Using Fick's Law, explain how rapid gaseous exchange takes place in a mammal
Gaseous exchange of oxygen and carbon dioxide (diffusion) occurs between the alveoli (tiny air sacs) and capillaries (microscopic blood vessels) inside the lung. Diffusion refers to the passive net movement of molecules from a high to low concentration, down a concentration gradient. The rate (speed) of diffusion is determined by Fick's Law (i.e. surface area * concentration gradient / length of diffusion pathway). The variables in the numerator are maximized and the variables in the denominator minimized, so the rate is as high as possible. I'll go through each variable separately:
- Alveoli form spherical sacs and the capillaries wrap around them, maximizing surface area
- If we allowed diffusion to continue with no changes to the environments of alveoli and capillaries, then the concentration gradient would decrease. Therefore adequate ventilation and circulation removes carbon dioxide and oxygen from the side where they're at their lowest concentration, maximizing concentration gradient.
- Both alveoli and capillaries have a thin wall (one cell thick) and are in close contact - this minimizes the distance the molecules have to travel for diffusion to take place.
These are examples of adaptations mammals use to maximize rapid gaseous exchange.
