What is blood plasma?

Human Physiology: What is the conceptual difference at a molecular level between the oxygen dissolved in blood plasma and the oxygen which exerts a partial pressure in the blood?

• I am teaching anesthesia residents and have never been clear on this myself. I was asked this today and couldn't think of how to describe it. Oxygen is either dissolved in minute amounts in plasma, or is bound to hemoglobin. But why oxygen content is measured as a simple concentration in gm/l and another physical quantity of oxygen is measured as a gaseous partial pressure has always eluded me. I believe it involves http://en.wikipedia.org/wiki/Henry%27s_Law:   where:   =  the equilibrium constant for the http://en.wikipedia.org/wiki/Solvation process =  partial pressure of gas in equilibrium with a http://en.wikipedia.org/wiki/Solution containing some of the gas =  the concentration of gas  in the liquid solution But at a molecular level in the bloodstream, what is the difference between a dissolved O2 molecule and one that exists as a gas exerting a partial pressure?

• Answer:

  This is a bit like the difference between temperature and heat capacity. Here partial pressure is like temperature. It's the force per area due to the number of impacts per second of oxygen molecules (the definition of pressure, but here just the pressure due to OXYGEN molecule impacts, ie, the oxygen partial pressure). so is just the intensity of oxygen molecule impacts , like rain. Some fluids and compounds store a lot more oxygen at a given oxygen partial pressure, and others less. These are the contents, but the intensity of the "rain" is the same. Some objects store more thermal energy for a given temperature, as for example water vs. air. The oxygen content (how much oxygen a volume of something contains) is related to this partial pressure number but varies from substance to distance. It's an oxygen holding capacity of substances per unit pressure. Clearly it won't be the same for plasma as for red cells, as for a given pressure, the red cells hold more (like water holds more "heat" at the same temperature). Red cells simply sop it up like little sponges. Or, if you are an electrically savvy person you can think of red cells as larger capacitors, which hold more charge for the same voltage. Voltage is here the analogous quantity to partial pressure, and now  *charge* is the thing that is analogous to oxygen content.