Respiration Lesson 3 and Airway Resistance (key factors affecting air flow) 1) What is the arterial blood pressure in a healthy 18 year old male? 2) What would his venous blood pressure be? 3) What is the pressure differential required to inflate the lungs? 4) Why is such a small pressure differential required to inflate the lungs compared to the pressure required for blood circulation? 5) What are the consequences for this in emphysema, asthma, pulmonary fibrosis, COPD (chronic obstructive pulmonary disease) Respiration Lesson 3 and Airway Resistance (key factors affecting air flow) Objectives Define Compliance and explain the factors that alter compliance and what their consequences are for air flow during breathing. 2. Define Resistance and explain the factors that alter resistance and what their consequences are for air flow during breathing. 3. Relate changes in lung compliance and airway resistance to the work required to breathe 1
Compliance is a measure of the distensibility of an object of how easily it can be stretched or deformed. Elastance is the inverse of compliance and refers to the tendency of an object to oppose stretch or distortion, as well as its ability to return to its original form after the distorting force is removed. Compliance = 1 / Elastance. Compliance = Volume / Pressure where Pressure is the change in transpulmonary pressure (i.e. P A - P PL ) The more the lung expands for any given change in pressure the greater it s compliance The greater the compliance the less change in pressure it takes to expand the lungs 2
The Static Compliance of the Lung Is Measured Using the Static Pressure Volume Curve of the Lungs Lung Volume TLC- V P V FRC- P C L = slope = V / P Transpulmonary Pressure The Static Compliance of the Lung Depends on Lung Volume is determined by two factors: Lung Tissue Elasticity Alveolar Surface Tension 3
Lung Tissue Elasticity The lungs are embedded in connective tissue that surrounds the airways and the alveoli The 2 key connective tissue fibers are: collagen fibers elastin fibers Collagen like a strong twine high tensile strength inextensible 4
Elastin like a weak spring low tensile strength extensible Lung Tissue Elasticity One of the factors determining the compliance of the lungs is the balance in composition between these two fibres and the consequences for lung tissue elasticity The two fibres together act like a rubber band, they allow stretch up to a point where the collagen fibres become taut and begin to resist further stretching this is why the static compliance of the lung changes with lung volume 5
Changes in the Compliance of the Lungs that are due to Loss or Addition of Connective Tissue Many changes in the Compliance of the Lungs are due to Loss or Addition of Connective Tissue aging loss of elastin & collagen fibers wrinkled skin lung compliance [floppy lungs] Changes in the Compliance of the Lungs that are due to Loss or Addition of Connective Tissue Healthy Lungs Emphysema: (the disappearing lung disease) loss of connective tissue (elastin and collagen) compliance - floppy lungs (do not return to resting volume as readily) Pulmonary fibrosis: (such as in asbestosis) increased connective tissue due to injury compliance - stiff lungs (takes more work to inflate) E.M. healthy lungs E.M. emphysematous lungs E.M. fibrotic lungs 6
Changes in the Compliance of the Lungs that are due to Loss or Addition of Connective Tissue high compliance (emphysema) Lung Volume normal low compliance (pulmonary fibrosis) Transpulmonary Pressure is determined by two factors: Lung Tissue Elasticity Alveolar Surface Tension 7
Alveolar Surface Tension Water molecules at the surface of a liquid-gas interface are attracted strongly to the water molecules within the liquid mass. This cohesive force is called surface tension. Surface tension makes it possible: for insects to walk on water maintain the shape of a droplet reduces the meniscus created by capillary action 8
Alveolar Surface Tension surface water molecules create substantial surface tension This tension resists expansion of the alveoli because the surface water molecules resist being pulled apart The greater the surface tension the lungs less compliant the Alveolar Surface Tension surface tension also results in inward recoil This tension causes the alveoli to tend to collapse and along with the rebound of the stretched elastin fibres produces the elastic recoil that passively returns the lungs to their resting volume during expiration 9
Alveolar Surface Tension In fact, if the alveoli were only lined with water, the surface tension would be so strong that the lungs would collapse Pulmonary surfactant secreted from Type II alveolar cells reduces alveolar surface tension 1) This increases compliance and reduces the work of inflating the lungs 2) It also prevents collapse and stabilizes the alveoli Alveolar Surface Tension Developing fetal lungs do not begin to synthesize surfactant until late in pregnancy As a consequence there may be insufficient surfactant in the lungs of infants born prematurely to prevent alveolar collapse. The resulting collection of symptoms that develop are known as neonatal respiratory distress syndrome (NRDS) Their lungs have low compliance, are stiff, require much effort to inflate and tend to collapse almost completely after each breath. This is also occurring at a time when the respiratory muscles are still weak. It is now treated by surfactant replacement and the administration of corticosteroids during pregnancy to hasten lung maturation 10
Alveolar Interdependence Since each alveolus is interconnected to other alveoli by connective tissue, when one starts to collapse, the surrounding alveoli by resisting expansion, will stabilize it and help keep it open Table 13-3 Opposing forces acting on the lungs Forces Keeping the Alveoli Open Forces Promoting Alveolar Collapse Transmural pressure gradient Pulmonary surfactant Alveolar interdependence Elasticity of pulmonary connective Tissue Alveolar surface tension 11
Respiration Lesson 3 and Airway Resistance (key factors affecting air flow) Objectives Define Compliance and explain the factors that alter compliance and what their consequences are for air flow during breathing. 2. Define Resistance and explain the factors that alter resistance and what their consequences are for air flow during breathing. 3. Relate changes in lung compliance and airway resistance to the work required to breathe Airway Resistance The other key Factor in Movement of Air In & Out of the Lungs Just as with the flow of blood in blood vessels: V Flow ( V ) = P / R P 1 P 2 P = P 1 - P 2 = pressure gradient = driving pressure the greater the resistance, the larger the P required to produce a given rate of flow The primary determinant of resistance is the radius of the conducting airways since R 1/ r 4 12
Airway Resistance The other key Factor in Movement of Air In & Out of the Lungs The primary source of resistance to air flow is friction between the air and the walls of the airways The total resistance to flow of air in the airways [R aw ] is very small Only a very small pressure gradient is required to produce a normal breath The smallest airways are still far larger than capillaries and the viscosity of air is much lower than that of blood Airway Resistance The other key Factor in Movement of Air In & Out of the Lungs Although bronchioles have a small radius individually, their parallel arrangement results in a large effective radius (total cross sectional area) and low resistance to flow of air. 13
Airway Resistance The other key Factor in Movement of Air In & Out of the Lungs Airway Resistance The other key Factor in Movement of Air In & Out of the Lungs In health, the mid size airways are the major site of resistance to flow of air. 14
Airway Resistance The other key Factor in Movement of Air In & Out of the Lungs Airway Caliber Can Be Regulated by Receptors on the Airway Smooth Muscle Cholinergic receptors stimulated by acetylcholine (Ach) from parasympathetic innervation of the airway smooth muscle bronchoconstriction Adrenergic receptors- stimulated by adrenaline from the adrenal medulla circulating in blood bronchodilation Airway Resistance The other key Factor in Movement of Air In & Out of the Lungs In health, the mid size airways are the major site of resistance to flow of air. In most common disease states, the smaller airways are the major site of resistance to flow of air because of a reduction in their luminal size. 15
Increases in the Resistance of the Airways due to Airway Narrowing Chronic Obstructive Pulmonary Disease Chronic Bronchitis: Long term inflammation of the lower respiratory airways leads to edematous thickening of the airways and overproduction of mucus Asthma: thickening of the walls of the airways due to histamine induced edema, increased secretion of mucus and airway hyperresponsiveness (bronchoconstriction) due to allergens, infections and irritants Emphysema: breakdown of alveolar walls and collapse of smaller airways due to enzymatic degradation due to chronic irritation or genetic inability to produce stabilizing enzymes and Airway Resistance (key factors affecting air flow) Work of Breathing Normally the lungs are highly compliant and airway resistance is low and so the act of breathing requires only about 3% of resting metabolism This can increase significantly: When pulmonary compliance is decreased When airway resistance in increased When elastic recoil is decreased When levels of ventilation are increased 16