The heart supplies blood to the body. Blood carried important materials which are required for the body to function and to keep healthy.

Humans have a double circulatory system. In the first circuit, deoxygenated blood is pumped to the lungs to be oxygenated, and then returns to the heart. In the second circuit, oxygenated blood is pumped around the body by the heart and blood vessels. The heart is located in the ribcage near the lungs. The heart continuously pumps in a consistent rhythm, and electrical impulses in the heart ensure it beats with a constant rhythm and rate. The heart rate can increase and decrease depending on how much blood the body requires, for example, when you’re exercising, the heart beats faster in order to pump more blood around the body, whereas when you’re resting, the heart beats slower.

The heart has four chambers, the small upper chambers are called atria, and the lower chambers are called ventricles. The chambers are: the right atria, left atria, right ventricle, and left ventricle. The heart also has four valves; these are like little doors that only open one way; this ensures that the blood is flowing in the right direction and doesn’t flow backward. The four valves are the tricuspid valve, pulmonary valve, mitral valve, and aortic valve.
The superior and inferior vena cava are large veins that deliver deoxygenated blood into the heart. The superior vena cava delivers blood into the heart from the superior part of the body; this includes the head, neck, arms, and chest. The inferior vena cava delivers blood into the heart from inferior parts of the body; this includes organs in the abdomen, legs, feet, etc.

The pulmonary vein carries oxygenated blood from the lungs to the heart. The aorta is the main artery that carries oxygenated blood away from the heart to the rest of the body. The pulmonary artery carries deoxygenated blood from the heart of the lungs.

Blood is transported around the body via blood vessels. Blood vessels play an important role in the body. They help to maintain the body’s blood pressure and deliver oxygen and nutrients to the cells around the body. They also help to transport waste products away.

Our body has different types of blood vessels, which all have different functions.

There are 3 types of blood vessels:

• Artery  
• Vein
• Capillary

Artery 

Arteries carry blood away from the heart. This blood contains nutrients and water. Arteries supply this blood to the rest of the body. Arteries also transport blood from the heart to the lungs to be oxygenated.

For example, the biggest artery of the heart is the aorta. This helps transport oxygenated blood to the body.

Arteries have thick muscular elastic walls that constrict and expand. They have a narrow lumen, and blood in the arteries flows at high pressure. Arteries have no valves, and they’re situated very deep into the skin. The thick walls of the arteries help to withstand the high pressure, and ultimately this means that valves aren’t required. Arteries branch into smaller vessels known as arterioles. Arterioles connect to capillaries, and their function is to transport blood to the capillary beds. Arterioles are smaller than arteries and bigger than capillaries.

Capillary 

We can think of capillaries as an exchange center. They are tiny blood vessels that allow oxygen, carbon dioxide, nutrients, and water to move in and out of the bloodstream. Capillaries are located between the arteries and veins. They allow the exchange of materials between the blood and the surrounding cells.

Capillaries receive oxygen and nutrients and release waste products to be sent away. Capillaries are found around the body.

For example, capillaries are found in the liver and kidneys.

Capillaries are one-cell thick. The thin walls help with the diffusion of nutrients and waste. They have a narrow lumen. Blood flowing through is at low pressure, and there are no valves. Capillaries are connected to arteries and veins.

Vein

Veins carry blood to the heart, so they transport deoxygenated blood, carbon dioxide, and other waste from the body back to the heart.

For example, the pulmonary vein and vena cava.

Veins have thin walls and a wide lumen. The blood flowing through the veins is at low pressure; therefore, the blood requires some help to ensure that the blood doesn’t flow backward. This is why there are valves. The valves ensure that the slow-moving blood only flows in one direction. Veins are located superficially on the skin. Veins branch into smaller vessels known as venules. Venules connect to capillaries, and their function is to transport blood away from the capillary beds. Venules are smaller than veins and bigger than capillaries.

How does blood flow through the blood vessels?

An artery transports blood away from the heart to the surrounding body, for example aorta of the heart. The blood then flows through the artery into the smaller branch called the arterial. The blood then flows through the capillary bed – this is where exchange occurs between the blood and the cells. The capillaries identify useful and waste products, so nutrients and oxygen from the blood are delivered to the surrounding tissue. The blood left will contain carbon dioxide and waste products. This is transported to the vein via its small branch called the venule. An example of a vein is the vena cava which transports deoxygenated blood to the heart.