The circulatory(blood)system. Food is digested in the stomach and small intestine, the nutrients are absorbed into the blood. Air is exchanged in the lung, the oxygen is absorbed into the red blood cells in the hemoglobin, carbon dioxide leaves the hemoglobin and is expelled when you exhale. Throughout the circulatory system the arteries narrow down into capillaries where the watery part of the blood (the plasma) carrying the nutrients and oxygen flows out through the capillary wall into the spaces between cells. Here the plasma is known as interstitial fluid, nutrients and oxygen are taken up through the cell wall, the waste and carbon dioxide are dissolved into the fluid which is collected into the lymphatic veins and are returned to the circulatory system veins which take blood to the lungs to rid itself of the carbon dioxide and to the liver and kidney to rid itself of waste.