Breathing and Exchange of Gases

SYNOPSIS

• Human Respiratory System
  
  
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• Respiratory Volumes and Capacities
  
  
  

  Terms	Description
  Tidal Volume (TV)	It is the volume of air inspired or expired during normal respiration. The tidal volume is approximately 500 ml.
  Inspiratory Reserve Volume (IRV)	It is the additional volume of air a person can inspire by a forcible inspiration. A person can inhale 2500 to 3000 ml of air over and above the tidal volume.
  Expiratory Reserve Volume (ERV)	It is the additional volume of air a person can expire by a forcible expiration. A person can expire 1000 to 1100 ml of air over and above the tidal volume.
  Residual Volume (RV)	Even after a forcible expiration, some volume of air remains in the lungs which is called the residual volume. About 1100 to 1200 ml air remains in the lungs.
  Inspiratory Capacity (IC)	It is the volume of air which a person can inspire after normal expiration. This includes tidal volume and inspiratory reserve volume (TV + IRV).
  Expiratory Capacity (EC)	It is the total volume of air a person can expire after a normal inspiration. This includes the tidal volume and expiratory reserve volume (TV + ERV).b
  Functional Residual Capacity (FRC)	It is the volume of air which is left in the lungs after normal expiration. It is the sum of expiratory reserve volume (ERV) and residual volume (RV).
  Vital Capacity (VC)	It is the maximum volume of air a person can breathe in after a forced expiration. This is the sum of expiratory reserve volume (ERV), tidal volume (TV) and inspiratory reserve volume (IRV).
  Total Lung Capacity (TLC)	It is the amount of air in the lungs after maximum inspiration. It is the sum of the vital capacity and the residual volume.

• Transport of Oxygen

• About 97% of oxygen is transported in the blood in the form of oxyhaemoglobin.
• Each haemoglobin molecule can combine with four oxygen molecules to form oxyhaemoglobin.
• Rest 3% is in the dissolved state in plasma.
• Factors such as H+ ion concentration, pO₂, pCO₂ and temperature interfere with the binding of oxygen with haemoglobin.

• CO₂ is carried by blood in three forms:
• As a simple solution: About 5–10% of CO₂ of the total blood dissolves in plasma and is carried as a simple physical solution.
• As bicarbonate ions: CO₂ diffuses into the blood and reacts with water to form carbonic acid. Carbonic acid dissociates into bicarbonate HCO₃^- and H+ ions. These bicarbonate ions are carried by the plasma.
• As carbamino-haemoglobin: CO₂ combines loosely with the globin part of the reduced haemoglobin to form carbamino-haemoglobin. When pO₂ is high and pCO₂ is low in alveoli, CO₂ dissociates from carbamino-haemoglobin; hence, CO₂ bound to haemoglobin is released in the alveoli.

• Regulation of Respiration

• The respiratory rhythm centre present in the medulla oblongata is responsible for the regulation of respiratory rhythm.
• The pneumotaxic centre present in the pons regulates or moderates the functions of the respiratory rhythm centre.
• A chemosensitive area is located adjacent to the rhythm centre and is highly sensitive to CO₂ and hydrogen ions.