Cardiovascular system

The cardiovascular system consists of the heart, blood vessels, and blood that is transported through the blood vessels. The system performs functions that are essential for life, pumping oxygen, nutrients, hormones, and cellular waste products around the body. The pump for the cardiovascular system is the heart, a relatively small, fist-sized muscular organ that can pump over 5 litres of blood through the body every minute.

The Heart

The heart is a muscular organ that supplies blood to the body by rhythmic contractions and thereby ensures blood flow to all organs. It functions like a pump and is divided into four chambers, known as the left and right atria and the left and right ventricles. The heart contracts and relaxes at each heartbeat. When the muscle contracts (systole), blood is pumped from the heart into the arteries. When the heart relaxes (diastole), it fills with blood.

Circulatory loops

The heart pumps blood through two primary circulatory loops in the human body: the pulmonary circulation and the systemic circulation. The pulmonary circulation transports blood from the right side of the heart to the lungs, where it is replenished with oxygen and carbon dioxide is removed. The systemic circulation pumps oxygenated blood from the left side of the heart and sends oxygen-rich blood to body tissues.

Blood vessels

There are three types of blood vessels:

  • Arteries—carry blood away from the heart
  • Capillaries—carry blood through the tissues
  • Veins—carry blood back to the heart


Arteries carry blood away from the heart. Every cell needs oxygen and nutrients to function. As a rule, the arteries function to carry blood rich in oxygen (oxygenated) and nutrients from the heart to where they are needed in the body tissues. Only the pulmonary arteries carry deoxygenated blood from the heart back to the lungs for replenishment with oxygen. Blood and oxygen are supplied to the heart tissue via the coronary arteries.

Two arteries carry blood directly from the heart:

  1. The aorta (main artery) transports oxygen-rich (oxygenated) blood from the left ventricle of the heart to the body; it is the largest artery in the human body, with a diameter of about 3 centimetres.
  2. The pulmonary artery carries oxygen-depleted (deoxygenated) blood from the right ventricle of the heart into the lungs. The blood returns, enriched with oxygen, back into the left atrium of the heart.

Arteries have thick muscular walls and carry blood under high pressure. Their structure keeps the blood pressure generated by the heart as stable as possible.

Arterioles are narrower arteries that branch off from the ends of arteries and carry blood to capillaries. Capillaries are the smallest of the body’s blood vessels and are what connect the arteries to the veins. The blood flows away from the heart via the arteries which divide up and branch out into arterioles and then branch further still into thin capillaries. After oxygen and nutrients have been delivered to the tissues, these capillaries then join and widen to become venules, which, in turn, widen and converge to become veins that return blood back to the heart.


Veins are the blood vessels that carry blood from tissues back to the heart. The veins of the systemic circulation transport deoxygenated blood, and those of the pulmonary circulation (from the heart to the lungs and back again) carry oxygenated blood. Deoxygenated blood is darker than oxygenated blood.

Blood is under lower pressure in the veins than in the arteries. Veins therefore have valves and require muscular movement to move blood through them and prevent it flowing back in the opposite direction.


The cardiovascular system is a closed system, i.e. blood should not leave the system. The blood coagulation system is designed to ensure that any injuries to the system are ‘plugged’ as quickly as possible. Platelets play a key role in the process of ensuring bleeding is stopped—a process known as haemostasis. Platelets are largely inactive in the blood until they come into contact with damaged tissue or a leak from a blood vessel—a wound. Once active, platelets become ‘sticky’ and are involved in the release of ‘clotting factors’ that begin to produce the protein fibrin to act as structure for the blood clot.