8.4- Transport Of Oxygen And Carbon Dioxide In The Blood Flashcards
Adaptations of erythrocytes.
- Biconcave shape- has a larger surface area available for diffusion of gases. Also helps them pass through narrow capillaries.
- Lose their nucleus by the time they enter the circulation- maximises amount of haemoglobin that can bind to the cell.
- No nucleus limits their life span. Only 120 days approx.
What is haemoglobin?
The red pigment that carries oxygen and gives erythrocytes their colour.
- is a very large globular conjugated protein made up of four peptide chains, each with an iron-containing haem prosthetic group.
- each haemoglobin molecule can bind to four oxygen molecules.
Oxygen binds loosely to haemoglobin forming oxyhemoglobin. Reaction is reversible.
Explain how oxygen is picked up.
- When erythrocytes enter the lung capillaries, O2 levels in the cells are low. This creates steep conc gradient between erythrocytes and air in alveoli.
- air moves into erythrocytes and bind to haemoglobin.
- (positive cooperativity.)
- as the O2 is bound to the haemoglobin, the free O2 conc in erythrocyte stays low, so a steep conc gradient is maintained until all the haemoglobin is saturated with O2.
- Hb has high affinity for oxygen.
What is meant by positive cooperativity?
The arrangement of the Hb molecule means that as soon as one oxygen molecule binds to a haem group, the molecule changes shape, making it easier for the next oxygen molecules to bind.
This is known as positive cooperativity.
Explain how oxygen is dropped of.
One the blood reaches body tissues:
- conc of O2 in cytoplasm of body cells is lower than in erythrocytes.
- so oxygen moves out of erythrocytes down a concentration gradient.
- once the first O2 molecule is released by the haemoglobin, the molecule again changes shape and it becomes easier to remove the remaining oxygen molecules.
- here haemoglobin has a low affinity for oxygen. There is also lower pH in tissues than lungs.
What is meant by partial pressure?
defined as the pressure of a single gas component in a mixture of gases.
-It corresponds to the total pressure which the single gas component would exert if it alone occupied the whole volume.
Explain the oxygen dissociation curve.
- % saturation of Hb plotted against the partial pressure of O2 (pO2).
- shows the affinity of haemoglobin for oxygen.
As pO2 increases, % saturation increases:
-at low pO2 (respiring cells), few haem groups are bound to oxygen so Hb doesn’t carry much O2. - at higher pO2, more haem group become bound to o2 making it easier for oxygen to be picked up due to positive cooperativity.
- once all haem group are bound, Hb is saturated and has high affinity for o2 (in lungs)
Explain the Bohr effect and its importance.
At high pCO2, Hb has lower affinity for oxygen so that it is easier go drop it off.
Important because:
- in active tissues with high pCO2, haemoglobin gives up O2 more easily to meet demands.
- in the lungs where CO2 levels in air are low, oxygen bunds to haemoglobin molecules easily.
Explain how fetal haemoglobin is different to adult haemoglobin.
When a fetus is developing, it is dependent on its mother for oxygen supply.
Fatal haemoglobin has a higher affinity for oxygen as some oxygen is lost from the bloodstream before it reaches the placenta.
- higher affinity allows maximum oxygen to be transferred.
Myoglobin.
Is a molecule with similar structure to Hb but has only one haem group.
- has a very high affinity for oxygen even at very low partial pressures.
- this means oxymyoglobin will only dissociate when O2 levels are low.
- found in muscle cells where it acts as a oxygen reserve.
3 ways in which CO2 can be transported from the tissues to lungs.
- 5% is carried dissolved in plasma.
- 10-20% is combined with amino groups in polypeptide chains of haemoglobin to form carbaminohaemoglobin.
- 75-85% is converted into hydrogen carbonate ions (HCO3^-) in the cytoplasm of RBCs.
Explain picking up of Co2 as hydrogen carbonate ions.
- CO2 reacts slowly with water to form carbonic acid. This is reversible and is catalysed by carbonic anhydrase which is present in cytoplasm of RBCs in high levels.
- the carbonic acid then dissociates to form hydrogen ions and hydrogen carbonate ions.
- the -ve HCO3- ions move out the erythrocytes into the plasma by diffusion down a conc gradient. -ve Cl- ions move into the erythrocytes which maintains the electrical balance of the cell.
= known as chloride shift.
Why is Co2 converted to hydrogen carbonate ions?
It allows the erythrocytes to maintain a steel conc gradient for carbon dioxide to diffuse from respiring tissues into the erythrocytes.
Explain how carbon dioxide is dropped off.
- when blood reaches the lung tissue where there is low conc of CO2, carbonic anhydrase catalyses the reverse reaction.
- carbonic acid is broken down into CO2 and water.
- hydrogen carbonate ions diffuse back into the erythrocytes and reacts with H+ ions to form more carbonic acid.
- when this is broken down by carbonic anhydrase it releases free carbon dioxide, which diffuses put of the blood into lungs.
- Cl- ions diffuse out of RBCs back into the plasma down an electrochemical gradient.
Role of haemoglobin in dropping off of CO2?
Haemoglobin in the erythrocytes acts as a buffer and prevents pH changes by accepting free H+ ions in a reversible reaction to form haemoglobinic acid.
- this causes haemoglobin to release all the oxygen it’s carrying.
