Ventilation: Physics of Breathing Flashcards
Learning outcomes
- Define the terms intrapleural pressure and intrapulmonary pressure
- Explain what causes intrapleural pressure
- Describe the processes involved in quiet inspiration and expiration
- Describe the processes involved in forced inspiration and expiration
- Define the term: work of breathing, and list what the work of breathing involves
- Define airway resistance and explain what determines it
- Define compliance and describe how it changes in different disease states
- Explain how alveolar surface tension is reduced in health and be able to describe conditions when it would be raised
- Describe the respiratory volumes measured by a spirometer
- Describe how pulmonary function tests can be used
What is the function of ventilation
• Function of ventilation is to provide O2 to the tissues and remove CO2
• Function achieved by:
– Pulmonary ventilation (movement of air from atmosphere to alveoli)
– Regulation of ventilation
– Matching of pulmonary blood flow to alveolar ventilation
– Movement of O2 and CO2 between alveoli and blood
– Transport of O2 and CO2 in blood and body fluids
• Non-respiratory functions
– Expulsion of foreign bodies
– Defence against infection/disease
Discuss alveolar ventilation
- Pulmonary ventilation renews air in gas exchange areas
- Alveolar Ventilation is the rate at which new air reaches these areas
- Some air that is breathed in never reaches gas exchange areas but fills respiratory passages (e.g. nose, pharynx, trachea) – Anatomic dead space air (about 150ml)
- Minute (total) ventilation rate (VE) = Freq x VT
• Alveolar Ventilation Rate (VA ) = Freq x (VT – VD)
Where VA = volume of alveolar ventilation per min Freq = frequency of respiration per min VT = tidal volume VD = dead space volume
• Alveolar ventilation is one of major factors determining O2 and CO2 concentrations in alveoli
Discuss the mechanisms of pulmonary ventilation
• Lungs can be expanded and contracted in 2 ways:
– Downward and upward movement of diaphragm to lengthen or shorten chest cavity
– Elevation and depression of the ribs to increase or decrease anterioposterior diameter of chest cavity
• Normal quiet breathing is accomplished entirely by 1st method
• During heavy breathing, normal elastic recoil not quick enough so need contraction of abdominal muscles too
What are the most important muscles in pulmonary ventilation
• Most important muscles that raise rib cage are: – External intercostals – Sternocleidomastoid (lift upward on sternum) – Anterior serrati (lift many ribs) – Scaleni (lift first two ribs)
• Most important muscles that lower rib cage are:
– Abdominal recti
– Internal intercostals
What are the static properties of the lungs
• Lung is elastic structure collapses like a balloon when no force to keep inflated
• Not attached to chest wall
• Lung floats in thoracic cavity surrounded by thin
layer of pleural fluid that acts as lubricant
• Chest wall also elastic
• Lymphatic drainage of excess fluid between lung pleural membrane and pleural surface of thoracic wall leads to suction effect – lungs held against thoracic wall
Discuss pressure changes during ventilation
- Intrapleural (Pleural) pressure is pressure of fluid in thin space between lung pleura and chest wall pleura – usually slight negative pressure
- IP pressure varies over length of lungs
- At start of respiration pleural pressure about -5 cm H2O
- During inspiration expansion of chest cage pulls lungs outward so negative pressure increases to about -7.5 cm H2O
- Air sucked into lungs
- Expiration process reversed
- Alveolar pressure is the pressure of air inside the lung alveoli
- When glottis open and no air flowing, pressure in all parts of respiratory tree is equal to atmospheric pressure (0 cm H2O)
- During inspiration and chest wall expansion, alveolar pressurees to about -1 cm H2O
- Pulls 0.5 L air into lungs
- During expiration opposite occurs
- Transpulmonary pressure is the pressure difference between that in the alveoli and that on the outer surfaces of the lungs
- It is a measure of the elastic forces that tend to collapse the lungs (recoil pressure)
Provide an overview of inspiration
• Change in volume leads to change in pressure
• Main muscle of inspiration - diaphragm. Contraction flattens domes.
Abdominal wall relaxes to allow abdominal contents to move downwards
• Role of the external intercostals – with first rib fixed, two movements, forward movement of lower end of sternum, and upward and outward movement of ribs
• Increases volume of thorax by about 500 ml – normal tidal volume
• Intrapleural pressure drops to approximately -7 mmHg
• Decreases intrapulmonary pressure by approximately 1 mmHg
• Accessory muscles in forced inspiration
– respiratory distress
– trapezius
Provide an overview of expiration
Quiet expiration:
– Passive – no direct muscle action normally
– Cessation (relaxation) of muscle contraction
– Elastic recoil – drives air out of lungs
– Thoracic volume decreases by 500 ml
– Intrapulmonary pressure increases
– Air moves down pressure gradient
Forced expiration:
– Contraction of abdominal walls, forces abdominal contents up against diaphragm, and internal intercostals – pull ribs downwards
What necessitates the work of breathing?
Energy is required to:
– contract the muscles of inspiration – in quiet breathing contraction of the diaphragm comprises 75% of energy expenditure
– stretch elastic elements
– overcome airway resistance
– overcome frictional forces arising from the viscosity of the lung and chest wall
– overcome inertia of the air and tissues
Discuss airway resistance in dynamic lung mechanics
• Most significant non-elastic source of resistance
F = ΔP (PB -PA)/R
• i.e. amount of air that flows is determined by change of pressure divided by resistance
• Airway Resistance (R) = 8Lƞ / πr4 (Poiseuille Law)
• So airway radius has greatest effect on airway
resistance
• Turbulent flow – likely to occur with high velocities and large diameter airways
• Greatest resistance to airflow is found in the segmental bronchi - cross sectional area relatively low and airflow high and turbulent
• At smallest airways, flow is laminar and resistance is small (there is a large total cross-sectional area due to large number of small airways combined)
Discuss compliance of the lung
- Static Compliance is the extent to which the lungs will expand for each unit increase in transpulmonary pressure (given time to reach equilibrium)
- The elastance of the lungs (measure of elastic recoil) is the reciprocal of compliance (E = 1/C). So high compliance means low elastic recoil
• Compliance diagram opposite is determined by 2 elastic forces:
– Elastic forces of the lung tissue itself
• determined mainly by elastin and collagen fibres among lung parenchyma
• deflated lungs, fibres are contracted and kinked
• expanded lungs, fibres become stretched and unkinked
– Elastic forces caused by surface tension of fluid that lines alveoli
Discuss compliance changes in lung diseases
• Pulmonary Fibrosis, a restrictive lung disease
– Disease process causes deposition of fibrous
tissue, so lungs become stiff
– Lung compliance is decreased, resulting in smaller than normal changes in lung volume for small changes in transpulmonary pressure
– Patients breath more shallowly and rapidly
– decreases in RV, FRC, TLC
• Emphysema, a chronic obstructive pulmonary disease (COPD)
– Common consequence of cigarette smoking
– Alveolar and capillary walls progressively
destroyed, particularly elastic tissue
– Lung compliance is increased, resulting in larger than normal changes in lung volume for small changes in transpulmonary pressure
– However, as airways tend to collapse on expiration, airway resistance is alsoed
– Patients breath more slowly and deeply
– increases in RV, FRC, TLC
• Chronic bronchitis, also a COPD
– Mucus and airway inflammation produce an increase in airway resistance
– increases in RV, FRC, TLC, however, compliance is normal
Discuss elastic forces due to surface tension
• Surface tension is a measure of the force acting to pull a liquid’s surface molecules together at an air-liquid interface
• In the lungs this results in the alveoli trying to force the air out of them so allowing the alveoli to collapse
• Law of Laplace states that the pressure (P) within a fluid-lined alveolus is dependent on the surface tension of the fluid (T) and the radius of the alveolus (r)
P = (2 x T) / r
• So if 2 alveoli are connected together but have different diameters, air will flow from smaller alveoli to larger alveoli
Discuss production of surfactant
- Lipid components enter Type II cell from bloodstream
- Secreted by Type II alveolar epithelial cells
• Surfactant is a complex mixture of phospholipids
– Dipalmitoylphosphatidylcholine (DPPC)
– proteins (surfactant apoproteins, SP- A, SP-B, SP-C & SP-D)
– ions (calcium)
• Part of DPPC molecule dissolves in fluid while rest spreads over surface of fluid
• Alveolar macrophages help in degrading surfactant, Type II cells take up rest and recycle or destroy it
