A basic understanding of the bodies’ processes is needed to grasp the physiological effects of diving and the application of diving medicine. The cardiovascular and respiratory systems are described here while the physiology of some other organs, such as the ear, are considered in later blog posts.
The Need for Energy
Energy is a fundamental requirement for all life processes. It is needed for growth, repair, movement and all the active functions of the body. The fuel for this energy comes from carbon compounds, which are incorporated in complex molecules in the food we eat. This is biochemically dismantled in the digestive tract into simple chemical compounds which are absorbed and carried by the blood stream to the cells. Here they undergo further biochemical processing until ultimately the carbon is combined with oxygen (O2), forming carbon dioxide (CO2) and releasing energy.
This is similar to the energy formation which takes place in an automobile engine or a fire, where carbon in fuel or wood is combined with O2 to produce energy. The body processes will only function under strict conditions of O2 availability, temperature and acidity.
The body needs a means of transferring food products to the cells, together with delivery of O2 and removal of CO2. This is performed by the blood, in the vascular system. It comprises arteries which take blood to the tissues, a vast network of microscopic capillaries that bring the blood into contact with all the cells of the body, and veins which return blood to the heart.
The blood is circulated through the blood vessels by a muscular pump – the heart, and the whole system is called the cardiovascular system. It brings O2 from the lungs to the cells and eliminates CO2 through the respiratory system.
The respiratory tract begins at the mouth and nose and ends in the microscopic air sacs called the alveoli, in the lungs.
The nose, apart from its decorative function, warms and humidifies the air that we breathe. It also filters large particles which might otherwise be inhaled. If the nose is bypassed by breathing through the mouth, a snorkel or scuba regulator, the lung then has to cope with drier, colder, unfiltered air.
After passing through the mouth or nose, the air then enters the throat where the larynx (or voice box) is situated. This is recognised as the “Adams Apple”. The larynx produces the sounds of speech as well as helping to protect the lungs from inhalation of foreign material.
A structure called the epiglottis closes over the opening and the vocal cords shut to prevent the foreign material from entering the lungs. If any material passes these structures, the cough reflex, activated by foreign material touching the inside of the air passages, may cause a coughing reaction which tends to expel whatever has been inhaled.
Below the larynx the air passes through a tube called the trachea. This is about as thick as the average snorkel and branches inside the chest into two tubes, the bronchi, which lead to the lungs. Those air passages are lined with cells covered with microscopic hairs (cilia) which move a sheet of secreted mucous slowly upwards towards the larynx. Small pieces of foreign material such as dust eventually find their way to the larynx, along with this mucous sheet. It is then either coughed-up or swallowed. The cilia may be damaged by smoking or infection, causing retention of mucous and inhaled material which may eventually obstruct the air passages.
The bronchi divide repeatedly into progressively smaller passages rather like the branches of a tree. These passages have encircling muscles in their walls which, by contraction or relaxation, can vary the diameter of the air passage.
In asthma the muscles of the small bronchi become oversensitive and overactive, causing excessive narrowing and obstruction of these air passages. This can occur in response to exercise, allergy, cold, infection, anxiety, smoking or other inhalants such as sea water. At the same time, the cells lining these passages produce excessive and thickened mucous. The combination of these factors causes airway narrowing which has serious repercussions for a diver.
The smallest branches of the bronchi end in bunches of microscopic air sacs called alveoli. The vast number of alveoli are packed together into the two sponge like organs, the lungs. There are about 300 million alveoli in the lungs and the combined surface area of all the alveoli in the lungs is equal to about half a tennis court. The alveoli are lined by a thin layer of fluid containing a detergent-like substance called surfactant. This acts as a wetting agent to prevent the alveoli from collapsing from surface tension.
The surfactant lining of the alveoli can be damaged in disease or by inhalation of water, leading to collapse of the lungs and serious respiratory difficulty.
Each alveolus is surrounded by a network of blood capillaries. These bring the blood into close contact with the air in the alveolus, with only the microscopically thin walls of the alveolus and capillary separating the two.
If the wall of an alveolus is ruptured, as it may be in pulmonary barotrauma (“burst lung”), then air from the alveolus is able to enter the blood stream where it may cause blockage of distant vessels such as those in the brain. This is called an air embolism.
The lungs occupy a cavity about the size of a football on each side of the chest. The lung is covered by a thin membrane coating, called the pleura, and the inside of the chest wall is lined by a similar membrane. Between the two pleural layers is a narrow space which contains a small amount of lubricating fluid to minimise friction as the lungs expand and contract during breathing. If the outer surface of the lung tears, as it may in pulmonary barotrauma, then air can enter this pleural space causing the lung to collapse. This disorder is called pneumothorax.
The chest wall which encloses the lungs is made up of ribs with muscles between them – known as intercostal muscles. At the base of the chest cavity lies a large thin dome shaped muscle called the diaphragm. When the diaphragm contracts, it flattens and has a piston like effect, reducing the pressure in the chest cavity and increasing the volume of the lungs. The reduced pressure draws air into the lungs through the air passages.
Contraction of the diaphragm is the main method of inhalation in the resting state. It is assisted by contraction of the muscles between the ribs which rotate the rib cage upwards and outwards, enlarging the chest cavity and reducing the pressure in the chest. A group of neck muscles which are attached to the rib cage can also assist respiration when maximal breathing is required.
At the end of inhalation the elasticity of lungs and rib cage causes the lungs and chest wall to contract and exhalation takes place. With quiet breathing, this does nor require muscular effort. With heavy breathing, exhalation can be assisted by the abdominal and chest muscles.
During quiet respiration in adult males, about 500 ml of air is moved in and out of the respiratory tract with each breath. The volume per breath is termed “tidal volume”. During extremely heavy exercise, the tidal volume can increase 10 fold, up to about 5 litres.
The total amount of air that can be held in the lungs (total lung capacity or TLC) in adult males is approximately 6 litres. Only about 10% of the air in the chest is exchanged with each breath during quiet respiration. The vital capacity (VC) is the maximum volume that can be exhaled in one breath, and the forced expiratory volume (FEV1.0) is the maximum volume that can be exhaled in one second.
The flow of air through the respiratory passages varies at different stages of respiration. It reaches a peak about midway through inspiration — and during quiet breathing this peak flow rate is approximates 30 litres per minute. This value increases during exercise to 600–700 litres per minute.
Any breathing system (such as a snorkel or demand valve) which the diver is using, should be capable of handling these large air flows without significant resistance. If this does not occur, then the diver must exert extra effort during respiration in order to overcome this resistance. This problem is compounded when the diver is breathing compressed air at depth because the increased density of the gas will further increase the resistance to airflow in both the equipment and the lungs.
Gas Uptake and Loss
Air, which contains approximately 21% oxygen (O2) and 78% nitrogen (N2), is inhaled into the alveoli where it is brought into contact with the blood in the capillaries. This blood contains a lower partial pressure of O2 than the air in the alveolus and a higher partial pressure of CO2, since it has just returned from the body, which has been using O2 and generating CO2. Consequently, there is a pressure gradient causing O2 to diffuse from the alveoli to the blood, and CO2 to diffuse from the blood to the alveoli, where it is then exhaled. There is no net movement of N2 since the N2 in the alveoli and in the blood is in equilibrium, except when diving, altitude exposure or breathing different gases.
If the diver breathes air (78% N2) or another inert gas such as helium, while descending or remaining underwater, this inert gas will pass from the alveoli to the blood because the partial pressure of the gas in the lungs is increasing as the diver goes deeper.
On ascent, the partial pressure of inert gas in the lungs will reduce, and this allows inert gas to move from the blood (returning from the tissues) to the alveoli, and be exhaled.
The partial pressures of CO2 and O2 in the blood are kept within very strict limits by a sensitive control system. There are sensors in the brain which detect small changes in the blood CO2. If this increases, then the sensor causes stimulation of the respiratory centre within the brain, leading to faster and deeper respiration to eliminate more CO2.
When a snorkel diver holds his breath, the CO2 level in his blood increases. This produces respiratory stimulation which compels the diver to take a breath — hopefully after he has had time to return to the surface.
The sensors for blood O2 pressure are in the carotid arteries which supply the brain. A reduction in the blood O2 level also leads to respiratory stimulation, but this effect is not as powerful as that caused by CO2 changes.
The ingenious habit of rolling tobacco into a tube of paper, setting fire to it and inhaling the smoke, sabotages the complex respiratory and circulatory process at several points.
As well as predisposing to lung cancer and emphysema, noxious tars in the smoke precipitate out in the bronchi producing chronic irritation, narrowing of the bronchi and cause a persistent outpouring of mucous. This ultimately results in chronic bronchitis. The tar also poisons the cilia, which conduct the mucous up the airway to the larynx, resulting in retention of old mucous in the lungs (smell the breath!).
Various toxins in the smoke ultimately cause destruction of the alveolar walls producing cavities in the lungs and destruction of the lung architecture, resulting in the disease called emphysema. This, combined with obstruction of the air passages, makes the smoking diver less physically fit and more liable to air trapping in the lungs and pulmonary barotrauma.
The carbon monoxide content of the smoke reduces the capacity of the blood to carry O2, thereby reducing oxygenation of the tissues.
Some of the chemical constituents of the smoke are absorbed into the blood stream producing changes in the walls of the blood vessels supplying the heart, brain and limbs. Ultimately these become obstructed. In later life this can cause heart attacks, strokes and peripheral vascular disease (gangrene).
Related articles by Unique Scuba
- Levels In Respiration (slideshare.net)