Gas TransferBioengineering 6000 CV PhysiologyGas Transfer and Lung FunctionBioengineering 6000 CV PhysiologyGas TransferOverview: Air Breathing• Goal: – Transfer of gases from environment to/from blood• Needs:– Bidirectional air movement– Protect lung surface– Manage water lossBioengineering 6000 CV PhysiologyGas TransferSummary of Mammalian Respiration• PO2 concentrations– lower in alveoli than room air• vapor pressure• dead space• PO2 and CO2 concentrations– reach equilibrium in alveoli and blood– reach equilibrium in tissue and blood• flow-limited diffusionBioengineering 6000 CV PhysiologyGas TransferFunctions of Respiratory System• Supply O2, remove CO2 • Eliminate heat• Moisten air – prevent alveoli from drying out– manage water loss• Increase venous return: Pressure is negative in chest• Maintain pH– Remove CO2 at rate it is produced to prevent dangerous fluctuations in pH• Defend against foreign matter– largest surface area exposed to atmosphere; 30 times that of the skin• Olfactory sensationBioengineering 6000 CV PhysiologyGas TransferVentilation • Condition air to BTPS– Warming to 37oC– Humidifying to 100%– Filter out particles• Nose breathing allows better conditioning• Mucus traps particles, immunoglobulins neutralize microorganisms• Smoking paralyzes ciliaBioengineering 6000 CV PhysiologyGas TransferMechanics of Ventilation•Airflow requires pressure gradients: Flow = ΔP/R• 60-75% of inspiration at rest is from diaphragm• Rib cage provides the remainder• Exhalation at rest is passive• 3-5% of body’s energy required for quiet breathing.1 2 3Bioengineering 6000 CV PhysiologyGas TransferPressure Changes During Breathing• Interpleural pressure < ambient• Pneumothorax:– Air enters interpleural space– Collapses lungBioengineering 6000 CV PhysiologyGas TransferAirway Resistance• Factors that determine resistance: R = kLη/r4– Length of airway (fixed)– Viscosity of air (varies slightly with T and humidity) – Radius of airway (control mechanism)• Radius varies and is main source of change– Trachea and bronchi: 90% of resistance– Bronchioles generally low resistance but can collapse• CO2 causes bronchodilation• Histamine is powerful constrictor• Parasympath. stimulation leads to constriction• Epinephrine stimulates β2 to cause dilationBioengineering 6000 CV PhysiologyGas TransferSurfactants• Tension in alveoli largely result of liquid lining inner surface• Without surfactants, air would all flow to larger alveoli and alveoli would stick together when folded• Surfactants reduce surface tension, adhesion, allow for easy inflation• More surfactant in smaller alveolixxxxxBioengineering 6000 CV PhysiologyGas TransferLung Volumes• Dead space:– Anatomical and physiological (alveolar volume not involved in gas exchange)– Long neck increases dead space –Results in elevated CO2 and reduced O2Bioengineering 6000 CV PhysiologyGas TransferGas Exchange• Exchange is (normally) rapid:– Equilibrium in both lungs and tissues• Pressures/concentrations drive exchange:–PO2 at sea level: 100 mmHg (in lungs)–PO2 at Everest: 53 mmHg• Alveolar membrane can reduce exchange:– Reduction in area– Increase in diffusion distance:• e.g., edema•CO2 affected less than O2Bioengineering 6000 CV PhysiologyGas TransferFunctional Anatomy: Mammalian Lung• Air ducts– Trachea to terminal bronchioles– Cartilage, smooth muscle– Cilia move mucus along ducts for cleaning• Respiratory portions– Respiratory bronchioles to alveoli – Alveoli interconnected by pores of Kohn (10 µm diameter)– Smooth muscle (point of regulation)– Number of terminal partitions increase (and size decreases) from amphibians to reptiles, to mammals– Smaller mammals have more respiratory surface (and more O2 uptake) per weight than large ones.Bioengineering 6000 CV PhysiologyGas TransferAirway BranchingSurface area: 50-100 m22Varies from 10-24Bioengineering 6000 CV PhysiologyGas TransferVentilation System StructureBioengineering 6000 CV PhysiologyGas TransferAlveolar Structure• Type I Alveoli– Diffusion– Larger with thin epithelium– No muscle but elastic (elastin)– Single cell between neighboring alveoli• Type II Alveoli– Smaller, thicker cells– Surface villi– Produce surfactants• Type III Alveoli– Rare, with mitochondria– Involved in uptake of NaClBioengineering 6000 CV PhysiologyGas TransferDiffusion Layers• Gas exchange via diffusion• 80-90% coverage by blood vessels• Capillary and alveolar endothelia fuse• Pores of Kohn connect alveoli and allow air movementBioengineering 6000 CV PhysiologyGas TransferHeat and Water Loss• In mammals, air is heated and humidified• Nose has extensive circulation to supply water• Conservation by:– Nose cools during inhalation from humidification– Absorbs heat from exhaled gas– Condensation in the nose retains water for next inhalation• Breathing through mouth removes more heat, but also moisture• Longer noses better at conserving waterBioengineering 6000 CV PhysiologyGas TransferVentilation in Birds• Instead of alveoli, birds have air capillaries (10 µm)--the parabronchi• Minimal breathing motion; instead, air sacs provide storage and pressure• Achieves gas exchange in both inhalation and exhalation• Parabronchi unidirectional flow by means of aerodynamic valving (turbulence alters resistance with direction)Bioengineering 6000 CV PhysiologyGas TransferVentilation in Frogs• Buccal cavity and lungs separated by glottis• Can open and close glottis and nares independently• Inhalation (and exhalation) stepwise• Allows mixing of pulmonary and fresh air Bioengineering 6000 CV PhysiologyGas TransferTracheal Gas Exchange• Tracheal walls very thin (40-70 nm)• Ends filled with fluid that regulates exchange• Insect flight wings have highest recorded O2 uptake of any tissueRest WorkTracheaTracheolesMuscle fibersBioengineering 6000 CV PhysiologyGas TransferTracheal Systems• Structure:– Air tubes that penetrate into the body, invaginations– Trachea have adjustable openings to conserve water, keep out dust– Trachea branch to level of individual cells, dead end into (but not inside) the cells– Air sacs to store gas and help insect float• Ventilation:– by diffusion and convention of gases, compression of air sacs– Opening and closing of trachea– Some species use trachea