Transport of Gases

TRANSPORT OF GASES

  • In the human body, oxygen (O2) and carbon dioxide (CO2) are crucial gases involved in cellular respiration and waste removal. 
  • These gases rely on the bloodstream as their primary mode of transport. Red blood cells (RBCs) play a significant role in carrying these gases, with a vast majority of oxygen and a significant portion of carbon dioxide being transported by them. 
  • Additionally, a small fraction of oxygen and carbon dioxide is carried in a dissolved state through the plasma, further contributing to the efficient exchange and distribution of these gases throughout the body. 
  • Understanding how oxygen and carbon dioxide are transported in the blood is essential for grasping the mechanisms underlying respiratory physiology and maintaining optimal cellular function. 
  • Approximately 97% of oxygen is transported by RBCs, primarily bound to hemoglobin molecules within these cells. 
  • The remaining 3% of oxygen is carried in a dissolved state through the plasma, contributing to the overall oxygen-carrying capacity of the blood. 
  • Around 20-25% of carbon dioxide is transported by RBCs, where it can bind to hemoglobin or be converted into bicarbonate ions. 
  • The majority (about 70%) of carbon dioxide is transported as bicarbonate ions (HCO3-) in the plasma. This conversion is facilitated by the enzyme carbonic anhydrase. 
  • Approximately 7% of carbon dioxide is carried in a dissolved state through the plasma, contributing to the overall removal of carbon dioxide from tissues to the lungs for exhalation.

 

Transport of Oxygen

 

  • Haemoglobin is an iron-containing pigment found in red blood cells (RBCs). 
  • Oxygen (O2) binds to haemoglobin in a reversible manner, forming oxyhaemoglobin. 
  • Each haemoglobin molecule can carry a maximum of four molecules of oxygen. 
  • Oxygen binding with haemoglobin is primarily influenced by the partial pressure of oxygen (pO2). 
  • Partial pressure of carbon dioxide (pCO2), hydrogen ion concentration (H+), and temperature also affect oxygen binding. 
  • A sigmoid curve representing the percentage saturation of haemoglobin with oxygen plotted against pO2.

  

 

 

  • Oxygen dissociation curve helps to study the effects of factors such as pCO2, H+ concentration, etc., on oxygen binding to haemoglobin. 
  • In the alveoli, where there is high pO2, low pCO2, lower H+ concentration, and lower temperature, conditions are favorable for the formation of oxyhaemoglobin. 
  • In tissues, where there is low pO2, high pCO2, high H+ concentration, and higher temperature, conditions favor the dissociation of oxygen from oxyhaemoglobin. 
  • Each 100 ml of oxygenated blood can deliver approximately 5 ml of oxygen to the tissues under normal physiological conditions.

 

Transport of Carbon Dioxide 

 

  • Carbon dioxide (CO2) is carried by haemoglobin as carbamino-haemoglobin, accounting for about 20-25% of CO2 transport. 
  • Binding of CO2 to haemoglobin is related to the partial pressure of CO2 (pCO2). 
  • The partial pressure of oxygen (pO2) also affects CO2 binding. 
  • In tissues with high pCO2 and low pO2, more binding of CO2 occurs. 
  • In the alveoli with low pCO2 and high pO2, dissociation of CO2 from carbamino-haemoglobin occurs, facilitating CO2 release. 
  • Role of Carbonic Anhydrase:

- Enzyme Concentration: Red blood cells (RBCs) contain high concentrations of carbonic anhydrase, while minute quantities are present in plasma. 

- Reaction Facilitation: Carbonic anhydrase catalyzes the conversion of CO2 into bicarbonate ions (HCO3-) and hydrogen ions (H+), and vice versa. 

  •  At tissue sites with high pCO2 due to cellular metabolism, CO2 diffuses into blood (RBCs and plasma) and forms HCO3- and H+ ions. 
  • At alveolar sites with low pCO2, the reaction proceeds in the opposite direction, leading to the formation of CO2 and water (H2O). 
  • CO2 trapped as bicarbonate at the tissue level is transported to the alveoli and released as CO2. 
  • Approximately 4 ml of CO2 is delivered to the alveoli by every 100 ml of deoxygenated blood under normal physiological conditions.

 

Regulation of Respiration:

 

  • Neural Control:

- Human beings possess the ability to regulate respiratory rhythm to meet the body's demands, primarily through the neural system. 

- Respiratory Rhythm Center: Located in the medulla region of the brain, this specialized center is responsible for regulating respiratory rhythm. 

- Pneumotaxic Center: Found in the pons region of the brain, this center can influence the functions of the respiratory rhythm center, adjusting the duration of inspiration and thereby altering respiratory rate. 

  • Chemoreceptors:

- Adjacent to the rhythm center, chemosensitive areas are highly sensitive to changes in carbon dioxide (CO2) and hydrogen ion concentrations. 

- Activation: Increased levels of these substances activate the chemosensitive area, signaling the rhythm center to make necessary adjustments in the respiratory process to eliminate them. 

  • Peripheral Receptors:

- Receptors associated with the aortic arch and carotid artery monitor changes in CO2 and H+ concentrations. 

- These receptors send signals to the rhythm center to initiate remedial actions in response to detected changes.

 

  • Role of Oxygen:

- Insignificant: Oxygen plays a minor role in regulating respiratory rhythm compared to CO2  and H+ concentrations.

 

Disorder of Respiratory System 

 

  • Asthma:

- Asthma is a respiratory disorder characterized by difficulty in breathing and wheezing due to inflammation of the bronchi and bronchioles. 

- Wheezing, shortness of breath, coughing, and chest tightness are common symptoms. 

- Causes: Triggers include allergens, irritants, exercise, and respiratory infections. 

  • Emphysema:

- Emphysema is a chronic disorder where the alveolar walls are damaged, leading to a decrease in the respiratory surface area. 

- Cigarette smoking is a major cause of emphysema, leading to the destruction of alveolar walls and loss of elasticity in lung tissue. 

- Symptoms: Shortness of breath, coughing, wheezing, and difficulty breathing are common symptoms. 

  • Occupational Respiratory Disorders:

- Certain industries, such as those involving grinding or stone-breaking, produce excessive dust that overwhelms the body's defense mechanisms. 

- Consequences: Prolonged exposure to such dust can lead to inflammation and fibrosis (proliferation of fibrous tissues), causing serious lung damage. 

- Preventive Measures: Workers in these industries should wear protective masks to prevent inhalation of harmful dust particles.