Cardiovascular system

What is the cardiovascular system?

The cardiovascular system, also known as the circulatory system, comprises the heart and blood vessels (arteries, veins and capillaries). It is essential for delivering oxygen and nutrients to the body’s organs and cells, as well as helping to eliminate waste products from the body, such as carbon dioxide (CO₂).

Components of the cardiovascular system

Heart

The heart is a muscle located in the center of the chest that acts like a pump to circulate blood throughout the body. It is made up of four cavities, separated by a central wall called the septum. These cavities are divided into two atria at the top and two ventricles at the bottom.

The right side of the heart receives oxygen-poor, CO₂-laden blood from the organs. This blood returns to the lungs to recharge with oxygen. Here’s the path taken by this blood:

It reaches the heart via the superior and inferior venae cavae, first entering the right atrium.
It then passes into the right ventricle, which sends it to the lungs via the pulmonary artery.

The left side receives oxygenated blood from the lungs and distributes it throughout the body. This blood takes the following route:

It returns from the lungs via the pulmonary veins and enters the left atrium.
It then passes into the left ventricle and is propelled throughout the body via the aorta, the largest artery.

This pumping of blood is made possible by the electrical activity of the heart, which triggers the contraction and relaxation of the heart’s muscle cells.

To find out more about how the heart works, read our articles How does the heart work? and The electrical activity of the heart.

Blood vessels

Blood vessels form a vast network that transports blood throughout the body. There are three main types, each with a specific function:

Arteries: these carry oxygen-rich blood from the heart to the organs and tissues. The main artery, known as the aorta, starts at the heart and divides into numerous arteries to supply blood to the whole body. The walls of the arteries are thick and elastic to withstand the high pressure of the blood.

Veins: these carry oxygen-poor blood from the organs back to the heart. Veins have thinner walls than arteries and contain small one-way valves that act like flaps to prevent blood from flowing backwards.

Capillaries: these are the smallest blood vessels. They connect arteries and veins and have extremely thin walls, allowing the exchange of gases (such as oxygen and CO₂), nutrients and waste products between the body’s cells and the blood.

How does the cardiovascular system work?

The cardiovascular system comprises two main circuits that work together to transport blood throughout the body: systemic circulation and pulmonary circulation.

Diagram of the human circulatory system showing the flow of blood between the heart, lungs, brain, arms, liver, intestines, kidneys, pelvis, and lower limbs. The diagram uses blue arrows to represent oxygen-poor blood and red arrows for oxygen-rich blood. Key blood vessels and organs are labeled, including the jugular vein, pulmonary artery and vein, superior and inferior vena cava, hepatic portal vein, renal veins, iliac vein, ascending arteries, aorta, mesenteric vein, and renal vein. The exchange of oxygen (O₂) and carbon dioxide (CO₂) at the lungs, kidneys, and body tissues is illustrated, with oxygen-rich blood traveling from the lungs to the heart and body, and oxygen-poor blood returning to the heart and lungs. A legend at the bottom explains the color coding for oxygen-rich and oxygen-poor blood.

Systemic circulation

Systemic circulation carries blood from the heart to all the body’s organs. Oxygen-rich blood leaves the heart via the aorta, the largest artery, and is distributed throughout the body to supply oxygen and nutrients to the cells. Once this oxygen has been used up, the blood returns to the heart, loaded with carbon dioxide, via the venae cavae.

Several main arteries supply blood to the vital organs:

The brain: this is supplied by four major arteries branching off from the aorta, i.e. the internal carotid arteries at the front of the neck and the cerebral arteries at the back of the neck.
The lower body: the aorta divides into femoral arteries to carry oxygenated blood to the legs and feet.

Pulmonary circulation

Pulmonary circulation transports blood between the heart and the lungs to allow it to recharge itself with oxygen. Oxygen-poor blood leaves the heart via the pulmonary artery and travels to the lungs, where it passes into tiny air spaces called pulmonary alveoli. Here, it recharges with oxygen and eliminates carbon dioxide. The oxygenated blood then returns to the heart via the pulmonary veins, ready to be distributed throughout the body.

Cartoon illustration of a cute, smiling heart character holding a small heart, standing cheerfully with big eyes and a friendly expression.

In systemic circulation, oxygen-rich blood flows through the arteries to reach the organs, while oxygen-depleted blood flows back to the heart through the veins.

For pulmonary circulation, it’s the other way round: oxygen-poor blood is sent to the lungs via the pulmonary artery to recharge with oxygen, then it returns to the heart via the pulmonary veins, rich in oxygen and ready to be distributed throughout the body.

Regulation of the cardiovascular system

For the cardiovascular system to function properly, it must adapt blood flow to the body’s needs, even under stress or exertion. This regulation is carried out by nervous, hormonal and local control systems.

Nervous regulation

The autonomic nervous system, made up of the sympathetic and parasympathetic systems, helps regulate heart rate and blood pressure:

The sympathetic system increases the heart rate and the strength of the heartbeat and constricts the blood vessels, thereby raising blood pressure. This is a useful response to stress or physical exertion.
The parasympathetic system reduces the heart rate, relaxes the blood vessels and promotes relaxation, thereby lowering blood pressure.

Baroreceptors are sensors located in the artery walls that detect variations in blood pressure. They then send information to the nervous system to adjust the heart rate and dilate the blood vessels, thus maintaining stable circulation.

Hormone regulation

Certain hormones also influence blood pressure and circulation:

Adrenalin and noradrenalin: released by the adrenal glands (or suprarenal glands, above the kidneys) in response to stress, these hormones increase heart rate and blood pressure.
Renin and angiotensin II: renin, produced by the kidneys, triggers the production of angiotensin II, a hormone that causes blood vessels to narrow and releases another hormone, aldosterone. Aldosterone helps the kidneys retain sodium and water, thereby increasing blood pressure (see diagram below).

Diagram illustrating the renin-angiotensin-aldosterone system (RAAS) and its effect on blood pressure. The image shows kidneys releasing renin, which leads to the production of angiotensin II. Angiotensin II causes blood vessels to narrow and stimulates the release of aldosterone. Aldosterone acts on the kidneys to retain sodium (Na) and water (H₂O), resulting in increased blood volume and blood pressure. Arrows indicate the flow and interactions between these components.

Regulation of blood pressure by the renin-angiotensin-aldosterone system

Local regulation

Blood vessels can adjust according to immediate needs. For example, they contract or dilate to maintain a stable blood flow depending on the pressure applied. The cells in the vessel walls also release chemical substances that act directly on the dilation or contraction of the blood vessels.

In conclusion, the cardiovascular system plays a crucial role in distributing oxygen and nutrients throughout the body and eliminating waste products. To keep this system working properly and reduce cardiovascular disease, it is essential to adopt healthy habits:

Eat a balanced diet,
Limit alcohol and avoid tobacco,
Exercise regularly,
Consult a doctor for regular check-ups.