This model shows a simplified version of the way the body processes a drug, in this case, aspirin. This model assumes that the body is one homogenous compartment with instantaneous drug distribution, hence one-compartment.
The green box is the stock that represents the mass of the aspirin that's immediately available in the plasma. Its initial value is the mass of two aspirin: 2 pills * 325 mg * 1000 μg/mg, the conversion factor to μg. The flow from aspirin in plasma represents elimination. It is proportional to the amount of aspirin in plasma, times an elimination constant, which is represented as a pink circular term. The rate of change of the drug leaving the system is proportional to the quantity of the drug in the system.
Let's take a closer look at what components go into elimination. The elimination constant has a red arrow going to it from the pink term "half life." In this model, the half-life is estimated at 3.2 hours. The relationship between the elimination constant and the drug's half-life is shown in Nova with the following equation: - Math.log(.5)/half_life.
The output from this model is shown as a graph of the term plasma concentration, which shows the amount of aspirin in plasma divided by the plasma volue, 3000 ml. The graph shows that the concentration of aspirin in the plasma is initially around 217 μg/ml, then decreases exponentially.