The heart receives all of the blood from around the body into the right atrium. The thoracic duct receives the lymph fluid from the lymph circulation and it empties into the anterior vena cava just before it empties into the upper right chamber or the heart (right atrium). Here the blood then goes through the tricuspid or right A / V valve to the right ventricle. The wall of this ventricle is quite thin because the pressure under which it works is very low as compared to the other side of the circulation and the heart.
The blood then goes through the pulmonary valve into the pulmonary artery where it goes through gradually smaller vessels until it travels through the capillaries that surround the air sacs (alveoli) of the lungs where the oxygen that is transported through through the respiratory system combines with the hemoglobin of the red blood cell and the dissolved carbon dioxide is evacuated through the air sacs of the lungs and exhaled through the larger and larger tubes of the lungs called bronchioles and bronchi and finally the trachea or wind pipe. The oxygenated blood is then bought through larger and larger veins from the pulmonary circulation to the pulmonary veins that connect and the well oxygenated blood is then emptied into the left upper chamber (left atrium) of the heart. The blood then travels through the bicuspid or mitral valve and enters the left lower chamber of the heart (the tricuspid valve) where it is then pumped out through the aortic valve into the aorta. Here the first vessels that receive the riches oxygenated blood are the coronary arteries, or the arms to the heart.
The heart does all of the work or circulation and so it needs the best blood that the body can produce. This is why the coronary arteries come off the aortic arch right after the blood is pumped from the left ventricle. The wall of the left ventricle is many times thicker than the wall of the right side of the right ventricle. The blood pressure is greatest here in the left ventricle because of the force that has to be moved by the lower left chamber of the heart to pump the blood back out to the entire body where the circulation becomes progressively further and further from the center of the heart and the size of the vessels become smaller and smaller in order to get the used blood back for resupplying the hemoglobin molecules with oxygen. The entire process of circulation takes approximately one minute from the right side of the heart back to the right side of the heart again. This process repeats itself thousands of times an hour and millions of times in a day. Without the proper circulation in all of the limbs and the extremities occurring regularly, the cells and tissues of the body will shrivel up and die. The blood also carries all of the nutrients the various cells of the body need and transports waste that is produced by the cells from normal functioning to the liver where it is detoxified and eliminated in the bile or in the urine as nontoxic waste through the bowel or through the kidneys and the urine.