Fick's Law of Gas Diffusion and Lung Cancer - Lecture Notes | BMS 360, Study notes of Biology

Download Fick's Law of Gas Diffusion and Lung Cancer - Lecture Notes | BMS 360 and more Study notes Biology in PDF only on Docsity! 24 April Figure 13 – 23 Oxygen diffusion along the length of the pulmonary capillaries quickly achieves diffusional equilibrium, unless disease processes in the lung reduce the rate of diffusion About 1 sec for RBC to flow through pulmonary capillary About ¼ sec for alveolar O2 to bind to hemoglobin (within 1/3 length of capillary) Reserve pulmonary capillary capacity At rest, capillaries are closed at the apex (top) of lungs due to low blood pressure (standing). These capillaries provide reserve capacity. Due to gravity, the transit time for blood is fastest at the lower lungs, and slower at the upper lungs. During exercise, the blood pressure in apex of lungs increases, the capillaries open, so more capillaries are participating in gas exchange. Fick’s Law of Gas Diffusion This law determines gas exchange across the respiratory membrane A greater partial pressure of the gas or a greater surface area results in a greater volume of gas that diffuses The thicker the membrane, the less gas diffusion Effect of Disease on Alveolar Gas Exchange Pulmonary edema: Increased pulmonary vein pressure causes fluid accumulation in interstitial space, lung cells and alveoli. Alveoli are compressed and respiratory membrane thicker, reducing gas exchange Emphysema: The loss of lung tissues and decrease in total number of working alveoli would decrease the total area available for diffusion, reducing gas exchange. Diffuse interstitial fibrosis: Alveolar cell walls thicken (fibrotic), the diffusing distance is increased, and gas exchange is reduced Lung cancer: Surface area available for gas diffusion can be substantially decreased Matching Ventilation and Perfusion Ventilation-perfusion inequality: This occurs when there is mismatch between alveolar airflow (ventilation) and pulmonary capillary blood flow (perfusion) Hypoxemia: Decreased pO2 in the pulmonary veins due to mismatching of ventilation-perfusion Hypoxemia can normally occur when standing, because gravity causes greater perfusion in the lower vs. upper lung. This explains why pO2 is 105 mm Hg in alveoli, whereas only 100 mm in the pulmonary vein Disease: Mismatch can be caused by diseases affecting lung compliance, airway resistance or vascular resistance. Clots may block blood supply Figure 13 – 24 Oxygen Transport in Blood Oxygen is carried in the blood in two forms Oxygen dissolved according to Henry’s Law (1.5%) In plasma and RBC cytosol The solubility of oxygen in water in about 3 mL/L This limits oxygen transport Hemoglobin is necessary to overcome this limitation Oxygen reversibly bound to hemoglobin (98.5) In erythrocytes is about 197 mL/L Greatly increases O2 carrying capacity of blood Figure 13 – 25 Hemoglobin is the gas-transport molecule inside erythrocytes An increase in the acidity of blood causes hyperventilation, independently of carbon dioxide levels Factors that Reduce Hb Affinity for Oxygen Figure 13 – 8 Increased temperature Exercise; less effect than below factors Increased 2,3-diphosphoglycerate (DPG): From RBC Example: chronic lung disease, anemia, congestive heart, high altitude – adaptive to low oxygen levels by greater unloading O2 to tissues Increased H+ concentration (lower pH) Alkalosis increases affinity Increased pCO2 CO2 creates more bicarbonate and H +, which reduces affinity Disorders of Oxygen Transport Resulting in Hypoxia Hypoxic hypoxia: Low arterial pO2 due to poor ventilation Typical causes: high altitude, alveolar hypoventilation, decreased lung diffusion capacity, abnormal ventilation-perfusion ratio Anemic hypoxia: Decreased total amount of O2 bound to hemoglobin Typical causes: blood loss, anemia (low [Hb] of altered HbO2 binding), carbon monoxide poisoning Ischemic hypoxia: Reduced blood flow Typical causes: heart failure (whole-body hypoxia), shock (peripheral hypoxia), thrombosis (hypoxia in a single organ) Histotoxic hypoxia: Failure of cells to use O2 because cells have been poisoned Typical causes: Cyanide and other metabolic poisons Carbon Dioxide Transport CO2 is transported in blood in three forms: The exact percentages can vary depending on conditions Dissolved (7 – 10%) Bound to Hb as carbaminohemoglobin (23 – 30%) Deoxyhemoglobin (Hb w/o oxygen) has a higher affinity for CO2 than O2 As the bicarbonate ion, HCO3 - (60 – 70%) Carbonic anhydrase, located in the RBC, converts CO2 plus water to carbonic acid, which dissociates to HCO3 - plus H+ Note that venous blood is more acidic than arterial blood Chloride shift: HCO3 - is transported out of RBC in exchange for Cl- The high pCO2 in tissues drives carbonic acid production by law of mass action