Respiratory System and Mechanics of Breathing II Flashcards
(42 cards)
PTotal =
Pelastic + Presistive = V/C + V(dot)R
Pulmonary Resistance (R) Composed of:
- frictional resistance of lung & chest wall tissue (~20%)
- airways resistance (~80%)
The movement of fluid (i.e. air) flowing through a rigid,
smooth bore tube is governed by
Poiseuille’s Law:
The total airway resistance to flow is the sum of all
the
resistances of the nose and mouth (a substantial portion
of the total) and of the 23 generations of the
tracheobronchial tree.
• Airway resistance is from the friction between
gas
molecules and between gas molecules and the walls.
• Airway resistance is important (~80%) and makes the
sliding of
lung tissue over each other (viscous tissue
resistance, ~20%) a minor issue.
Turbulent flow requires a much greater
driving pressure
• Doubling the length (l) of
the airways doubles the
airway resistance.
• But halving the radius (r)
increases the resistance by
sixteen-fold.
Poiseuille’s Law
- Gives the relationship for laminar flow in a cylindrical tube.
- It states that the rate of flow is due to the difference in pressure (DP) between the two ends:
- i.e. Flow is proportional to DP.
Intermediate sized
airways contribute
most
of the total resistance.
• Total cross-sectional
area increases towards
the periphery, whereas
total airflow is
constant.
• Flow is therefore more
laminar in the
small
airways.
Flow is:
• effort dependent at
• effort independent at
high lung volumes.
low lung volumes.
Work is proportional to ΔP x ΔV
The work of inspiration consists of three different
fractions:
(i) compliance work, or elastic work i.e. that required
to expand the lungs against elastic forces (recoil).
(ii) tissue resistance work, i.e. that required to
overcome the viscosity of the lung and chest wall
structures.
(iii) airway resistance work, i.e. that required to move
air through the airways into the lungs.
0AECD0 Insp. work done overcoming
elastic forces.
ABCEA Insp. work done overcoming
viscous (airway + tissue) resistance.
AECFA Work done on expiration to overcome
airway (+ tissue) resistance.
Emphysema
destruction of lung tissue
around alveoli makes them
collapse on expiration. creating larger air spaces instead of many small ones.
Bronchitis
Increased mucous buildup in
the airways.
Obstructive disease
e.g.
Resistive work greatly increased (ABCEA) insp
COPD (i.e. emphysema and bronchitis.)
Obstruct airways, resulting in increased work of breathing especially during
expiration.
Restrictive disease
e.g.
Elastic work increased (0AECD0)
fibrosis, pulmonary congestion.
Restrict lung expansion, resulting in a decreased lung volume, an
increased work of breathing, and inadequate ventilation and/or oxygenation.
The Work of Breathing
Rapid shallow breathing ¯
• Slow deep breathing ¯
decr elastic work but incr frictional (viscous) work.
decr frictional work but incr elastic
work.
During INCREASED ventilation to levels of ~50 L min-1:
Total cost of breathing =
total VO2 - resting VO2
