All cells in the body need to have oxygen and nutrients delivered to them, and they need their wastes, including carbon dioxide, removed. These are the main roles of the circulatory system.

This system delivers both oxygen from the lungs and nutrients, using the blood, which travels to all cells in the body through a network of arteries. The arteries eventually end up at very tiny blood vessels called capillaries. The capillaries have very thin walls where the blood flows more slowly. Through them, in the small intestine, the blood gathers food nutrients which will get delivered to every cell.

Waste products are also brought into the capillaries from various organs. After these transfers through the very thin capillaries, these vessels lead back to larger and larger veins as the deoxygenated blood returns to the lungs for exhalation, where the carbon dioxide is dumped and oxygen absorbed. The blood also travels to the organs that remove wastes from the blood and eliminate them from the body. For example, excess water is filtered out by the kidneys, and toxins are removed from the blood by the liver.

The heart pumps blood around the body. It sits inside the chest, in front of the lungs and slightly to the left side. The heart is actually a double pump made up of four chambers, with the flow of blood going in one direction due to the presence of the heart valves. The contractions of the chambers make the sound of heartbeats.
On the right side of the heart is an upper chamber called the atrium that takes in deoxygenated blood that is loaded with carbon dioxide. The blood is squeezed down into the right lower chamber, the ventricle, and is then pumped through the pulmonary artery to the lungs where the carbon dioxide is replaced with oxygen.

On the left side of the heart, the newly oxygenated blood comes back to the heart, this time entering the left upper atrium. It is then pumped into the left lower ventricle, and into the aorta, the artery that starts the blood on its journey around the body once more.
You can see an animation of this process on our heart page.

The heart sounds include the movement of blood in and out of the four chambers. We hear the sounds of the valves between chambers opening and closing in synchronization between left and right. It isn't actually the sound of the valves closing which we can hear; the noise is created as the closure of the valves causes a vibration in the heart walls which travels through surrounding tissues until it reaches the chest wall, where it can be heard with a stethoscope. It makes a lub - dub' sound.

 


The blood vessels have a range of different sizes, depending on their role.
  • Arteries
    Oxygenated blood is pumped from the heart along muscular arteries. They divide like tree branches until they are very slender. The largest artery is the aorta, which exits the left lower ventricle, taking the newly oxygenated blood on its way around the body. The only artery that picks up deoxygenated blood is the pulmonary artery, which runs between the lungs and heart.

  • Capillaries
    The arteries eventually divide down into the smallest blood vessel, the capillary. Capillaries are so small that blood cells can only move through them one at a time. Oxygen and food nutrients pass from these capillaries to the cells. Capillaries are also connected to veins, so wastes from the cells are transferred back into the blood.

  • Veins
    Veins have one-way valves instead of muscles, to stop blood from running back the wrong way. Generally, veins carry deoxygenated blood from the body back to the heart, where it can be sent to the lungs. The exception is the network of pulmonary veins, which take oxygenated blood from the lungs to the heart.

Blood pressure refers to the amount of pressure inside the circulatory system as the blood is pumped around.


Common Circulatory System Problems:
  • Aneurysm – a weak spot in the wall of an artery
  • Atherosclerosis – a narrowing of the arteries caused by plaque deposits
  • Heart disease – lack of blood supply to the heart because of narrowed arteries that feed the heart oxygen
  • High blood pressure – often caused by obesity
  • Varicose veins – problems with the valves that stop blood from running backwards.



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