Lecture 5 (How much air actually reaches the alveoli?)

What does ventilation depend on?

How much air is inhaled, where that air goes, and how much fresh air reaches the alveoli

Lung Volumes

Individual volumes of air within the lungs

Tidal volume (V_T or TV)

Air moved during a normal breath

How much air is moved during tidal volume?

~500mL

Inspiratory reserve volume (IRV)

Extra air you can breathe in after a normal inspiration

Expiratory reserve volume (ERV)

Extra air you can breathe out after a normal expiration

Residual volume

Air remaining in the lungs after a maximal expiration

Lung capacities

The combination of two or more lung volumes

Inspiratory capacity

Maximum amount of air you can breathe in after a normal expiration

VT + IRV

Inspiratory capacity (IC)

Functional residual capacity

Air remaining in the lungs after a normal expiration

ERV + RV

Functional residual capacity (FRC)

Vital capacity

Maximum amount of air you can breathe out after a maximal inspiration

IRV + VT + ERV

Vital capacity (VC)

Total lung capacity

Total amount of air in the lungs after a maximal inspiration

VC + RV

Total lung capacity (TLC)

Why does residual volume (RV) matter?

Prevents the lungs from emptying completely, helps maintain alveolar patency (keeps them open, the way they should be), allows continuous gas exchange between breaths

Why does functional residual capacity matter?

Represents the resting lung volume, acts as an oxygen reservoir between breaths, helps stabilize blood gases, and helps maintain alveolar patency

Why do physiotherapists need to know lung volumes and capacities?

To understand normal breathing, understand how disease affects breathing, and guide physiotherapy management

Understanding normal breathing:

Learn how air moves into and out of the lungs, and understand how much air the lungs can hold and use

Understand how disease affects breathing:

Recognize how conditions change RV, FRC, VC, and TLC. Relate these changes to symptoms and functional limitations

Guide physiotherapy management:

Inform assessment and treatment decisions, and understand the effects of positioning, exercise, and breathing interventions

Minute Ventilation (V_E)

Total air moved into and out of the lungs each minute (depends on tidal volume and respiratory rate). It tells us how much air moves, not how much reaches the alveoli

Does all the air inhaled reach the alveoli?

No, some air remains in the conducting airways. No gas exchange occurs in the conducting airways (there is about 150mL of space in the conducting airways )

Anatomical dead space

The 150mL of air in the conducting airways, it does not participate in gas exchange and is deemed wasted ventilation

Can air reach the alveoli by another route?

Yes, by collateral ventilation. Air normally travels through the bronchi and bronchioles to reach the alveoli because that is the lower resistance pathway

What are the different types of collateral ventilation pathways?

Pores of Kohn, canals of Lambert, and channels of Martin

Pores of Kohn

Connections between alveolus

Canals of Lambert

Connection between bronchioles and alveolus

Channels of Martin

Connections between bronchioles

Collateral channels

Have higher resistance, so normally little air flows through them

What happens when an airway is narrowed or blocked?

Air may redistribute through collateral channels into under-ventilated alveoli

Clinical tip for when airflow is limited

Slow, deep breathing with an inspiratory hold may promote redistribution of ventilation, including through collateral pathways

If air reaches the alveoli, does gas exchange always occur?

No, not all air that reaches the alveoli participates in gas exchange

Alveolar Dead Space

Air reaches the alveoli, but gas exchange does not occur. The alveoli are ventilated but no perfusion occurs as blood flow is absent

Alveolar dead space in healthy lungs

It is minimal, most ventilated alveoli are also perfused

Alveolar dead space in diseased lungs

It may increase

Clinical example of alveolar dead space

Blood clot in pulmonary artery (pulmonary embolism) that blocks blood flow, alveoli continue to receive air (ventilated) but no effective gas exchange occurs, this is alveolar dead space

Physiological Dead Space

Total ventilation that does not participate in gas exchange, in healthy lungs it is about the same amount as anatomical dead space

Anatomical dead space + alveolar dead space

Physiological dead space

Apparatus dead space

Adds anatomical dead space, some exhaled air is rebreathed (meaning less fresh air reaches the alveoli per breath). We compensate by increasing ventilation, especially tidal volume (deeper breaths)

What happens if the apparatus adds too much dead space?

Compensation may be insufficient, increased amount of CO₂=hypercapnia= can cause headache, breathlessness, confusion, or dizziness

Alveolar ventilation

The amount of fresh air that reaches the alveoli each minute and is available for gas exchange, how much useful ventilation reaches the gas-exchanging region of the lungs

(VT-dead space) x RR

Alveolar ventilation (VA)

What best reflects the portion of ventilation available for gas exchange?

Alveolar ventilation

If three patterns have the same minute ventilation but different VT and RR, will they deliver the same amount of fresh air to the alveoli?

No, the pattern of breathing affects alveolar ventilation because dead space must be filled with every breath

Why does alveolar ventilation matter to physiotherapists?

Deep breathing, inspiratory hold/SMI, mobilization & exercise, and positioning

Deep breathing: (alveolar ventilation)

Increased tidal volume, results in more fresh air reaching the alveoli

Inspiratory hold/ SMI: (alveolar ventilation)

Sustains lung inflation, promotes air redistribution and alveolar recruitment

SMI

sustained maximal inspiration

Mobilization & exercise: (alveolar ventilation)

Increased ventilatory demand, deeper breaths and greater diaphragm excursion

Positioning: (alveolar ventilation)

Optimizes diaphragm mechanics and regional lung expansion

What does increased alveolar ventilation support?

Increase O₂ uptake, increase CO₂, and improved gas exchange

Why isn’t alveolar air the same as atomospheric air?

It is continuously being modified, O₂ is removed from the alveoli in the blood and CO₂ is added to the alveoli from the blood. Inspired air becomes fully humidified in the airways.

Fresh inspired air mixes with the large volume of air that is already present in the lungs, why?

Due to the functional reserve capacity, the lungs do not empty completely between breaths, so each new breath mixes with a large volume of air already in the lungs

Why is there so much nitrogen in the lungs?

Nitrogen makes up most of inspired air and is not normally exchanged with the blood, so it remains a major gas in the alveoli

What acts as a passive (inert) filler gas in the lungs?

Nitrogen

Why is the composition of alveolar air ideal for gas exchange?

Because its composition creates the concentration gradients needed for O₂ uptake and CO₂ removal

O₂ concentration is high in the alveoli and low in the capillaries, what way does the gradient flow?

From alveoli to the capillary

CO₂ concentration is low in the alveoli and high in the capillaries, which way does the gradient flow?

From the capillary to the alveoli

What is the symbol for partial pressure?

P

Partial pressure (P)

The pressure contributed by an individual gas in a mixture

What direction do gases move in terms of their pressure?

From high partial pressure to low partial pressure

PA

Alveolar (in the alveoli) partial pressure

Pa

Arterial (in the arterial blood) partial pressure

Pv

Venous (in venous blood) partial pressure