Introductory Anatomy: Respiratory System
Dr D.R.Johnson, Centre for Human Biology
The word respiration describes two processes.
Internal or cellular respiration is the process
by which glucose or other small molecules are oxidised to produce
energy: this requires oxygen and generates carbon dioxide.
External respiration (breathing) involves simply the stage
of taking oxygen from the air and returning carbon dioxide to
The respiratory tract, where external respiration occurs,
starts at the nose and mouth. (Description of respiratory tract
from nose to trachea here from overheads) (There is a brief complication
where the airstream crosses the path taken by food and drink
in the pharynx: air flows on down the trachea where food normally
passes down the oesophagus to the stomach. )
The trachea (windpipe) extends from the neck into the thorax,
where it divides into right and left main bronchi, which enter
the right and left lungs, breaking up as they do so into smaller
bronchi and bronchioles and ending in small air sacs or alveoli,
where gaseous exchange occurs.
The lungs are divided first into right and left, the left being
smaller to accommodate the heart, then into lobes (three on the
right, two on the left) supplied by lobar bronchi.
Bronchi, pulmonary arteries and veins (which supply deoxygenated
blood and remove oxygenated blood), bronchial arteries and veins
(which supply oxygenated blood to the substance of the lung itself)
and lymphatics all enter and leave the lung by its root (or hilum).
Lymph nodes blackened by soot particles can often be seen here
and the substance of the lung itself may be blackened by soot
in city dwellers or heavy smokers.
Each lobe of the lung is further divided into a pyramidal bronchopulmonary
segments. Bronchopulmonary segments have the apex of the
pyramid in the hilum whence they receive a tertiary bronchus,
and appropriate blood vessels. The 10 segments of the right lung
and eight of the left are virtually self contained units not
in communication with other parts of the lung. This is of obvious
use in surgery when appropriate knowledge will allow a practically
bloodless excision of a diseased segment.
Gaseous exchange relies on simple diffusion. In order to provide
sufficient oxygen and to get rid of sufficient carbon dioxide
there must be
- a large surface area for gaseous exchange
- a very short diffusion path between alveolar air and
- concentration gradients for oxygen and carbon dioxide
between alveolar air and blood.
The surface available in an adult is around 140m2 in an adult,
around the area of a singles tennis court. The blood in the alveolar
capillaries is separated from alveolar air by 0.6* in many places
(1* = one thousandth of a mm) . Diffusion gradients are maintained
- ventilation (breathing) which renews alveolar air, maintaining
oxygen concentration near that of atmospheric air and preventing
the accumulation of carbon dioxide
- the flow of blood in alveolar capillaries which continually
brings blood with low oxygen concentration and high carbon dioxide
Haemoglobin in blood continually removes dissolved oxygen
from the blood and binds with it. The presence of this tennis
court, separated from the outside air by a very narrow barrier
imposes demands on the respiratory tract.
- varies in temperature. At the alveolar surface it must be
at body temperature
- varies from very dry to very humid. At the alveolar surface
it must be saturated with water vapour
- contains dust and debris. These must not reach the alveolar
- contains micro-organisms, which must be filtered out of the
inspired air and disposed of before they reach the alveoli, enter
the blood and cause possible problems.
It is easy to see that the temperature and humidity of inspired
air will increase as it passes down a long series of tubes lined
with a moist mucosa at body temperature. The mechanisms for filtering
are not so obvious.
The respiratory tract, from nasal cavities to the smallest bronchi,
is lined by a layer of sticky mucus, secreted by the epithelium
assisted by small ducted glands. Particles which hit the side
wall of the tract are trapped in this mucus. This is encouraged
by: (a) the air stream changing direction, as it repeatedly
does in a continually dividing tube. (b) random (Brownian)
movement of small particles suspended in the airstream.
The first of these works particularly well on more massive particles,
the second on smaller bits
Once the particles have been sidelined by the mucus they have
to be removed, as indeed does the mucous. This is carried out
by cilia on the epithelial cells which move the mucous continually
up or down the tract towards the nose and mouth. (Those in the
nose beat downwards, those in the trachea and below upwards).
The mucus and its trapped particles are and bacteria are then
swallowed, taking them to the sterilising vat of the stomach.
The length of the respiratory tract helps in both bringing the
air to the right temperature and humidity but hinders the actual
ventilation, as a long tract has a greater volume of air trapped
within it, and demands a large breath to clear out residual air.
The entry of food and drink into the larynx is prevented by the
structure of the larynx and by the complicated act of swallowing.
The larynx is protected by three pairs of folds which close off
the airway. In man these have a secondary function, they vibrate
in the airstream to produce sounds, the basis of speech and singing.
Below the larynx the trachea is usually patent i.e. open, and
kept so by rings of cartilage in its walls. However it may be
necessary to ensure that this condition is maintained by passing
a tube (endotracheal intubation) to maintain the airway, especially
post operatively if the patient has been given a muscle relaxant.
Another common surgical procedure, tracheotomy, involves a small
transverse cut in the neck. If this is done with anatomical knowledge
no major structure is disturbed and the opening may be used for
a suction tube, a ventilator, or in cases of tracheal obstruction
as a permanent airway.
Ventilation and perfusion
The gills of fish and the lungs of birds allow water and air
receptively to flow continually over the exchanging surface.
In common with all mammals humans ventilate their lungs by breathing
in and out. This reciprocal movement of air is less efficient
and is achieved by alternately increasing and decreasing the
volume of the chest in breathing. The body's requirements for
oxygen vary widely with muscular activity. In violent exercise
the rate and depth of ventilation increase greatly: this will
only work in conjunction with increase in blood flow, controlled
mainly by the rich innervation of the lungs.. Gas exchange can
be improved by breathing enriched air, which produces significantly
reduced times for track events. Inadequate gas exchange is common
in many diseases, producing respiratory distress.
Mechanism of breathing
In order to grasp the way in which we breathe we have to grasp
the following facts:
Each lung is surrounded by a pleural cavity or sac, except where
the plumbing joins it to the rest of the body, rather like a
hand in a boxing glove. The glove has an outer and inner surface,
separated by a layer of padding. The pleura, similarly, has two
surfaces, but the padding is replaced by a thin layer of fluid.
Each lung is enclosed in a cage bounded below by the diaphragm
and at the sides by the chest wall and the mediastinum (technical
term for the bit around the heart). It is not usually appreciated
that the lung extends so high into the neck. A syringe inserted
above a clavicle may pierce the lung.
Breathing works by making the cage bigger: the pleural layers
slide over each other and the pressure in the lung is decreased,
so air is sucked in. Breathing out does the reverse, the cage
collapses and air is expelled. The main component acting here
is the diaphragm. This is a layer of muscle which is convex above,
domed, and squashed in the centre by the heart. When it contracts
it flattens and increases the space above it. When it relaxes
the abdominal contents push it up again. The proportion of breathing
which is diaphragmatic varies from person to person. For instance
breathing in children and pregnant women is largely diaphragmatic,
and there is said to be more diaphragmatic respiration in women
than in men.
The process is helped by the ribs which move up and out also
increasing the space available. The complexity of breathing increases
as does the need for efficiency. In quiet respiration,
say whilst lying on ones back, almost all movement is diaphragmatic
and the chest wall is still. This will increase thoracic volume
by 500-700ml. The expansion of the lung deforms the flexible
walls of the alveoli and bronchi and stretches the elastic fibres
in the lung. When the diaphragm relaxes elastic recoil and abdominal
musculature reposition the diaphragm again.
Deeper respiration brings in the muscles of the chest
wall, so that the ribs move too.
We must therefore understand the skeleton and muscular system
of the thoracic wall.
The 12 pairs of ribs pass around the thoracic wall, articulating
via synovial joints with the vertebral column - in fact two per
rib. The ribs then curve outwards then forwards and downwards
and attach to the sternum via the flexible costal cartilages.
The first seven pairs of ribs (true ribs) attach directly, the
next five hitch a lift on each other and the last two float i.e.
are unattached. Costal cartilages are flexible. The first rib
is rather different, short, flattened above and below and suspended
beneath a set of fairly hefty muscles passing up into the neck,
the scalene muscles. Between the ribs run two sets of intercostal
muscles, the external intercostals running forward and downwards,
the internal intercostals running up and back. These two muscle
sheets thus run between ribs with fibres roughly at right angles.
When they contract each rib moves closer to its neighbours. Because
the lowest ribs float, and the first rib is suspended from the
scalene muscles contraction of the intercostal muscles tends
to lift rib two towards rib 1, and so on. The ribs are all, therefore
pulled up towards the horizontal, increasing anteroom-posterior
and lateral thoracic diameters.
These movements are sometimes divided intopump handle movements,
the rib abducting on its vertebral joints and bucket handle
movements, the rib rotating on its axis around anterior and
posterior attachments: these are not necessarily helpful.
With more and more effort put into deeper and deeper breathing
the scalene muscles of the neck contract, raising the first rib
and hence the rest of the cage, then other neck muscles and even
those of the upper limb become involved. A patient with difficulty
in breathing often grips a table edge in order to stabilise the
limbs so that their muscles can be used to help in moving the
The lungs sometimes fail to maintain an adequate supply of air.
The earliest cases of this are seen in infant respiratory distress
syndrome. In premature infants (less than about 2 lbs or 37 weeks
the cells which make surfactant are not yet active. Surfactant
reduces the surface tension in the fluid on the surface of the
alveoli, allowing them to expand at the first breath, and remain
open thereafter. The sacs either fail to expand, or expand then
collapse on expiration and result in laboured breathing. In adults
a similar syndrome is due to accidental inhalation of water,
smoke, vomit or chemical fumes.
Acute bronchitis is due to infection of the bronchial tree, which
may have impaired function due to fluid accumulation. Pneumonia
involves the lung proper. Lung cancers a malignancy that may
spread to other tissues via the lymphatics in the lung roots.
Return to Human Biology Course Notes
This page is maintained by Steve