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Respiratory System Core Principles & Physics of the Respiratory System Kinetic theory - pressure is the force per unit area exerted by the impact of gas molecules colliding with the walls of the container Boyle's Law - at a constant temperature, the volume of the gas varies inversely with the pressure to which it is subjected P1V1 = P2V2 Charles's Law - provided that pressure remains constant, the volume of a gas is directly proportional to the absolute temperature
Partial Pressure (P) The total pressure exerted by a mixture of gases is equal to the sum of the separate pressures that each gas would exert if they were in the container alone Dalton's Law of Partial Pressure Ptotal = Pa + Pb + Pc Dry atmospheric air - PB = PN2 + PO2 + PCO2
Fick's Law of Diffusion Net flux = - D.A.DP X
Flux NOT Flow
- Down gradient D Diffusion coefficient A Surface area of membrane
DP Partial Pressure gradient X Thickness of membrane
Composition of air at sea level Nitrogen - 79% Oxygen - 21% CO2 - 0.03% Water - 0.5%
1atm = 760mmHg = 101.3kPa 1kPa ~ 7.5mmHg 1mmHg ~ 1.36 cm H2O
Respiratory Quotient RQ = Volume of Carbon Dioxide Produced 200ml/min = 0.8 Volume of Oxygen Consumed 250ml/min 1 litre of oxygen consumption equates to approximately 21,000 Joules of energy released from foodstuffs, thus allowing the amount of chemical energy released to be calculated from the oxygen consumption
1 Basal Metabolic Rate The energy required to maintain vital functions such as pumping of the heart and active transport channels
Airways & Alveoli The Tracheobronchial Tree Trachea divides into 2 bronchi Right bronchus is wider than the left and makes a smaller angle with the trachea Then divides into ?
4 lobar bronchi 16 segmental bronchi and from there to Terminal bronchioles, Respiratory bronchioles, Alveolar ducts and Alveolar sacs 23 generations of divisions results in 8 million alveolar sacs Each sac has about 17 alveoli These alveoli account for around 250-300 million, the rest arise from alveolar ducts Total surface area ~ 75m2 As the airways reduce in size, the supporting cartilage that holds the airways open during exhalation, disappears and the amount of smooth muscle increases Bronchioles consist of smooth muscles, collagen and reticular fibres for support. There is no cartilage present and smooth muscle occupies about 20% of the walls As divisions progress the smooth muscle layer gets thinner The airways are lined with epithelium containing ciliated and goblet cells, with submucosa containing mucus secreting cells. This ensures that air is adequately humidified and that foreign bodies are removed from the airways From the trachea down to the smallest bronchioles, the sole purpose is conduction and humidification but beyond this, the purpose changes to gas exchange and so the smooth muscle walls disappear
Alveoli Type I pneumocyte - flat thin cells on the alveolar surface Type II pneumocyte - secretes surfactant Macrophage - scavenge foreign bodies Fibroblasts - lay down collagen
Airway resistance Q = [?]P/R A majority of the resistance comes from the upper airways and medium-sized bronchioles
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