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Respiratory Physiology
Pulmonary ventilation
(breathing)
Gas exchange between
lungs and blood
Transport of
gases in blood
Gas exchange between
blood and tissues
Cellular Respiration
Anatomy of the Respiratory System
Conducting airways
(Nasal passages, pharynx,
trachea, bronchii,
bronchioles)
Inspired air is warmed and
humidified in these tubes.
Moistening of air is essential
to prevent drying out of
alveolar linings.
Photomicrograph of Tracheal Epithelium
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Defence mechanisms
Respiratory system is largest area of the body in
direct contact with the environment.
Large particles filtered out in hairs in nasal passages
Respiratory airways lined with mucus to trap foreign
objects
Cilia move mucus upwards towards throat to be
swallowed
Coughs and sneezes
Alveolar macrophages scavenge within the alveoli
Function of the alveoli
Exchange of gases between air and blood by diffusion
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Alveoli
Site of gas exchange
300 million alveoli/lung (tennis court size)
Rich blood supply- capillaries form sheet
over alveoli
Alveolar pores
Type I alveolar cells – make up wall of alveoli
Single layer epithelial cells
Type II alveolar cells – secrete surfactant
Alveolar macrophages
Resin cast of pulmonary
blood vessels
Scanning electron
micrograph of capillaries
around alveoli
Pulmonary Circulation is low-pressure, low-resistance
Ventilation-perfusion matching: blood flow through the pulmonary
circulation is matched to ventilation
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Structures of the Thoracic Cavity
Chest wall – air tight, protects lungs
Skeleton: rib cage;sternum; thoracic vertebrae
Muscles: internal/external intercostals; diaphragm
Lungs are surrounded by pleural sac
Role of Pressure in pulmonary ventilation
Air moves in and out of lungs by bulk flow
Pressure gradient drives flow (air moves from high to low
pressure)
Atmospheric pressure = Patm (760mmHg at sea level)
Intra-alveolar pressure = Palv
Pressure of air in alveoli
During inspiration = negative (less than
atmospheric)
During expiration = positive (more than
atmospheric)
Difference between Palv and Patm drives ventilation
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Atmospheric Pressure
760 mm Hg at sea level
Decreases as altitude increases
Increases under water
Other lung pressures given relative to atmospheric (set Patm = 0 mm Hg)
Intrapleural Pressure
Pressure inside pleural sac
Always negative under normal conditions
Always less than Palv
Varies with phase of respiration
At rest, -4 mm Hg
Negative pressure due to elasticity in lungs and chest wall
Lungs recoil inward
Chest wall recoils outward
Opposing pulls on intrapleural space
Surface tension of intrapleural fluid holds wall and lungs
together
Pneumothorax
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Mechanics of Breathing
Movement of air in and out of lungs due to pressure gradients
Mechanics of breathing describes mechanisms for creating pressure
gradients
Boyle’s Law (pressure and volume are inversely related)
The lungs follow the movement of the rib cage
Forces for Air Flow
Flow =
Patm – Palv
R
Force for flow = pressure gradient
Atmospheric pressure constant (during breathing cycle)
Therefore, changes in alveolar pressure creates/changes gradients
Muscles of Respiration
Inspiratory muscles increase volume of thoracic cavity
Diaphragm & external intercostals
Expiratory muscles decrease volume of thoracic cavity
Internal intercostals & abdominal muscles
Expiration is generally passive (no muscles required): elastic recoil
Active expiration requires expiratory muscles
Contraction of expiratory muscles creates greater and faster
decrease in volume of thoracic cavity
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Inspiration and Expiration
Figure 17.11b
Factors affecting ventilation
Compliance
Airway resistance
Lung Compliance:
Ease with which lungs can be stretched
Larger lung compliance means easier lung inflation
Factors Affecting Lung Compliance:
Elasticity: if lungs are less elastic, they are less compliant
eg in restrictive lung diseases such as fibrosis (asbestosis, radiation fibrosis)
Surface tension of lungs: the greater the tension, the less compliance
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Surface Tension in Lungs
Thin layer of fluid lines alveoli
Surface tension due to attractions between water molecules
Force for alveoli to collapse or resist expansion
To Overcome Surface Tension
Surfactant secreted from type II cells
Surfactant: detergent that decreases surface tension
Surfactant increases lung compliance
Makes inspiration easier
Airway Resistance
Like blood vessels, the resistance of the airways affects air flow
Airway radius affects airway resistance
Disease states:
Asthma – caused by spasmic contractions of smooth muscle of bronchioles.
Histamine is a bronchoconstrictor
Chronic obstructive pulmonary disease (COPD)
COPD (special scholarship topic)
A common, progressive, lung disease.
2 main forms:
Chronic bronchitis: long-term mucus-producing cough
Emphysema: progressive destruction of the lung tissue
Most people with COPD have a combination of both conditions.
Major cause is smoking
Diagnosed by spirometry. Symptoms include cough, breathlessness (dyspnea)
Treatment: bronchodilators, steroids, anti-inflammatory drugs. No cure.
Significant inflammatory component
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Extrinsic control of airway resistance
Autonomic nervous system
Sympathetic
Relaxation of smooth muscle
Bronchodilation
Parasympathetic
Contraction of smooth muscle
Bronchoconstriction
Hormonal Control
Adrenaline
Relaxation of smooth muscle
Bronchodilation
Spirometry
A pulmonary function test
Method of measuring lung volumes
Can be used diagnostically
Dependent upon patient effort
Used to measure several lung volumes, including tidal volume (VT) - the
volume of a normal breath (approx. 500ml)
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Lung Volumes and Capacities
Pulmonary Function Tests: Forced Vital Capacity (FVC)
Maximum volume inhalation followed by fast exhalation
Pulmonary Function Tests: Forced Expiratory Volume (FEV)
FEV1 = percent of FVC that can be exhaled within 1 second
Normal FEV1 = 80%
FEV1 < 80% can indicate obstructive pulmonary disease
Abnormal Spirograms associated with Obstructive &Restrictive Lung Diseases
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Minute Ventilation
Total volume of air entering and leaving
respiratory system each minute
Minute ventilation = VT x RR
Normal respiration rate = 12 breaths/min
Normal VT = 500 mL
Normal minute ventilation =
500 mL x 12 breaths/min = 6000 mL/min
Anatomical Dead Space
Air in conducting zone does not participate
in gas exchange
Thus, conducting zone = anatomical dead space
Dead space volume (DSV) approximately 150 mL
Alveolar Ventilation
Volume of air reaching gas exchange areas per minute
Alveolar Ventilation = (VT x RR) – (DSV x RR)
Normal Alveolar Ventilation =
(500 mL/br x 12 br/min) – (150 mL/br X 12 br/min) =
4200 mL/min
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Diffusion of gases across respiratory
membrane
Gas composition of air
Composition of Air
79% Nitrogen
21% Oxygen
Trace amounts of carbon dioxide, helium, argon, etc.
Water vapour can be a factor depending on humidity
Diffusion of Gases
Gases diffuse down pressure gradients
High pressure  low pressure
In gas mixtures, gases diffuse down partial pressure gradients
High partial pressure  low partial pressure
A particular gas diffuses down its own partial pressure gradient
Presence of other gases irrelevant
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Partial Pressures of Oxygen and
Carbon Dioxide
Oxygen transport in the blood
Oxygen not very soluble in plasma
Thus only 1.5% arterial blood oxygen is dissolved in plasma
Other 98.5% arterial blood oxygen transported by haemoglobin - a
protein present in red blood cells
Each haemoglobin protein can bind 4 oxygen molecules
Haemoglobin located in
red blood cells
4 globins
2 alpha
2 beta
4 haem groups
Hb + O2  Hb.O2
Hb = deoxyhaemoglobin
Hb.O2 = oxyhaemoglobin
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Haemoglobin-Oxygen dissociation curve
Note: Haemoglobin has
greater affinity for
carbon monoxide (CO)
than for oxygen
Prevents oxygen from
binding to haemoglobin:
CO is poisonous
Carbon Dioxide Transport Mechanisms
Some is transported dissolved in plasma
Some is transported bound to haemoglobin
Most is converted to bicarbonate ions by red blood cells, then
transported into plasma
Carbonic anhydrase converts carbon dioxide
and water to carbonic acid
CA
CO2 + H2O  H2CO3  H+ + HCO3-
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Control of breathing
Respiratory muscles controlled from medulla oblongata
Factors which influence ventilation:
Arterial PCo2 (most important; monitored by central
chemoreceptors)
Arterial PO2 (monitored by peripheral chemoreceptors; only
responsive when level falls below 60mmHg)
Arterial pH (a consequence of Pco2; monitored by peripheral
chemoreceptors)
Increased arterial PCo2 results in increased ventilation as a
direct result of stimulation of the central chemoreceptors,
as CO2 diffuses across the blood-brain barrier.
Summary
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