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Transport of Gases


Introduction:

  • One of the important functions of blood is transportation.
  • Blood transports vitamins, nutrients, hormones and gases within the body.
  • Blood is the medium of transportation of oxygen and carbon dioxide in the body.
  • Blood transports oxygen from the lungs to the body cells and carbon dioxide back from cells to lungs.

Topics covered:

  • Transport of Oxygen
  • Hb-oxygen Dissociation Curve
  • Transport of Carbon
  • Chloride Shift
  • Haldane Effect

Transport of Oxygen

Introduction:

  • Oxygen is transported in the body by the following means:
    - 3% of O2 is carried in a dissolved form through the plasma.
    - Around 97% of O2 is transported by RBCs in the blood as oxyhaemoglobin.

Detailed Explanation:

i. In a dissolved form through the plasma
- 3% of O2 is carried in a dissolved form through the plasma

ii. Transportation of Oxygen as Oxyhaemoglobin

  • Haemoglobin (Hb): - It is made up of two parts - haem (iron) and globin (protein)
    - It is red coloured iron-containing pigment present in RBCs.
    - It binds with O2 in a reversible manner to form oxyhaemoglobin and transports it.
    - A molecule of Hb can carry four molecules of oxygen.
    - A Hb molecule has four polypeptide chains each having a haem group containing an iron atom.
    - It is the iron with which oxygen atoms are attached.
  • O2 binds with Hb at the lungs surface and dissociates at the tissues.
  • A healthy adult has around 15 g of haemoglobin per 100 ml.
  • 1g haemoglobin carries 1.32ml oxygen.
  • Therefore,100 ml blood carries around 20 ml oxygen.
  • Under normal physiological conditions, 5 ml of oxygen is transported to tissues by 100 ml of blood.
  • During strenuous conditions/ exercise, when there is a shortage of oxygen then around 15 ml of oxygen is transported by every 100 ml of haemoglobin.

1

haemoglobin-molecule

haemoglobin-molecule-1

Hb-Oxygen Dissociation Curve

Definition:

  • “A graphical representation of the relationship between pO2 and percentage saturation of Hb is known as Hb-O2 dissociation curve”.
  • The percentage of Hb that is bound with O2 is called percentage saturation of Hb.

Detailed Explanation:

  • A sigmoid curve or ‘S-shaped’ curve is obtained when percentage saturation of haemoglobin with O2 is plotted against the pO2.
  • This curve is useful in studying the effect of factors like pO2., pCO2, etc. on binding of O2 with Hb.
    partial-pressure-of-oxygen
  • It can be seen in the graph that Hb gets saturated to about 50% when pO2 is 25 mm Hg.
  • It means that 50% of blood is saturated with oxygen.
  • The partial pressure at which Hb saturation is 50% is called P50.
  • Factors affecting the binding of oxygen molecules with haemoglobin are:
  • Partial pressure of O2 (Primary factor)
  • Partial pressure of CO2
  • Hydrogen ion concentration
  • Temperature
  • O2-dissociation curve can shift either left or right
  • Shift to left - Shifting of curve towards the left indicates the association of oxygen with Hb which occurs in alveoli
  • Shift to right - Shifting of curve towards the right indicates the dissociation of oxygen from Hb which occurs in tissue
  • Shifting of curve towards left or right depends on the following conditions:
     
    Shift to Left Shift to Right
    High partial pressure of oxygen Low partial pressure of oxygen
    Low partial pressure of carbon dioxide High partial pressure of carbon dioxide
    Less hydrogen ion concentration and high pH High hydrogen ion concentration and less pH (more acidic)
    Low temperature High temperature
    Foetal haemoglobin Increase in 2,3-bisphosphoglycerate (2,3-DPG)
  • Bohr’s Effect - A rise in pCO2 or fall in pH value decreases oxygen affinity towards haemoglobin, raising the P50 value, this is called Bohr’s effect.

Transport of Carbon dioxide
 

Introduction:

  • CO2 produced is transported in the blood by the following means:
    - In dissolved form through plasma (Around 7%)
    - By RBCs as carbaminohaemoglobin (Around 20-25%)
    - As bicarbonate ions (Around 70%)
  • Under normal conditions, around 4ml of CO2 is delivered to the alveoli by every 100 ml of deoxygenated blood.

Explanation:

I. In dissolved form through plasma:

  • gets dissolved in blood plasma and is carried to the lungs in soluble form.
  • Solubility of is quite high, therefore 7% of it gets transported in dissolved form whereas only 3% O2 is transported in dissolved form.

ii. By RBCs as carbaminohaemoglobin (HbCO2):

  • CO2 in RBCs form reversible compound - carbaminohaemoglobin (HbCO2)
  • CO2 binds with amino group of protein that is a part of Hb
  • High pCO2 and low pO2 supports more binding of CO2 with Hb in tissues
  • Low pCO2 and high pO2 are responsible for dissociation of CO2 from HbCO2 in alveoli
  • This is how CO2 bound with Hb in tissues is delivered to alveoli and is exhaled out.

iii. As bicarbonate ions:

  • Carbonic anhydrase, an enzyme plays a vital role in the transportation of CO2
  • This enzyme is present in high concentration in RBCs and in small concentration in plasma
  • CO2 binds with water in RBCs to form H2CO3 (carbonic acid) in presence of carbonic anhydrase
  • Carbonic acid is an unstable compound, it dissociates quickly into hydrogen ion (H+) and bicarbonate ion (HCO3-)
  • At tissue sites -
    - pCO2 is high due to catabolism
    - High pCO2 supports the diffusion of CO2 into blood (RBCs and Plasma)
    - Here, H+ and HCO3- ions are formed
    - HCO3- quickly diffuses into plasma, where they are carried to the lungs
  • At alveolar sites (lungs) -
    - pCO2 is low
    - Reaction proceeds in opposite direction, CO2 and H2O are formed
    Thus, CO2 is converted to bicarbonate in tissue level and is transported to alveoli where it is released as CO2

Chloride Shift

  • It is also known as the Hamburger phenomenon
  • It is exchange of chloride ion and bicarbonate ion across the membrane of RBCs
  • At tissue level, CO2 reacts with H2O to form H2CO3 in presence of carbonic anhydrase in RBC
  • H2CO3 being a highly unstable compound dissociates into H+ and HCO3
  • HCO3- diffuse out from RBCs to plasma
  • As HCO3- moves out from RBC, this increases the positive charges in the RBC
  • In RBCs, positive charges are balanced by diffusion of chloride (Cl-) ion from plasma.
  • Chloride shift maintains the electrical neutrality of the cell.

chloride-shift

Haldane Effect

  • It is related to release of CO2 from the deoxygenated blood into alveoli.
  • At the level of alveoli, pO2 is high while pCO2 is low, this allows movement of oxygen into RBCs and formation of oxyhaemoglobin.
  • Oxyhaemoglobin behaves as strong acid and releases excess H+ ions
  • The Cl-HCO3 pump reverses
  • Chloride ions are pumped out of RBCs and bicarbonate ions diffuse back into the RBCs
  • H+ ions bind with HCO3 to form H2CO3
  • H2CO3 dissociates into CO2 and H2O facilitated by carbonic anhydrase
  • This is also called the reverse chloride shift

erythrocyte-in-lung-capillary

Frequently Asked Questions - FAQs

Q1. What is the effect of pCO2 on oxygen transport?
Ans.

  • At the tissue level, high pCO2 and low pO2 facilitates dissociation of oxygen from oxyhaemoglobin
  • At the alveoli level, high pO2 and low pCO2 facilitates the formation of oxyhaemoglobin

Q2. Hb has a higher affinity for which gas O2 or CO2?
Ans.

  • Hb has a higher affinity for O2 than CO2

Q3. What causes the oxygen haemoglobin dissociation curve to shift to the left?
Ans.

  • Shifting of the curve towards the left indicates the association of oxygen with Hb which occurs due to high pO2 and low pCO2, less hydrogen ion concentration and high pH.

Q4. What percentage of CO2 is transported by plasma?
Ans.

  • 7% of CO2 is transported through plasma in dissolved form
  • 70% as bicarbonate ions are transported through plasma

Q5. What is the other name for chloride shift?
Ans.

  • Chloride shift is also known as the Hamburger phenomenon

Q6. When does chloride shift occur?
Ans.

  • HCO3- diffuse out from RBCs to plasma
  • As HCO3- moves out from RBC, this increases the positive charge in the RBC
  • In RBCs, positive charge is balanced by diffusion of chloride (Cl-) ion from plasma
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