Download Respiratory System

Collin County Community College
BIOL. 2402
Anatomy & Physiology
WEEK 9
Respiratory System
1
Lung Physiology
Factors affecting Ventillation
1. Airway resistance
Flow = Δ P / R
Most resistance is encountered at the medium sized
bronchioli ( mostly smooth muscle, no cartilage )
Diameter is affected by :
• Symp. System : epi and norepi result in dilation
• ParaSymp. System : can result in constriction
• Irritants, histamine also cause constriction
• Any blockage of passageways also increases resistance
( such as thick mucus )
2
1
Lung Physiology
2. Lung Compliance
Compliance = Δ V / Δ P
Lung compliance is a
measure of the
lung’s “stretchability.”
When compliance is
abnormally low, the work
of breathing is increased.
3
Lung Physiology
Decreased Lung Compliance can be due to
• reduced lung elasticity
• reduced thoracic cage flexibility
• reduced production of surfactant
In general, the higher the compliance the better since less
pressure changes are needed to inflate the lungs.
Diseases that cause reduced lung compliance are referred
to as Restrictive Lung Diseases. Those patients require
more energy and effort to breathe !
4
2
Lung Physiology
An extremely high compliance
is not good either !
This because
transpulmonary pressure
never gets a chance to
become large enough to
exert its physiological
effect.
With such a high
compliance, the lungs
might fail to hold
themselves open,
and are prone to collapse.
5
Lung Physiology
3. Surfactant
6
3
Lung Physiology
Surface Tension
The attraction of the
water molecules to each
other resists expansion of
the bubble (resists an
increase in surface area).
The surface area tends to
shrink as small as
possible.
If alveoli were lined with water alone, they would collapse.
It would require more transpulmonary pressure to open up
the alveoli, thus resulting in a lower compliance
7
Lung Physiology
Law of La Place as it applies to alveoli
Collapsing Inward Pressure = 2 . Surface Tension / radius
Translation :
The smaller a bubble, the greater the collapsing pressure
If two bubbles of different diameter are connected to each
other, and they are not lined with surfactant, the smaller
one will collapse and the air will flow into the larger one.
8
4
P = 2 .T/ r
In the absence of surfactant,
the attraction between
water molecules (H-bonds)
can cause alveolar collapse.
By reducing the surface
tension of water,
surfactant helps prevent
alveolar collapse.
9
Lung Physiology
During normal quiet breathing, respiratory muscles work to expand
the lungs and overcome airway resistance, while expiration is a
passive process
On average, however, inhalation only expends 3 % of total body
energy due to high compliance of lungs and low airway resistance.
More energy will be needed when
• pulmonary compliance decreases ( pulmonary fibrosis)
• airway resistance increases (obstructive lung disease)
• elastic recoil decreases (emphysema)
• there is a need for increased ventilation
A person with obstructive lung disease may for example expend 30%
of total body energy just to inhale.
10
5
Respiratory Volumes
Respirometer or a Spirometer
11
Respiratory Volumes
• Maximum Lung volume ~ 5700 ml of air
• However, during quiet breathing the lungs are not close
to being completely filed or completely emptied.
• Lungs operate at half capacity during normal breathing.
• The lungs can never be completely deflated, even when
lungs are emptied forcefully.
• About 1,200 ml remains behind and provides continued
gas exchange between alveoli and blood during
heavy exercise.
• If lungs emptied all the way, the body would experience
dramatic fluctuations in O2 and CO2 .
12
6
• The tidal volume is the amount of air moved in (or out) of the
airways in a single breathing cycle.
• Inspiratory and expiratory reserve volumes, are, respectively, the
additional volume that can inspired or expired.
• All three quantities sum to the lung’s vital capacity.
• The residual volume is the amount of air that must remain in the lungs to
13
prevent alveolar collapse.
Restrictive lung diseases reduce IRV
Obstructive lung diseases reduce ERV
FEV1 = amount expelled in 1 sec when lungs
are fully filled
Healthy people: FEV1 ~ 80% of V.C.
14
7
Respiratory Volumes
Concept of Dead Space
• the air located in the conducting zones
• air that will not take part in gas exchange
• ~ 150 ml
• So if Tidal Volume is 500 ml, then only (500 -150) ml takes
part in gas exchange
• This 350 ml mixes with the 2400 ml already in the lungs
• Each tidal volume breath only replaces 12 % of lung volume.
350 of (2400 + 350)
15
Respiratory Volumes
“Fresh” inspired air is diluted by the “stale” air remaining
in the lungs from the previous breathing cycle.
16
8
Respiratory Volumes
Ventilation Rates
Minute Ventilation rate (MVR)
• The amount of air inhaled every minute
• MVR = breathing rate x Tidal volume
Alveolar Ventilation rate (AVR)
• The amount of air that takes part in gas exchange
• AVR = breathing rate x (Tidal volume - Dead space)
17
Respiratory Volumes
Increased O2 demand and Ventilation
For the heart : CO = HR x SV
For the lungs : AVR = BR x (TV - Dead space)
Increasing TV and/or BR will increase air supply
More specifically, the body tries to make ventilation more
efficient by increasing the fraction of useful air.
A useful ratio is to analyze what fraction of the inspired air is
actually used by the alveoli in gas exchange.
AVR / MVR = (TV - Dead space) / TV
18
9
Respiratory Volumes
AVM/MVR
A = Fast shallow respiration :
0/6000 = 0
B = Normal respiration :
4200/6000 = 0.70
C = Slow deep respiration :
5100/6000 = 0.8519
Respiratory Volumes
• Shallow faster breathing lowers the efficiency of ventilation
since dead space becomes a larger fraction of tidal volume
• Increasing TV makes ventilation more efficient. So, it’s
better to increase TV than to increase BR.
• The body in general tries to regulate both TV and BR
closely in order to match respiration with oxygen
demands.
Normally, dead space is fixed. But in certain conditions it can
become altered.
Example:
• breathing through a tube extends dead space
• why are snorkels short and fat ?
20
10
• Snorkel 1 cm in diameter and 0.5 meter long
• Volume = πr2 . L = (3.14)(1)2 .50 = 157 cm3
• Total Dead Volume = 150 + 157 = 307 ml
TV = 150
BR = 12
MVR = 6000
AVR = 12 x ( 500-307) = 2316
Normal
AVR/MVR = 0.70
Now
AVR/MVR = 0.39
21
Pulmonary Pathologies
22
11
Gas Exchanges
Provision of O2 to the tissues is accomplished by 4 main
processes.
Pulmonary Ventilation
movement of air in and out of the lungs.
External Respiration
gas exchange between lungs and blood
Transport of the gases
transport via blood between lungs and tissues
Internal Respiration
gas exchange between blood and tissues.
Resp. System
CardioVasc. System
23
Properties of Gases
Dalton’s Law
Total pressure of a mixture of gases is equal to the sum of
the partial pressure of each of the gases in the mixture
PT = P1 + P2 + P3 + …..
The partial pressure of each gases is directly proportional to
the percentage of that gas in a mixture.
For example, if gas 1 is 30 % of the mixture , then the
partial pressure of P1 = 0.3 PT
24
12
Properties of Gases
Gases in Air and their Partial Pressures
Atmospheric Pressure = 760 mm Hg
Gas
%
Pp
(mm Hg)
N2
78.6
597
O2
20.9
159
CO2
0.04
0.3
H2O
0.46
3.7
25
Properties of Gases
During gas exchange, gases diffuse from an “air mixture”
into a liquid medium. The amount that will diffuse and
dissolve is determined by Henry’s Law !
Henry’s Law
The amount of gas that will dissolve in a liquid is
proportional to it’s partial pressure in the gas mixture above
the liquid.
Dissolved ml of a gas in a liquid = (K . Pi)/ 760
Where K = solubility coefficient
Pi = Partial pressure of gas in question
26
13
Properties of Gases
Dissolved ml of a gas in a liquid = (K . Pi)/ 760
• Solubility coefficient for following gases is such that
K C02 = 20 . K O2
K N2 = 0.5 K O2
• This means that the relative diffusion rates of these gases
into body fluids are
N2 to O2 to C02 = 0.5 to 1 to 20
27
Properties of Gases
ml of gas dissolved in 100 ml blood at pulmonary vein :
N2 : 1.25
O2 : 0.29
C02 : 2.62
Even though the diffusion coefficient for Nitrogen is 1/2 of
that of Oxygen, there is thus more Nitrogen dissolved in
blood than Oxygen . Why ?
The answer is the same reason why Nitrogen becomes a
lethal element during scuba diving and prolonged exposure
to higher atmospheric pressures ( similar as opening a bottle
of soda ! ).
28
14
Exchange of gases
External Respiration
It is the Exchange of gases at the level of the alveoli
(between lungs and capillary blood)
Internal Respiration
Exchange of gases at the level of the tissues
Exchange of gases between capillary blood and tissue cells
Driving force is simple diffusion
29
Exchange of gases
30
15
Composition of Atmospheric vs Alveolar air
Lungs
Air
Gas
%
Pp
%
(mmHg)
Pp
(mmHg)
N2
78.6
597
74.9
569
O2
20.9
159
13.7
104
CO2
0.04
0.3
5.2
40
H2O
0.46
3.7
6.2
47
These Differences are due to :
• gas exchange in the alveoli
• humidification (increased Pp of water)
• mixture with residual volumes in the lungs
31
External Respiration
Factors that influence gas exchange
1. Pressure gradient
Alveoli
Equilibrium
occurs within
0.25 sec
PO2 = 104
PCO2 = 40
O2
CO2
PO2 = 104
PO2 = 40
O2
PCO2 = 46
CO2
PCO2 = 40
32
16
External Respiration
2. Thickness of Respiratory Membrane
The thicker the membrane,
the harder diffusion
3. Surface Area
The larger the area, the more
gases can diffuse
Fick’s Diffusion Equation
Flux = D. A. (C2-C1) / x
Time to diffuse 1 µm = 0.5 msec
Time to diffuse 1 cm = 14 hrs
33
External Respiration
4. Ventilation-Perfusion coupling
Normally, ventilation in the alveoli is matched perfectly
with blood flow through the alveolar capillaries.
34
17
External Respiration
• If alveoli are not functioning properly, and ventillation of those
alveoli is lower than normal, the alveoli will end up with low
oxygen levels and high CO2 levels.
• Thus capillary blood PO2 will also drop, and capillary PCO2 increases.
• The drop on O2 results in vasoconstriction of the capillaries.
The result is less blood flow through that section of the lungs.
Also, blood is diverted to better perfused sections of the lungs. The
result is a better matched ventilation - perfusion !
35
36
18
External Respiration
Local Pulmonary Controls
Gas Composition
Bronchioli
Pulmonary arteries
PCO2 increases
Dilate
(Constrict)
PCO2 decreases
Constrict
(Dilate)
PO2 increases
(Constrict)
Dilate
PO2 decreases
(Dilate)
Constrict
Responses in brackets indicates a weak response
37
External Respiration
The ratio of O2 consumption to ventilation determines alveolar PO2
The ratio of CO2 production to ventilation determines alveolar PCO2
38
19
Internal Respiration
Pressure gradient
Tissue cells
PO2 < 40
PCO2 > 46
O2
CO2
PO2 = 40
PO2 = 104
O2
CO2
PCO2 = 40
PCO2 =3946
External Respiration
Changes in the concentration of dissolved gases are indicated as the
blood circulates in the body. Oxygen “disappears” at active cells as it is
converted to water; active cells release carbon dioxide as a byproduct
40
of fuel catabolism.
20
Internal/External Respiration and Metabolism
AVR = 4000 ml/min
pO2 in air = 20.9 %
How much oxygen is delivered to the alveoli for exchange ?
20.9 % of 4000 = 840 ml/min
How much oxygen is actually exchanged ?
pO2 in exhaled air = 14.75 % = 590 ml/min
840 - 590 = 250 ml O2 enters blood stream / min
This amount is what tissues in general require at rest
(basal metabolism) = 250 ml O2 / min
41
21