Detaile Notes-2 – Chapter 14 -Breathing and Exchange of Gases

14.3 EXCHANGE OF GASES

1. Primary Sites of Gas Exchange:

   • Alveoli are the primary sites of gas exchange in the respiratory system.

2. Exchange between Blood and Tissues:

   • Exchange of oxygen (O2) and carbon dioxide (CO2) also occurs between blood and tissues.
   • This exchange happens by simple diffusion, primarily based on pressure/concentration gradient.

3. Factors Affecting Diffusion Rate:

   • Solubility of gases and thickness of the membranes involved in diffusion are important factors affecting the rate of diffusion.

4. Partial Pressures and Concentration Gradient:

   • Partial pressure, represented as pO2 for oxygen and pCO2 for carbon dioxide, is the pressure contributed by an individual gas in a mixture of gases.
   • There is a concentration gradient for oxygen from alveoli to blood and blood to tissues, and for CO2 in the opposite direction, from tissues to blood and blood to alveoli.

5. Solubility and Diffusion:

   • The solubility of CO2 is 20-25 times higher than that of O2, making the diffusion of CO2 much higher per unit difference in partial pressure compared to O2.
   • The diffusion membrane, composed of the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement substance between them, has a total thickness much less than a millimeter.

6. Favorable Factors for Diffusion:

   • All factors in the body are favorable for the diffusion of O2 from alveoli to tissues and CO2 from tissues to alveoli due to the concentration gradients and the thin, efficient diffusion membrane.

14.4 TRANSPORT OF GASES

1. Blood as the Medium of Transport:

   • Blood serves as the primary medium of transport for both oxygen (O2) and carbon dioxide (CO2) in the body.

2. Oxygen Transport:

   • Approximately 97% of oxygen is transported by red blood cells (RBCs) in the blood.
   • The remaining 3% of oxygen is carried in a dissolved state through the plasma.

3. Carbon Dioxide Transport:

   • Around 20-25% of carbon dioxide is transported by red blood cells.
   • The majority of carbon dioxide, about 70%, is transported as bicarbonate ions.
   • Approximately 7% of carbon dioxide is carried in a dissolved state through the plasma.

14.4.1 Transport of Oxygen

1. Role of Hemoglobin:

   • Hemoglobin is a red-colored iron-containing pigment present in red blood cells (RBCs).
   • Oxygen (O2) can bind with hemoglobin in a reversible manner to form oxyhemoglobin.
   • Each hemoglobin molecule can carry a maximum of four molecules of oxygen.

2. Factors Affecting Oxygen Binding:

   • The binding of oxygen with hemoglobin is primarily related to the partial pressure of oxygen (pO2).
   • Other factors such as partial pressure of carbon dioxide (pCO2), hydrogen ion (H+) concentration, and temperature can also influence this binding.

3. Oxygen Dissociation Curve:

   • When the percentage saturation of hemoglobin with oxygen is plotted against pO2, a sigmoid curve is obtained.
   • This curve, known as the Oxygen Dissociation Curve, is highly useful in studying the effect of factors like pCO2, H+ concentration, etc., on the binding of oxygen with hemoglobin.

4. Conditions in Alveoli and Tissues:

   • In the alveoli, where there is high pO2, low pCO2, lesser H+ concentration, and lower temperature, the conditions are favorable for the formation of oxyhemoglobin.
   • In the tissues, where there is low pO2, high pCO2, high H+ concentration, and higher temperature, the conditions are favorable for the dissociation of oxygen from oxyhemoglobin.

5. Delivery of Oxygen to Tissues:

   • Every 100 ml of oxygenated blood can deliver around 5 ml of oxygen to the tissues under normal physiological conditions.

14.4.2 Transport of Carbon dioxide

1. Binding to Hemoglobin:

   • Carbon dioxide (CO2) is carried by hemoglobin as carbamino-hemoglobin, accounting for about 20-25% of CO2 transport in the blood.
   • The binding of CO2 to hemoglobin is related to the partial pressure of CO2 (pCO2).

2. Effect of Partial Pressures:

   • High pCO2 and low pO2 in tissues favor the binding of carbon dioxide to hemoglobin.
   • Conversely, low pCO2 and high pO2 in the alveoli lead to the dissociation of CO2 from carbamino-hemoglobin.

3. Role of Carbonic Anhydrase:

   • Red blood cells (RBCs) contain a high concentration of the enzyme carbonic anhydrase, which facilitates the conversion of CO2 and water to carbonic acid (H2CO3), which further dissociates into bicarbonate ions (HCO3-) and hydrogen ions (H+).
   • This reaction occurs in both directions, depending on the prevailing partial pressures of CO2 and O2.

4. Transport and Release of CO2:

   • At tissue sites where pCO2 is high due to cellular metabolism, CO2 diffuses into the blood (both RBCs and plasma) and forms bicarbonate ions and hydrogen ions.
   • At the alveolar sites where pCO2 is low, the reverse reaction occurs, leading to the release of CO2 and water.
   • CO2 trapped as bicarbonate at the tissue level is transported to the alveoli, where it is released as CO2 during expiration.

5. Amount of CO2 Delivered to Alveoli:

   • Every 100 ml of deoxygenated blood delivers approximately 4 ml of CO2 to the alveoli.

14.5 REGULATION OF RESPIRATION

1. Respiratory Rhythm Center:

   • The regulation of respiratory rhythm is primarily controlled by a specialized center located in the medulla region of the brain called the respiratory rhythm center.

2. Pneumotaxic Center:

   • Another center present in the pons region of the brain, known as the pneumotaxic center, can modulate the functions of the respiratory rhythm center.
   • Neural signals from this center can shorten the duration of inspiration, thereby altering the respiratory rate.

3. Chemosensitive Area:

   • Adjacent to the rhythm center, there is a chemosensitive area highly sensitive to changes in carbon dioxide (CO2) and hydrogen ion (H+) concentrations.
   • Increased levels of these substances can activate this area, which then signals the rhythm center to make necessary adjustments in the respiratory process to eliminate these substances.

4. Role of Receptors:

   • Receptors associated with the aortic arch and carotid artery can also detect changes in CO2 and H+ concentrations.
   • These receptors send signals to the rhythm center, prompting remedial actions to adjust the respiratory rhythm accordingly.

5. Role of Oxygen:

   • The role of oxygen in the regulation of respiratory rhythm is relatively insignificant compared to CO2 and H+.
   • Oxygen levels do not have a major direct effect on respiratory rhythm regulation.

14.6 DISORDERS OF RESPIRATORY SYSTEM

1. Asthma:

   • Asthma is characterized by difficulty in breathing, often accompanied by wheezing.
   • It occurs due to inflammation of the bronchi and bronchioles, which are the airways leading to the lungs.
   • Symptoms can include shortness of breath, chest tightness, coughing, and wheezing.
   • Asthma attacks can be triggered by various factors such as allergens, respiratory infections, exercise, and stress.

2. Emphysema:

   • Emphysema is a chronic respiratory disorder characterized by damage to the alveolar walls in the lungs.
   • This damage leads to a decrease in the respiratory surface area available for gas exchange.
   • Cigarette smoking is one of the major causes of emphysema, as it leads to chronic inflammation and destruction of lung tissue.
   • Symptoms of emphysema include shortness of breath, coughing, wheezing, and fatigue.

3. Occupational Respiratory Disorders:

   • Certain industries, such as those involving grinding or stone-breaking, can produce a significant amount of dust.
   • Prolonged exposure to such dust particles can overwhelm the body’s defense mechanisms, leading to inflammation and damage to the lungs.
   • Chronic exposure may result in fibrosis, which is the proliferation of fibrous tissues in the lungs, causing serious lung damage.
   • Workers in such industries should wear protective masks to minimize exposure to harmful dust particles and prevent respiratory disorders.