In this example there is an inflow into the compartment of 50 units (e.g. mg - a unit is represented by a "*") per time interval. (e.g. minute, hour, day.) The outflow from the compartment is the loss of X% of the amount of the molecule in the compartment per time interval. An equilibrium will be attained when inflow and outflow are equal in quantity (i.e. are in balance.)

This equilibrium amount can be calculated, or it can be visualized using the following demonstration.

Select the % loss and initial amount of the molecule. Then click Setup. After each click of the Go One Time Interval button, the display shows the resulting amount of the molecule in the compartment.

	Select the % loss: 5% 10% 20% Select the initial amount: 1 100 200 300 400 500 600 700 800 900 1000
	Last Time Interval Amt.: This Time Interval Amt.: Time Interval:

• Does the initial amount have any effect on the equilibrium reached?
• What does affect the equilibrium which is reached?
• For what parameter combinations does it take a longer/shorter time to approach equilibrium?
• When, for any set of parameters, is the approach faster, when is it slower?
• How could one calculate the equilibrium algebraically? Note: X is the %loss, and N is the amount (the inflow is fixed at 50.)
  • What is the loss (outflow) in one time interval?
    • Hint: To multiply by a %, divide it by 100 (i.e. change it to decimal fraction) before proceeding.
  • What is the inflow each time interval?
  • What is the change in amount in one time interval?
  • What is the relationship between the loss and inflow at equilibrium?
  • Write the equation showing this relationship.
• What does it mean to approach equilibrium "from above" or "from below"?
  • Can you illustrate this using the exercise above?
• The inflow in this exercise is fixed at 50 units/time interval.
  • Does this limit the demonstration of compartment equilibrium?
    Hint: Is it the amount of inflow, or the ratio of inflow to other parameters which determines the equilibrium?
• The outflow in this model is chosen to be a % of the amount in the compartment. So it is a "linear function" of the amount.
  • Could other functions be chosen?
  • Advanced Question: Are there functions which would keep the compartment from reaching an equilibrium value?