Respiratory 2

• Lung compliance


  How easily lung stretch (distensibility)
• Compliance


  ∆V / ∆P - The change in lung volume from a change in distending pressure
• High compliance


  • Large ∆V for a given ∆P
• Low compliance


  • Small ∆V for a given ∆P
• Gravity causes weight of lung (80% water) to pull down on alveoli
• Gravity pulling down on lungs (think slinky) causes alveoli in apex to be more expanded
• Compliance of the base > apex; More tidal volume goes to base
• Pulmonary fibrosis


  • Collagen deposition due to lung injury - High elastic recoil
• Emphysema


  • Disappearing lung tissue - Low elastic recoil
• Surfactant


  1. Lowers surface tension - Increases compliance of lung making it easier to inflate<br>2. Keeps alveoli dry - Lowers surface tension, less inward pressure pulling fluid from capillaries
• Boyle's Law: the pressure & volume of a gas are inversely related (P1V1 = P2V2)
• Henry's Law: The amount of a gas that dissolves into a fluid is related to the solubility of the gas, the temperature, and the partial pressure of the gas
• Dalton's Law: The total pressure of a gas mixture is equal to the sum of the pressures that each gas exerts independently
• Inspiration begins

  Ambient air brought into airways warmed & humidified<br>Water vapor = a gas (PH2O = 47 mm Hg)<br>Partial pressure of other gases diluted
• PO2 in a humidified mixture

  PO2 trachea = (PB-PH2O) x FO2
• Total ventilation


  VE = VT x f
• Both VT and f increase during max exercise, but depth of breathing increased more because of anatomical dead space
• Anatomical dead space


  Part of inspired air that remains in conducting airways and is useless for gas exchange
• Alveolar ventilation (VA)


  VA = (VT -VD) x f
• Inverse relationship between VA and PaCO2
• Systemic circulation: Left ventricle -> aorta -> rest of body
• Pulmonary circulation: Right ventricle -> main pulmonary artery -> lungs
• The lungs receive the entire right ventricular cardiac output
• Bronchial circulation arises from the aorta and nourishes conducting airways (& parenchyma) up to terminal bronchioles
• Blood from bronchial circulation (deoxygenated) mixes with O2-enriched blood in the pulmonary vein; contributes to the small A-a O2 difference
• Pulmonary blood flow is high (5 L/min) but at low pressure
• Pulmonary circulation


  • Low resistance - Pulmonary arteries shorter, in dilated state; pulmonary arterioles thin walled with less smooth muscle and lower resting tone, more distensible;; Enormous number of capillaries in a unique arrangement to create sheets of blood flowing past alveoli
• 3 Factors That Alter Pulmonary Vascular Resistance

  • Changes in blood flow (perfusion) - Increased flow leads to recruitment and distention, decreasing resistance<br>2. Changes in lung volume - Resistance lowest at FRC<br>3. Changes in local O2 concentration - Hypoxia causes vasoconstriction
• Pulmonary vasculature is NOT significantly regulated by the autonomic nervous system
• Increase in cardiac output (e.g. exercise)


  Pulmonary blood flow increases, pulmonary vascular resistance decreases due to recruitment and distention of capillaries
• Decrease in cardiac output (e.g. heart failure)


  Pulmonary blood flow decreases, pulmonary vascular resistance increases
• Pulmonary vascular resistance is lowest at FRC and increases at lower and higher lung volumes
• Hypoxia (low O2 in alveoli) and/or hypoxemia (low O2 in blood) trigger vasoconstriction in the pulmonary vasculature
• In an upright person, blood flow is highest near the base and lowest near the apex of the lungs
• Bulk flow


  How gas moves in airways from trachea to alveoli - Due to mass movement, occurs when there are differences in total pressure
• Diffusion


  How gas moves from air to liquid, and liquid to air - Gases moving due to their individual partial pressure gradients
• O2 in blood in 2 forms: physically dissolved and bound to hemoglobin
• Dissolved O2 is directly proportional to PO2, but bound O2 does not contribute to PO2
• Total O2 content in arterial blood is ~20 ml O2/100 ml blood
• Partial pressure gradients (∆P)

  Major determinant of rate of diffusion