Respiratory ventilation & mechanics Flashcards
Describe the muscles involved in inspiration & forced exhalation
Inspiration: diaphragm, external intercostal muscles, scalene & sclernocleidomastoid
Forced exhalation: abdominal muscles, internal intercostal muscles
Define the FRC
Functional residual capacity, the volume of air left in the lungs after a normal exhalation. The outward recoil of the chest wall is balanced by the inward recoil of the lungs. There is no pressure gradient at this point.
Describe the mechanism of inspiration
- Air flows into the lungs down a pressure gradient
- The thorax expands stimulating the contraction of the inspiratory muscles
- Alveolar pressure drops as alveolar volume increases in thorax
- Lungs stretch to fill the expanded throax
- The pleural pressure becomes more negative
Describe the mechanism of expiration
- Air flows out of the lungs via a pressure gradient, pressure in lungs is greater than ATM so air flows out
- The thorax decreases in size & inspiratory muscles relax
- As alveolar volume decreases, pressure in the alveoli increases
- Lungs decrease in size as they recoil
The pleural pressure becomes more positive
Describe the factors influencing ventilation
- Compliance
- Airway resistance
- Muscle strength & fatigue
Define lung compliance & factors affecting it
LC refers to the lungs ability to expand and stretch when pressure is applied. Defined as a change in lung volume per unit change in transpulmonary pressure.
LC is influenced via:
1. elastic properties (35-50%)
2. Surface tension /surfactant ( 35-45%)
3. Lung volume & airway resistance ( 5-10%)
4. Chest wall mechanics (10-20%)
Low lung compliance means the lungs are stiffer and more difficult to expand (as in conditions like pulmonary fibrosis), while high compliance means the lungs are more easily expanded, but can be less efficient at holding air (seen in conditions like emphysema).
Describe the compliance equation
Explains how easy it is for the lungs to expand. Defined as a change in lung volume per unit of transpulmonary pressure.
Equation=
compliance= change in lung volume/ change in pressure
The higher the value of compliance, the less pressure required to inflate the lungs for a given volume
UNITS: L/kPa
Describe the influence of lung elasticity on compliance
Elasticity plays a role in influencing lung compliance, Elasticity refers to the ability of the chest wall & lung tissue to return to their original shape after expanding
Describe the elastance equation
Elastance=1/compliance
So if elastance is high, then compliance is low (vice versa)
Lung elastance is determined by the collagen & elastin fibres present in the lung parenchyma
Describe the effects of high/low elasticity on compliance
Too high elasticity results in the lungs having low compliance as more pressure is required to expand the lungs & they tend to recoil quicker & resist expansion
e.g., pulmonary fibrosis
Too low of elasticity results in high compliance, lungs are able to fully expand, but struggle to recoil. Less force is needed to inflate the lungs however struggle to recoil & expel air, e.g., emphysema
Describe the recoil action of the lung elasticity
The lungs will contract & recoil until no air remains on their own. However due to the thoraic cavity the respiratory muscles are limited
Define surface tension
ST refers to the force that acts on the interface between air & liquid that lines the alveoli. These water molecules are attracted to each other, which causes them to form a “skin” on the surface, exerting inward pressure. Water molecules are more attracted to those adjacent or below them. The cohesive forces between water molecules reduce the interface area.
Describe Laplace law
As surface tension in the lungs aims to reduce volume to bare minimum. The pressure in an alveoli bubble is dependent on the surface tension and radius of the bubble (laplace law)
Pressure inside a alveolus= 2 x ST/ radius
Describe the effects of high/low surface tension
If pressure was equal in all alveoli, the smallest alveoli would have the greatest pressure this would cause unstable lungs, this is prevented by the Prescence of surfactant.
When surface tension is high, without sufficient surfactant the lungs become stiffer, and compliance decreases. This results in the lungs needing greater pressure to expand.
When surface tension is low, due to the presence of sufficient surfactant, the lungs become more compliant, and the alveoli are able to expand more easily. This allows for smoother, more efficient breathing with less effort required to inhale.
Describe the function of lung surfactant
They are produced by type 2 alveolar cells and aim to decrease the cohesive forces between water molecules allowing the alveoli to expand easily & prevent them from collapsing
ST is reduced by sufficient surfactant causing the lungs to become more compliant.
Describe the components
& properties of lung surfactant
Properties: Surfactant has hydrophilic & hydrophobic properties, possess a polar head (hydrophilic) and a non-polar tail (hydrophobic). Surfactant moves via their hydrophobic/hydrophilic properties into the liquid to form a thin film, this reduces the surface tension as forces are prevented from being created between water molecules.
Components: 90% phopsholipid, 10% protein. DPPC makes up of 50% of surfactant
DPPC =Dipalmitoylphospatidylcholine
Describe the surface tension in a small alveoli
In small alveoli, ST is lower as it contains surfactant, whereas large alveoli contain less surfactant therefore experience more ST
This enables the larger alveoli to have a greater pressure than the smaller alveoli, so air flows from large to small alveoli
How does surfactant stabilize the alveoli
Pressure in the small alveoli is stabilised by surfactant reducing ST.
As the alveoli expand surfactant is reduced & ST rises. When pressure exceeds alveoli is will empty via pressure gradient from large to small alveoli. This slows rate of expansion and allows all alveoli to inflate similarly in size. Surfactant helps prevent the alveoli from collapsing, especially at low lung volumes, ensuring they remain open for gas exchange.
Surfactant helps balance the pressure across alveoli of different sizes, preventing smaller alveoli from collapsing into larger ones, promoting uniform inflation.
Describe the effects on lung volume on compliance
Compliance increases with lung volume & increases with age due to structural changes in elastin fibres
What conditions does lung altered compliance help identify
COPD,
emphysema: high compliance due to damage to elastin fibres
pulmonary fibrosis: low compliance, due to scarring
atelectasis
NRDS
Ageing: high compliance due to loss of elastic recoil from loss of elastin & collagen
Define NRDS & the symptoms
Refers to newborn respiratory distress syndrome, if affects primarily premature infants as their is an insufficient production of lung surfactant. This makes it difficult for the baby to breathe properly as lack of surfactant results in high ST, alveoli are more prone to collapse
Symptoms: Cyanosis, Hypoxemia, rapid & shallow breathing (tachypnea), chest retractions
Describe airway resistance
Formed by airway resistance which is resistance to flow (80%), alveolar resistance (resistance to expansion) (20%) or inertia of respiratory system.
Airway resistance accounts for 80% of total resistance
Measured in: kPa.L-1.s-1
Airway resistance is affected via: lung volume, airway diameter & air viscosity
Describe the airway resistance equation
Airway resistance=pressure difference driving the airflow/the airflow
Describe the factors that influence airway resistance
Type of flow: laminar, turbulent or transitional
(Turbulent flow found in larger airways causes greater airway resistance than laminar flow)
Airway calibre: smooth airways & dynamic flow
Lung volume: R decreases as LV increases
Define poiseuilles law
Poiseuille’s law states that the volumetric flow rate of a fluid through a cylindrical vessel is directly proportional to the pressure difference across the vessel and the fourth power of the vessel’s radius, and inversely proportional to the fluid’s viscosity and the length of the vessel.
Q = ΔPπr^4/8ηl
Describe the airway calibre equation
Resistance= 8xnxI/ πxr^4
n=viscosity of gas
l=length of tube
r=radius
What happens to the resistance if the radius of the alveoli halves?
If radius of alveoli halves, then resistance increase 16-fold (x16)
Describe the effect of muscle strength & fatigue
Respiratory muscles can work continuously without rest. Although disease puts more strain on the respiratory pump as well as decreasing the capacity in which the respiratory muscles can generate pressure.
If the respiratory muscles are over-worked for a long period of time, they will fatigue, this may cause the respiratory pump to fail resulting in hypercapnic ventilatory failure
