Download Respiratory System

Functions of the Respiratory System
1. Provides extensive gas exchange surface area
between air and circulating blood
2. Moves air to and from exchange surfaces of lungs
3. Protects respiratory surfaces from outside
environment
4. Produces sounds
5. Participates in olfactory sense
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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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Respiratory System – conducting and respiratory zone
• Conducting passageways carrying air to and from the alveoli
• Upper respiratory passages filter and humidify incoming air
• Lower passageways include delicate conduction passages and
alveolar exchange surfaces
• The conducting passageways of the respiratory system (nasal cavity,
trachea, bronchi and bronchioles) are lined by ciliated
pseudostratified columnar ET, which includes mucus-secreting
goblet cells.
• Because the passage of air depends on wide open passageways, the
larger respiratory passages (trachea and bronchi) are supported by
rings of cartilage.
• The respiratory regions are lined with simple squamous ET
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Organization of the respiratory system
Figure 23.1
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The primary bronchi
• Trachea branches in the mediastinum into right and left
primary bronchi
• Bronchi enter the lungs at the hilus
• Have C-shaped cartilaginous supporting rings
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The bronchial tree
• System of tubes formed from the primary bronchi and
their branches
• Primary bronchi branch into secondary or lobar bronchi
• Secondary bronchus goes to each lobe of the lungs
• Secondary bronchi branch into tertiary (segmental)
bronchi
• Both secondary and tertiary bronchi are covered by
overlapping plates of cartilage and not rings
• Cartilage in walls decrease and smooth muscle
increase with branching
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The bronchioles
• Branching of the bronchi that are 1 mm or less in
diameter and lack cartilage. Bronchioles are surrounded
by smooth muscle that allows the change of diameter.
• Ultimately bronchioles branch into the final branch of the
conducting division - terminal bronchioles with a
diameter of 0.3-0.5 mm
• Terminal bronchiole becomes respiratory bronchioles –
the beginning of the respiratory division
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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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Pleurae
• Thin, double-layered serosa
• Parietal pleura
• Covers the thoracic wall and superior face of the
diaphragm
• Continues around heart and between lungs
• Visceral, or pulmonary, pleura
• Covers the external lung surface
• Divides the thoracic cavity into three chambers
• The central mediastinum
• Two lateral compartments, each containing a lung
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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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Gas exchange and transport
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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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Haldane Effect
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Figure 22.23