The heart and the circulatory system

The heart’s four chambers function like a double-sided pump. The heart needs to work at just the right rhythm. It is divided into right and left halves. These halves are further divided into atria and ventricles. The right ventricle pumps blood to the lungs whereas the left ventricle pumps blood to everywhere else in the body. For this reason, the left ventricle has the thickest muscle mass of all the heart's chambers.

Veins carry blood both to the right and left atria at the same time. The cycle begins when the two atria contract, which pushes blood into the ventricles. After this, the ventricles contract, which forces the blood out of the heart.

The valves between the atria and the ventricles prevent blood from returning to the ventricles. When the heart rests between its beats, the valves between the ventricles and arteries prevent the blood from flowing back into the heart. However, there are no valves between the atria and the veins.

In order to understand the function of the circulatory system properly, it is important to remember the following three things:

• The heart has two halves. There is no movement of blood between them.
• There are three main types of blood vessels. Arteries carry blood from the heart, whereas veins carry blood to the heart. The capillaries are small blood vessels that are located between cells and larger blood vessels.
• During one heartbeat, all the following things happen:

As the heart rests, the inferior vena cava and the superior vena cava carry carbon dioxide-rich blood from all over the body to the right atrium. At the same time, oxygen-rich blood is carried from the lungs to the left atrium. From the atria, blood flows immediately to the right and left ventricles.

Pulmonary arteries carry the blood into the lungs where carbon dioxide is replaced by oxygen. This takes place in the pulmonary capillaries, where carbon dioxide is removed from the blood and oxygen is bound to red blood cells. This oxidised blood returns to the heart via the pulmonary veins.

From the left ventricle, the blood, which is now under a high pressure, exits the heart via the aorta. The arteries that branch from the aorta carry the blood everywhere in the body. Nutrients and oxygen are released to all cells of the body through the thin walls of capillary blood vessels.

The blood flow from the left ventricle of the heart to the right atrium is called systemic circulation. The circulation of blood in the lungs (right ventricle → lungs → left atrium) is called pulmonary circulation.

Cell respiration uses the oxygen transported by the blood and produces toxic carbon dioxide. The inferior and superior vena cava transport blood containing carbon dioxide into the right atrium.

Arteries are the blood vessels that transport blood away from the heart. When the ventricles of the heart contract, blood is transported into the arteries. Arteries have thick muscular walls, which help them withstand high pressures caused by blood leaving the heart.

The largest artery is the aorta, which begins from the heart's left ventricle. The aorta branches above the heart into several arteries, which continue to branch into thousands of smaller arterioles throughout the body.

Arterioles connect with even smaller blood vessels called capillaries. You can see these capillaries, for example, in the white of your eyes. Capillaries distribute oxygen and nutrients into the surrounding cells.

All harmful substances produced in the body are transported from the intercellular space back to the capillaries for further transport.

Veins are the blood vessels that transport blood to the heart. They carry oxygen-poor blood, and therefore appear as blue through the skin. Little pressure remains by the time blood leaves the capillaries and enters the venules. The flow of blood back to the heart is referred to as venous return. It is dependent on muscle action.

Muscle contractions that occur near the veins help to squeeze the blood through venous valves. The purpose of these valves is to prevent blood from flowing in the wrong direction.

The lymphatic system consists of lymphatic vessels and lymph nodes. Blood plasma leaks into the body's tissues through the thin walls of capillary blood vessels. The purpose of the lymphatic system is to remove these fluids and materials from tissues, returning them to the bloodstream via lymphatic vessels. The lymphatic system also plays a key role in transporting fats into your cells from the intestines.

The structure of the lymphatic vessels resembles that of the blood vessels, as the lymphatic vessels also have valves that cause lymph to eventually flow forward instead of travelling backwards. The contraction of your muscles and the pressure inside the arteries acts as a kind of pump that helps the fluid move around your body. Exercise can help the lymphatic system to function more effectively.

The lymph passes through lymph nodes before returning to the bloodstream via veins located near the heart. Lymph nodes can be located, for example, in the armpits and the groin. They are rich in white blood cells that fight microbes that may have entered your lymphatic system. This explains why your doctor may test your neck to see if your lymph nodes are swollen when you are sick.