Download Lect 20 Resp 1

Major respiratory topics
Respiratory Physiology 1
1. Pulmonary ventilation & work of
breathing
2. Gas exchange
3. Gas transport in the fluids
4. Regulation of respiration
Major functions of the respiratory
system
1. Provide oxygen
2. Eliminate carbon dioxide
3. Regulate pH, in coordination with the
kidney
All of the above require gas exchange
4. Sound production (speech)
5. Angiotensin II
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The relaxation/contraction
of circular smooth muscle
lining these “airways’”
determines how easily
airflow can occur
(bronchodilation vs.
bronchoconstriction).
Most gas exchange
occurs in the
~8,000,000
alveolar sacs.
The challenge:
Get as much air, exposed to as much blood
as possible to allow for efficient gas
exchange
The strategy (in mammals at least):
Coordination of blood circulation, gas
exchange, and matching blood and gas
flow in/around the alveoli
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1. Pulmonary Ventilation
• The thorax works as a pump to move gas
in and out of the airways
– Inhalation involves active, muscular
expansion of the thorax
– Exhalation is passive elastic recoil of the
lungs and thorax
Each lung is surrounded by the pleural sac
Fluid in the pleural sac provides lubricant to allow the thoracic wall
to move over the lungs
Muscular aspects of inhalation and
exhalation
Thorax is a closed container whose volume can be
changed.
Inspiration: diaphragm, and intercostal rib muscles contract
→↑ thoracic volume →↓pressure in the lung
Expiration: diaphragm, and intercostal rib muscles relax →
thoracic volume returns to rest →pressure returns to rest
The intrapleural fluid acts like a glue to hold the lung and
thoracic wall together
Boyle’s law :
pressure of a fixed number of
molecules is related to the volume
of a container in which they are placed.
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Relaxation phase:
1. The lungs have a lot of elastic tissue and
are normally stretched out
•
Therefore have a tendency to collapse
2. The surface tension of fluid in the alveoli
also tends to cause collapse
The normal intra-pleural pressure is about -4mmHg
Airflow (F) is a function of the pressure differences
between the atmosphere (Patm) and the alveoli (Palv),
divided by airflow resistance (R).
Air enters the lungs when Palv < Patm
Air exits the lungs when Palv > Patm
Changes in the pressure of the intrapleural fluid (Pip) affect
the pressure in the alveolus. The difference between
atmospheric pressure (Patm) and alveolar pressure (Palv)
is the major pressure driving ventilation.
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Lung Volume is due to transpulmonary
pressure (PTP) = PALV - PIP
Note: by convention Patm is designated 0mmHg;
it’s really 760mmHg at sea level
Patm = 0
Patm = 0
Diaphragm and Intercostal Muscles Contract
Thorax Expands
↓ Intrapleural Pressure (PIP < PAtm)
Lungs Expand
↓ Alveolar Pressure (PALV < PAtm)
PALV = 0
PALV = 0
PIP = -4
At the end of expiration
Air Flows into the Lungs
PIP = -7
At the end of inspiration
Pressure changes during one cycle
2
Palv
Patm
0
-2
PIP
Lung Compliance
• Lung volume a function of transpulmonary
pressure (PTP)
• How much the volume changes is called
lung compliance
-4
Transpulmonary
-6
inhalation
CL =
ΔVol
ΔVol
=
Δ( Palv − Pip ) ΔPTP
exhalation
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• Compliance ≈ stretchiness
• eg. less compliance, harder to expand
Increased
compliance
Normal compliance
Lung Volume
(mL)
• If the lung has reduced compliance, it
takes a higher PTP to achieve the same
lung volume
• Fibrosis – reduced compliance
Reduced compliance
Transpulmonary Pressure
Factors affecting compliance
• Relationship between pressure,
surface tension and radius
+
O-H+
water
-
alv
-O
+H
•
Inside surface of alveoli are lined with
fluid
Surface tension of the fluid in the alveoli
resists expansion
H
+H
•
H+
P=
2T
radius
Compare 2 alveoli, radiusA>radiusB
T is constant, PA<PB
O-H+
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What helps lungs overcome
surface tension?
• Surfactant:
– a mixture of protein and phospholipid
secreted by Type II alveolar cells
– Works like a detergent to reduce surface
tension
– Secretion is stimulated by stretch, associated
with deep breaths
P=
radB
A
• Aside:
With Surfactant
B
→ Large alveoli have more surface tension
2T
radius
No Surfactant
radA
• Surfactant also helps balance the volume in alveoli
• Small alveoli have higher concentration of surfactant
than larger alveoli
radB
radA
A
B
If TA=TB and radA>radB, then PB>PA
With surfactant
If TA>TB, and radA>radB, then PB≈PA
Thus air will flow B→A and
B will collapse
Thus alveoli are stable
– Premature babies, sometimes their Type II
alveoli cells are not mature enough to secrete
surfactant
– Therefore surface tension is huge and must
generate large transpulmonary pressure to
inflate lungs
– Treatment: administer surfactant to the alveoli
thorugh the trachea and mechanical
ventilation to generate pressure.
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Airway resistance
• Bronchioles have smooth muscle
Remember:
Flow (Q) = Δ pressure / resistance
Resistance =
8 ⎛ Lη ⎞
π ⎜⎝ r 4 ⎟⎠
– Therefore contraction and dilation of
bronchiole smooth muscle can regulate
airflow
– However, under most normal conditions the
airways represent a low resistance pathway to
flow.
– When does this change dramatically? asthma
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