Respiratory physiology
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Respiratory physiology
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The goals of respiration are to provide oxygen to the tissues
and to remove carbon dioxide.
To achieve these goals, respiration can be divided into four
major functions:
Pulmonary ventilation: gas exchange between the
atmosphere and lungs
External respiration -gas exchange between the lungs and
blood (O2 loading and CO2 unloading).
Transport of respiratory gases –via movement of blood O2
from the lungs is transported to the cell and tissues.
Internal respiration –gas exchange between the capillaries
and the tissues (O2 unloading and CO2 loading).
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Breathing
Breathing, or pulmonary ventilation, consists of two phases
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Inspiration – air flows into the lungs
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Also known as inhalation
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Expiration – gases exit the lungs
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Also called exhalation
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Breathing ….cont’d
Breathing accomplished by two ways:
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By downward and upward movement of the diaphragm
to lengthen or shorten the chest cavity,
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By elevation and depression of the ribs to increase and
decrease the anteroposterior diameter of the chest cavity.
Normal quiet breathing is accomplished almost entirely by the
first method and assisted by the second method
During inspiration, contraction of the diaphragm pulls the
lower surfaces of the lungs downward.
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Breathing ….cont’d
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During expiration, the diaphragm simply relaxes, and it
compresses the lungs and expels the air.
How rib cage taking part in breathing ?
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In resting position, the ribs slant downward
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But when the rib cage is elevated, the ribs project almost
directly forward,
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so that the sternum also moves forward, away from the spine,
increase the anteroposterior thickness of the chest cavity.
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Breathing…. cont’d

Muscles that raise the chest cage are known as inspiratory
muscle

The most important muscles that raise the rib cage are the
external intercostals, but others that help are the
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Sternocleidomastoid muscles, which lift upward on the
sternum;
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Anterior serrati, which lift many of the ribs; and
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Scaleni, which lift the first two ribs.
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Breathing…. cont’d
The muscles that pull the rib cage downward during expiration
are mainly the
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Internal intercostals.
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Abdominal recti, which have the powerful effect of
pulling downward on the lower ribs and other abdominal
muscles also compress the abdominal contents upward
against the diaphragm
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Pressures Relationships
Pressure differences between the alveoli and the environment are
the driving “forces” for the exchange of gases that occurs during
ventilation.
At rest just before an inhalation, the air pressure inside the lungs
is the same as the pressure of the atmosphere
Respiratory pressure is always described relative to atmospheric
pressure (Patm)
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Pressures Relationships …. cont’d
Atmospheric pressure
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pressure exerted by the air surrounding the body
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Negative respiratory pressure is less than Patm
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Positive respiratory pressure is greater than Patm
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Lung Pressures
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Intrapulmonary pressure:
 Intra-alveolar pressure (pressure in the alveoli).
 pressure of the air inside the lung alveoli
Intrapleural pressure:
 Pressure in the intrapleural space.
 pressure of the fluid in the pleural cavity.
 Pressure is negative, due to lack of air in the intrapleural space.
Transpulmonary pressure:
 Pressure difference across the wall of the lung.
 Intrapulmonary pressure – intrapleural pressure.
 Keeps the lungs against the chest wall.
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Mechanics of breathing
Inspiration
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Inspiration is initiated by the neurally induced contraction of
the diaphragm and the “inspiratory” intercostal muscles
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Contraction of external intercostal muscles → elevation of ribs
& sternum → ↑ antero-posterior dimension of thoracic cavity
→the volume of the lungs increase → lowers air pressure in
lungs → moving of air into lungs
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Inspiration …. cont’d
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Contraction of diaphragm → movement of the diaphragm
downward → ↑ vertical dimension of thoracic cavity → ↑ the
volume of the lungs increase → lowering of air pressure in the
lungs → movement of air into the lungs:
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As the diaphragm and external intercostals contract and the
overall size of the thoracic cavity increases →
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Increase in intrapulmonary volume → intrapulmonary pressure
drops about (-1mmHg) below atmospheric pressure.
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Inspiration …. cont’d
Now this pressure difference between the atmosphere and the
alveoli leads
Air flows from the atmosphere (higher pressure) into the lungs
(lower pressure) until pressures equalizes
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Inspiration
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Inspiration( quit normal breathing)
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Intra alveolar pressure falls from 0 to -1 mmH2O
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Intrapleural pressure falls further to -7 mmH2O
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Transpulmonary pressure raises to +6 mmH2O
Net= 0.5(500ml) of air gushs to the lungs in 2-3 seconds.
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Expiration
At the end of inspiration, the nerves to the diaphragm and
inspiratory intercostal muscles decrease their firing,
So the diaphragm and external intercostals muscle relax →
decrease the size of chest cavity → decrease intrapulmonary
volume →
Intrapulmonary pressure rises (+1 mm Hg) above atmospheric
pressure.
Gases flow out of the lungs down the pressure gradient until
pressures equalize
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Expiration
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Expiration:(quit normal breathing)
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Intra alveolar pressure raise from -1 to +1 mmH2O
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Intrapleural pressure raise from -7 to -4 mmH2O
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Transpulmonary pressure falls to +5 mmH2O
Net= 500ml of air is exhaled from the lungs.
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Abdominal and Thoracic Breathing
Downward movement of diaphragm displacement of abdominal
viscera and abdominal wall abdominal breathing
Movement of chest wall thoracic breathing
In pregnancy and the movement of the diaphragm is limited and
breathing becomes mainly thoracic.
During deep breathing both abdominal and thoracic breathing are
equal in magnitude.
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Lung Compliance
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It is the tendency of the lungs to expand. This is made possible
by negative pleural pressure and surfactant which prevent
alveoli from collapsing.
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Specifically, the measure of the change in lung volume that
occurs with a given change in transpulmonary pressure
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Lung Compliance…..
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The total compliance of both lungs together in the normal
adult human being averages about 200 milliliters of air per
centimeter of water transpulmonary pressure
Compliance are determined by the elastic forces of the lungs.
These can be divided into two parts:

elastic forces of the lung tissue itself

elastic forces caused by surface tension of the fluid that
lines the inside walls of the alveoli and other lung air
spaces.
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Lung Compliance…..
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The elastic forces of the lung tissue are determined mainly by
elastin and collagen fibers interwoven among the lung
parenchyma
In deflated lungs, these fibers are in an elastically contracted
and kinked state;
When the lungs expand, the fibers become stretched and
unkinked, thereby elongating and exerting even more elastic
force.
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Lung Compliance
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The elastic forces caused by surface tension are much more
complex
The tissue elastic forces tending to cause collapse of the airfilled lung represent only about one third of the total lung
elasticity
Whereas the fluid-air surface tension forces in the alveoli
represent about two thirds
The fluid-air surface tension elastic forces of the lungs also
increase tremendously when
 the substance called surfactant is not present in the alveolar
fluid
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Lung compliance…. cont’d
Determinants of Lung Compliance
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Stretchability of the lung tissues, particularly their elastic
connective tissues.
However, an important determinant of lung compliance is not
the elasticity of the lung tissues, but
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The surface tension at the air-water interfaces within the
alveoli.
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Lung compliance…. cont’d
Lung collapse is overcome by:
I.
Surfactant,
II.
Distending pressure of alveoli, and
III.
Mechanical interdependence of alveoli.
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Factors That Diminish Lung Compliance
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Scar tissue or fibrosis that reduces the natural pliability of the
lungs
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Blockage of the smaller respiratory passages with mucus or fluid
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Reduced production of surfactant (surfactant decreases alveolar
surface tension and prevents tendency of lung collapse)
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Decreased flexibility of the thoracic cage or its decreased ability
to expand (restictive lung diseases e.g., emphysema)
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lung compliance …. Cont’d
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Deep inspiration Expansion of alveolisurface area of fluid
 dispersion of surfactant↓surfactant effect surface
tension
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Expirationrecoil of alveoli to smaller size↓ fluid surface
areacondensed surfactant surfactant activity↓surface
tension
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Surface tension during inspiration  prevention of alveolar
over distension and rupture.
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Surface tension during expiration  prevention of lungcollapse.
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Surface Tension
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A force acting across an imaginary line 1 cm long in the surface
of liquid lining alveoli and respiratory bronchioles.
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The surface of the alveolar cells is moist, and so the alveoli can
be pictured as air-filled sacs lined with water.
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At an air-water interface, water molecules have strong attraction
for one another, this force is known as surface tension.
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As a result, the water surface tends to contract, this force air out
of the alveoli,
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Surface Tension…. cont’d
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This causes the alveoli tends to collapse
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An alveolus tending to collapse pulls away from its neighbouring
alveoli thus increasing stress on their walls.
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The neighbours pull its wall out and prevent its collapse.
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This is how pulling forces of the alveoli keep them all open.
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Surfactant
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Surfactant is a complex mixture of several phospholipids,
proteins, and ions
 Important components are the phospholipid
dipalmitoylphosphatidylcholine, surfactant apoproteins, and
calcium ions.
Surfactant is a surface active agent in water, which means that
it greatly reduces the surface tension of water.
 Reduces attractive forces of hydrogen bonding by becoming
interspersed between water molecules.
As alveoli radius decreases, surfactant’s ability to lower
surface tension increases
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Action of surfactant
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Reduced surface tension
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 distensibility of lungs
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Reduced recoil force of lungs
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Prevention of alveolar collapse
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Reduction in the work of breathing
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Work” of Breathing
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Under resting conditions, the respiratory muscles normally
perform “work” to cause inspiration but not to cause expiration
The work of inspiration can be divided into three fractions:
 to expand the lungs against the lung and chest elastic forces,
called compliance work or elastic work
 to overcome the viscosity of the lung and chest wall
structures, called tissue resistance work
 to overcome airway resistance to movement of air into the
lungs, called airway resistance work.
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Energy Required for Respiration
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During normal quiet respiration, only 3 to 5%of the total
energy expended by the body is required for pulmonary
ventilation
During heavy exercise, the amount of energy required can
increase as much as 50-fold, especially if the person has any
degree of
increased airway resistance
 decreased pulmonary compliance.

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A simple method for studying pulmonary ventilation is to
record the volume movement of air into and out of the lungs, a
process called spirometry
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Pulmonary Volumes and Capacities
Lung Volumes
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Tidal volume (TV) – air that moves into and out of the lungs
with each normal breath (approximately 500 ml)
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Inspiratory reserve volume (IRV) – air that can be inspired
forcefully beyond the tidal volume (3000 ml)
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Expiratory reserve volume (ERV) -forceful volume of
expiration (1100ml)
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Residual volume (RV) – air left in the lungs after maximal
expiration (1200 ml)
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Pulmonary Capacities
Inspiratory Capacity
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It’s the sum of TV and IRV
Functional Residual capacity
It’s the sum of ERV and RV
Vital Capacity (VC) – The total amount of exchangeable air
(TV + IRV + ERV)
Total lung Capacity (TLC) – Sum of all lung volumes
(approximately 5800 ml in males)
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Methods of measuring lung volumes
and capacities
1.wet spirometry
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This method uses a volume
displacement spirometer with a waterseal type.
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Technique for measuring lung volumes
and capacities
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Volumes and capacities measured using
this method include: TV, IRV, ERV,
VC and IC
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Fig. Lung volumes and capacities measured using wet spirometer
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Timed vital capacity(FEV1)
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Dry Spirometry (Vitalography)
 Is used to record the forced expiratory spirogram.
 It is the fraction of vital capacity which can be expired at
the end of one second using maximal expiratory effort.
 It is the volume of air an individual can expire maximally
as far as possible after maximal inspiration during the
first one second.
A simple vitalograph test for FEV1 can be performed to
identify both obstructive and restrictive conditions.
Normal FEV1 is about 80% of the VC.
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Timed vital Capacity(FEV1)…. Cont’d
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People with obstructive lung diseases charaterized by
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Increased airway resistance
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FEV1 which is less than 80 percent of the vital capacity
People with restrictive lung diseases are characterized by
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Normal airway resistance but impaired respiratory
movements =>
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Because of abnormalities in the lung tissue, the pleura, the chest
wall, or the neuromuscular machinery.
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Timed vital Capacity(FEV1)…. Cont’d
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Restrictive lung diseases
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Characterized by a reduced vital
capacity
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but a normal ratio of FEV1 to
vital capacity.
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Timed vital Capacity(FEV1)…. Cont’d
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Restrictive lung diseases
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Characterized by a reduced vital
capacity
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but a normal ratio of FEV1 to
vital capacity.
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Pulmonary ventilation rate
The total amount of air moved into the respiratory passages
each minute.
Is equal to the tidal volume multiplied by the respiratory rate:
Pulmonary ventilation rate depends upon 2 factors:
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The size of each breath (tidal volume: TV)
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The number of breaths/minute (respiratory frequency: BR)
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PV = BR X TV
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Pulmonary ventilation rate…. Cont’d
The normal tidal volume is about 500 milliliters, and
The normal Breathing rate is about 12 breaths/minute.
Therefore, ventilation rate averages about 6 L/min.
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Alveolar ventilation
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The total volume of fresh air entering the alveoli per minute is
called the alveolar ventilation or
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The rate at which new air reaches these areas is called alveolar
ventilation.
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It is equal to the respiratory rate times the amount of new air
that enters these areas with each breath.
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Alveolar ventilation (ml/min) =(Tidal volume - Dead space) x
Respiratory rate)
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