Lecture 16 blood and oxygen association curves. The simplest way to move respiratory gases is by dissolution. Diffusion is metabolically free, random movement of molecules. Oxygen will move along a gradient in the blood to the active tissue where oxygen is being consumed. However, the quantities of gas that can be moved around in this way will be limited by the solubility of the gas concerned. Solubility of o2 in water (especially warm salty water) not enough to provide o2 to active tissues. Many organisms use respiratory pigments to bind o2 and transport it to tissues. Air diffuses rapidly and easily into the blood across blood capillaries. Oxygen will enter blood system in solution, however respiratory pigment grabs onto oxygen bound and no longer in solution. Partial pressure of oxygen in the blood decreases, and this allows more to diffuse across and more to bind to pigment.