Respiratory Volumes and Capacities
Tidal Volume (TV)
The volume of air inspired or expired during a normal respiration is called Tidal Volume. It is approximately 500 ml in a healthy man. This means that a healthy adult can inspire or expire about 6 to 8 litre of air per minute.
Inspiratory Reserve Volume (IRV)
Additional volume of air, a person can inspire by a forcible inspiration is called Inspiratory Reserve Volume. This is about 2500 ml to 3000 ml in normal adult.
Expiratory Reserve Volume (ERV)
Additional volume of air, a person can expire by a forcible expiration is called Expiratory Reserve Volume. This is about 1000 ml to 1100 ml in a normal adult.
Residual Volume (RV)
The volume of air remaining in the lungs even after a forcible expiration is called Residual Volume. This is about 1100 ml to 1200 ml in a normal adult.
Inspiratory Capacity (IC)
The total volume of air a person can inspire after a normal expiration is called Inspiratory Capacity. This includes the tidal volume and inspiratory reserve volume, i.e. IC = TV + IRV.
Expiratory Capacity (EC)
The total volume of air a person can expire after a normal inspiration is called Expiratory Capacity. EC = TV + ERV.
Functional Residual Capacity (FRC)
The volume of air which remains in the lungs after a normal expiration is called Functional Residual Capacity. FRC = ERV + RV.
Vital Capacity (VC)
The maximum volume of air a person can breathe in after a forceful expiration is called Vital Capacity. This is also defined as the maximum volume of air a person can breathe out after a forceful inspiration. VC = ERV + TV + IRV.
Total Lung Capacity
Total volume of air accommodated in the lungs at the end of a forced inspiration is called Total Lung Capacity. Total Lung Capacity = VC + RV = (ERV + TV + IRV) + RV.
Exchange of Gases
Alveoli are the main sites of exchange of gases. However, exchange of gases also occurs between blood and tissues. The exchange of O2 and CO2 at these sites happens by simple diffusion which is mainly based on pressure/concentration gradient. Some important factors which can affect the rate of diffusion are; solubility of gases and thickness of the membranes involved in diffusion. Pressure contributed by an individual gas in a mixture of gases is called partial pressure. It is represented as pO2 for oxygen and pCO2 for carbon dioxide.
Transport of Gases
Blood is the medium of transport for O2 and CO2. About 97% of oxygen is transported by RBCs. The remaining 3% of oxygen is carried in a dissolved state through the plasma. About 20-25% of carbon dioxide is transported by RBCs. About 70% of carbon dioxide is carried as bicarbonate and about 7% is carried in a dissolved state through plasma.
Transport of Oxygen
Oxygen can bind with haemoglobin in a reversible manner to form oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four molecules of oxygen. Binding of oxygen with haemoglobin is mainly related to the partial pressure of O2. Partial pressure of CO2, hydrogen ion concentration and temperature are the other factors which can interfere with this binding. When percentage saturation of haemoglobin with O2 is plotted against pO2, we get a sigmoid curve. This curve is called Oxygen Dissociation Curve. This curve is very useful in studying the effect of factors; like pCO2, H+ concentration, etc. on binding of O2 with haemoglobin.
Transport of Carbon dioxide
The binding of carbon dioxide with haemoglobin is related to partial pressure of CO2. The partial pressure of O2 is a major factor which can affect this binding. In the tissues, pCO2 is higher than pO2 and hence more binding of carbon dioxide occurs at the tissue level. In the alveoli, pCO2 is lower than pO2 and hence dissociation of carbamino-haemoglobin takes place in the alveoli.
RBCs contain a very high concentration of the enzyme; carbonic anhydrase. Minute quantities of the same enzyme are present in plasma as well. This enzyme facilitates the following reaction.
CO2 + H2O ⇄ H2CO3 ⇄ HCO3- + H+
At the tissue site, partial pressure of CO2 is high due to catabolism. At this level, CO2 diffuses into blood and forms bicarbonate and hydrogen ions.
At the alveolar site, pCO2 is low. At this level, the reaction proceeds in the opposite direction and thus carbon dioxide and water are formed. Thus, carbon dioxide trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO2. Every 100 ml of deoxygenated blood delivers about 4 ml of CO2 to the alveoli.