Respiration, Part I

pulmonary and systemic circuitsBreathing and respiration aren’t the same thing, but they go hand in hand. Breathing is a simple mechanical process whereby air enters and exits the lungs due to volume and hence, pressure changes.

Respiration* is a more complex process of delivering oxygen to the tissues for cell metabolism and carbon dioxide to the lungs for removal. The image at left shows blood being oxygenated at the lungs (red) and traveling through the heart to the systemic circuit. The gas exchange at the alveoli/blood interface is called external respiration. Gas exchanges between blood and tissue cells is termed internal respiration. Deoxygenated blood (blue) flows from the tissues in the systemic circuit back to the heart, the lungs, and CO2 is exhaled. As seen in the illustration, the cardiovascular system (the heart and blood vessels) plays a mechanical role in transporting gases, but partial pressures are also involved.


The total pressure exerted by a mixture of gases is the sum of the pressures exerted independently by each gas in the mixture.

The pressure exerted by each gas, its partial pressure, is directly proportional to its percentage in the total gas mixture (Marieb 745).

The air you breathe is a mixture of gases, composed primarily of nitrogen (N2), oxygen (O2), carbon dioxide (CO2), and water vapor (H2O). Each gas exerts a pressure as if the other gases were not present, and these partial pressures (written Pgas) can be summed to determine the total pressure (760 mm Hg at sea level). As the amounts of gases in a mixture vary, their partial pressures will vary, as they are ratios of the total pressure. Thus, if you add oxygen to a mixture, its partial pressure will increase (given a constant volume). If you were to increase or decrease the pressure on this system, the partial pressures of the gases would increase or decrease, respectively, as well. I will discuss this more later, but now look at this table:



(at sea level)


(at sea level)

(mm Hg)
(mm Hg)
N2 596 78.4 569 74.9
O2 160 21.05 104 13.7
CO2 0.3 0.04 40 5.2
H2O 3.7 0.49 47 6.2
TOTALS 760 mm Hg 100.0% 760 mm Hg 100.0%

We know already that gases flow from areas of higher pressure to those of lower pressures by diffusion. This is how smells migrate across a room. Looking at the table, then, you can see that nitrogen and oxygen gases will rush into the lungs and carbon dioxide and water vapor will rush out. There exist steep partial pressure gradients between the atmosphere and the lung alveoli which insure this. Because blood is a liquid and air is composed of gases, it is important to mention Henry’s law:


The amount of gas that will dissolve in a liquid varies directly with the pressure above that liquid (Hein 277). Or,

When a mixture of gases is in contact with a liquid, each gas will dissolve in the liquid in proportion to its partial pressure (Marieb 745).

High pressures force gas into solution/dissolution. However, solubilities and temperatures also come into play when considering Henry’s law. So – even though a huge PN2 gradient may exist between the air and plasma, the fact is, nitrogen is barely soluble at all.

* My discussion of “respiration” does not venture into cellular respiration, the metabolic process of ATP generation. Sorry.

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