They are enzymes. They are manufactured by the pancreas. They help stabilize fat-water emulsions. A hormone produced by the epithelial lining of the stomach is a. The lining of the alveoli is only one cell thick.
Gas exchange takes place across moist membranes.
The total exchange surface area is relatively large. The concentration of CO 2 is higher in the air than in the alveolar capillaries. The gases move across the exchange membranes by diffusion. The osmotic pressure of the interstitial fluid is greater than that of the blood. The osmotic pressure of the interstitial fluid is greater than the hydrostatic pressure of the blood.