Force and resistance in physiology.

Physiology is a discipline grounded in fundamental physical principles, and frequent referral to these principles is important for effective teaching. In particular, some concepts related to mechanical aspects of physiological phenomena are easier to grasp if one realizes that the development of any physical force requires resistance. This consideration is illustrated with four examples. (1) Osmotic pressure develops into mechanical pressure (turgor) only when osmotic inflow is resisted by stiff cell walls. (2) Intrapleural pressure has a simple mechanical analogy of two unbalanced weights on a block. (3) Independence of muscle tension of the total length of the fiber (the fact often omitted in standard courses) results from the pulling by neighboring sarcomeres in opposite directions. 4) Arterial pressure and its regulation by various factors is readily visualized by focusing on the mechanical tension within arterial walls. The latter approach has been was compared in class to the more traditional method based on the total peripheral resistance. The results showed that our approach resulted in an improved students' understanding.