Exchange & Transport of Gases | Biology A-Level - A Level PDF Download

Exchange of Gases

Exchange of gases takes place at two sites:

(i) Alveoli to blood

(ii) Between blood and tissues

• Exchanges of gases occur by simple diffusion due to pressure/ concentration gradient, solubility of the gases and thickness of membrane.
• Pressure contributed by individual gas in a mixture of gas is called partial pressure represented by pC and p .
• Partial pressure of Oxygen and carbon dioxide at different part involved in diffusion varies from one part to another and moves from higher partial pressure to lower partial pressure.
• Solubility of C is 20-25 times more than solubility of  , so C diffuse much faster through membrane.
• Diffusion membrane is three layered thick, that is alveolar squamous epithelium, endothelium of alveolar capillaries and basement substance between them.

Transport of Gases In Blood

1. Transport of Oxygen 

• As much oxygen  comes in the blood from air, it is approximately 3% dissolves in the blood plasma. Remaining 97% oxygen combines with haemoglobin to form oxyhaemoglobin. 
• One molecule of haemoglobin combines with 4 molecules of oxygen. Haemoglobin is made up of 4 units. Every unit of it, reacts with one molecule of oxygen. 1 gm of haemoglobin transports 1.34 ml of oxygen. 100 ml (1 dL) of blood contains normally 15 gm of haemoglobin, so 100 ml blood transports approximately 20 ml of oxygen. 
• Oxygen does not oxidise haemoglobin. Formation of oxyhaemoglobin is a process of oxygenation. The valency of iron is 2 in oxyhaemoglobin.
• Some gases (e.g. Ozone) oxidise haemoglobin. This oxidised haemoglobin is called Methaemoglobin .This is a colourless compound. 
• This type of gases are environmental pollutant. At the time, oxyhaemoglobin reaches upto the tissues it dissociates. O₂ freed from it goes into the tissue fluid from blood. In place of it, CO₂ from tissue fluid comes into blood. 
• Gaseous exchange between blood and tissue is called internal respiration or tissue respiration. It is also done by simple diffusion. In a conducting cycle blood gives its 25% O₂ to tissues.
• Dissociation of oxyhaemoglobin is affected by so many factors –

1. Low partial pressure of oxygen :- Combination of oxygen with haemoglobin is a reversible reaction. Low partial pressure of O₂ activates dissociation of Oxyhaemoglobin.

2. High concentration of CO₂ :- High concentration of CO₂ also activates the dissociation of oxyhaemoglobin. The effect of CO₂ concn on dissociation of oxyhaemoglobin is called Bohr's effect.

3. Low pH value of tissue fluid - Acidity activates dissociation of oxyhaemoglobin. The effect of pH on dissociation of oxyhaemoglobin is called Root effect.

• Oxyhaemoglobin Dissociation Curve
  

  A graph is plotted between O2 concentration and percentage saturation of haemoglobin with oxygen (we get a sigmoid curve), this curve is called Dissociation curve.
• Dissociation curve is sigmoid curve. As the concentration of  CO2 increases, saturation of haemoglobin with oxygen decreases. At higher CO2 concentration, dissociation curve shifts towards right side. This effect is called Bohr's effect.
• The meaning of right side shifting of dissociation curve is that , O2 is readily dissociating from oxyhaemoglobin.

Shift to Left 

Shift to Right 

Hb can not take up O₂ beyond a saturation level of 97%.

Hb is 50% saturated with O₂ at 30 mm Hg

P₅₀ value – PO₂ at which the Hb is 50% saturated with O₂. Higher the P₅₀ lower is the affinity of Hb for O₂. 2, 3 diphosphoglycerate (2, 3 DPG) – a susbtance formed during glycolysis. ­  2, 3, DPG will cause shift to right.

Transport of O₂  

a) Normally Activity : 5ml O₂/ 100 ml is delivered normally.

b) During Heavy Exercise: Muscle cell use O₂ at a rapid rate.

• Interstitial fluid PO₂ falls  as low as 15mm Hg.

• Only 4.4 ml O₂ remains bound to Hb.

• 19.4 - 4.4 = 15 ml O₂ /100 ml blood is delivered to muscle. Also Cardiac output can reach maximum 7 times the (normal) value. Therefore O₂ delivery maximum limit which we can achieve  is 20 -21 times the normal.

2. Transport of Carbon Dioxide 

The blood transports carbon dioxide comparatively easily because of its higher solubility. There are three ways of transport of carbon dioxide by which 100 ml of blood manages to deliver about 4 to 4.2 ml of carbon dioxide to the alveoli.

(a) In Dissolved State : Approximately 5-7 per cent of carbon dioxide is transported, being dissolved in the plasma of blood. Hence 0.3 ml of carbon dioxide is transported per 100 ml of blood plasma.

(b) In the Form of Bicarbonate : Carbon dioxide produced by the tissues, diffuses passively into the blood stream and passes into the red blood corpuscles, where it reacts with water to form carbonic acid (H₂CO₃). This reaction is catalysed by the enzyme, carbonic anhydrase, found in the erythrocytes, and takes less than one second to complete the process. Immediately after its formation, carbonic acid dissociates into Hydrogen (H+) and bicarbonate (HCO₃^–) ions.

The oxyhaemoglobin (HbO₂) of the erythrocytes is weekly acidic and remains in association with K⁺ ions as KHbO₂. The hydrogen ions (H⁺) released from carbonic acid combine with haemoglobin after its dissociation from the potassium ions.

The majority of bicarbonate ions (HCO₃^–) formed within the erythrocytes diffuse out into the plasma along a concentration gradient. H⁺ combine with haemoglobin to form the haemoglobinic acid (H.Hb).

In response, chloride ions (Cl^–) diffuse from plasma into the erythrocytes to maintain the ionic balance. Thus, electrochemical neutrality is maintained. This is called Chloride shift or Hamburger Phenomenon. The chloride ions (Cl^–) inside RBC combine with potassium ions (K⁺) to form potassium chloride (KCl), whereas hydrogen carbonate ions (HCO₃^–) in the plasma combine with Na⁺ to form sodium hydrogen carbonate (NaHCO₃). Nearly 70 per cent of carbon dioxide is transported from tissues to the lungs in this form.

(c) In combination with amine group of protein : Besides the above two methods, carbon dioxide reacts directly with the amine radicals (NH₂) of haemoglobin molecule and forms a carbaminohaemoglobin (HHb.NHCOOH) molecule. This combination of carbon dioxide with haemoglobin is a reversible reaction. Nearly 23 percent of carbon dioxide is transported through this mode.

Release of Carbon Dioxide in the Alveoli of Lung

• When the deoxygenated blood reaches the alveoli of the lung, it contains carbon dioxide as dissolved in plasma, as carbaminohemoglobin, and as bicarbonate ions. 
• In the pulmonary capillaries, the carbon dioxide dissolved in plasma diffuses into alveoli. Carbaminohemoglobin also splits into carbon dioxide and haemoglobin. 
• For the release of carbon dioxide from the bicarbonate, a small series of reverse reactions takes place. When the haemoglobin in the pulmonary blood takes up oxygen, the H+ is released from it.
• Then the Cl– and HCO3– ions are released from KCl in RBC, and NaHCO3 in the plasma, respectively. 
• Then HCO3– reacts with H+ to form H2CO3. This H2CO3, ultimately, then splits into carbon dioxide and water in the presence of carbonic anhydrase enzyme and carbon dioxide is released into lungs.
• When bicarbonates and carbamino compounds reach in the lungs, then they dissociate. Thus CO2 is formed.
• This dissociation is stimulated by oxyhaemoglobin. 
• This CO₂ freed from blood goes into atmosphere. The effect of oxyhaemoglobin on the dissociation of these compounds is known as Haldane Effect. 
• In this reaction oxyhaemoglobin acts like a strong acid i.e, it frees H⁺ in the medium. These H⁺ combine with bicarbonates and thus their dissociation is stimulated. In this way transportation of CO₂ is completed.