Module 2 – Exchange and Transport Lesson 10

Title: Carriage of O2 and CO2 Lesson objectives: Can I explain the different affinities for oxygen? Grade C/B Can I explain the Bohr Effect? Grade C/B Starter: Quick question on RBC..

RBC…. Which of these functions could, or could not, be carried out by a red blood cell? In each case, justify your answer: Protein synthesis Cell division Lipid synthesis Active transport

Haemoglobin… Haemoglobin is a protein which, when combined with oxygen, forms oxyhaemoglobin. Has four subunits, each made of a polypeptide chain and a haem (iron atom Fe2+) group. The iron ion attracts and holds the oxygen molecule = affinity/attraction. Hb + 4O2 = HbO8

Partial pressure…. Oxyhaemoglobin releases oxygen for aerobic respiration = dissociation. Haemoglobin takes up and releases oxygen depending on the amount of oxygen in the tissues. This is measured by the pressure that it contributes to a mixture of gases = partial pressure(pO2)/oxygen tension (kPa).

Oxygen dissociation curve The shape of the curve is due to the way in which oxygen binds with haemoglobin. Each haemoglobin molecule can bind with four oxygen molecules. The first molecule binds with difficulty and distorts the shape of the haemoglobin molecule. This change means the subsequent oxygen molecules bind progressively more easily.

Explanation…. Low oxygen tension/partial pressure – haemoglobin does not combine easily with oxygen. Why? This difficulty then accounts for the low saturation level of haemoglobin at low oxygen tension/partial pressure. As the oxygen tension rises – one O2 molecule diffuses into the haemoglobin molecule and associates with the haem group.

Explanation continued… The distortion or conformational change of the haemoglobin allows more O2 molecules to diffuse and associate – hence the curve steepens. It is a little harder for the fourth O2 molecule to combine – the curve levels off just before 100% saturation.

Oxygen dissociation curves: Myoglobin Myoglobin is a globular protein similar to haemoglobin; however, it has just one haem-globin unit. It is mostly found in skeletal muscle and is the pigment responsible for the characteristic red colour of ‘meat’. It has a much higher affinity for oxygen than haemoglobin. As a result, myoglobin acts as an oxygen store in muscle and will only release oxygen when levels in the tissue become very low, e.g. during prolonged or strenuous exercise. Myoglobin Llama haemoglobin Human foetal haemoglobin Human maternal haemoglobin

Adult vs foetal haemoglobin… Foetal haemoglobin has a higher affinity for oxygen as it must be able to absorb O2 from the mother’s blood fluid. This reduces the oxygen tension within the blood fluid which makes the maternal haemoglobin release O2. At each partial pressure of O2, foetal haemoglobin is more saturated than adult haemoglobin.

Carbon dioxide CO2 is transported in 3 ways: 5% dissolved in plasma directly 10% as carbaminohaemoglobin 85% as hydrogencarbonate ions HCO3

Hydrogencarbonate ions…. CO2 + H20 H2CO3 This happens in the RBC forming carbonic acid with the help of carbonic anhydrase. The carbonic acid dissociates releasing hydrogen ions and hydrogencarbonate ions. H2CO3 HCO3 + H+

Gas exchange: in the tissues capillary tissue fluid respiring cells red blood cell plasma lungs 10% 5% CO2 HbCO2 H2O + CO2 CO2 carbonic anhydrase lungs H2CO3 85% H+ + HCO3- HCO3- Cl- H+ + HbO2 O2 HbH

Explanation…. The chloride shift maintains the charge inside the RBC as the chloride ions (Cl-) move from the plasma into the RBC, whilst the HCO3 diffuse out of the RBC into the plasma. Haemoglobin acts as a buffer to prevent the H+ making the RBC too acidic. The H+ are taken up by the haemoglobin to produce haemoglobinic acid.

The Bohr Effect Oxyhaemoglobin begins to dissociate and release O2. The H+ ions compete for space taken up by the O2 so when CO2 is present, the H+ ions displace the O2 on the haemoglobin. As a result the oxyhaemoglobin releases more O2 to the tissues. IOW – due to a high partial pressure of CO2, haemoglobin releases O2.

Dissociation curves showing the Bohr effect A – low partial pressure of CO2 C – high partial pressure for CO2 The lower the partial pressure of O2, the less saturated the haemoglobin.

To do…. Complete Q3