What causes air to flow into the lungs (inspiration)?
Diaphragm and external intercostals contract → thoracic volume increases → alveolar pressure drops below atmospheric pressure → air flows in to equalize pressure.
What causes air to flow out of the lungs (expiration)?
Diaphragm and intercostals relax → thoracic volume decreases → alveolar pressure rises above atmospheric → air flows out of lungs.
Why is normal expiration considered passive?
Elastic lung tissue recoils naturally after inspiration → reduces thoracic volume → air exits without muscle effort.
What causes forced expiration (like during exercise or coughing)?
Abdominal and internal intercostal muscles contract → push diaphragm upward → thoracic volume decreases further → more air is expelled.
What is the difference between Costal breathing and Diaphragmatic breathing?
Diaphragmatic breathing uses the diaphragm to breath
Costal breathing uses the intercoastal muscle to breath
Both require thought
What happens during forced inspiration?
Accessory muscles (sternocleidomastoid, scalenes, pectoralis minor) lift the rib cage → expand thoracic volume → draw more air in for increased oxygen demand.
Tidal volume (TV)
Amount of air that normally
enters the lungs during quiet
breathing (~500ml)
Inspiratory reserve volume (IRV)
Produced by a deep inhalation, beyond a normal
tidal inspiration and represents the extra volume
that can be brought into the lungs during a forced
inspiration
Expiratory reserve volume (ERV)
Amount of air you can forcefully exhale past a normal tidal expiration
Residual volume (RV)
The air left in the lungs if you exhale as much air as
possible.
* The residual volume makes breathing easier by preventing the alveoli from collapsing
What is the normal Vital capacity (VC)?
- Typically, is between 3000 and 5000 ml
Inspiratory capacity (IC)
the maximum amount of air that can be inhaled past a
normal tidal expiration (TV and IRV)
Functional residual capacity (FRC)
The amount of air that remains in the lung after a normal tidal expiration (ERV and RV)
Why do the lungs always contain some air even after maximal exhalation?
Residual volume prevents alveolar collapse → keeps alveoli partially inflated → makes the next breath easier.
What causes differences in lung capacity between individuals?
Body size, age, sex, and conditioning affect total lung capacity → larger or fitter individuals typically have higher capacities.
Why is residual volume important?
It maintains alveolar inflation and continuous gas exchange between breaths → prevents airway collapse.
What causes anatomical dead space?
Air in conducting airways (trachea, bronchi) doesn’t reach alveoli → no gas exchange occurs there.
dorsal respiratory group (DRG) is responsible for what part of breathing?
stimulating the diaphragm and intercostal muscles to contract, resulting in inspiration
ventral respiratory group (VRG) is responsible what part of breathing?
forced breathing, as the neurons in the VRG stimulate the
accessory muscles involved in forced breathing to contract, resulting in forced inspiration
What is the effect of alveolar (physiological) dead space?
Damaged or fluid-filled alveoli can’t exchange gases → reduces respiratory efficiency.
What triggers the brain to adjust breathing rate and depth?
Chemoreceptors detect ↑CO₂ (↓pH) → medulla oblongata increases diaphragm and intercostal activity → faster, deeper breathing removes CO₂ and raises pH.
What happens when CO₂ levels are low?
↓CO₂ → ↓H⁺ (higher pH) → reduced chemoreceptor firing → medulla decreases respiratory stimulation → slower, shallow breathing lets CO₂ build back up.
Why is CO₂ a stronger respiratory driver than O₂?
CO₂ directly affects blood pH → brainstem chemoreceptors are more sensitive to pH changes → ventilation adjusts quickly to CO₂ fluctuations.
What causes breathing to increase at high altitude?
Low O₂ (<60 mmHg) activates peripheral chemoreceptors → respiratory center increases rate and depth of breathing → enhances O₂ uptake despite low pressure.
