i love my dad

A conversation with my dad:

Me: *takes his blood pressure*
Him: Why do we need to measure both blood pressure and heart rate?
Me: It controls the volume of blood that enters the kidneys.
Him: So we already have P and V, and T is more or less constant in the body. So what's the n?
Me: All the solutes in the blood. RBCs, WBCs, platelets, proteins...
Him: Great. *leaves room*
Me: How did I know that he was talking about PV=nRT?