Respiratory physiology

• Pulmonary Circulation


  • Short route circulation compared to systemic circulation
  • Carries deoxygenated blood
  • High compliance and low pressure circulation
• Pulmonary artery


  Carries deoxygenated blood
• Mixed venous blood


  Blood with different oxygen saturation and content from different organs, mixed in the right ventricle
• Pulmonary circulation is a high compliance and low pressure circulation

  
• High compliance of pulmonary circulation

  • 24 times more compliant than systemic circulation, to accommodate entire blood circulating
  • 15-18% of total blood volume present in the lungs at any time
• Low pressure in pulmonary circulation

  • Left ventricular pressure 5 times more than right ventricular pressure
  • Pulmonary artery systolic pressure 25mmHg, diastolic 8mmHg, mean 15mmHg
  • Pulmonary capillary pressure 7mmHg, pulmonary capillary wedge pressure 6-8mmHg
• Hypoxia in systemic circulation

  Leads to vasodilation
• Hypoxia in pulmonary circulation

  Leads to vasoconstriction
• Zones of blood flow in the lungs

  Not anatomical but functional, due to interplay of hydrostatic factor (gravity) and alveolar pressure

  
• Zone I


  • No blood flow, P(alv) > P(art) > P(vein)
• Zone II


  • Intermittent blood flow, P(art) > P(alv) > P(vein)
• Zone III


  • Continuous blood flow, P(art) > P(vein) > P(alv)
• Pulmonary circulation has lower resistance compared to systemic circulation
• Pulmonary resistance is lowest at Functional Residual Capacity (FRC)
• Pulmonary capillary bed contributes 40% of total pulmonary vascular resistance, unlike systemic circulation where arterioles contribute maximally
• Pulmonary Vascular Resistance (PVR)

  Calculated using Ohm's law: R = DP/Q, where DP is pressure difference and Q is flow (cardiac output)

  
• Normal PVR is 1mmHg/L/min
• Pulmonary Hypertension (PH)


  Sustained elevation of mean pulmonary arterial pressure above 25mmHg at rest or 30mmHg after exercise, in absence of elevated left atrial pressure
• Types of Pulmonary Hypertension

  • Primary PH (idiopathic)
  • Secondary PH (due to other underlying diseases)
• Diffusing Capacity of Respiratory Membrane

  Volume of gas that will diffuse through the membrane per minute for a pressure difference of 1mmHg
• Factors affecting diffusion rate across respiratory membrane

  • Thickness of membrane
  • Surface area available
  • Pressure difference
  • Solubility of gases
  • Molecular weight of gases
  • Temperature

  
• Diffusion Limited Carbon Monoxide (DLCO)


  Measure of diffusing capacity using carbon monoxide, which does not reach equilibrium in the pulmonary capillaries
• Steps in measuring DLCO

  1. Give measured amount of CO via face mask

  2. Patient holds breath for 10 seconds, then exhales

  3. Measure amount of CO in exhaled air, compute diffusing capacity
• Conditions altering DLCO


  • Decreased - Decreased membrane surface area, increased membrane thickness, pulmonary hypertension, anaemia
  • Increased - Exercise, asthma, pulmonary haemorrhage, polycythemia, mild left heart failure
• Diffusing capacity of oxygen is 1.23 times that of carbon monoxide
• Normal alveolar-capillary oxygen gradient during quiet breathing is 11mmHg
• Diffusion capacity for oxygen equals 230ml, which is the resting body's oxygen consumption rate
• Oxygen and carbon dioxide are perfusion limited gases, unlike carbon monoxide which is diffusion limited
• Ventilation-Perfusion (V/Q) Ratio


  Ratio of amount of air reaching alveoli (ventilation) to amount of blood flow to alveoli (perfusion)

  
• Normal V/Q ratio is 0.8
• V/Q mismatch


  • Differences in ventilation and perfusion in different lung zones due to gravity
  • Lung diseases reducing ventilation like pneumonia, COPD, pulmonary edema
• Apex of the lung

  • Large alveoli
  • Decreased pulmonary intravascular pressure due to less blood flow hence less perfusion
  • Less ventilation because of large alveoli (less compliance)
• Base of the lungs

  • Smaller size alveoli
  • More blood flow (more perfusion)
  • More ventilation because alveoli are able to expand more
• The differences in the lung are found in a normal person and is due to gravity (hydrostatic forces) in upright posture

  
• Lung diseases that lower reduce the V/Q ratio by reducing ventilation
• Lung diseases that lower reduce the V/Q ratio by reducing ventilation

  • Pneumonia- Lung consolidation and less ventilation
  • Chronic Obstructive Pulmonary Disease (COPD) e.g. bronchitis- reduced bronchial aperture and mucus clogging reduces ventilation leading to reduced V/Q ratio
  • Pulmonary Oedema- fluid overload in the lung reduces ventilation
• Diseases that increase V/Q ratio by reducing perfusion

  • COPD can also cause destruction of pulmonary capillaries supply the alveoli hence reduced perfusion
  • Pulmonary embolism – Blood clot occludes the pulmonary artery leading to compromised perfusion
• V/Q Ratio mismatch Spectrum

  • V/Q= 0 (Perfusion without Ventilation). This represents wasted perfusion and is referred to as Absolute Physiologic Shunt
  • V/Q= 1 (Normal Ventilation and Normal Perfusion)
  • V/Q= ∞ (Ventilation without Perfusion). This represent wasted Ventilation and is referred to as Absolute Physiologic Dead Space

  
• The total pressure of a mixture of gases is equal to the sum of the partial pressures of the individual component gases
• When air is inspired into the lungs, it is warmed, moistened and saturated with water vapor at 37 degrees celsius (PH2O = 47mmHg)