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1.4 Oxygen Carriage, Oxygen Transport, Oxygen Cascade from atmosphere to the Cell

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Now, we know that one of the major functions of blood is transport of oxygen. We should know how.

Oxygen can be transported in two forms in the blood. It can be dissolved in the blood plasma or it can be transported by Hemoglobin.

Oxygen doesn't really want to dissolve in plasma. Only about 1.5% of our total blood oxygen is dissolved here. The actual amount is around 3ml per 1000 liters of blood. That's kind of negligible.

Almost 98.5% of our Blood oxygen is bound to hemoglobin. Each molecule of Hemoglobin can bind to 4 Oxygen molecules.

An important number to remember here is the Huffner's Number which is how much oxygen volume can be bound to 1 gram of Hemoglobin. It is 1.34 ml of Oxygen bound to 1 gram of Hemoglobin. I memorize it by thinking that Huffner didn't want to make our lives too complicated so he just wrote 1 2(dot) 3 and then 4 so 1.34. Easy actually.

Now, we need to do some math first. We would like to know much oxygen is present per liter of blood. We need to use the following formula:

the saturation percentage of hemoglobin with oxygen, the volume of oxygen per gram of hemoglobin, and the grams of hemoglobin per liter of blood

I will use normal physiological values to perform a sample calculation. The saturation of hemoglobin normally is around 99% with oxygen, the volume of oxygen per gram of hemoglobin is 1.34 ml/g which is the Huffner's number and the average amount of hemoglobin in an individual is 130 to 160 g/l but I will just use 150 as an average. So:

99% x 1.34 x 150 = ~200ml of oxygen / liter of blood

We can also extrapolate this to the total amount of oxygen in our total volume of blood. It would be 200 ml x 5 liters of blood = 1000 ml or 1 l of oxygen in 5 liters of blood.

Now, all of this previous story happened in the blood. But how did this oxygen even get from the air into our blood?

Lets start with the air and the concentration of oxygen in it and I will move deeper and deeper into the respiratory system until we reach the mitochondrion of each cell. It will be exciting.

First lets start with the partial pressure of oxygen in the air. Side note: the total pressure of atmospheric air is 101 kPa and partial pressure of a gas is the fractional contribution of this gas to the total pressure.
Anyways, the partial pressure of oxygen in the atmosphere is 21 kPa. Side Side Note: The total pressure is 101 kPa so its almost also 21% (21/101 x 100). This is why from now on, I will write percent instead of kPa because they can essentailly be used interchangably here. 😄 Enough side notes, now I promise the real deal is the following.

  1. Okay, so the air has 21% oxygen content.
  2. When this air reaches the trachea, the oxygen percentage reduces to 20% because water vapour is mixed.
  3. Then in the alveoli, the oxygen percentage reduces to 14% becuase there is a lot more carbon dioxide, so the percent of pure oxygen is lower.
  4. Next, when this oxygen is being carried into the red blood cells, it reduces to 13% becuase passive diffusion can't be fully effecient.
  5. Now, the percentage reduces to just 6% in the tissues as the Hemoglobin doesn't release all the Oxygen.
  6. Lastly, in the mitochondria, the oxygen content is just 1% because the rest of it is being used in the electron transport chain as soon as it arrives into the mitochondrion.

And this sums up this topic.