During inspiration Ptp expands lungs because it is greater than elastic recoil inward until elastic recoil inward becoming too great Then resp muscles relax Pip becomes less negative and Ptp becomes smaller and recoil inward contracts the lungs At end of normal expiration when lungs are at rest the still positive Ptp opposes further inward elastic recoil of the lungs and holds the lungs open 22 1 Expiration Between breaths when Palv Patm negative Pip which keeps Ptp positive keeps the lungs partially expanded In forced expiration the accessory muscles raise intrapulmonary pressure by 10 fold compared to quiet expiration Forceful contraction of abdominal muscles increases abdominal pressure from below pushing diaphragm into thorax which decreases the thoracic cavity vol and increases pressure in thoracic cavity 22 2 Respiratory Cycle 22 3 Pneumothorax Presence of air in pleural cavity turning the potential space of the pleural cavity negative pressure into an air filled space positive pressure Without negative intrapleural pressure the normally positive transpulmonary pressure which was holding the lung open is eliminated and the lung recoils and collapses 22 4 Resistance to Airflow Bronchiole diameter primary control over resistance to airflow bronchodilation increase diameter stimulated by sympathetic nerves epinephrine During exercise increase breathing rate using bronchodilation and increase blood flow in blood vessels to skeletal lung and heart tissue using vasodilation while use vasoconstriction to reduce blood flow to other tissues bronchoconstriction decrease diameter triggered by airborne irritants cold air parasympathetic stimulation histamine protect the lungs extreme bronchoconstriction may occur in anaphylactic shock or asthma 22 5 Resistance to Airflow Pulmonary compliance stretchability ease of change in lung volume relative to a given pressure change a given change in Ptp The greater the lung compliance the more the lungs expand for the less elastic the connective tissue the less it can stretch a low compliance means greater than normal Ptp must be developed across the lung to produce a given lung expansion The lungs in two diff people may have the same Ptp but lung will expand less in the one with lower pul compliance i e less distensible Compliance can be reduced by degenerative lung diseases black lung disease and TB stiffen lungs by thickening lungs tissues with scar tissue 22 6 Alveolar Surface Tension thin film of water needed for gas exchange creates surface tension that acts to collapse alveoli and distal bronchioles and decrease lung compliance pulmonary surfactant great alveolar cells decreases surface tension by disrupting the hydrogen bonding in water surfactant is composed of amphiphilic proteins and phospholipids which are partially hydrophobic and has hydrophilic ends like soap The surfactant molecules spread over the surface of the water film floating in the water layer The surfactants resist compression as the alveolus deflates retards and halts the collapse Surfactant reduces the cohesive force btw water molecules on the alveolar surface It lowers surface tension which increases lung compliance and makes it easier to expand the lungs 22 7 Composition of Air Inhaled Air consists of about 79 nitrogen about 21 oxygen and very small amounts of water vapor carbon dioxide and minor gases The total atmospheric pressure is a sum of the contributions of each gas in the mixture a principle known as Dalton s law The pressure each gas exerts is independent of the pressure the other gases each exert 22 8 Composition of Air Inhaled The separate contribution of each gas is called its partial pressure which is symbolized by P followed by the gas formula e g PN2 The partial pressure of a gas is proportional to its concentration The partial pressure of a gas in a mixture is the fractional concentration of the gas times the total pressure of all gasses Gas A s of total moles gas A s of total pressure If at sea level pressure is 760 mmHg and nitrogen is 78 65 of air then PN2 is simply 786 x 760 mmHg 597 mmHg or 78 6 of total pressure is due to PN2 22 9 Composition of Air Alveolar Alveolar air is humidified Oxygen in alveolar air is diluted PO2 is about 2 3 of inhaled air CO2 is enriched from the air left behind PCO2 of alveolar air is more than 100 times of inhaled air 22 10 Alveolar Gas Exchange Diffusion is responsible for the net movement of O2 and CO2 across the respiratory membrane in alveoli while O2 and CO2 is transported btw lungs and tissues by bulk flow Henry s law states at a constant temperature the amount of a given gas dissolved in a liquid at a particular temperature is proportional to the pressure of that gas above the liquid The greater the partial pressure of the gas above the liquid the more gas can dissolve in the liquid The blood is said to unload CO2 and load O2 The greater the PO2 in alveolar air the more O2 is loaded into blood and the greater the PCO2 in the blood the more CO2 is unloaded from blood into alveolar air 22 11 Alveolar Gas Exchange Each gas diffuses down its pressure gradient until the partial pressure of each gas is equal to its partial pressure in water Which is in the water film on the surface of alveoli and the blood plasma in surrounding capillaries The diffusion of one gas does not influence the diffusion of another gas Partial pressure and solubility of a gas determine diffusion rate of gas and gas exchange rate between blood and alveolar air 22 12 Alveolar Gas Exchange 22 13 Factors Affecting Gas Exchange the pressure gradient of oxygen in alveoli to oxygen in blood after leaves tissues is about 2 5 fold difference pressure gradient of carbon dioxide in blood after leaves tissues to carbon dioxide in alveoli is 15 greater pressure gradients differ with changing circumstances High elevation reduce the gradient of O2 Decrease alveolar ventilation rate decreases the gradients of O2 and CO2 as alveolar O2 amount decrease while alveolar CO2 amount increases Increased tissue O2 consumption rate also decrease the gradient of O2 and CO2 gases have different solubilities in water with carbon dioxide about 20 times as soluble as oxygen even though the pressure gradient of oxygen is greater than of carbon dioxide equal amounts of oxygen and carbon dioxide are exchanged because CO2 is much more soluble 22 14