Ventilation: Physics of Breathing Flashcards
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 of ventilation
- Expulsion of foreign bodies
- Defence against infection/disease
Alveolar Ventilation
the rate at which new air reaches these area
Dead space air
(about 150ml)
Some air that is breathed in never reaches gas exchange areas but fills respiratory passages (e.g. nose, pharynx, trachea
Alveolar Ventilation Rate (VA ) =
Freq x (VT – VD)
4200ml/min = 12breath/min x (500ml – 150ml)
abbreviations
VA, volume of alveolar ventilation per min
Freq, frequency of respiration per min
VT, tidal volume
VD, dead space volume
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
how is normal breathing accomplished
entirely by method 1
During heavy breathing
During heavy breathing, normal elastic recoil not quick enough so need contraction of abdominal muscles too
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
Intrapleural (Pleural) pressure
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
intrapleural pressure values
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
alveolar pressure mechanism
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 pressure decreases 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)
Overview of Inspiration
- Change in volume leads to change in pressure
- Main muscle of respiration - diaphragm. - Contraction flattens domes. Abdominal wall relaxes to allow abdominal contents to move downwards
- Role of the intercostals – externals – 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
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
when is energy required in the work of breathing
- 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
resistance in vessels
+ 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 is relatively low and airflow is high and turbulent
+ At the smallest airways, flow is laminar and the resistance is small (there is a large total cross-sectional area due to large number of small airways combined)
Static Compliance
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
