Transcript alveolar wall

Major Functions of the Respiratory System
• To supply the body with oxygen and dispose of carbon
dioxide
• Respiration – four distinct processes must happen
• Pulmonary ventilation (breathing):
movement of air into and out
of the lungs
• External respiration: O2 and CO2
exchange between the lungs
and the blood
• Transport: O2 and CO2
in the blood
• Internal respiration: O2 and CO2
exchange between systemic blood
vessels and tissues
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Respiratory
system
Circulatory
system
Respiratory System – conducting and respiratory zone
• Consists of the respiratory and conducting zones
• Respiratory zone:
• Site of gas exchange
• Consists of respiratory bronchioles, alveolar ducts, and
alveoli
• Conducting zone:
• Conduits for air to reach the sites of gas exchange
• Includes all other respiratory structures (e.g., nose,
nasal cavity, pharynx, trachea)
• Respiratory muscles – diaphragm and other muscles that
promote ventilation
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Organization of the respiratory system
Figure 23.1
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Respiratory Zone
• Defined by the presence of alveoli; begins as terminal
bronchioles feed into respiratory bronchioles
• Respiratory bronchioles lead to alveolar ducts, then to
terminal clusters of alveolar sacs composed of alveoli
• Approximately 300 million alveoli:
• Account for most of the lungs’ volume
• Provide tremendous surface area for gas exchange
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Respiratory Membrane
• This air-blood barrier is composed of:
• Alveolar and capillary walls
• Their fused basal laminas
• Simple squamous ET (type I) – most of the cells in
the alveolus wall and are part of the respiratory
membrane (allow gas exchange)
• Septal cells (type II ) – about 5% of the alveolar wall.
The septal cells secret surfactant – a lipoprotein
secretion that reduces the surface tension in the
alveolus
• Alveolar Macrophage (dust cells) - patrol epithelium
and engulf foreign particles
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Red blood
cell
Nucleus of type I
(squamous
epithelial) cell
Alveolar pores
Capillary
O2
Capillary
CO2
Alveolus
Alveolus
Type I cell
of alveolar wall
Macrophage
Endothelial cell nucleus
Alveolar
epithelium
Fused basement
membranes of the
Respiratory alveolar epithelium
membrane and the capillary
Red blood cell
endothelium
Alveoli (gas-filled in capillary
Type II (surfactantCapillary
air spaces)
secreting) cell
endothelium
(c) Detailed anatomy of the respiratory membrane
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Figure 22.9c
Breathing or 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
• Consists of two phases
• Inspiration – air flows into the lungs
• Expiration – gases exit the lungs
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Pressure Relationships in the Thoracic Cavity
• Respiratory pressure is always
atmospheric pressure
described
relative to
• Atmospheric pressure (Patm)
• Pressure exerted by the air surrounding the body
• Negative respiratory pressure is less than Patm
• Positive respiratory pressure is greater than Patm
• Intrapleural pressure (Pip) – pressure within the pleural cavity
• always less than intrapulmonary pressure and atmospheric
pressure
• Intrapulmonary pressure (Ppul) – pressure within the alveoli
• eventually equalizes itself with atmospheric pressure
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Intrapleural Pressure and Pressure Relationships
• Negative Pip is caused by opposing forces
• Two inward forces promote lung collapse
• Elastic recoil of lungs decreases lung size
• Surface tension of alveolar fluid reduces alveolar size
• One outward force tends to enlarge the lungs
• Elasticity of the chest wall pulls the thorax outward
• If Pip = Ppul the lungs collapse
• (Ppul – Pip) = transpulmonary pressure
• Keeps the airways open
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Intrapleural pressure
• As a result of the relationship
between the lungs and the pleurae
(lungs pull the visceral pleura in),
the intrapleural pressure is below
atmospheric pressure – average of 4 mmHg
• This pressure (or the fluid bond
between the pleurae) prevents the
collapsing of the lungs due to there
elasticity
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Pulmonary ventilation – inspiration and expiration
• Pulmonary ventilation depends on volume changes in
the thoracic cavity
• Volume changes lead to pressure changes
• Pressure changes lead to flow of gases
• Boyle’s Law - The volume of a fixed amount of gas
is inversely proportional to the total amount of
pressure applied.
• If the pressure doubles, the volume shrinks to half.
• In the lungs – if lungs volume increase the pressure
decreases (intrapulmonary pressure)
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Modes of breathing
• Quiet breathing – eupnea
• Inhalation is active and exhalation is passive
(relaxation of muscles)
• Forced breathing – hyperpnea
• Both inhalation and exhalation involve muscle
contraction – both active
• Inhalation involve muscles like the pec. minor,
sternocleidomastoid and more
• Exhalation involves the internal intercostals and
abdominal muscles among others
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Physical Factors Influencing Ventilation
• 3 factors influence pulmonary ventilation
• Airway Resistance
• Alveolar Surface Tension
• Lung Compliance
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Airway Resistance
• As airway resistance rises, breathing movements become more
difficult
• Resistance is usually insignificant because of
• Large airway diameters in the first part of the conducting
zone
• Progressive branching of airways as they get smaller,
increasing the total cross-sectional area
• Severely constricted or obstructed bronchioles:
• Can prevent life-sustaining ventilation
• Can occur during acute asthma attacks which stops
ventilation
• Epinephrine release via the sympathetic nervous system dilates
bronchioles and reduces air resistance
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Alveolar Surface Tension
• Surface tension – the attraction of liquid molecules to one
another at a liquid-gas interface
• The liquid coating the alveolar surface is always acting to
reduce the alveoli to the smallest possible size
• Surfactant, a detergent-like complex, reduces surface
tension and helps keep the alveoli from collapsing
• Normally, surfactant synthesis starts at about the 25th
week of fetal development and production reaches
optimal levels at 34th week
• Premature babies with insufficient surfactant can be
treat with aerosol administration with artificial
surfactant until lungs mature
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Lung Compliance
• Compliance is the indication of the lungs expandability
• The ease with which lungs can be expanded
• Factors that diminish lung compliance
• Scar tissue or fibrosis that reduces the natural elasticity
of the lungs
• Blockage of the smaller respiratory passages with
mucus or fluid
• Reduced production of surfactant
• The mobility of the thoracic cage – changes cause to
the articulations of the ribs or to the muscles involved.
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Respiratory Volumes
• Used to assess a person’s respiratory status
• Tidal volume (TV)
• Inspiratory reserve volume (IRV)
• Expiratory reserve volume (ERV)
• Residual volume (RV)
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Respiratory Capacities
• Inspiratory capacity (IC) equals TV plus IRV
• Maximum amount of air (about 3.5 liters) a person can
breath in
• Functional residual capacity (FRC) equals the ERV plus RV
• Amount of air remains in the lungs at the end of normal
expiration
• Vital capacity (VC) equals IRV+ERV+TV
• Maximum amount of air a person can expel from the lungs
after filling with inspiratory capacity
• Total lung capacity (TLC) equals VC+RV
• Maximum volume to which the lungs can be expanded
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Dead Space
• Some of the inspired air does not contribute to the gas
exchange in the alveoli
• Anatomical dead space – volume of the conducting
respiratory passages (150 ml)
• Alveolar dead space – alveoli that cease to act in
gas exchange due to collapse or obstruction
• Total dead space – sum of alveolar and anatomical
dead spaces
• On expiration, the air in the anatomical dead space is
expired first
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Pulmonary Function Tests
• Spirometer – an instrument used to evaluate respiratory
function
• Spirometry can distinguish between:
• Obstructive pulmonary disease – increased airway resistance
by narrowing or blocking airways (ex. Asthma)
• Restrictive disorders – reduction of pulmonary compliance
thus limiting inflation of lungs.
• Caused by any disease that produces pulmonary fibrosis
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Nonrespiratory Air Movements
• Most result from reflex action
• Examples include: coughing, sneezing, crying, laughing,
hiccupping, and yawning
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Respiratory physiology is a series of integrated processes
• External respiration
• Exchange of gases between blood and the external
environment
• Internal respiration
• Exchange of gases between blood and interstitial
fluid
• Transport of oxygen and carbon dioxide
• To understand the above processes, first consider
• Physical properties of gases
• Composition of alveolar gas
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Basic properties of gases
• Dalton’s Law of Partial Pressures
• Total pressure exerted by a mixture of gases is the
sum of the pressures exerted independently by each
gas in the mixture (as if no other gases were present)
• The separate contribution of each gas in a mixture is
called partial pressure (symbolized with P)
• The partial pressure of each gas is directly
proportional to its percentage in the mixture
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Table 22.4
Composition of air in alveoli
• The composition of air we breath is not the composition in
the alveoli:
• The air is humidified by the contact with the mucus
membrane – so PH2O is >10 times higher than the
inhaled air
• Freshly inspired air is mixed with residual air left from
previous breathing cycle
• That causes the oxygen to be diluted and CO2 to be
higher
• Alveolar gas exchanges O2 and CO2 with blood
• As a result, PO2 of alveolar air is about 65% of that of the
inhaled air and PCO2 is >130 higher
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Alveolar gas exchange Diffusion between liquid and gases
(Henry’s law)
• When a mixture of gases is in contact with a liquid,
each gas will dissolve in the liquid in proportion to its
partial pressure
• The greater the concentration of a particular gas, the
more and the faster that it will go into a solution
• The amount of gas that will dissolve in a liquid also
depends upon its solubility:
• Carbon dioxide is the most soluble
• Oxygen is 1/20th as soluble as carbon dioxide
• Nitrogen is practically insoluble in plasma
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External Respiration: Pulmonary Gas Exchange
• Factors influencing the movement of oxygen and carbon
dioxide across the respiratory membrane (what is the
respiratory membrane?)
• Partial pressure gradients and gas solubility
• Matching of alveolar ventilation and pulmonary
blood perfusion
• Structural characteristics
membrane
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of
the
respiratory
Partial Pressure Gradients and Gas Solubility
• The partial pressure oxygen (PO2) of venous blood is 40
mm Hg; the partial pressure in the alveoli is 104 mm Hg
• This steep gradient allows oxygen partial pressures to
rapidly reach equilibrium (in 0.25 seconds)
• this is one third of the time a RBC is in the pulmonary
capillary (0.75 seconds)
• Although carbon dioxide has a lower partial pressure
gradient (45 mm Hg in the blood and 40 mm Hg in the
alveoli; a gradient of 5 mm Hg):
• It diffuses in equal amounts with oxygen because it is
20 times more soluble in plasma than oxygen
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Surface Area and Thickness of the Respiratory Membrane
• The amount of gas that moves across a tissue is
• proportional to the area of the sheet
• inversely proportional to its thickness
• Respiratory membranes:
• Are only 0.5 to 1 m thick, allowing for efficient
gas exchange
• Have a total surface area of about 60 m2 (40 times
that of one’s skin)
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Internal Respiration
• The factors promoting gas exchange between systemic
capillaries and tissue cells are the same as those acting in
the lungs
• The partial pressures and diffusion gradients are
reversed
• PO2 in tissue is always lower than in systemic
arterial blood
• PO2 of venous blood draining tissues is 40 mm Hg
and PCO2 is 45 mm Hg
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Oxygen Transport: Role of Hemoglobin
• Molecular oxygen is carried in the blood:
• Bound to hemoglobin (Hb) within red blood cells
• Dissolved in plasma (O2 has low solubility in water and only
1.5% is dissolved in plasma)
• Each Hb molecule binds four oxygen atoms in a rapid and
reversible process
• The hemoglobin-oxygen combination is called oxyhemoglobin
(HbO2)
• Hemoglobin that has released oxygen is called reduced
hemoglobin/deoxyhemoglobin (HHb)
• Saturated hemoglobin – when all four hemes of the molecule are
bound to oxygen
• Partially saturated hemoglobin – when one to three hemes are
bound to oxygen
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The Oxygen-Hemoglobin Saturation Curve

PO2 of 40 mm Hg –
(average in the tissues)
Hb is 75% saturated

only 25% of the O2 is
unload from Hb in
resting conditions
of 60-70 mm Hg –
Hb is 90% saturated
PO2
PO2 of 20 mm Hg – Hb
is only 30% saturated
 Ex. – active muscle;
relatively high
percentage of O2
released with small
decrease in Po2

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Factors That Increase Release of Oxygen by Hemoglobin
• As cells metabolize glucose, carbon dioxide is released
into the blood causing:
• Increases in PCO2 and H+ concentration in capillary
blood
• Declining pH (acidosis), which weakens the
hemoglobin-oxygen bond (Bohr effect)
• Metabolizing cells have heat as a byproduct and the rise in
temperature increases BPG synthesis
• All these factors ensure oxygen unloading in the vicinity of
working tissue cells
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Carbon Dioxide Transport
• Carbon dioxide is transported in the blood in three forms
• Dissolved in plasma – 7 to 10%
• Chemically bound to hemoglobin – 20% is carried
in RBCs as carbaminohemoglobin
• Bicarbonate ion in plasma – 70% is transported as
bicarbonate (HCO3–)
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Carbon Dioxide Transport
• In areas with high PCO2, carbon dioxide leaves the cell, diffuses
through the interstitial fluid and enters a capillary.
• Most of it enters an erythrocyte that contains an enzyme,
carbonic anhydrase which catalyses the following reaction:
• CO2 + H20 -----> H2CO3 -----> H+ + HCO3-- .
• The bicarbonate ion leaves the red blood cell (against
concentration gradient) and travels to the lungs in the plasma of
the blood.
• In exchange, Cl- moves from plasma into RBCs to maintain the
electrical balance between plasma and RBC (chloride shift)
• It often combines with Na+ present in the plasma to form
sodium bicarbonate which plays a role in maintaining the
homeostasis of blood pH.
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Tissue cell
Interstitial fluid
CO2
CO2
CO2 (dissolved in plasma)
CO2 + H2O
Slow
H2CO3
HCO3– + H+
HCO3–
Cl–
CO2
Fast
CO2
CO2 + H2O
H2CO3
Carbonic
anhydrase
CO2
CO2 + Hb
HbCO2 (Carbaminohemoglobin)
Red blood cell
HbO2
O2 + Hb
CO2
CO2
HCO3– + H+
Cl–
HHb
Binds to
plasma
proteins
Chloride
shift
(in) via
transport
protein
O2
O2
O2 (dissolved in plasma)
Blood plasma
(a) Oxygen release and carbon dioxide pickup at the tissues
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Figure 22.22a
Alveolus
Fused basement membranes
CO2
CO2 (dissolved in plasma)
CO2
CO2 + H2O
Slow
H2CO3
HCO3– + H+
HCO3–
Fast
CO2
H2CO3
CO2 + H2O
Carbonic
anhydrase
CO2
CO2 + Hb
Red blood cell
–
HCO3 +
H+
HbCO2 (Carbaminohemoglobin)
O2 + HHb
HbO2 + H+
Cl–
Cl–
Chloride
shift
(out) via
transport
protein
O2
O2
O2 (dissolved in plasma)
Blood plasma
(b) Oxygen pickup and carbon dioxide release in the lungs
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Figure 22.22b
Haldane Effect
• The amount of CO2 that can be transported in the blood is
influenced by Hb saturation with O2.
• The lower the amount of Hb-O2 the higher the CO2 carrying
capacity (Haldane effect):
• Deoxyhemoglobin has higher affinity to CO2
• Deoxyhemoglobin buffers more H+ thus promoting
conversion of CO2 to HCO3• At the tissues, as more carbon dioxide enters the blood:
• More oxygen dissociates from hemoglobin (acidosis Bohr effect)
• More carbon dioxide combines with hemoglobin, and
more bicarbonate ions are formed
• This situation is reversed in pulmonary circulation
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