Carbon dioxide transport in blood

Definitions & Key takeaways

Carbon dioxide is produced as a by-product of cellular metabolism and is transported in the blood to the lungs, to be expelled from the body through exhalation. The transport of carbon dioxide in the blood occurs through three main mechanisms. First, there is a portion of carbon dioxide that is directly dissolved in the plasma, which is the liquid part of blood. The next part of carbon dioxide is bound to hemoglobin, what's called carbaminohemoglobin. Most of the amount of carbon dioxide is chemically dissolved in the plasma as bicarbonate ions (HCO3-).

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Introduction0:00–0:24

Carbon dioxide is made as a waste product by cells and blood helps to transport that carbon dioxide from the tissues to the lungs where we can breathe it out.
Now, to facilitate this blood has three important mechanisms to move carbon dioxide. Around first, a small amount of carbon dioxide is dissolved in the plasma, which is the liquid portion of blood.

Plasma-dissolved CO20:24–1:33

Now to calculate the concentration of dissolved carbon dioxide, you can multiply the partial pressure of carbon dioxide measured in millimeters of mercury with the solubility of carbon dioxide.
The solubility of carbon dioxide is the amount of carbon dioxide that can be dissolved in blood and it turns out that in 100 mL of blood, 0.07 mL of carbon dioxide is dissolved per millimeter of mercury of carbon dioxide in venous blood.
The equation becomes dissolved. Carbon dioxide equals the venous partial pressure of carbon dioxide in millimeters of mercury times 0.07 mL of carbon dioxide per millimeters of mercury per 100 mL of blood.
And if we plug in the partial pressure of carbon dioxide in the veins, which is about 45 millimeters of mercury, we get 3.15 mL of carbon dioxide in 100 mL of blood.
This works out to be about 5% of the total carbon dioxide transported by the blood, but it can go up to 10%. Now, another 10 to 20% is transported.
A second way, carbon dioxide binds directly to the terminal amino acids of each of the four globin chains. In a hemoglobin protein, hemoglobin is the most abundant protein in the red blood cells.

Hemoglobin-bound CO21:33–2:33

And each hemoglobin can hold on to four molecules of carbon dioxide. When hemoglobin is bound to carbon dioxide, it's called carbaminohemoglobin.
Now, carbaminohemoglobin alters the shape of the hemoglobin molecule slightly and it decreases hemoglobin's affinity for oxygen.
And this is called the bore effect. It leads to slightly more oxygen becoming unbound and getting dropped off in tissues full of carbon dioxide.
This causes a shift to the right in the oxygen hemoglobin dissociation curve. But the majority of carbon dioxide about 70 to 80% is transported a third way which involves turning carbon dioxide into a bicarbonate ion to get there carbon dioxide first undergoes a chemical reaction with water to form carbonic acid as a weak acid, carbonic acid, easily dissociates into hydrogen ions and bicarbonate ions.

Bicarbonate2:33–4:36

And these reactions are reversible and can happen in the opposite direction as well. And while this reaction can also happen in the plasma, it is sped up in the red blood cell by the enzyme carbonic anhydrase to produce a large amount of bicarbonate ions and hydrogen ions.
As bicarbonate ion levels build up in the red blood cell. The bicarbonate ions begin moving down their concentration gradient and flowing into the plasma.
The shift in charge this creates is balanced by facilitated diffusion. An ionic exchange between bicarbonate and chloride called the chloride shift.
So chloride comes into the red blood cell as bicarbonate goes into the plasma and here bicarbonate plays an important role in maintaining physiologic hydrogen ion levels in the blood, which ultimately affect ph levels.
It does this through Licia tier's principle. In other words, when the hydrogen ion concentration gets low, the equation moves to the right, producing more bicarbonate and hydrogen ions conversely, when there are lots of hydrogen ions around the bicarbonate will bind to a hydrogen ion and form carbonic acid which then splits into water and carbon dioxide.
Ultimately, the concentration of hydrogen ions doesn't shift too quickly in either direction. And that means the PH stays stable.
Now, carbon dioxide waste ultimately needs to make its way from the tissues to the lungs and be removed from the body through expiration.
So in the lungs release of carbon dioxide from the blood occurs because of a decrease in partial pressure of carbon dioxide and increase in partial pressure of oxygen compared to the tissues.
This means that all the processes working to pick up carbon dioxide in the tissues are reversed in the lungs as a result of higher partial pressure of oxygen, more oxygen diffuses into the red blood cells and binds with hemoglobin at its heme group.
Now, even though oxygen doesn't compete with carbon dioxide and hydrogen for binding sites, it does alter hemoglobin's shape slightly.

Pulmonary gas exchange4:36–6:48

And this reduces hemoglobin's binding affinity for carbon dioxide and hydrogen. This is called the Haldane effect.
And it means that as more oxygen binds to hemoglobin, more carbon dioxide and hydrogen becomes unbound. Now, when there's a rise in hydrogen, more bicarbonate is needed to bind with it.
And so bicarbonate re enters the red blood cell from the plasma once again by slowing down its concentration gradient, except this time going into the red blood cell with all this extra carbon dioxide building up, partial pressure of carbon dioxide increases and drives carbon dioxide diffusion down its partial pressure gradient from the red blood cell into the plasma.
And from there, it goes into the alveoli where partial pressure of carbon dioxide is 40 millimeters of mercury, partial pressure of carbon dioxide in the alveoli and blood equilibrate about 20 times faster than oxygen.
This is largely because carbon dioxide is about 23 times more soluble than oxygen. This is important because even when there is a build up of fluid in the lungs, like in pulmonary edema and it's difficult for oxygen to diffuse into the blood carbon dioxide can still easily diffuse out of the blood.
All right, as a quick recap. There are three ways for carbon dioxide to move around in the blood dissolved directly into the blood plasma bound to hemoglobin.
As carbaminohemoglobin and chemically modified and dissolved in the plasma. As bicarbonate ions, the processes of carbon dioxide pick up in the tissues are reversible in the lungs.
This leads to carbon dioxide expiration.

Review6:48–6:16