ABSTRACT
Passive transports are dissipative processes where the flux is directed against a certain gradient. By contrast, biology displays a number of examples where the flux takes place along the gradient. Active transport across the corneal endothelium avoids swelling of the cornea and contributes to maintain its transparency. Active transports require energy. The physiological activities are optimized at the standard pH. While the pHs in plasma and interstitium are very similar, ranging from 7.35 to 7.45, the cytoplasm is slightly more acid, with a pH from 7.1 to 7.3. About 1/200 of the flow ejected by the heart crosses the capillaries to the tissue, that is about 20 liters per day. Out of these 20 liters, 18 liters are re-absorbed by the capillaries while 2 liters enter first the lymphatic circulation and latter re-enters the capillaries. The couplings are important only when the characteristic times of the underlying physical processes are of like order.
