8.4 Transport of O2 & CO2 in blood

Erythrocytes adaptations

• Have biconcave shape

  • Gives large SA for gaseous exchange

  • Helps them pass thru narrow capillaries

• Contain O2 carrying pigment haemoglobin

• Mature ones have no nucleus

  • More room for maximum amount of haemoglobin

• In cytoplasm contains enzyme carbonic anhydrase involved in carriage of CO2 in blood

How long can RBCs last?

120 days

Where are erythrocytes fomed?

Red bone marrow continuously

Haemoglobin

• Red pigement that carries oxygen

• Gives erythrocytes their color

• Water soluble

• Large globular protein

  • Made of 4 peptide chains, each w. an iron-containing haem prosthetic group

  • 2 alpha subunits, 2 beta subunits

• 300 million molecules in each RBC

How many oxygens can bind to 1 haemoglobin molecule

4

Each haem binds to 1 O2

Oxygen binding to haemoglobin

• Oxygen binds quite loosely to haemoglobin forming oxyhaemoglobin

• Reaction is reversible

How does oxygen fit onto haemoglobin?

The arrangement of the haemoglobin molecule means that as soon as 1 O2 molecule binds to a haem group, the molecule changes shape,

• Makes it easier for the next O2 molecules to bind

How is oxygen carried from lungs?

• When erythrocytes enter capillaries in lungs, O2 levels in cells are relatively low

  • Makes steep conc. gradient between inside of erythrocytes & air in alveoli

• O2 moves into erythrocytes & binds w. haemoglobin

• As O2 is bound to haemoglobin, free O2 conc. in erythrocyte stays low

  • Maintains steep diffusion gradient until haemoglobin completely saturated w. O2

Lungs and Oxygen

• High pO2

  • Haemoglobin in RBCs is rapidly loaded w. O2

• 10-12kPa

• High affinity

• High conc. of O2

When blood reached body tissues, how is O2 let off?

• Conc. of O2 in cytoplasm of body cells is lower than in erythrocytes

• O2 moves out of erythrocytes down a conc. gradient

• Once 1st O2 molecule is released by the haemoglobin, the molecule changes shape

  • Makes it easier to remove remaining O2 molecules

Oxygen dissociation curve

• Shows affinity of haemoglobin for O2

• Curve levels out at the highest partial pressures of O2

  • All haem groups are bound to O2

  • Haemoglobin is saturated & cannot take up any more

Respiring tissues & O2

• Low pO2

• 2-4kPa

• Low affinity (drops off O2)

  • A relatively small drop in O2 levels in respiring tissues means O2 is released rapidly from haemoglobin to diffuse into the cells

  • Effect is enhanced by relatively low pH in tissues compared w. lungs

Low pO2

• pCO2 is high

• Occurs in respiring tissues

• Haemoglobin has low affinity at low pO2 in order to drop off O2 at respiring tissues

Bohr effect

At higher partial pressures of CO2, haemoglobin gives up O2 more easily

Why is the Bohr effect important in the body?

• In active tissues w. a high partial pressure of CO2, haemoglobin gives up its O2 more readily

• In lungs where proportion of CO2 in air is relatively low, O2 bind to haemoglobin molecules easily

Foetal haemoglobin

• Has higher affinity for O2 than maternal haemoglobin

  • Oxygenated blood from mum runs close to deoxygenated blood in placenta

  • If blood of fetus had same affinity for O2 as blood of mum, little or no O2 would be transferred to blood of fetus

  • Having higher affinity allows fetal haemoglobin to remove O2 from maternal blood as the bloods run past each other

What is fetal haemoglobin made from?

2 alpha

2 gamma

3 ways in which CO2 is transported from tissues to lungs

1. About 5% is carried dissolved in plasma

2. 10-20% is combined w. amino groups in polypeptide chains of haemoglobin to form carbaminohaemoglobin

3. 75-85% is converted into hydrogen carbonate ions in the cytoplasm of RBCs

CO2 + H2O

• In blood plasma CO2 reacts slowly w. H20 to form carbonic acid

• Carbonic acid then dissociated to form H+ ions & hydrogen carbonate ions (HCO3-)

Why is the reaction between CO2 & H2O faster in cytoplasm of RBCs?

• Due to high levels of carbonic anhydrase enzyme

  • Catalyzes reversible reaction

Chloride Shift

• Negatively charged HCO3- ions move out of erythrocytes into plasma by difffusion, down conc. gradient

• Negatively charge chloride ions (Cl-) move into erythrocytes

What is the point of the chloride shift?

Maintains electrical balance of the cell

Reason for removing CO2 & converting into HCO3- ions

Erythrocytes maintain a steep conc. gradient for CO2 to diffuse from respiring tissues into erythrocytes.

What does carbonic anhydrase do when blood reaches lung tissue?

Here, there is a relatively low conc. of CO2

• Carbonic anhydrase catalyzes reverse reaction

• Breaks down carbonic acid into CO2 + H2O

• HCO3- ions diffuse back into erythrocytes & react with H_ ions to form more carbonic acid

What happens when more carbonic acid is broken down by carbonic anhydrase

• It releases free CO2

• This diffuses out of blood into lungs

• Chloride ions diffuse out of RBCs back into plasma down an electrochemical gradient

How does haemoglobin play a role in transporting CO2?

• Acts as a buffer

• Prevents changes in pH by accepting free H+ ions in a reversible reaction to form haemoglobinic acid

Myoglobin is an O2 binding molecule found in muscles. From the graph, explain the differences between the 2 curves.

• Myoglobin in muscles has higher oxygen affinity than haemoglobin in blood

• So muscles can take oxygen from haemoglobin in blood

• Enables muscles to get extra oxygen when they are contracting during exercise