# Total amount of CO₂ / Oxygen in Bloodstream in Humans

How much carbon dioxide and oxygen from/for respiration are in the bloodstream at any one time? (mass per litre of blood or similar)

And would there be much more aside from the blood in tissue, e.g. muscle, brain, etc.

So far the figures I have are as follows (please double check for me, I dont have a clue)

O₂ molecules per red blood cell: 1 billion. Red blood cells per micro litre of blood in males: 4.7 to 6.1 million. Molecules per mole: 6 x 10^23. Therefore moles of O2 per litre of blood: approx 0.01 (correct?) This seems quite low, since an inhalation is typically 0.5L and about a quarter of the oxygen is absorbed (oxygen by volume, 5% of the air) (1 thousandth of a mole of oxygen absorbed per inhalation?)

Thanks

• This is little bit uncommon to use physical units for blood oxygen. Normally one just cares for the oxygen bound to haemoglobin, and doesn't consider the diluted oxygen. And trying to use your numbers I come to 0.8232 mole, that seems to be incorrect too... Jul 11, 2012 at 5:56

Your calculation for O2 is along the right direction, but missing a factor I think.

Each red blood cell holds millions of molecules of hemoglobin, each hemoglobin molecule, when saturated (at the lung), holds four O2 molecules. So this is off by a factor of millions at least. Wikipedia estimates that hemoglobin makes up about 35% of the total weight of blood. We can use this figure and an average of 4.5 l of blood for a human being ( numbers vary, but it will be within 30%).

4.5l blood * 1.060 g/ml density of blood * 1000 ml/l water * 35% = 1667 g hemoglobin

4 molecules O2/molecule hemoglobin * 1667 g hemoglobin / (64000 g/mol molecular weight of hemoglobin) = 0.104 moles O2

so I get:

0.104 moles O2 * 30 g/mol for O2 = 3.12 g O2 in the blood at a time.

In the muscle and tissues it gradually depletes so that all four molecules will usually end up somewhere in your tissue. So this is all assuming that counting all the hemoglobin bound O2 will approximate the count of O2 in the body.

We know this is a crude estimate as myoglobin in mammal muscle will store O2 for use later, and the average hemoglobin is not in the lungs and will have given up some of its O2. It also doesn't count the O2 which diffuses into the blood and is carried by the water. This is about 0.035 g / l, which is why we need hemoglobin - total blood saturation would carry about 1.15 g of O2. Hemoglobin multiplies the oxygen carrying capacity of blood by 100 fold.

CO2 is much more soluble in water (blood/tissue) than O2 is- about 1g/l. There is I think only 2 CO2 binding sites for hemoglobin, which means it carries out only half the CO2 oxygen that it carries in. Hemoglobin is is only considered to account for 10% of the total CO2 carrying capacity of humans. So calculating CO2 respiration by hemoglobin is not a good way to estimate blood CO2.

To get the answers you're looking for, you need a couple of figures to start with: (1) The amount of O2 dissolved in blood plasma: about 0.3 ml O2 per 100 ml plasma; (2) The amount of O2 bound to hemoglobin in blood: about 20 ml O2 per 100 ml blood. - Ref: https://www.ncbi.nlm.nih.gov/books/NBK54103/ (which also sources the figures used elsewhere here)

Blood makes up about 7% w/v of the human body, so a 70 Kg (154 lb) person will have about 4.9 litres of blood. 5 litres of blood would hold around 1 litre of O2 bound to the hemoglobin. However, around half of the blood will be venous, not arterial, and that only has about 75% O2 saturation, so the actual amount in arterial plus venous blood will be about 1/8 less than that calculation. Obviously, the figure will vary significantly with body weight.

At normal pressure, the arterial hemoglobin is effectively saturated with O2. However, if the ambient pressure increases, then the amount of O2 dissolved in the plasma will increase proportionately (Henry's Law). Similarly, breathing a gas with a greater O2 fraction will increase the amount of dissolved O2 in a linear fashion. Nevertheless, O2's low solubility means that the dissolved O2 is only about 1/60 of that bound to hemoglobin, so unless that is severely compromised (e.g. CO poisoning), increasing the pressure or fraction of O2 has little effect on the supply to the body.

As for the O2 stored in the body, each myoglobin molecule only binds 1 molecule of O2, compared with 4 molecules of O2 per molecule of hemoglobin, so it will contribute a much smaller amount to the total. Of course, that will vary again with body weight but also with the amount of muscle and other type of tissue making up the body.