Respiratory System: gas exchange and pulmonary circulation (lecture 15)

Partial Pressure

• Atmospheric pressure is the sum of the pressure of all gases in a mixture, according to Dalton's Law.
• Partial pressure is the contribution of one gas to the total pressure.
• At sea level, atmospheric pressure measures 760 mm Hg.
• Oxygen constitutes 20.9% of the atmosphere.
• The partial pressure of oxygen (PO₂) is 159 mm Hg, representing 20.9% of atmospheric pressure.
• Gases move from areas of high pressure to areas of low pressure.
• Atmospheric pressure decreases with altitude due to less air weight pressing down.
• Lower partial pressure of oxygen at high altitudes can cause breathing difficulties; supplemental oxygen helps.

Alveolar Gas Exchange

• The epithelium lining the lumen is coated with a water film.
• Gases must dissolve in water to enter tissues and blood.
• Alveolar air enters the water film before reaching the respiratory membrane, which separates air from the bloodstream.
• Gases diffuse into water if their partial pressure is greater than that of the water.
• Higher PO₂ in alveolar air results in increased O₂ uptake by the blood.
• Blood arriving at the alveolus has a higher PCO₂ than the air, causing it to release CO₂ into the alveolar air.
• Achieving 100% blood saturation is nearly impossible, despite blood leaving alveolar capillaries being highly saturated.
• Anastomosis is the convergence or joining of two structures.
• Pulmonary veins anastomose with bronchial veins, mixing oxygen-rich pulmonary blood with oxygen-poor systemic blood.

Changes in Partial Pressures & the Pulmonary Circuit

• Inspired air contains a PO₂ of 159 mm Hg and a PCO₂ of 0.3 mm Hg.
• Alveolar air contains a PO₂ of 104 mm Hg and a PCO₂ of 40 mm Hg.
• Expired air contains a PO₂ of 116 mm Hg and a PCO₂ of 32 mm Hg.
• Deoxygenated blood has a PO₂ of 40 mm Hg and a PCO₂ of 46 mm Hg.
• Oxygenated blood has a PO₂ of 95 mm Hg and a PCO₂ of 40 mm Hg.

Gas Exchange Efficiency

• Key factors influencing efficiency include pressure gradients, gas solubility, membrane thickness and area, and ventilation-perfusion coupling.
• CO₂ is 20 times more soluble in water than O₂.
• Solubility refers to the ability of a substance to dissolve in water.
• The membrane is approximately 0.5mm thick, which prevents little obstacle to diffusion
• Ventilation-perfusion coupling ensures blood flow is matched to oxygen supply.
• Poor ventilation leads to low PO₂, causing vasoconstriction, while increased ventilation leads to high PO₂, causing vasodilation.
• A healthy lung has about 70m² of respiratory membrane for gas exchange.
• Alveolar capillaries contain only about 100ml of blood at any given time, spreading the blood thinly.
• Certain pulmonary diseases reduce the alveolar surface area, leading to low blood PO₂.
• Emphysema and lung cancer are examples of diseases that reduce the alveolar surface area for gas exchange.

Gas Exchange

• Alveoli are composed of thin simple squamous cells, which facilitate gas exchange.
• Blood vessels surround alveoli, allowing efficient O₂ and CO₂ transfer.
• During inspiration, alveoli have high O₂ and low CO₂ concentrations compared to deoxygenated blood returning from the body.
• The concentration difference drives O₂ diffusion from alveoli into the blood and CO₂ from the blood into alveoli.
• Oxygen moves from the air, across the water film lining the alveoli, through the alveolar epithelium, and into the blood.
• Oxygen binds to hemoglobin within RBCs and is transported to body tissues.
• At the tissues, high oxygen concentration in blood contrasts with the tissues undergoing aerobic metabolism (using O₂ to produce CO₂ + H₂O).
• Blood possesses a high O₂ and low CO₂ concentration, while tissues have low O₂ and high CO₂ concentrations.
• Oxygen unbinds from hemoglobin and diffuses into tissue cells when pH is low and CO₂ is high.
• CO₂ diffuses out of tissue cells and enters the blood.
• CO₂ binds to hemoglobin inside RBCs.
• Within RBCs, CO₂ + H₂O ⇄ H₂CO₃ ⇄ H⁺ + HCO₃⁻ occurs, converting carbon dioxide and water into carbonic acid, then bicarbonate and hydrogen ions.
• Carbonic anhydrase assists in the conversion of carbonic acid to bicarbonate.
• HCO₃⁻ leaves RBCs to buffer against low pH.
• Cl⁻ enters RBCs to maintain electrical balance (Chloride shift).
• Back in the lungs where temperatures are colder.
• Alveoli have high O₂ and blood has low O₂.
• Alveoli have low CO₂ and blood has high CO₂.
• HCO₃⁻ enters RBCs, and Cl⁻ exits (reverse Chloride shift).
• The equation reverses to regenerate CO₂ + H₂O: CO₂ + H₂O ⇄ H₂CO₃ ⇄ H⁺ + HCO₃⁻.
• CO₂ diffuses out of RBCs and into alveoli.
• Oxygen diffuses from alveoli into the blood.
• This cycle repeats roughly three times per minute.

Pulmonary Circulation

• Deoxygenated blood enters the right atrium of the heart from the superior vena cava, inferior vena cava, and coronary sinus.
• Atrial contraction pushes blood into the right ventricle.
• Ventricular contraction sends blood through the pulmonary trunk to the pulmonary arteries and the lungs.
• The blood travels from the lungs to the pulmonary veins, which lead to the left atrium.
• Contraction of the left atrium moves blood to the left ventricle and then to the aorta.
• The aorta delivers blood to systemic and coronary circulation.