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how are erythrocytes adapted
have a biconcave shape which has a larger surface area than a simple disc structure or a sphere, increasing the surface area available for diffusion of gases
helps them to pass through narrow capillaries
they are formed continually in the red bone marrow
they lose their nuclei to maximise the amount of haemoglobin that fits into the cell (also limiting their lifespan)
haemoglobin
the red pigment that carried oxygen and gives red blood cells their colour
a large globular protein conjugated protein made up of 4 peptide chains, each with an iron containing haem prosthetic group
there are about 300 millions haemoglobin molecules in each red blood cells
each haemoglobin molecule can bind to four oxygen molecules
oxygen binds to haemoglobin forming oxyhaemoglobin in a reversable reaction
carrying oxygen (air into blood)
when the erythrocytes enter capillaries in the lungs, the oxygen levels in the cells are relatively low
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 and one oxygen molecule binds to a 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 erythrocyte stays low, so a steep diffusion gradient is maintained until all of the haemoglobin is saturated with oxygen
carrying oxygen (blood into body tissues)
the concentration of oxygen in the cytoplasm of the body cells is lower than in the erythrocytes
as a result, oxygen moves out of the erythrocytes down a concentration gradient
once the first oxygen molecule is released by the haemoglobin, the molecule again changes shape and it becomes easier to remove the remaining oxygen molecules
oxygen dissociation curve
the percentage saturation haemoglobin in the blood is plotted against the partial pressure of oxygen
once the first molecule of oxygen is attached, the shape of the haemoglobin changes meaning other oxygen molecules are added rapidly
the curve levels out at the high partial pressures of oxygen because all the haem groups are bound to oxygen so the haemoglobin is saturated and cannot take up any more
at a high partial pressure of oxygen in the lungs the haemoglobin in the red blood cells is rapidly loaded with oxygen. A small drop in oxygen levels in the respiring tissues means that oxygen is released rapidly from the haemoglobin to diffuse into the cells
this effect is enhanced by the relatively low PH in the tissues compared with the lungs
graphs with an oxygen dissociation curve
effect of carbon dioxide on partial pressure of oxygen
as the partial pressure of carbon dioxide rises, haemoglobin gives up oxygen more easily. This change is known as the Bohr effect
in active tissues with a high partial pressure of carbon dioxide, haemoglobin gives up its oxygen more readily
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 in the uterus it is completely dependant 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 of no oxygen would be transported to the blood of the fetus,. However, fetus haemoglobin has a higher affinity for oxygen as each point along the dissociation curve so it removed oxygen from the maternal blood
3 ways in which carbon dioxide is transported from the tissues to the lungs
5% carried dissolved in the plasma
10-20% is combined with the amino groups in polypeptide chains of haemoglobin to form a compound called carbaminohaemoglobin
75-85% is converted into hydrogen carbonated ions in the cytoplasm of the red blood cells
how does carbon dioxide form hydrogen ions
carbon dioxide reacts with water to form carbonic acid
this reaction happens slowly in the blood plasma but in the cytoplasm of red blood cells there is the enzyme carbonic anhydrase which catalyses this reactions
the carbonic acid then dissociates to form hydrogen carbonate ions and hydrogen ions
chloride shift
negatively charged hydrogen carbonate ions move out of the erythrocytes into the plasma by diffusion down a concentration gradient
negatively charged chloride ions move into the erythrocytes which maintains the electric balance of the cell
How does carbon dioxide affect oxygen unloading
most of the CO2 from respiring tissues diffuses into red blood cells. Here it reacts with water to form carbonic acid, catalysed by the enzyme carbon anhydrase
the carbonic acid dissociates to give hydrogen ions and hydrogen carbonate ions
this increase in H+ ions causes oxyhaemoglobin to unload its oxygen so that haemoglobin can take up the H+ ions. This forms a compound called haemoglobinic acid
the HCO3- ions diffuse out of the red blood cells and are transported in the blood plasma. To compensate for the loss of HCO3- ions from the red blood cells, chloride ions diffuse into the red blood cells. This is called the chloride shirt and it maintains the balance of charge between the red blood cell and the plasma
when the blood reaches the lungs the low p CO2 causes some of the HCO3- and H+ ions to recombine into CO2
then CO2 diffuses into the alveoli and is breathed out
