Respiratory System Gas Exchange PDF

Respiratory system gas exchange D/Suzan Mustafa Hazzaa Professor of clinical physiology Physiology department Menoufia University  Oxygen carriage by the blood Oxygen is carried by the blood in two forms: Physical form. Chemical combination. 1. Physical form (Dissolve...

Respiratory system gas exchange D/Suzan Mustafa Hazzaa Professor of clinical physiology Physiology department Menoufia University  Oxygen carriage by the blood Oxygen is carried by the blood in two forms: Physical form. Chemical combination. 1. Physical form (Dissolved Oxygen 1-2% = 0.3ml/dl): A small portion of oxygen is dissolved directly in the plasma. Although the amount of oxygen dissolved in plasma is small, it plays a crucial role in determining the partial pressure of oxygen (PaO₂) in the blood and supports gas exchange across the alveoli and tissues. 2. Chemical combination (Hemoglobin-Bound Oxygen 98-99%): Oxygen binds to hemoglobin in the red blood cells, forming oxyhemoglobin (HbO₂). Most oxygen in the blood is transported this way, and it is essential for efficient oxygen delivery to tissues. Mechanisms of Oxygen Transport: Oxygen Bound to Hemoglobin (Hb): Hemoglobin Structure: Hemoglobin consists of four subunits, each containing a heme group with an iron (Fe²⁺) atom that can bind one oxygen molecule (O₂). Each hemoglobin molecule can carry up to four oxygen molecules. Oxygen Saturation: The percentage of hemoglobin molecules fully saturated with oxygen is referred to as oxygen saturation (SaO₂). Under normal conditions, oxygen saturation in arterial blood is typically around 95-100%. Hemoglobin's affinity for oxygen increases as more oxygen molecules bind. When oxygen is bound to hemoglobin, the compound is called oxyhemoglobin. In this form, oxygen is transported from the lungs to the tissues. In tissues where oxygen levels are low, hemoglobin releases oxygen, converting back to deoxyhemoglobin. Oxygen Transport Process: In the Lungs: Alveolar Gas Exchange: Oxygen diffuses from the alveoli (where oxygen partial pressure (PaO₂) is high) into the pulmonary capillaries due to the difference in oxygen partial pressure between the alveoli and the blood forming oxyhemoglobin. In the Tissues: In the low oxygen partial pressure in the tissue, hemoglobin releases oxygen, which diffuses into the tissues where it is needed for cellular respiration. Factors Affecting Oxygen Binding and Release: 1. Partial Pressure of Oxygen (PaO₂): Which is determined by the amount of oxygen dissolved in plasma, and this influences the binding of oxygen to hemoglobin. A higher PaO₂ results in more oxygen binding to hemoglobin. 2. Ph (Bohr Effect): In areas where CO₂ levels are high (such as active tissues), the pH drops, leading to a decreased affinity of hemoglobin for oxygen, causing more oxygen to be released to the tissues. 3. Temperature: Higher temperatures (such as during exercise) reduce hemoglobin’s affinity for oxygen, enhancing oxygen delivery to active tissues. 4. 2,3-Diphosphoglycerate (2,3-DPG): It is produced by red blood cells and reduces hemoglobin’s affinity for oxygen, thus promoting oxygen release to tissues. Carbon dioxide carriage by the blood Carbon dioxide is carried by the blood in two forms: Physical form. Chemical combination. 1. Physical form (Dissolved carbon dioxide 1-2% = 0.3ml/dl): A small portion of CO₂ is transported dissolved directly in the plasma. CO₂ is more soluble in blood than oxygen, so a larger fraction of it can dissolve. This dissolved CO₂ contributes to the partial pressure of carbon dioxide (PaCO₂) in the blood and drives CO₂ diffusion from tissues into the blood and from the blood into the alveoli for exhalation. 2. Chemical combination: Carbon dioxide (CO₂) is transported in the chemically bound form as carbamino compounds bounded to hemoglobin (30%) or bicarbonate (63%) ions. a. Chemically bounded to hemoglobin (carbaminohemoglobin): Unlike oxygen, CO₂ does not bind to the iron atom of the heme group but rather to the globin protein portion of hemoglobin. This process is reversible: in tissues, where CO₂ levels are high, CO₂ binds to hemoglobin; then in the lungs, where CO₂ levels are low, it is released and exhaled. Haldane Effect: Hemoglobin's ability to carry CO₂ is influenced by its oxygenation state. When hemoglobin releases oxygen in the tissues, its affinity for CO₂ increases. This is known as the Haldane effect, which facilitates CO₂ uptake in tissues and release in the lungs, where oxygen is more abundant. b. Bicarbonate Ions (HCO₃⁻): The majority of CO₂ in the blood is transported in the form of bicarbonate ions (HCO₃⁻). This process occurs through the following steps: In red blood cells, CO₂ reacts with water (H₂O) under the influence of the enzyme carbonic anhydrase to form carbonic acid (H₂CO₃). Carbonic acid is unstable and quickly dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻). CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ The bicarbonate ions diffuse out of red blood cells into the plasma, where they are transported to the lungs. To maintain electrical neutrality, chloride ions (Cl⁻) move into the red blood cells in exchange for bicarbonate ions. This is called the chloride shift or Hamburger effect. In the lungs, the process reverses: bicarbonate ions re-enter red blood cells, recombine with hydrogen ions to form carbonic acid, and carbonic anhydrase catalyzes the breakdown of carbonic acid back into CO₂ and water. The CO₂ is then exhaled. Thank you 14