8.4 Transport of oxygen and carbon dioxide in the blood Flashcards
how are erythrocytes specialised to their function of transporting oxygen
Erythrocytes have a biconcave shape – this gives them a large surface area to volume ratio allowing oxygen to diffuse in and out rapidly.
Erythrocytes have no nuclei – by the time mature erythrocytes enter circulation they have lost their nuclei, which maximises the amount of haemoglobin that can fit into the cells. It also limits their life, so they only last 120 days in the bloodstream.
Erythrocytes contain haemoglobin – the red pigment that carries oxygen and gives them their colour. Haemoglobin is a very large globular conjugated protein made up of four peptide chains, each with an iron-containing haem prosthetic group. Haemoglobin is a protein with a quaternary structure. Each haemoglobin molecule can bind to four oxygen molecules and there are 300 million haemoglobin molecules in each red blood cell.
oxyhaemoglobin reaction
The oxygen binds quite loosely to the haemoglobin forming oxyhaemoglobin. The reaction is reversible
haemoglobin + oxygen —-> oxyhaemoglobin (reversible reaction)
Hb + 4O2 —-> Hb(O2)4 (reversible reaction)
carrying oxygen
When the erythrocytes enter the capillaries in the lungs, the oxygen levels in the cells are relatively low.
This makes a steep concentration gradient between the inside of the erythrocytes and the air in the alveoli.
Oxygen moves into the erythrocytes and binds with the haemoglobin.
The arrangement of the haemoglobin molecule means that as soon as one oxygen molecule binds to haem group, the molecule changes shape, making it easier for the next oxygen molecule to bind. This is known as positive cooperativity.
Because the oxygen is bound to the haemoglobin, the free oxygen concentration in the erythrocytes stays low, so a steep diffusion gradient is maintained until all the haemoglobin is saturated with oxygen.
When the blood reaches the body tissues, the concentration of oxygen in the cytoplasm of the body cells is lower than in the erythrocytes.
So, oxygen moves out of the erythrocytes down a concentration gradient.
Once the first oxygen molecule is released by haemoglobin, the molecule again changes shape and reduces the affinity of the haem groups for oxygen, so it becomes easier to remove the remaining oxygen molecules.
why is an oxygen dissociation curve needed
An oxygen dissociation curve helps us understand how the blood carries and releases oxygen
how is an oxygen dissociation curve plotted
The percentage saturation haemoglobin in the blood is plotted against the partial pressure of oxygen.
what does an oxygen dissociation curve show
Oxygen dissociation curves show the affinity of haemoglobin for oxygen.
oxygen dissociation curve
A change in the partial pressure of oxygen in the surroundings makes a significant difference to the saturation of haemoglobin with oxygen.
Because, once the first oxygen molecule becomes attached the change in the shape of the haemoglobin molecules mean other oxygen molecules are added rapidly.
The curve levels out at the highest partial pressures of oxygen because all the haem groups are bound to oxygen so the haemoglobin is saturated and cannot take up any more.
Oxygen is loaded in regions of high partial pressure of oxygen (alveoli) and is unloaded in regions of low partial pressure of oxygen (respiring tissues).
This means that at the high partial pressure of oxygen in the lungs the haemoglobin in the erythrocytes is rapidly loaded with oxygen due to haemoglobin having a high affinity for oxygen.
A relatively small drop in oxygen levels in the respiring tissues means oxygen is released rapidly from the haemoglobin to diffuse into cells.
The affinity of haemoglobin for oxygen depends on the partial pressure of oxygen.
the effect of carbon dioxide
At higher partial pressures of CO2, haemoglobin gives up oxygen more easily.
This change is known as the Bohr effect.
This means that carbon dioxide causes the oxygen affinity of haemoglobin to decrease.
the bohr effect
The Bohr effect is that as the proportion of carbon dioxide increases, the oxygen dissociation curve for haemoglobin shifts to the right. This means that carbon dioxide causes the oxygen affinity of haemoglobin to decrease.
The Bohr effect is important in the body because as a result:
In active tissues with a high partial pressure of carbon dioxide, haemoglobin gives up its oxygen more easily because the haemoglobin now has a lower oxygen affinity
In the lungs where the proportion of carbon dioxide in the air is relatively low, oxygen binds to the haemoglobin molecules easily.
fetal haemoglobin
When a fetus is developing, it’s completely dependent on its mother to supply it with oxygen.
Oxygenated blood from the mother runs close to the deoxygenated fetal blood in the placenta.
If the blood of the fetus had the same affinity for oxygen as the blood of the mother then little or no oxygen would be transferred to the blood of the fetus.
However, fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin at each point along the dissociation curve.
So it removes oxygen from the mothers blood as they move past each other.
how is carbon dioxide transported
Carbon dioxide is transported from the tissues to the lungs in 3 different ways:
- About 5% is carried dissolved in the plasma
- 10-20% is combined with the amino groups in the polypeptide chains of haemoglobin to form a compound called carbaminohaemoglobin
- 75-85% is converted into hydrogen carbonate ions (HCO3-) in the cytoplasm of the red blood cells
chloride shift
reversible reaction
transporting carbon dioxide
