A hemoglobin molecule can carry one oxygen or one carbon monoxide molecule. Suppose that the two types of gases arrive at rates 1 and 2 and attach for an exponential amount of time with rates 3 and 4, respectively. Formulate a Markov chain model with state space {+, 0, -} where + denotes an attached oxygen molecule, - an attached carbon monoxide molecule, and 0 a free hemoglobin molecule and find the long run fraction of time the hemoglobin molecule is in each of its three states.

I don't know how to set up the Markov chain.