Pulmonary Ventilation And lung capacity
BY RANA DAS
External respiration can be divided into4 major functional eventsVentilationDiffusionTransport of O2 and CO2 in the blood, body fluids, to and from the cellsRegulation of ventilation
PULMONARY VENTILATIONBOYLES LAWGas pressure in closed container is inversely proportional to volume of containerPressure differences and Air flows
Mechanics of pulmonary ventilation Diaphragm which increase and decrease the vertical diameter of the chest cavity.
Intercostal muscles affect the anteroposterior diameter of the chest cavity by moving the ribs.
Internal intercostal muscle (downward and backward) lower the ribs and sternum reducing the anteroposterior diameter
External intercostal muscle (downward and forward) raise the ribs and sternum increasing the anteroposterior diameter of the thoracic cavity
ExpirationInspirationIncreased vertical diameterIncreased A-P diameterExternal intercostals contractedInternal intercostals relaxedAbdominals contractedElevated rib cageDiaphragmatic contraction
Inspiration/InhalationDiaphragm & Intercostal musclesIncreases volume in thoracic cavity as muscles contractVolume of lungs increasesIntrapulmonary pressure decreases (758 mm Hg)
Expiration/ExhalationMuscles relaxVolume of thoracic cavity decreasesVolume of lungs decreasesIntrapulmonary pressure increases (763 mm Hg)Forced expiration is active
Factors that influence pulmonary air flowF = P/RDiameter of airways, esp. bronchiolesSympathetic & Parasympathetic NS
Various pressure in the lungsPleural pressure is the pressure of fluid in the narrow space between the visceral and parietal pleura, normally slightly negative pressure
The normal pleural pressure at the beginning of inspiration is 5cm of H2O (it reach about 7.5cm of H2O due to movement of the chest cage)
The pleural pressure at the beginning of expiration is7.5cm of H2O to reach 5cm of H2O
Alveolar pressureAlveolar pressure: is the pressure inside the lung alveoli
During inspiration: 1cm of H2O (this slight negative pressure is enough to move about 0.5 liter of air into the lungs in the first 2 second of inspiration)
During expiration: it rises to about +1cm of H2O (this forces 0.5 liter of inspired air out of the lungs during the 2 to 3 seconds of expiration
Inspiration and expiration
The work done in breathing
the work of inspiration can be divided into 3 fractions:The work required to expand the lungs against its elastic forces called compliance work or elastic work.The work required to overcome the viscosity of the lung and chest wall structures called tissue resistance work.The work required to overcome airway resistance called airway resistance work.Work energy required for respiration:during normal quiet respiration = 2 to 3% of the total work energy ( to 50 fold in exercise, airway resistance).
Pulmonary volumes and capacities Tidal volume is the volume of air inspired or expired with each normal breath = 500ml in young adult man.
Inspiratory reserve volume is the extra volume of air that can be inspired over and beyond the normal tidal volume = 3000ml.
Expiratory reserve volume is the extra amount of air that can be expired by forceful expiration after the end of a normal tidal expiration ~ 1100ml.
Residual volume is the extra volume of air that still remain in the lungs after the most forceful expiration ~ 1200ml.
The pulmonary capacities
Inspiratory capacity is the volume of air inspired by a maximal inspiratory effort after normal expiration = 3500ml = inspiratory reserve volume + tidal volume.
The functional residual capacity is the volume of air remaining in the lungs after normal expiration = 2300ml = expiratory reserve volume + residual volume.
The vital capacity is the volume of air expired by a maximal expiratory effort after maximal inspiration ~ 4600ml = inspiratory reserve volume + tidal volume + expiratory reserve volume.
Total lung capacity is the maximum volume of air that can be accommodated in the lungs ~ 5800ml = vital capacity + residual volume.
Minute respiratory volume is the volume of air breathed in or out of the lungs each minute = respiratory rate x tidal volume = 12 X 500ml = 6000ml/min.
All lung volume and capacity are about 20 to 25% less in women than in men and are greater in athletic persons than in small and asthenic persons.
Does Ventilation Limit Aerobic Capacity for Average Person?If inadequate breathing capacity limited aerobic capacity, ventilatory equivalent for oxygen would decrease.Actually, healthy person tends to over-breathe in relation to VO2.In strenuous exercise, decreases arterial PCO2 & increase Alveolar PO2.
Work of BreathingAcute effects of 15 puffs on a cigarette during a 5-minute period3 fold increase in airway resistanceLasts an average 35 minutesSmokers exercising at 80%Energy requirement of breathing after smoking was 14% of oxygen uptakeEnergy requirement of breathing no cigarettes was only 9%.