Mechanism Of Respiration

RESPIRATORY MECHANISM Inspiration and expiration INSPIRATION: z It is an active process. z Diaphragm is the main muscle involved in inspiration. z External intercostal muscles are also involved. EXPIRATION: z It is a passive process. z It is due to elastic recoil of the lungs and thoracic wall. Pleural pressure: Different types of pressure 1. Intrapleural pressure- pressure within the pleural space. It is also called intra thoracic pressure. 2. Intrapulmonary pressure – pressure within the airways. 3. Transmural pressurez z

Transpulmonary pressure: Pressure difference between the intrapleural pressure and intra pulmonary pressure. Transthoracic pressure: Pressure difference between the intrapleural pressure and atmospheric pressure.

Pressure changes during respiration: 1. Intrapleural pressure: At the start of quiet inspiration, it is -2.5 mmHg. At the end of inspiration, It becomes -6 mmHg. 2. Intrapulmonary pressure: At the start and at the end of inspiration as well as expiration, the intrapulmonary pressure is equal to the atmospheric pressure. At the peak of inspiration, it is -3mmHg. At the peak of expiration it is +3mmHg.

Lung volumes 1. Tidal volume: Volume of air breathed in and out of lungs during quiet respiration. 2. Inspiratory reserve volume: Maximum volume of air that can be inspired after normal tidal inspiration. 3. Expiratory reserve volume: Maximum volume of air that can be expired after normal tidal expiration. 4. Residual volume: Volume of air that remains in lungs after forceful expiration. 5. Closing volume: Lung volume at which airways begin to close off.

6. Inspiratory capacity: Maximum Volume of air that can be inspired after end of tidal expiration. IC = TV + IRV. 7. Expiratory capacity: Maximum volume of air that can be expired after end of tidal inspiration. EC = TV + ERV. 8. Vital capacity: Maximum volume of air that can be expelled forcefully following a maximal inspiration. VC = TV +IRV=ERV. 9. Functional residual capacity: Volume of air that remains in the lung after end of tidal respiration FRC = RV + ERV. 10. Total lung capacity: VC + RV 11. Forced vital capacity: Maximum volume of air that can be breathed out forcefully and rapidly after a maximum inspiration. It comprises of: FEV1, FEV2 and FEV3 FORCED VITAL CAPACITY: FEV1 FEV2 Fev3

volume of FVC expired in first second volume of FVC expired in first 2 seconds volume of FVC expired in first 2 seconds of expiration

80% OF FVC 95 % OF FVC 97- 100% of FVC.

NORMAL VALUES: 1. TIDAL VOLUME, TV 2. IRV 3. ERV 4. RV 5. Inspiratory capacity 6. Expiratory capacity 7. Vital capacity 8. FRC 9. TLC

= 500 ml. = 2000 – 3200 ml. = 750 - 1000 ml. = 1200 ml. = 2500- 3700 ml. = 1250 – 1500 ml. = 4800 ml in males 3200 in females. = 2500 ml. = 6000 ml. 6 L.

METHODS FOR MEASURING LUNG VOLUMES AND CAPACITY 1. Spirometry: It can measure all lung volumes except residual volume, functional residual capacity and total lung capacity. 2. Single breath nitrogen method:

• It measures anatomical dead space. • Measures closing volume • Measures residual volume.(RV) 3. Helium dilution method: measures FRC. FACTORS AFFECTING VITAL CAPACITY: 1. 2. 3. 4. 5.

VC in males > females. VC decreases in advancing age. VC is greater in standing position as compared to sitting or lying. VC decreases in pregnancy. VC decreases in diseases of respiratory system.

Obstructive lung disease Restrictive lung disease Expiration is obstructed. e.g. Chest expansion is restricted e.g. bronchial asthma , emphysema. Kyphoscoliosis. VC is Normal VC decreased FEV1 decreased Normal TLC Normal/ Increased Normal / Decreased. COMPLIANCE: 1. 2. 3. 4. 5. 6.

Change in lung volume per unit change in airway pressure. Inversely related to stiffness and elasticity in lungs. Depends on lung size and airway resistance. Compliance increases in asthma, emphysema. Decreases in pulmonary edema, pulmonary fibrosis and congestion. Compliance of lungs alone is greater than compliance of lungs and thorax due to presence of large no. of elastic fibers of in the lung tissue.

ALVEOLAR SURFACE TENSION: Lungs have the tendency to recoil inwards and chest wall has tendency to expand outwards. The tendency of lungs to recoil back from chest wall is regulated by 2 forces: 1. Recoil of elastic tissue of lungs. 2. Surface tension within the alveoli.

Law of Laplace: P = 2T/ R. P is distending pressure, T is surface tension, R is radius. SURFACTANT: 1. Consists mainly of Dipalmitoyl phosphatidylcholine lipid. 2. Produced by granular pneumocytes, type II alveolar cells. 3. Action: Prevents development of surface tension in the alveoli between fluid and air. 4. Surface tension is inversely related to surfactant in alveoli--more is the surfactant, lesser is the surface tension. 5. Surfactant prevents pulmonary edema by reducing surface tension. 6. Factors affecting surfactant: Decreasing surfactant 1. 100% O2 inhalation. 2. Smoking 3. Blockage of main bronchus. Increasing surfactant 1. Cortisol 2. Thyroid hormones

APPLIED: HYALINE MEMBRANE DISEASE: Disease in newborn infants due to deficiency of surfactant. Surfactant is less so, the surface tension is more in the lungs of these infants, leading to collapsing of alveoli. WORK OF BREATHING: Elastic work Nonelastic work

65% 35%

Non elastic has 2 components: Airway resistance Viscous resistance.

Factors affecting airway resistance: 1. Total crossectional area . – Has inverse relation. Airway resistance is high in conducting zone and low in respiratory zone. Factors affecting work of breathing: 2. Increased in pathological diseases of lung, bronchial asthma, emphysema, CCF. 3. Increases during exercise.

ALVEOLAR VENTILATION: Amount of air ventilating the alveoli per minute. AV = TV X RESPIRATORY RATE. DEAD SPACE: Amount of air in the respiratory passage, that does not take part in gaseous exchange TWO TYPES: ANATOMICAL - Volume of air present between nose and mouth upto terminal bronchioles where gaseous exchange dooes not take place. Normal = 150 ml. PHYSIOLOGICAL DEADSPACE = anatomical dead space + volume of air in the alveoli that does not take part in respiration.

VENTILATION / PERFUSION RATIO: Ratio of alveolar ventilation to pulmonary perfusion. Alveolar ventilation( 4L/min) / pulmonary blood flow (5L/ min) = 0.8

V/P RATIO is more THAN 0.8 in the apical portion of lung. It is less than 0.8 in basal portion of lung.

Factors effecting V/P RATIO: Any factor that alters alveolar ventilation and pulmonary blood flow alters the V/P ratio. Causes of altered V/P ratio: 1. Bronchial asthma 2. Emphysema. 3. Interstitial disease of lung. 4. CCF Causes of altered pulmonary blood flow: 1. Anatomical shunt. 2. Pulmonary embolism. 3. Any valvular disease of heart. 4. Increased pulmonary vascular resistance.

DIFFUSION CAPACITY OF LUNGS: Amount of gas that crosses respiratory membrane per minute per mm Hg difference in partial pressure of gas on two sides of the membrane. Normal value, for O2 is 20 – 30 ml/ mm Hg at rest. Factors affecting Decreased by: 1. Blockage of pulmonary blood flow 2. Airway blockage. 3. Pulmonary edema.

Increased by 1. Exercise.