2_ Lung mechanics_2_per_page

The Respiratory System
Lecture 2: Lung Mechanics
Human form & Function - Physiology 08-09
Mikel Egaña
Trinity College Dublin
Copyright © 2005 Pearson Education, Inc., publishing as Benj amin Cummings.
Lecture Outline
I.
Forces for Pulmonary Ventilation
II.
Factors Affecting Pulmonary Ventilation
III. Lung Volumes and Capacities
Physiology 08-09. Respiratory System. Lecture 2.
1
I. Forces for Pulmonary Ventilation
•
Interrelationships among pressures inside and outside
the lungs are important in ventilation
•
Three different pressure considerations important in
ventilation
•
Atmospheric (barometric) pressure
•
Intra-alveolar pressure (intrapulmonary pressure)
•
Intrapleural pressure (intrathoracic pressure)
•
Mechanics of Breathing
Physiology 08-09. Respiratory System. Lecture 2.
Atmospheric Pressure
•
760 mm Hg at sea level
•
Decreases as altitude increases
•
Normally other lung pressures given relative to atmospheric
(set Patm = 0 mm Hg)
Intra-alveolar Pressure
•Pressure of air in alveoli (not closed system, opened)
•Varies with phase of respiration
–During inspiration = negative
(less than atmospheric)
–During expiration = positive
(more than atmospheric)
•Difference between Palv and Patm
drives ventilation
Physiology 08-09. Respiratory System. Lecture 2.
2
Intrapleural Pressure
•
•
Pressure inside pleural sac (closed system)
–
Always negative under normal conditions
–
Always less than Palv
Varies with phase of respiration
–
•
At rest, 756 (or -4) mm Hg
Negative pressure due to elasticity in lungs
and chest wall
–
Lungs recoil inward
–
Chest wall recoils outward
–
Opposing pulls on intrapleural space
–
Surface tension of intrapleural fluid hold wall
and lungs together
–
Sub-atmospheric P: due to vacuum in the pleural cavity
Physiology 08-09. Respiratory System. Lecture 2.
Pulmonary Pressures at Rest
FRC = Functional
Residual Capacity =
volume of air in lungs
between breaths (defined
as rest); Palv = Patm
Physiology 08-09. Respiratory System. Lecture 2.
Sherwood 6th Ed., Fig 13-8
3
Pneumothorax (punctured lung)
Physiology 08-09. Respiratory System. Lecture 2.
Sherwood 6th Ed., Fig 13-9
Mechanics of breathing:
Forces for Air Flow
Flow =
Patm – Palv
R
•
•
•
•
•
Atmospheric pressure constant (during
breathing cycle)
Therefore, changes in alveolar pressure
create/change gradients
Boyle’s Law: pressure is inversely
related to volume in an airtight container
(closed system)
Thus – can change alveolar pressure
by changing its volume
R = resistance to air flow
– Resistance related to radius of
airways and mucus
Boyle’s Law
Sherwood 6th Ed., Fig 13-10
Physiology 08-09. Respiratory System. Lecture 2.
4
Determinants of Intra-Alveolar Pressure
Factors determining intra-alveolar pressure:
–
Quantity of air in alveoli
–
Volume of alveoli
Inspiration: lungs expand – alveolar
volume increases
Palv decreases
Pressure gradient: air into lungs
Quantity of air in alveoli rises
Palv increases
Expiration: lungs recoil – alveolar
volume decreases
Palv increases
Pressure gradient: air out of lungs
Quantity of air in alveoli decreases
Palv decreases
Germann & Stanfield 3rd Ed., Fig 16.10
Physiology 08-09. Respiratory System. Lecture 2.
Breathing cycle pressures
(opened
system)
(closed system:
follows Boyle’s law)
Physiology 08-09. Respiratory System. Lecture 2.
Sherwood 6th Ed., Fig 13-14
5
Anatomy of the Respiratory Muscles
Physiology 08-09. Respiratory System. Lecture 2.
Netter’
Netter’s Atlas, fig 5.5
Respiratory Muscle Activity During Inspiration
Physiology 08-09. Respiratory System. Lecture 2.
Sherwood 6th Ed., Fig 13-12
6
Respiratory Muscle Activity During Expiration
Physiology 08-09. Respiratory System. Lecture 2.
Sherwood 6th Ed., Fig 13-12
II. Factors Affecting Pulmonary Ventilation
•
•
Lung Compliance
–
Ease with which lungs can be stretched
–
The less compliant the lungs are, the more work is
required to produce a given degree of inflation
–
Affected by elasticity (elastic recoil) and surface
tension of lungs (alveoli) (type II cells produce
surfactant to decrease surface tension)
Airway Resistance
–
Affected by: passive forces, contractile activity of
smooth muscle and mucus secretion
–
Increased in pathologies (later
(later lecture)
lecture)
Physiology 08-09. Respiratory System. Lecture 2.
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Work of Breathing
•Normally requires 3% of total energy expenditure for quiet
breathing
•Lungs normally operate at about “half full”
•Work of breathing is increased in the following situations
•
When pulmonary compliance is decreased
•
When airway resistance is increased
•
When elastic recoil is decreased
•
When there is a need for increased ventilation
Physiology 08-09. Respiratory System. Lecture 2.
III. Lung Volumes and Capacities: Spirometer
Sherwood 6th Ed., Fig 13-19
Physiology 08-09. Respiratory System. Lecture 2.
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Sherwood 6th Ed., Fig 13-18
Spirograph
Physiology 08-09. Respiratory System. Lecture 2.
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