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SCAN0095 - Humidification of air by conducting passages The...

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Unformatted text preview: Humidification of air by conducting passages The mixing of alveolar gas that occurs with each breath External Respiration: Pulmonary Gas Exchange Factors influencing the movement of oxygen and carbon dioxide across the respiratory membrane Partial pressure gradients and gas solubilities Matching of alveolar ventilation and pulmonary blood perfusion Structural characteristics of the respiratory membrane Partial Pressure Gradients and Gas Solubilities The partial pressure oxygen (P02) of venous blood is 40 mm Hg; the partial pressure in the alveoli is 104 mm Hg This steep gradient allows oxygen partial pressures to rapidly reach equilibrium (in 0.25 seconds), and thus blood can move three times as quickly (0.75 seconds) through the pulmonary capillary and still be adequately oxygenated Partial Pressure Gradients and Gas Solubilities Although carbon dioxide has a lower partial pressure gradient: It is 20 times more soluble in plasma than oxygen It diffuses in equal amounts with oxygen Partial Pressure Gradients Oxygenation of Blood Ventilation-Perfusion Coupling Ventilation 0 the amount of gas reaching the alveoli Perfusion 0 the blood flow reaching the alveoli Ventilation and perfusion must be tightly regulated for efficient gas exchange Changes in Pm in the alveoli cause changes in the diameters of the bronchioles Passageways servicing areas where alveolar carbon dioxide is high dilate Those serving areas where alveolar carbon dioxide is low constrlct Ventilation—Perfusion Coupling Surface Area and Thickness of the Respiratory Membrane Respiratory membranes: Are only 0.5 to 1 mm thick, allowing for efficient gas exchange Have a total surface area (in males) of about 60 m2 (40 times that of oneOs skin) Thicken if lungs become waterlogged and edematous, whereby gas exchange is inadequate and oxygen deprivation results 4, Decrease in surface area with emphysema, when walls of adjacent alveoli break through Internal Respiration The factors promoting gas exchange between systemic capillaries and tissue cells are the same as those acting in the lungs The partial pressures and diffusion gradients are reversed P02 in tissue is always lower than in systemic arterial blood PC2 of venous blood draining tissues is 40 mm Hg and Pcm is 45 mm Hg Oxygen Transport Molecular oxygen is carried in the blood: Bound to hemoglobin (Hb) within red blood cells Dissolved in plasma Oxygen Transport: Role of Hemoglobin Each Hb molecule binds four oxygen atoms in a rapid and reversible process The hemoglobin-oxygen combination is called oxyhemoglobin (HbOz) Hemoglobin that has released oxygen is called reduced hemoglobin (HHb) Hemoglobin (Hb) Saturated hemoglobin 0 when all four hemes of the molecule are bound to oxygen Partially saturated hemoglobin 0 when one to three hemes are bound to oxygen The rate that hemoglobin binds and releases oxygen is regulated by: P02, temperature, blood pH, Pm, and the concentration of BPG (an organic chemical) These factors ensure adequate delivery of oxygen to tissue cells Influence of P02 on Hemoglobin Saturation Hemoglobin saturation plotted against POZ produces a oxygen-hemoglobin dissociation curve 98% saturated arterial blood contains 20 ml oxygen per 100 ml blood (20 vol %) As arterial blood flows through capillaries, 5 ml oxygen are released The saturation of hemoglobin in arterial blood explains why breathing deeply increases the P02 but has little effect on oxygen saturation in hemoglobin Hemoglobin Saturation Curve Hemoglobin is almost completely saturated at a Pm of 70 mm Hg Further increases in PC,2 produce only small increases in oxygen binding Oxygen loading and delivery to tissue is adequate when Pi,2 is below normal levels Hemoglobin Saturation Curve ...
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