Internal respiration takes place in the tissues of the body with the exchange of gases between systemic capillaries and surrounding tissue fluid, according to the McGraw-Hill College Division. In contrast, external respiration is the exchange of gases between air in the alveoli of the lungs and blood in pulmonary capillaries.
Internal respiration cleanses the tissue fluid of carbon dioxide in exchange for the oxygen carried by hemoglobin molecules in the blood. As Dr. Hugh Potter of Union County College explains, carbon dioxide diffuses across capillary walls to the blood due to the partial pressure of the gas. Gases follow a net movement along a pressure gradient from high to low concentrations. Oxygen in the blood follows this same law, diffusing into the surrounding tissue fluid as a result of low oxygen concentrations in the tissue. Within the cells of tissue, the delivered oxygen is used to carry out aerobic cellular respiration. Carbon dioxide is formed as a by-product of this biochemical process, notes Carl Rod Nave of Georgia State University.