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 it.

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 blood
• 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 by

• 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 concentration

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.
Outside air:

• 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 wall
• 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.

Mucus
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

Cilia
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.

Length
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.

Protection
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 thoracic wall.

Problems.
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.

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