Transcript Module XIII

Chapter 23
The Respiratory
System:
Physiology
Respiratory System Anatomy
Functionally, the respiratory system is divided into the
conducting zone and the respiratory zone.
The conducting zone - nose, pharynx, larynx,
trachea, bronchi, bronchioles and terminal
bronchioles.
The respiratory zone is the main site of gas exchange
and consists of the respiratory bronchioles, alveolar
ducts, alveolar sacs, and alveoli.
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Functions of Respiratory System
The respiratory system functions to supply the body with
oxygen and dispose off carbon dioxide
Four processes accomplish this:
Pulmonary ventilation – moving air into and out of
the lungs
External respiration – gas exchange between the lungs
and the blood
Internal respiration – gas exchange between blood and
tissues
Transport of oxygen and carbon dioxide between the
lungs and tissues- by blood
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Pulmonary ventilation
Pulmonary ventilation is the movement of air between
the atmosphere and the alveoli
Inspiration – air flows into the lungs
Expiration – air flows out of the lungs
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Pressure Relationships in the Thoracic
Cavity
Respiratory pressures are described relative to
atmospheric pressure
Atmospheric pressure
Pressure exerted by the air surrounding the body
At sea level the atmospheric pressure is 760mmHg=
1atm
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Pressure Relationships in the Thoracic Cavity
Intrapulmonary pressure– pressure within the alveoli
Intrapulmonary rises & falls with the phases of
breathing, but always equalizes itself with atmospheric
pressure- 760mmHg
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Pressure Relationships in the Thoracic Cavity
Intrapleural pressure– pressure within the pleural cavity
Intrapleural pressure is less than intrapulmonary
pressure= 756mmHg
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Pulmonary Ventilation
A mechanical process that depends
on volume changes in the thoracic
cavity
Volume changes lead to pressure
changes, which lead to the flow of
gases to equalize pressure
Boyle’s law – the pressure of a gas
varies inversely with its volume
The larger the volume the lesser the
pressure- V ∝ 1/P
Volume = 1 liter
Pressure = 1 atm
Volume = 1/2 liter
Pressure = 2 atm
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Pulmonary Ventilation
Muscles of inspiration ( inhalation):
Diaphragm ( primary muscle of inspiration)
External intercostals
Normal expiration is a passive process
Muscles of forced expiration (exhalation):
Internal intercostals
Abdominal muscles
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The recruitment
of accessory
muscles
depends on
whether the
respiratory
movements are
quiet (normal),
or forced
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Inspiration
Inspiratory muscles contract: diaphragm
descends, rib cage rises
Thoracic cavity volume increases
Lungs stretched- intrapulmonary
volume increases
Intrapulmonary pressure drops by
2mmHg
Air flows into lungs down the pressure
gradient, till intrapulmonary pressure
equalizes atmospheric pressure
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Expiration
Inspiratory muscles relax; diaphragm
rises, rib cage descends
Thoracic cavity volume decreases
Elastic lungs recoil passively
Intrapulmonary volume decreases
Intrapulmonary pressure rises by
2mmHg
Air flows out of the lungs, down the
pressure gradient, till intrapulmonary
pressure equalizes atmospheric
pressure
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Factors affecting Pulmonary Ventilation
3 factors affect the ease with which we ventilate:
Surface tension of alveolar fluid
Lung compliance
Airway resistance
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Factors affecting Pulmonary Ventilation
1. The surface tension of alveolar fluid causes the alveoli to
assume the smallest possible diameter
The alveoli would collapse each expiration
o
Surfactant reduces tension- prevents the collapse of
alveoli
o
Clinical connection: Infant respiratory
distress syndrome ( IRDS)
o
.
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Factors affecting Pulmonary Ventilation
2. Lung compliance means the ease with which lungs and chest
wall expand.
Related to two main factors
Elasticity of the lung tissue
Surface tension of the alveoli
Lungs of healthy people have a high compliance
Compliance is decreased in:
Lung fibrosis, IRDS, intercostal muscle paralysis,
emphysema
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Factors affecting Pulmonary Ventilation
3. Airway resistance
Gas flow is inversely proportional to resistance (friction)mainly determined by diameter of airways
The smaller the diameter the more the resistance
Sympathetic stimulation dilates bronchi & decreases
resistance
Airway resistance increases in:
Asthma attacks, chronic bronchitis-when bronchioles are
constricted -decreases ventilation
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Measuring VentilationVentilation can be measured using spirometry.
Lung volumes and Capacities can be measured
Old and new spirometers used to measure
ventilation.
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Lung Volumes
Tidal Volume (VT) is the volume of air inspired (or
expired) during normal quiet breathing (500 ml).
Inspiratory Reserve Volume (IRV) is the volume
inspired during a very forced inhalation (3100 ml –
height and gender dependent).
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Lung Volumes
Expiratory Reserve Volume (ERV) is the volume expired
during a forced exhalation (1200 ml).
Residual Volume (RV) is the air still present in the lungs
after a force exhalation (1200 ml).
o
The RV is a reserve for mixing of gases but is not
available to move in or out of the lungs.
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Lung Capacities
Inspiratory capacity: Is the total volume of air that can be
inspired after a tidal expiration
IC=TV+IRV
Functional residual capacity: Is the volume of air that remains
in the lungs at the end of normal tidal expiration
FRC= RV+ ERV
Vital Capacity (VC) : the total amount of exchangeable air
Is all the air that can be exhaled after maximum inspiration.
It is the sum of the inspiratory reserve + tidal volume +
expiratory reserve (4800 ml)
Total lung capacity- Is the sum of all lung volumes-6000ml
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A graph of spirometer volumes and capacities
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Forced vital capacity (FVC)– the volume of air forcibly &
rapidly expelled after taking a deep breath
Forced expiratory volume (FEV1) – the volume of air
expelled during 1sec (healthy person can expel 80% of
FVC in 1sec) in the FVC test
COPD decreases FEV1, because it increases resistance
to flow of air
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Only about 350 ml of the tidal volume reaches the
respiratory zone – the 150ml remains in the conducting
zone (called the anatomic dead space).
If a single VT breath = 500 ml, only 350 ml will exchange
gases at the alveoli.
o
With a respiratory rate of 12/min, the minute
ventilation rate= 12 x 500 = 6000 ml/min.
o
The alveolar ventilation rate(volume of air/min that
actually reaches the alveoli) = 12 x 350 = 4200ml/min.
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Respiration
Respiration is the exchange of gases.
External respiration (pulmonary) is gas
exchange between the alveoli and the blood.
Internal respiration (tissue) is gas exchange
between the systemic capillaries and the tissues
of the body.
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Exchange of O2 and CO2
The respiratory system depends on the medium of the
earth’s atmosphere to extract the oxygen necessary for
life.
The atmosphere is composed of these gases:
Nitrogen (N2)
79%
Oxygen (O2)
21%
Carbon Dioxide (CO2)
0.04%
Water Vapor
variable, but on average
around 1%
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Exchange of O2 and CO2
Using gas laws we can understand the principals of
respiration
Dalton’s Law states that each gas in a mixture of
gases exerts its own pressure- its partial pressure Pp.
Total pressure is the sum of all the partial
pressures.
The partial pressure of each gas is directly
proportional to its percentage in the mixture
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Exchange of O2 and CO2
The partial pressures determine the direction of
movement of gases
Each gas diffuses across a permeable membrane
from high to low partial pressure
There is a higher PO2 in the alveoli than in
the pulmonary capillaries O2 moves from the
alveoli into the blood.
Since there is a higher PCO2 in the
pulmonary capillaries CO2 moves into the
alveoli
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Exchange of O2 and CO2
Henry’s law deals with gases and solutions:
The quantity of a gas that will dissolve in a
liquid is proportional to the partial pressures of
the gas and its solubility.
Increasing the partial pressure of a gas in
contact with a solution will result in more
gas dissolving into the solution
How much it dissolves also depends on
solubility
CO2 is 24 times more soluble in blood (and
soda !) than O2
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Clinical connections
Hyperbaric oxygen- high pressures of O2 are used to treat
anaerobic bacterial infections such as tetanus, gangrene
Decompression sickness (“the bends”)
Air is mostly N2, but very little dissolves in blood due to its
low solubility
Insoluble N2 is forced to dissolve into the blood and tissues
because of breathing compressed air in scuba diving
o
By ascending too rapidly, the N2 bubbles out of the
tissues and blood
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Alveolar air is different in composition from
Atmospheric air
The atmosphere is mostly oxygen and nitrogen, while
alveoli contain in comparison more carbon dioxide and
less oxygen
These differences result from:
Gas exchanges in the lungs
Mixing of alveolar air that remains, with newly inspired air
Atmospheric air:
Alveolar air:
PO2 = 159 mmHg
PO2 = 105 mmHg
PCO2 = 0.3 mmHg
PCO2 = 40 mmHg
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External Respiration (Pulmonary gas exchange)
O2 diffuses down its steep PO2 gradient in the alveoli
(105mmHg) to pulmonary capillary blood (40mmHg)
CO2 diffuses down its gentler PCO2 gradient from pulmonary
capillary blood ( 45mmHg) to alveoli (40mmHg)- exhaled
Blood in the pulmonary veins entering the left atrium has:
PCO2 40mmHg
PO2 100mmHg (due to mixing of blood from bronchial
veins)
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Internal Respiration
As in gas exchange between blood & alveoli, the gas
exchange between blood & tissue cells occurs by
simple diffusion, driven by partial pressure gradients
Tissue cells constantly use O2 & produce CO2
PO2 in tissue is 40mmHg- O2 moves into tissues from
blood capillaries
PCO2 is 45 mm Hg in tissues- CO2 moves into blood
PO2 of venous blood draining tissues is 40 mm Hg and PCO2
is 45 mm Hg
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CO2 exhaled
O2 inhaled
Atmospheric air:
PO2 = 159 mmHg
PCO2 = 0.3 mmHg
Alveoli
Alveolar air:
PO2 = 105 mmHg
PCO2 = 40 mmHg
CO2 O
2
Pulmonary capillaries
To lungs
(a) External respiration:
pulmonary gas
exchange
Deoxygenated blood:
PO2 = 40 mmHg
PCO2 = 45 mmHg
To right atrium
(b) Internal respiration:
systemic gas
exchange
Systemic capillaries
CO2
O2
Systemic tissue cells:
PO2 = 40 mmHg
PCO2 = 45 mmHg
To left
atrium
To tissue cells
Oxygenated blood:
PO2 = 100 mmHg
PCO2 = 40 mmHg
Factors affecting gas exchange
Factors influencing the movement of oxygen and carbon
dioxide across the respiratory membrane
Partial pressure gradients and gas solubilities
Surface area for gas exchange & thickness of the
respiratory membrane
Matching of alveolar ventilation (airflow) to alveoli
and pulmonary perfusion (blood flow)
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Partial pressure gradients and gas solubility
The more the partial pressure differences, the more is the
rate of gas diffusion
During exercise greater differences in PCO2 and PO2
between alveolar air and pulmonary blood- greater
rate of gas diffusion
Decreased alveolar PO2 at high altitudes – decreases
oxygen diffusion
Solubility:
CO2 diffuses out faster compared to O2 diffusing in
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Surface area & respiratory membrane
Respiratory membranes are only 0.5 to 1 m thickallows efficient gas exchange
Thicken in pulmonary edema- gas exchange is
inadequate
The greater is the surface area, the more gases can be
exchanged- normally huge
Decrease in surface area:
o
emphysema, when walls of adjacent alveoli break
o
mucus, tumors block gas flow into alveoli
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Ventilation-Perfusion Matching
Ventilation and perfusion must be matched for efficient
gas exchange
In the lungs, pulmonary vasoconstriction occurring in
response to hypoxia diverts pulmonary blood from
poorly ventilated areas of the lungs to well-ventilated
regions
pulmonary vasodilation in response to increased
ventilation
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Transport of O2
In the blood, some O2 is dissolved in the plasma as a gas
(only about 1.5%)
Most O2 (about 98.5%) is carried attached to Hb.
Oxygenated Hb is called oxyhemoglobin (Hb-O2)
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Transport of O2
The amount of Hb saturated with O2 is called percent
saturation of hemoglobin
Each Hb molecule can carry 1 to 4 molecules of O2. Blood
leaving the lungs has Hb that is almost fully saturatedthe percent saturation is close to 98%
Partially saturated hemoglobin –
when 1-3 heme groups are bound to oxygen
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Factors affecting saturation of Hb
Most important factor is PO2
The relationship between the amount of PO2 in plasma and
the saturation of Hb is called the oxygen-hemoglobin
dissociation curve.
The higher the PO2 dissolved in
the plasma, the higher the Hb.
saturation
•
With PO2 100mmHg in
arterial blood saturation is 98%
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PO2 and percent saturation contd.
In the venous blood at PO2 40mmHg
-percent saturation is 75%
- only 25% has O2
been unloaded to tissues
With PO2 between 60-100mmHg, Hb is
90% or more saturated with oxygen
So even with PO2 as low as 65mmHg
Hb saturation is not so low(important for those with lung diseases
or living at high altitudes
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PO2 and percent saturation
contd.
Between 40 and 20mmHg a
small decrease in PO2
causes a large drop in Hb
saturation -with release of
oxygen
In actively contracting muscles
PO2 may drop to 20mmHg –
saturation 35%- with oxygen
release to muscles
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Transport of O2
Measuring hemoglobin
saturation is common in
clinical practice- done
by Pulse oximeters
3660 Group,
Inc/NewsCom
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Factors influencing the affinity of Hb binding with
O2 -Affect percent saturation of Hb
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Bohr Effect
Metabolically active tissues produce H+
H+ bind to Hb- change its shape- decreasing affinity of Hb
for oxygen- enhancing unloading of O2 to tissues
The pH decrease shifts the O2–Hb saturation curve “to the
right”
This is called the Bohr effect
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Transport of CO2
CO2 is transported in the blood in three different forms:
1. 7% is dissolved in the plasma, as a gas.
2. 70% is transported as bicarbonate ions (HCO3–)
through the action of an enzyme called carbonic
anhydrase.
o
CO2 + H2O
H2CO3
H+ + HCO3-
3. 23% is attached to Hb (to the amino acids) as
carbaminohemoglobin( HbCO2)
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Transport of CO2
At the level of tissues: Carbon dioxide
diffuses into RBCs, combines with
water to form H2CO3, (catalyzed by
carbonic anhydrase), which quickly
dissociates into hydrogen ions and
bicarbonate ions
Cl–)
Bicarbonate diffuses from RBCs into
the plasma
The chloride shift – to balance the
outrush of negative bicarbonate ions
from the RBCs, chloride ions (Cl–)
move into the erythrocytes
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Transport of CO2
At the lungs, these processes are reversed
Cl–)
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Control of Respiration- Respiratory Center
The medullary rhythmicity area, has centers that control
basic respiratory rythm
The inspiratory center
stimulates the diaphragm
via the phrenic nerve, and
the external intercostal
muscles via intercostal nerves.
Inspiration normally lasts about 2s.
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Control of Respiration-Respiratory Center
Expiration is a passive process- nerve impulses cease for about
3 sec, causing relaxation of inspiratory myscles
The expiratory center is inactive during quiet breathing
During forced exhalation,
however, impulses from this
center stimulate the internal
intercostal and abdominal
muscles
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Control of Respiration
Other sites in the pons help the medullary centers
The pneumotaxic center limits inspiration to prevent
hyperexpansion of lungs
The apneustic center prolongs
inhalation
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Chemoreceptor Regulation of Respiration:
Central chemoreceptors in medulla only sensitive to PCO2
Peripheral chemoreceptors sensitive to PCO2, PO2, arterial pH
PCO2 levels rise (hypercapnia) stimulate both the central &
peripheral chemoreceptors
Respiratory center stimulated
Hyperventilation – increased rate and depth of breathing
occurs in response to hypercapnia- CO2 flushed out
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Chemoreceptors
Medulla oblongata
Central chemoreceptors
Internal carotid
artery
glossopharyngeal nerve
(cranial nerve IX)
Carotid body
Carotid sinus
vagus nerve
(cranial nerve X)
Peripheral Chemoreceptors
Arch of aorta
Aortic bodies
Heart
Chemoreceptor Regulation of Respiration
Fall in pH:
Acidosis may occur due to:
Carbon dioxide retention, other metabolic conditions
e.g. accumulation of lactic acid
Increased ventilation in response to falling pH is
mediated by peripheral chemoreceptors
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Chemoreceptor Regulation of Respiration
Arterial PO2 levels are monitored by the aortic and
carotid body peripheral chemoreceptors
Substantial drops in arterial PO2 (to 60 mm Hg) are
needed before oxygen levels become a major stimulus to
increase ventilation (hypoxic drive)
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Chemoreceptor Regulation of Respiration
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Control of Respiration
Other brain areas also play a role in respiration:
The cerebral cortex has influence over breathing.
Stretch receptors in lungs sense overinflationinhibitory signals are sent to the medullary
inspiration center to end inhalation and allow
expiration (Herring Breuer reflex)
Emotions (limbic system) affect respiration.
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Diseases
Asthma is a disease of hyper-reactive airways (the major
abnormality is constriction of smooth muscle in the
bronchioles
It presents as attacks of wheezing, coughing, and excess
mucus production.
It typically occurs in response to allergens
Bronchodilators and antiinflammatory corticosteroids
are mainstays of treatment.
Pulse Picture Library/CMP mages /Phototake
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Diseases
Chronic Obstructive Pulmonary Diseases
They are diseases caused by cigarette smoking
Chronic bronchitis is caused by chronic irritation and
inflammation
Patients have cough with sputum
Emphysema : destruction of elastic tissue
with enlargement of air spaces
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