External respiration is the exchange of gases between the air sacks uh the alvi of the lungs and the blood capillaries in the lungs. Oxygen goes from an area of high partial pressure this would be the alvei to an area of lower partial pressure uh the capillaries of the lungs. The partial pressure of oxygen in the alvei is say 104 that in the deoxxygenated blood arriving at the capillaries is 40. So oxygen will go from an area of high partial pressure to low from the air sacks into the blood capillaries of the lungs. Carbon dioxide will move in the opposite direction although it also moves from an area of high partial pressure to low. Its high partial pressure is in the deoxxygenated blood arriving at the lungs capillaries where its partial pressure is say 45 uh millimeters of mercury. It has a lower partial pressure in the alvoli, the air sacks of the lungs, say 40 millimeters of mercury. So when blood reaches the lungs, carbon dioxide goes from an area of high partial pressure, the deoxxygenated blood arriving in the lungs to the airsacks. So carbon dioxide will leave the blood and enter the airsacks to be exhaled. Obviously both of these processes are occurring simultaneously. So while oxygen is going from the air sacks into the blood where it binds hemoglobin uh carbon dioxide is leaving the blood and going to the airsack so that it can be exhaled. So oxygen and carbon dioxide are moving in opposite directions. In internal respiration oxygen will move from an area of high partial pressure to low but at this time it will leave the blood and go to uh the surrounding tissues. The partial pressure of oxygen in oxygenated blood is say around 104 um millimeters of mercury and that in tissues can be 40 or if this is an exercising tissue 20 or lower. And so oxygen will leave the blood going from an area of high partial pressure to low. Uh the portion of the oxygen which is traveling as a gas that's around 1.5% can simply go directly to the surrounding tissues. Otherwise uh it leaves hemoglobin to become a gas before leaving uh the blood. So it always travels as a gas and it can leave hemoglobin uh to become the gaseous oxygen which then leaves the blood. Carbon dioxide travels in the opposite direction. uh it has a high concentration in the tissues where it may be say 45 uh millimeters of mercury and a lower concentration in the uh deoxxygenated blood where or the oxygenated blood where it may be 40 millime of mercury. uh if the tissue has been exercising then the concentration of carbon dioxide in the tissue would be much higher but in any case carbon dioxide will leave the tissue and diffuse into the blood going from an area of high partial pressure to low. Obviously both of these processes are occurring simultaneously so that uh oxygen is leaving the blood and going into the surrounding tissues at the same time as carbon dioxide is going from the tissues into the blood. And so in internal respiration we see oxygen going from the blood to the tissues and carbon dioxide going from the tissues into the blood.