This file has a description of how to solve the first problem in the diffusion in osmosis problem set. This is a fixed law problem. You should understand the terms in the fixed law equation. J is the rate of solute transfer across the membrane. T is the permeability coefficient for the solute. A is the area of the membrane. C1 and C2 are the concentrations of the solute on the two sides of the membrane. For this problem, what is J? Carefully read the question. Considering exercise, the concentration of oxygen in the blood returning to the lungs, C2, decreases twofold. Concentration of oxygen in the alveolar air spaces of the lungs is the same during exercise and while at rest. When using fixed law to describe the movement of oxygen from the alveolar lie into the blood, which of C1, P and J is expected to increase during exercise. For this problem, J is the rate of movement of oxygen from the alveolar air spaces in the lungs to the blood of the vasculature. For this problem, what is C2? C2 is the concentration of oxygen in the blood returning to the lungs. For this problem, what is C1? C1 is the concentration of oxygen in the alveolar air spaces of the lungs. The problem states that the concentration of oxygen in the alveolar air spaces is the same during exercise and while at rest. We can also eliminate P, the permeability coefficient for oxygen. Permability coefficient depends on the molecular features of oxygen molecules such as size, and the ease with which oxygen dissolves in the lip bilayer of cell membranes. These properties do not change during exercise, so the value of P does not change during exercise. The solution to problem 1 is choice A, J. J will increase during exercise. The problem states that during exercise, the concentration of oxygen in the blood returning to the lungs decreases twofold. The concentration of oxygen in the blood returned to the lungs is lower during exercise because skeletal muscle tissue uses oxygen at a greater rate and there is less oxygen returned to the lungs by the blood. Think about fixed law. Fix law states that the rate of movement of oxygen from the lungs to the blood depends on the concentration difference of oxygen between the alveolar air spaces and the blood delivered to the lungs. Since C2 decreases during exercise, concentration difference C1-C2 will increase. Evening gradient from the lung air to the blood is increased, therefore the rate of movement of oxygen into the blood will be increased during exercise. So choice A, J, is correct. The rate of movement of oxygen molecules from the alveolar air spaces to the blood will increase during exercise compared to when you are at rest.