Arteries are larger vessels than veins. Arteries function like pipes to carry fresh oxygenated blood away from the heart toward all the tissues and organs of the body. Veins are smaller vessels that also function like pipes to carry old blood that needs to get more oxygen back to the heart. The oxygen depleted blood enters the right side of the heart called the right atrium then it’s pumped down to the right ventricle which pumps it up though the Pulmonary artery to the lungs to get oxygen and then travels back through pulmonary veins which enter the left side of the heart called the left ventricle to then be pumped back up the aorta with fresh oxygen rich blood. Then the cycle starts all over again. So, you have your Pulmonary circulation which circulates the oxygen depleted blood through and from your lungs, and you have your Systematic circulation which takes the blood to and from the rest of the body.
