internal Structure of the Heart

As previously mentioned, the heart is made up of four chambers. There are two atria at the top of the heart which receive blood and two ventricles at the bottom of the heart which pump blood out of the heart. The septum divides the left and right sides of the heart. In order to make sure that blood flows in only one direction (forward), and to prevent backflow of blood, there are valves between the atria and ventricles (atrioventricular valves). These valves only open in one direction, to let blood into the ventricles, and are flapped shut by the pressure of the blood when the ventricles contract.

The tricuspid valve is situated between the right atrium and the right ventricle while the bicuspid/ mitral valve is found between the left atrium and the left ventricle. Strong tendinous cords (chordae tendineae) attached to valves prevent them from turning inside out when they close. The semi-lunar valves are located at the bottom of the aorta and pulmonary artery, and prevent blood from re-entering the ventricles after it has been pumped out of the heart.

The Heart and Associated Blood Vessels

The internal structure of the mammalian heart.

Video: The Heart GCSE Science Revision

This video shows the passage of blood through the heart and around the body.

In the previous topics, we have discussed pulmonary and systemic circulation, and we have described the four chamber structure of the heart as well as some of the major arteries and veins that transport blood towards and away from the heart. In order to summarise all this information, study the flow diagram below which describes the passage of deoxygenated blood through one full cycle.

The Heart and Associated Blood Vessels

Flow diagram depicting movement of blood from the heart through the circulatory system. The blue boxes represent deoxygenated blood, the purple boxes represent capillary networks where gaseous exchange occurs and the red boxes represent stages at which the blood is oxygenated.


Memory trick: the tRI cuspid valve is found on the RIght side of the heart.

Optional Video: How the Heart Actually Pumps Blood

For most of history, scientists weren’t quite sure why our hearts were beating or even what purpose they served. Eventually, we realized that these thumping organs serve the vital task of pumping clean blood throughout the body. But how? In this TED-Ed video, Edmond Hui investigates how it all works by taking a closer look at the heart’s highly efficient ventricle system.

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