08 Respiration and Circulation - part 03 - Mechanism of respiration

 


08 Respiration and Circulation - part 03 - Mechanism of respiration


Mechanism of respiration :
  • Respiration is a biological process involving exchange of gases between the atmosphere and the lungs.
  • It results in the formation of ATP. 
  • It includes the following processes:
  1. Breathing
  2. External respiration
  3. Internal respiration
  4. Cellular respiration
1. Breathing :
  • Physical process by which gaseous exchange takes place between the atmosphere and the lungs. 
  • Involves inspiration and expiration. 
  • Both these steps involved parts of  -  
  1. Thoracic cage
  2. The ribs
  3. Sternum and 
  4. The intercostal muscles and muscles of the diaphragm.
Inspiration : 
  • During inspiration, the atmospheric air is taken in to the lungs. 
  • Occurs due to the pressure gradient formed between the lungs and the atmosphere. 
  • Active process in which the diaphragm becomes flat and goes downward, the external intercostal muscles contract.
  • So the ribs and sternum move upward and outward. 
  • This leads to an increase in the thoracic volume and a decrease in pressure of thorax and the lungs.
  • To equalize the low pressure inside the lungs, air from the atmosphere rushes into lungs. This is inspiration.
Expiration : 
  • During expiration, the thorax contracts causing air to be exhaled. 
  • Diaphragm relaxes and is pushed upwards. 
  • It becomes dome shaped. 
  • The intercostal muscles also relax pulling the rib cage inward and downward. 
  • This causes a decrease in thoracic volume and leads to increase in pressure in the thorax and the lungs as compared to the atmospheric pressure. 
  • So air from the lungs rushes out. This is expiration.
  • One inspiration and one expiration is one breath.
B. External respiration/ Exchange of gases at the alveolar level :
  • An alveolus consists of a layer of simple squamous epithelium resting on a basement membrane.
  • Intimately associated with a dense network of capillaries. 
  • The capillary wall is also made up of simple squamous epithelium resting on a thin basement membrane. 
  • Both the layers have similar structure and are thin walled. 
  • Together they make up the respiratory membrane through which gaseous exchange occurs i.e. between the alveolar air and the blood.
  • Diffusion of gases will take place from an area of higher partial pressure to an area of lower partial pressure until the partial pressure in the two regions reaches equilibrium.
  • The partial pressure of carbon-dioxide of blood entering the pulmonary capillaries is 45 mmHg 
  • Partial pressure of carbondioxide in alveolar air is 40 mmHg. 
  • Due to this difference, carbon dioxide diffuses from the capillaries into the alveolus.
  • Similarly, partial pressure of oxygen of blood in pulmonary capillaries is 40 mmHg
  • In alveolar blood it is 104 mmHg. 
  • Due to this difference oxygen diffuses from alveoli to the capillaries.
Pulmonary volumes and capacities (Normal values) :
Lung Volumes :

1. Tidal volume (T.V.) : 
  • It is the volume of air inspired or expired during normal breathing.
  • It is 500 ml.
2. Inspiratory reserve volume (IRV) : 
  • The maximum volume of air, or the extra volume of air, that is inspired during forced breathing in addition to T.V. 
  • Its value is 2000 to 3000ml.
3. Expiratory reserve volume (ERV) : 
  • The maximum volume of air that is expired during forced breathing after normal expiration. 
  • Its value is 1000 to 1100ml.
4. Dead space (DS) : 
  • The volume of air that is present in the respiratory tract (from nose to the terminal bronchioles), but not involved in gaseous exchange. 
  • It is 150 ml.
5. Residual volume (RV) : 
  • The volume of air that remains in the lungs and the dead space even after maximum expiration. 
  • It is 1100 to 1200ml.
Lung capacities :

Total Lung capacity : 
  • The maximum amount of air that the lungs can hold after a maximum forcefull inspiration (5200 to 5800ml).

Vital capacity (VC) : 
  • The maximum amount of air that can be breathed out after a maximum inspiration. 
  • It is the some total of TV, IRV and ERV and is 4100 to 4600ml.
C. Internal respiration :
  • The two main components of blood involved in transport of the respiratory gases- CO2 and O2, are the RBCs and the plasma.
i. Transport of oxygen :
  • Of the total oxygen transported only 3% is transported in a dissolved state by the plasma. 
  • The remaining 97% is bound to the haemoglobin (Hb) present in the RBCs.
  • Haemoglobin acts as the respiratory carrier. 
  • It has a high affinity for O2 and combines with it to form oxyhaemoglobin. 
  • Theoretically, one molecule of Hb has 4 Fe++, each of which can pick up a molecule of oxygen (O2).
  • Hb + 4O2 → Hb (4O2)
  • Oxyhaemoglobin is transported from lungs to the tissues where it readily dissociates to release O2.
  • Hb (4O2) → Hb + 4O2
  • However, the degree of saturation of Hb with O2 depends upon the O2 tension i.e. ppO2.
  • 100% saturation is rare.
  • Maximum saturation of 95 to 97% is at ppO2 in alveoli (100 mmHg).
  • Degree of saturation decreases with the drop in ppO2. This begins the dissociation of HbO2.
  • At 30 mmHg of ppO2, only 50% saturation can be maintained.
  • The relationship between HbO2 saturation and oxygen tension (ppO2) is called oxygen dissociation curve. 
  • This oxygen - haemoglobin dissociation curve is a sigmoid curve and it shifts towards the right due to - 
  1. increase in H+ concentration
  2. increase in ppCO2 
  3. rise in tempreature and 
  4. rise inDPG (2, 3 diphosphoglycerate), formed in the RBCs during glycolysis. 
  • It lowers the affinity of haemoglobin for oxygen.
Bohr effect : 
  • It is the shift of oxyhaemoglobin dissociation curve due to change in partial pressure of CO2 in blood.

Haldane effect : 

  • Oxyhaemoglobin functions as an acid. 
  • It decreases pH of blood. 
  • Due to increase in the number of H+ ions, HCO-3 changes into H2O and CO2.
  • In the alveoli where ppO2 is high and ppCO2 is low, oxygen binds with haemoglobin.
  • But in the tissues, where ppO2 is lower and ppCO2 is high, haemoglobin does not hold as much O2.
  • It releases O2 for diffusion into the tissue cells.
Carbon monoxide poisoning :
  • Affinity of haemoglobin for carbon monoxide is about 250 times more, than for oxygen. 
  • In the presence of carbon monoxide, haemoglobin readily combines to form a stable compound carboxyhaemoglobin.
  • The haemoglobin is blocked by carbon monoxide, preventing oxygen from binding with haemoglobin. 
  • Thus, less haemoglobin is available for oxygen transport depriving the cells of oxygen. This is carbon monoxide poisoning.

ii. Transport of CO2 :
  • Carbon dioxide is readily soluble in water and is transported by RBCs and plasma in three different forms.
a. By plasma in solution form (7%) :
  • Only 7% of CO2 is transported in a dissolved form as carbonic acid (which can breakdown into CO2 and H2O).
b. By bicarbonate ions (70%) : 
  • Nearly 70% of carbon dioxide released by the tissue cells diffuses into the plasma and then into the RBCs.
  • In the RBCs, CO2 combines with water in the presence of a Zn containing enzyme, carbonic anhydrase to form carbonic acid.
  • Carbonic anhydrase enzyme is found in the RBCs and not in the plasma.
  • The rate of formation of carbonic acid inside the RBC is very high as compared to its formation in the plasma.
  • Carbonic acid being unstable almost immidiately dissociates into HCO-3 and H+ in the presence of the enzyme carbonic anhydrase (CA) leading to large accumulation of HCO-3 inside the RBCs.
  • It thus moves out of the RBCs. This would bring about inbalance of the charge inside the RBCs. 
  • To maintain the ionic balance between the RBCs and the plasma, Cl- diffuses into the RBCs. 
  • This movement of chloride ions is known as chloride shift or Hamburger’s phenomenon.
  • HCO3- that comes into the plasma joins to Na+ / K+ forming NaHCO3 / KHCO3 (to maintain pH of blood).
  • H+ is taken up by protein (haemoglobin).
  • These H+ ions might be expected to lower blood pH, but they are buffered by haemoglobin by the formation of deoxyhaemoglobin (reduced haemoglobin).
  • At the level of the lungs in response to the low partial pressure of carbon dioxide (ppCO2) of the alveolar air, hydrogen ion and bicarbonate ions recombine to form carbonic acid and under the influence of carbonic anhydrase yields carbon dioxide and water.
c. By red blood cells (23%) : 
  • Carbon dioxide binds with the amino group of the haemoglobin and form a loosely bound compound carbaminohaemoglobin. 
  • This molecule readily decomposes in region where the partial pressure of carbon dioxide (ppCO2) is low (alveolar region), releasing the carbon dioxide.
D. Cellular Respiration :
  • It is the last step taking place inside the cell where food is oxidized and ATP is generated. 
  • It can be shown by two steps:
  1. Oxidation
  2. Phosphorylation
1. Oxidation : 
  • Breaking down of complex organic molecules into simple inorganic molecules with release of heat energy.
                    C6H12O6 + 6O2  → 6CO2 + 6H2O + 686 Kcal
2. Phosphorylation : 
  • It involves trapping the heat energy in the form of high energy bond of ATP molecule. 
  • ATP is used to carry out vital life processes and so is called as energy currency of the cell.
                          ADP + iP + 7.3 Kcal → ATP

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