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.