The available surface area, in my eyes, takes into consideration the blood flow to that area. So if you put it into a muscle, the muscle depends on what the muscle is doing. If it's a more active muscle, it's going to get a greater blood flow than it takes it away. You've got five liters of blood coming out of the heart every minute. That five liters isn't distributed evenly throughout the tissues. Your brain gets 20%, kidneys get 20%, muscles get about 25%. And lungs 100? Well, from the right hand side, yeah, exactly. But it's not really. It's just passing through the train station of the lungs to pick up