Oxygen binding capacity and oxygen content

Definitions & Key takeaways

Hemoglobin's oxygen binding capacity refers to the maximum amount of oxygen that can be bound to hemoglobin (Hb) which is the main protein found inside red blood cells. The oxygen binding capacity of hemoglobin is affected by various factors, including pH, temperature, and the presence of other chemicals such as carbon dioxide and bicarbonate.

Blood's oxygen content is the amount of oxygen in a certain volume of blood, typically 100mL. It is influenced by several factors, including the concentration of hemoglobin, the partial pressure of oxygen, and the amount of oxygen that is bound to hemoglobin.

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Introduction0:00–0:45

Oxygen content is the amount of oxygen in a certain volume of blood, typically 100mL. Oxygen binding capacity is the maximum amount of oxygen that can be bound to hemoglobin, abbreviated as Hb, which is the main protein found inside of red blood cells, which is a main component of blood.
As it turns out, there are two major ways for oxygen to move around in the blood. The majority of oxygen is bound to hemoglobin inside red blood cells, and a small amount is dissolved directly in the blood plasma.
So the oxygen content of blood is the sum of these two, oxygen content equals hemoglobin-bound oxygen plus dissolved oxygen.
Now, if you just wanted to calculate dissolved oxygen, you’d do that by multiplying the partial pressure of oxygen, measured in mmHg, with the solubility of oxygen.

Dissolved oxygen0:45–1:32

And the solubility of oxygen is the amount of oxygen that can be dissolved in 100mL of blood, and it has a constant value of 0.003 mL of O2, per mmHg per 100mL of blood.
So the equation becomes dissolved oxygen equals partial pressure of oxygen in mm of mercury times 0.003. So if we plug in a physiologic arterial pressure of O2 of 100 mmHg, we get 0.3 ml of oxygen in 100ml of blood.
Now, that’s not enough oxygen to meet the metabolic demands of the body. So that’s where hemoglobin comes to the rescue.

Hemoglobin-bound oxygen1:32–2:49

The oxygen binding capacity of hemoglobin, is the maximum amount of oxygen in milliliters that 1 gram of hemoglobin can bind, multiplied by the number of grams of hemoglobin in 100mL of blood.
Each hemoglobin molecule can carry up to four molecules of O2, that’s one oxygen molecule for each of the four hemoglobin subunits.
Now, because each red blood cell carries a few hundred million hemoglobin proteins, that means that each red blood cell carries over a billion O2 molecules.
So that for each gram of hemoglobin there’s 1.34 mL O2 carried around. Now, normally the concentration of hemoglobin in the blood is 15 g/100 mL, so multiplying that out we get 20.1mL O2 /100 mL.
So the amount of oxygen in the blood bound to hemoglobin is actually quite a bit higher than the amount of oxygen dissolved in the blood, or 0.3mL O2/100mL!

Hemoglobin saturation2:49–3:50

But hemoglobin isn’t always 100% saturated or bound by oxygen. In other words, a hemoglobin might have none bound, and be 0 % saturated, it might only have one oxygen bound and three open spots for more that haven’t bound yet, meaning that particular molecule would be 25% saturated, or two filled spots and two open spots—50%, 3 spots filled and one spot open—75%, or all spots filled and 100% saturated.
And the amount of oxygen that’s bound to hemoglobin is directly proportional to the amount of oxygen dissolved in the blood.
For example, at a partial pressure of 25mmHg, hemoglobin proteins might be a total of 50% saturated; and at a partial pressure of 100mmHg, they might at a total of 98% saturated, meaning most are fully saturated.
So, going back to the original formula for oxygen content, and plugging in some numbers like a blood hemoglobin of 15 g/100 mL and 98% for the percent saturation, we get 19.7mL of oxygen from Hemoglobin bound oxygen and 0.3ml from dissolved oxygen - a total of 20 mL of oxygen per 100mL of blood.

Oxygen content3:50–5:11

So that’s the oxygen content of the blood, it doesn’t tell us how much oxygen is actually delivered to tissues. Oxygen delivery is the oxygen content multiplied by the cardiac output, which is the amount of blood that’s being pumped out by the heart each minute.
Cardiac output itself is calculated as the stroke volume, the volume of blood pumped per beat from the left ventricle of heart, expressed as mL per heartbeat; multiplied by the heart rate in beats per minute.
So if we assume a heart rate of 70 beats per minute and a stroke volume of about 70 mL per heart beat we have 4900 ml per minute as the cardiac output, or more commonly 4.9 L / min.
And we said there’s 20mL of oxygen per 100ml of blood, so that’s an oxygen delivery 980mL of oxygen per minute to the body!
So, to recap: The oxygen content of blood is the amount of oxygen that’s carried in the blood as dissolved oxygen and hemoglobin bound oxygen.

Review5:11–5:29

Oxygen content can then be multiplied by the cardiac output to determine the oxygen delivery, which is the amount of oxygen that reaches tissues each minute.