The cardiovascular system is also sometimes called the circulatory system, and it is made up of your heart and the pathways blood takes through your body: arteries, veins, and capillaries. The flow of blood through your body is vital to the survival of all of the cells in your body – they need fuel to survive, and the blood delivers it! The blood also takes waste products from cells, which is filtered out in the liver.
Your heart is one big muscle responsible for pumping blood all day, every day. Every minute, your heart pumps about 5 liters of blood, which is the size of 5 big bottles of soda – and your body only contains 5-6 liters of blood! Blood becomes oxygenated in the lungs by the respiratory system, and the left side of your heart pumps it all over your body, where cells take the oxygen and replace it with carbon dioxide. (Read more about this process in the Respiratory System) The de-oxygenated blood travels back to the heart, where the right side pumps it through the lungs to replace the carbon dioxide with oxygen, starting the cycle over again.
Your blood contains many nutrients besides carbon dioxide or oxygen, like amino acids, proteins, iron, or even viruses or bacteria that get into your body. Doctors can tell a lot about your health by doing tests on what is in your blood. Don’t be worried if they take some of your blood – you can make more! Most of your blood cells are created in your bone marrow.
The blood you see when you scrape your knee or hunt an animal is red because of hemoglobin, the molecule that binds iron to your red blood cells, erythrocytes. Red blood cells carry oxygen and carbon dioxide through your body. However, your blood also contains white blood cells, which are part of your immune system. White blood cells, leukocytes, fight foreign objects in your blood that might be harmful to your health by engulfing them or producing antibodies to do the work. White blood cells also play a role in allergies!
When you break your skin, you might start to bleed because you have also broken some of the capillaries near the surface of your skin. To stop your cut from bleeding forever, certain parts of your blood clot, or clump together, to block up the leak, allowing your skin to heal.