Respiratory air flow is determined by the pressure difference between mouth and alveoli. Flow results from either an upstream rise (positive pressure breathing) or a downstream fall in pressure (negative pressure breathing).
Pressure gradients during normal respiratory cycle:
A) ATM
B) < ATM
C) Inspiration
Pressure gradients during normal respiratory cycle:
Positive pressure - increase atmospheric pressure relative to lung pressure - air forced into lungs
Negative pressure - decrease lung pressure relative to atmospheric pressure - air drawn into lungs
Humans are negative pressure breathers under normal circumstances.
Assisted breathing utilises positive pressure - if individuals are unconscious we increase the outside pressure to force air into their lungs.
Inspiration - lungs at lower pressure than atmosphere therefore air is forced into lungs
Expiration - increased pressure inside the lungs to create a gradient and force air out
Barometric (atmospheric) pressure ~ 750 mmHg
Atmospheric pressure is the pressure in the atmosphere.
Alveolar pressure is the pressure in the alveoli
Two layers of pleura
parietal pleura is connected to chest wall (ribcage)
visceral pleura is connected to lung tissue
Intrapleural cavity - space between two pleural layers containing pleural fluid which provides lubrication and surface tension to pull the two layers together - as the chest wall goes out the parietal membrane goes out and pulls the visceral membrane with it.
Intrapleural pressure - pressure between two pleural membranes
The difference between the alveolar and intrapleural pressures is the transpulmonary pressure.
Intrapleural pressure is always negative (lower than atmospheric) due to counter recoil of chest wall and alveoli - the ribcage wants to pull outwards and alveoli want to pull inwards. By extending and pulling the two membranes apart, you decrease the pressure inside that chamber - therefore negative in relation to the atmosphere.
Boyle's law - if you increase the space without changing the volume, you will decrease the pressure (and vice versa)
Inspiration
inspiratory muscles contract
diaphragm goes from dome to flat shape
Ribs come up and out
Therefore thoracic cavity size increased.
Physiology of inspiration
parietal membrane pulled out with chest wall, due to attraction the visceral membrane is also pulled out but not to the same degree
intrapleural pressure decreases because the space gets bigger
transpulmonary pressure increases (difference between alveolar and intrapleural pressures)
Bigger Ptp - bigger alveoli
therefore alveolar pressure decreases - creates a gradient between the atmosphere and the alveoli so air goes IN
Expiration is passive under normal conditions i.e. unless there is exercise or respiratory distress
Expiration
relaxation of inspiratory muscles
chest wall recoils
space between visceral and parietal membranes decreases
intrapleural pressure becomes less negative
transpulmonary pressure decreases (difference between alveolar and intrapleural pressures)
alveoli become smaller
increase alveolar pressure
create gradient - positive to force air out of lungs
Collagen and elastin fibres help the lung to recoil
alveolar interdependence means inner alveoli open as well as outer. The outer alveoli are affected by the change in intrapleural pressure. This pulls the next layer and then the next layer etc. Alveolar parenchyma in between is pulled.
Pneumothorax
Pleural seal broken, with pathway either:
inwards through lung tissue
outwards through chest wall
no connection between chest wall and lungs
lung tissue wants to come in, chest wall wants to go out, so they separate
lungs recoil and collapse
elastic recoil of alveoli
Work of Breathing: 2 factors to overcome
resistance
compliance
resistance
resistance of respiratory tract to airflow during inspiration and expiration
poiseuille's law of fluid dynamics
look at inspiration, expiration - usually expiration issue
compliance
measure of the lung's ability to stretch and expand (distensability of elastic tissue)
not the elastic properties/elasticity
Resistance becomes a problem with:
airway narrowing (e.g. asthma, croup)
asthma is predominantly an expiratory problem
increasing respiratory rate
eventually the body will try to increase volume of breath but first will increase respiratory rate as it is easier
children are at greater risk with reduced resistance because they have smaller airways and higher resting respiratory rates.
Obstructive pulmonary disease affects resistance. Larger to smaller airway diameter.
compliance
lung tissue thickens through disease process e.g. fibrosis
restriction to ability to expand
expandability of lungs (and chest wall)
low compliance - fibrosis
high compliance - emphysema
value varies as lung inflates
low compliance is bad as you need to put more work in
Restrictive pulmonary disease affects compliance. Reduced ability of chest wall to expand
Understanding and quantifying resistance in conduction zone is difficult.
branching of airways, narrowing of airways, dispensible, compressible - all lead to dynamic resistance
airflow within the conduction zone also changes (laminar, turbulent, transitional)
Presuming air flows through a rigid, smooth bored tube - governed by Poiseuille's law
dP = V x R1
(dP = pressure difference, V = airflow, R1 = resistance)
POISEUILLE'S LAW
Resistance is directly proportional to viscosity of fluid and the length of tube and inversely proportional to the fourth power of the radius of the tube
change in diameter of airway has a massive impact on resistance