Ventilation Basics
This is the constant movement of air into and out of the lungs.
pulmonary ventilation
This pressure is ~760 mmHg at sea level and remains relatively constant.
atmospheric pressure (Patm)
This muscle contracts and flattens to increase thoracic volume.
the diaphragm
This describes how easily the lungs expand (ΔV/ΔP).
pulmonary compliance
This is the amount of air moved in or out during a normal breath.
tidal volume (VT)
This term describes one full inhalation and exhalation sequence.
respiratory cycle
This pressure must always remain lower than alveolar pressure to prevent collapse.
intrapleural pressure (Pip)
These muscles elevate ribs “up and out” during inspiration.
external intercostal muscles
the tendency of lungs to return to their original shape.
elastance (elastic recoil)
This is the extra air that can be inhaled after a normal inspiration.
inspiratory reserve volume (IRV)
Air flows in the lungs due to this principle, always moving from high to low.
pressure gradient
The equation Ptp = Palv − Pip defines this pressure.
transpulmonary pressure
These muscles are primarily used during forced expiration.
internal intercostal muscles
This substance reduces alveolar surface tension and prevents collapse.
surfactant
This volume remains in the lungs after maximal expiration and cannot be measured by spirometry.
residual volume (RV)
This law explains why increasing thoracic volume decreases pressure.
Boyle’s Law
During inspiration, alveolar pressure changes in this way relative to atmospheric pressure.
decreases below atmospheric pressure
This process is mostly passive due to elastic recoil of the lungs.
expiration
According to the Law of Laplace, pressure in an alveolus is related to surface tension and this variable.
radius
This capacity equals TV + IRV.
inspiratory capacity (IC)
This change directly drives airflow during breathing.
change in thoracic volume altering intrapulmonary pressure
During expiration, intrapleural pressure does this.
becomes less negative
During inspiration, thoracic volume and alveolar pressure change in this paired way.
thoracic volume increases and alveolar pressure decreases
This is the main determinant of airway resistance according to Poiseuille’s law.
airway radius
This equation calculates alveolar ventilation.
VA = RR × (VT − dead space)