A. Erythrocytes (= red blood cells)

 diagram of the erythrocytes


  • do have a plasma membrane
  • do NOT have a nucleus
  • are biconcave flat cells
  • the shape is caused by the cytoskeleton
  • the shape is very flexible so it can move through smaller capillaries
  • contains hemoglobin (Hb)







phtograph of several erythrocytes


B. Flexibility:

isotonoc, hypotonic and hypertonic



are very flexible. Remember (Unit 2) that they can expand and shrink very easily with variations in salt concentrations. 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 is the task of the spleen.


The narrowest capillaries in the body are in the spleen. Inside these spleen capillaries, the erythrocytes have to do a lot of bending while being pushed through these narrow vessels. If they rupture, that is then the end of the erythrocytes. 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!


C. Why are erythrocytes biconcave?

diagram of a biconcave and a spherical erythrocyte

Why is it useful that erythrocytes have a biconcave shape (= more or less flat) and not spherical?

This diagram shows what happens when an erythrocyte is placed in a hypotonic solution -> the erythrocyte swells and becomes a sphere. The membrane, of course, has not increased but only the volume has increased. But in a sphere, the diffusion distance from the surface of the membrane to the haemoglobin in the middle of the sphere is much longer than in the biconcave situation. And a longer diffusion distance means longer difusion times.

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 much helped with shorter distances between the plasma membrane and the Hb molecules.

In addition, a sphere cell is much less flexible than a biconcave cell. This is very important when the erythrocyte has to flow through very narrow capillaries.



D. Diffusion in the lungs

diagram of oxygen diffusion in the lungs

Diffusion of oxygen in the lungs:

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 (O; 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 is filled with erythrocytes which contain little oxygen as they have been used by the metabolism in 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

  1. diffusion from the alveoli, through the alveolar membrane, to the interstitial fluid
  2. from the interstial fluid, through the endothelium, into the blood plasma
  3. from the plasma, through the plasma membrane, into the erythrocytes.

Note that both the alveolar membrane and the capillary membrane consists of cells (alveolar cells and endothelial cells respectively). Therefore the oxygen molecules have to pass through the plasma membranes of both cells. Therefore, the oxygen molecules have to pass, in total, five plasma membranes (2 membranes in the alvelor cells + 2 in the endothelial cells + 1 erythrocyte membrane).

Note that the oxygen molecule is dissolved in the interstial fluid and in the plasma but, in the erythrocytes, it is bound to the hemoglobine molecule.

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 of 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.


E. Diffusion in the tissues

Diffusion of oxygen in the peripheral tissues:

This diagram shows the pathway of oxygen molecules from the blood to the individual cells in tissues.

In contrast to the situation in the lungs, in the tissues in the body, the concentration of oxygen is highest in the erythrocytes and lowest in the cells. It is lowest in the cells because their metabolism consumes oxygen. Therefore, there will be:

  1. diffusion from the erythrocytes, through its membrane, to the plasma
  2. from the plasma, through the endothelium, into the interstitial fluid
  3. from the interstitial fluid, through the plasma membrane of the tissue cells, into those cells.


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, opening more capillaries, solves the problem.

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, and therefore needs more oxygen (to make more ATP), more capillaries open, thereby shortening the diffusion time from the capillaries to the individual cells.

Another, long term solution, is to create new capillaries. This can happen when you exercise your muscles a lot. This will increase the size of your muscles (=hypertrophy) and create new blood (= angiogenesis; from angio = blood vessel and genesis = creation).


F. Other functions of erythrocytes:

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, whewhere our lungs exhale it.

Some of the CO2 is bound to haemoglobin while another portion is converted to bicarbonate as shown in the diagram:


  1. This conversion of CO2 to bicarbonate ion occurs in the erythrocytes.
  2. An enzyme (carbonic anhydrase) helps in this conversion.
  3. This enzyme is located in the erythrocytes, NOT in the plasma.
  4. The CO2 diffuses into the erythrocytes.
  5. With the help of carbonic anhydrase, it binds CO2 to water to form carbonic acid.
  6. Carbonic acid is not stable and splits into 1 hydrogen ion and 1 bicarbonate ion.
  7. The bicarbonate ion (and the hydrogen ion) diffuse back into the plasma.

In summary; the erythrocytes transports CO2 bound to Hb and converts it into bicarbonate ion that is then transported in the plasma. 2014