The cell membrane is a thin semi-permeable membrane that surrounds the cytoplasm of a cell. Its function is to protect the integrity of the interior of the cell by allowing certain substances into the cell, while keeping other substances out.
Sometimes organisms need to move molecules from an area where they are highly concentrated to an area where the molecules are less concentrated. This transport is much more easily done than moving molecules from a low concentration to a high concentration. To go from a high concentration to a low concentration, in essence the molecules need to only “spread” themselves, or diffuse, across the membrane separating the areas of concentration n the human body, this action occurs in the lungs. You breathe in air, and oxygen gets into the tiniest air sacs of the lungs, the alveoli. Surrounding the tiniest air sacs of the lungs are the tiniest blood vessels capillaries. The capillaries in the lungs, called pulmonary capillaries, contain the lowest concentration of oxygen in the body, because by the time the blood gets to the tiniest vessels, most of the oxygen has been used up by other organs and tissues. So, the tiniest air sacs of the lungs have a higher concentration of oxygen than do the capillaries. That means that the oxygen from the alveoli of the lungs can spread across the membrane between the air sac and the capillary, getting into the bloodstream.
Each cell exists in a fluid environment and the