Full of oxygen, a blood cell leaves the left ventricle through a major artery known as the This artery branches and becomes smaller as it extends away from the heart. The blood cell continues through the arterioles and into the These are the smallest blood vessels and the site of gas exchange between the blood and the body tissues. The blood cell then travels back to the heart through It reaches the largest vein, where all blood returns, known as the Cava. It then enters the heart's right travels through a valve into the right and is pumped through the artery to the Here it can release carbon dioxide and pick up oxygen in another round of gas exchange. The blood cell returns to the heart through the vein and enters the left It travels through a valve into the left ventricle to begin the cycle again.​