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A. Erythrocytes (= red blood cells, also called RBC’s):
- do NOT have a nucleus
- do have a plasma membrane
- are biconcave flat cells
- the shape is caused by the cytoskeleton
- the shape is very flexible so it can move through smaller capillaries
- contains haemoglobin (Hb)
Link: Osmosis in A.2.3. Passive Transport Systems.
But as they get older, flexibility is reduced (wear and tear) mainly because the cell machinery no longer replaces old pieces (no nucleus remember?). So, in due time, erythrocytes will die. This occurs in the spleen.
This happens on average 120 days after they emerged from the bone marrow. The spleen is like an exam for the erythrocytes. If they pass the exam, then they go back to the circulation for another few cycles. If they fail -> death!
This diagram shows what happens when an erythrocyte is placed in a hypotonic solution -> the erythrocyte swells and becomes a sphere. The surface area of the membrane, of course, has not increased but the volume has increased.
In other words, the biconcave shape has a very favourable surface:volume ratio whereas a sphere has a much more unfavourable surface:volume ratio.
Diffusion of oxygen is therefore much promoted with shorter distances between the plasma membrane and the Hb molecules.
This diagram shows the pathway of oxygen molecules from the lungs to the blood. In the lungs, air from outside is transported to small airspaces at the end of the bronchial system called the alveoli.
One alveolus is depicted in the diagram (blue). The space inside is filled with air, which is a mixture of nitrogen (N2; 80%), oxygen (O2; 19%) and a few more gases.
In the blood vessel, which runs as close as possible to the wall of the alveolus, blood is pumped from the heart. This blood contains erythrocytes that have little oxygen as they have been used by the metabolism in the rest of the body.
Therefore the concentration of oxygen in the erythrocytes is low while the concentration of oxygen in the alveolus is high. Therefore, there will be oxygen diffusion:
- from the alveoli, through the alveolar membrane, to the interstitial fluid
- from the interstial fluid, through the endothelium, into the blood plasma
- from the plasma, through the plasma membrane, into the erythrocytes.
Note that both the alveolar membrane and the capillary membrane consist of cells. Therefore the oxygen molecules have to pass through the plasma membranes of both cells. Thus, the oxygen molecules passes, in total, five plasma membranes (2 membranes in the alveolar cells + 2 in the endothelial cells + 1 erythrocyte membrane).
This diagram shows the pathway of oxygen molecules from the blood to the individual cells in tissues.
Therefore, there will be oxygen diffusion:
- from the erythrocytes, through its membrane, to the plasma
- from the plasma, through the endothelium, into the interstitial fluid
- from the interstitial fluid, through the plasma membrane of the tissue cells, into those cells.
4.
In contrast to the diffusion in the lungs, the distance that the oxygen molecules have to travel is often much longer. That is because it is impossible to provide every cell with its own capillary! This therefore limits the metabolism of the tissue. In some cases, such as in skeletal muscles, the problem is solved by opening additional capillaries. (Link B.5.6. Special Circulations)
For example, this happens when the muscle exercises. In a normal resting muscle, about 80-90% of the capillaries are closed. But when the muscle exercises, more capillaries open, thereby shortening the diffusion time from the capillaries to the individual cells.
Another, long term solution, is to create new capillaries, especially when you exercise a lot. This will create new blood vessels (= angiogenesis; from angio = blood vessel and genesis = creation).
The amount of oxygen that can be dissolved in plasma is very small and not sufficient for keeping us alive. That is the reason why we have erythrocytes; they are like batteries that can pack (=store) a lot more oxygen for transportation to the tissues where they are needed. Without (sufficient) erythrocytes, our tissues will literally suffocate and die because of lack of oxygen.
Other transport functions:
Erythrocytes are also used to transport other compounds in blood. One of these is carbon dioxide (=CO2). This CO2 is formed by the metabolism in active cells in the body. And this CO2 has to go back from the tissues to the lungs, where our lungs exhale it.
Some of the CO2 is bound to haemoglobin while another portion is converted to bicarbonate as shown in the diagram:
This conversion of CO2 to bicarbonate ion occurs in the erythrocytes.
- An enzyme (carbonic anhydrase) helps in this conversion.
- This enzyme is located in the erythrocytes, not in the plasma.
- The CO2 diffuses into the erythrocytes.
- With the help of carbonic anhydrase, it binds CO2 to water to form carbonic acid.
- Carbonic acid is not stable and splits immediately into 1 hydrogen ion and 1 bicarbonate ion.
- The bicarbonate ion (and the hydrogen ion) diffuses back into the plasma.
In summary; the erythrocytes transports CO2 bound to Hb and also converts CO2 into bicarbonate ion that is transported in the plasma.
