Chapter 40 complete- Guyton book 1st .pptx

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1. Chapter 40 - Guyton and hall text book of physiology Dr. ALI ZAIN MBBS, FCPS

2. *Diffusion Rate (D) = (ΔP × A × S) / (d × √MW)* - ΔP = Partial pressure difference between two ends - A = Cross-sectional area - S = Solubility of the gas - d = Distance of diffusion - MW = Molecular weight of the gas 2.⁠ ⁠*Partial Pressure Basics:* - Gas molecules hit surfaces creating pressure—this is partial pressure for each gas. - Higher gas concentration means higher partial pressure. - More soluble gas means lower partial pressure. - Henry’s Law: Partial Pressure = (Concentration of dissolved gas) / (Solubility coefficient) 3.⁠ ⁠*Solubility Note:* - CO₂ is about 20 times more soluble than O₂, so it diffuses easier despite its weight. 4.⁠ ⁠*Example partial pressures in air:* - Atmospheric air total pressure = 760 mmHg - N₂ = 597 mmHg, O₂ = 159 mmHg, CO₂ = 0.3 mmHg, H₂O = 3.7 mmHg - Humidified air adjusts these values slightly. Water vapor pressure changes with temperature: •⁠ ⁠O₂ = 5 mmHg •⁠ ⁠Body temperature = 47 mmHg •⁠ ⁠100°C = 760 mmHg

3. •⁠ ⁠The left side shows a detailed cross-section drawing of the respiratory membrane between an alveolus and a capillary. It labels these layers: 1. Fluid & surfactant layer 2. Alveolar epithelium 3. Epithelial basement membrane 4. Interstitial space 5. Capillary basement membrane 6. Capillary endothelium 7. Red blood cell (RBC) inside the capillary •⁠ ⁠The notes say the membrane thickness is about 0.2 micrometers and the surface area is roughly 70 square meters. •⁠ ⁠On the right side, the Factors Affecting Rate of Gas Diffusion are listed: 1. Thickness of membrane (increased in edema, fibrosis) 2. Surface area of membrane (decreased in emphysema) 3. Diffusion coefficient of gas (CO₂ 20× faster than O₂) 4. Partial pressure difference •⁠ ⁠At the bottom right, the Respiratory Unit (Respiratory Lobule) 1. Respiratory bronchiole 2. Alveolar ducts 3. Atria 4. Alveoli

4. Diffusing Capacity: •⁠ ⁠It defines diffusing capacity as the volume of gas that diffuses through the membrane each minute per 1 mmHg pressure difference. *Oxygen:* •⁠ ⁠Diffusing capacity = 21 ml/min/mmHg •⁠ ⁠Mean pressure difference = 11 mmHg •⁠ ⁠So, 21 × 11 = 230 ml/min oxygen diffuses normally •⁠ ⁠During exercise, diffusing capacity rises to 65 ml/min/mmHg due to better capillary opening and improved ventilation/perfusion. *Carbon Dioxide:* •⁠ ⁠Small pressure difference (~<1 mmHg) between alveoli and blood makes it hard to measure. •⁠ ⁠Diffusing capacity is ~20 times greater than oxygen’s. •⁠ ⁠So, 21 × 20 = 400–450 ml/min/mmHg normally. •⁠ ⁠During exercise, it jumps to 1200–1300 ml/min/mmHg.

5. •⁠ ⁠To measure diffusing capacity of any gas, we need: 1. Alveolar partial pressure 2. Pulmonary capillary blood partial pressure 3. Rate of gas uptake by blood •⁠ ⁠We use *Carbon Monoxide* because: - It has zero partial pressure inside pulmonary capillary blood (binds rapidly to hemoglobin) - This allows easy calculation of diffusing capacity •⁠ ⁠CO diffusing capacity = 17 ml/min/mmHg •⁠ ⁠Oxygen diffusing capacity is 1.23 times CO’s, so: - Oxygen diffusing capacity = 17 × 1.23 = 21 ml/min/mmHg