Physical training is very important before attempting to free dive. Once you are in good physical shape, your body is more efficient in using and transporting oxygen. Your cells actually continue to function with less oxygen present. Therefore, a healthy physical lifestyle is synonymous with being a safe free diver.

There are many ways to train to hold your breath longer and they all focus on one governing principle -lowering your heart rate. Each diver must come up with their own way to lower their heart rate that works best for them. In my experience, lowering your heart rate requires both physical commitment and strong mental concentration with the helpful aid of the diving reflex. The phenomenon known as the diving reflex or the mammalian response occurs from the immersion of ones' face in cold water, which causes the heart to slow down automatically. Personally, I am really interested in learning more about the physics and physiology of the underwater environment and its impact on the human body. I found Lawrence Martin's online book, Scuba Diving Explained-Physiology and Medical Aspects of Scuba Diving, to be very helpful.

The Physics of Respiration
Respiration is the process by which living organisms convert energy to sustain life. In humans, cells are able to harness energy in a process known as metabolism, which can result from aerobic respiration (with oxygen) or anaerobic respiration (without oxygen). Aerobic respiration is a fast process in which cells harness energy from nutrients using oxygen while producing carbon dioxide and water as by products (Krebs cycle). In contrast, anaerobic respiration is a relatively slow process, without the use of oxygen, in which energy is derived through a process known as glycolysis. Muscles, which can occasionally function without oxygen, produce lactic acid as a by-product of anaerobic respiration. Elevated levels of blood lactate beyond a tolerance level results in muscle fatigue or cramping. Due to the limited energy yield during anaerobic respiration, the body can only function for short periods of time without using oxygen. For example, brain cells can only survive for 5 minutes without oxygen; after that the cells literally poison themselves with a toxic cascade of chemical reactions. However, some free divers have been known to hold their breath for over seven minutes and survive. This is due to the fact that when holding ones' breath, the body continues to use oxygen supplied by the lungs (aerobic respiration) until at some point, due to the depletion of oxygen, anaerobic respiration becomes the dominant source of energy. Since free divers rely on both aerobic and anaerobic respiration, it would be beneficial to understand the processes governing breathing and gas exchange as it relates to the physics of respiration. Basic anatomy, the physics of the lung and gas exchange are discussed in the following paragraphs.

Physics of the Lung
During breathing air enters by the nose and mouth and passes through the glottis to the trachea and then by two main bronchi and their branches (bronchioles). Then the air passes to the terminal or respiratory bronchioles. The whole thing is like a tree with branching taking place repeatedly into pairs of smaller bronchioles. The terminal bronchiole gives rise to ten to about twenty respiratory bronchioles, each of which widens into alveolar ducts with many hundreds of alveoli. The alveoli ducts often have several major partitions, which are called the alveolar sacs. This is where the pulmonary capillary network responsible for oxygen gas transfer is physically located in the lung. There is no appreciable gas exchange in the lung up to and including the terminal bronchioles. Hence, this volume of the (upper) part of the lung is termed the anatomic dead space.

Gas Exchange
During the breathing cycle, the volumes of the alveolar ducts and the alveoli increase and decrease in equal proportions. The gaseous exchange of oxygen and carbon dioxide across the alveolar-capillary membrane occurs solely by a process known as diffusion and is governed by Ficks Law. Diffusion depends on the difference in partial pressure of the gas across the membrane (i.e. the pressure gradient) and also on the area and thickness of the membrane. Furthermore, gas exchange in the lungs work on oxygen tension (i.e. partial pressure), not the percentage or concentration of gases. The total capillary surface area, that is the area actually doing the gas exchange (i.e., the air - tissue - blood interface), is estimated to be about 90 square meters for the average adult. This large capillary area exposes air and blood to an enormous surface area for adequate oxygen diffusion into the blood stream and diffusion of carbon dioxide into the lung volume for subsequent exhalation.

Oxygen Transportation
Oxygen is carried in the blood in red blood cells attached to a chemical called hemoglobin. However, the number of red blood cells and the hemoglobin count are not synonymous. It is possible in many diseases to have the normal number of red cells, but also to have a low hemoglobin count. Conditions like this normally indicate iron deficiency. The average red cell count in an adult male is 5.5 million per mm3 and 4.8 million per mm3 for an adult female. In athletes, the average red cell count may be higher, because the body has an amazing ability to adapt to the high oxygen demands that occur as a result of intensive training and exercise.

Control of Breathing
The respiratory centre responsible for rhythmic respiration is located in the pons and the upper Medulla of the brain. This centre can be divided into an inspiratory centre and an expiratory centre in the Medulla, an apneustic centre in the lower and midpons and a pneumotaxic centre in the rostral-most part of the pons. This respiratory centre is very sensitive to the partial pressure of carbon dioxide (pCO2) in the arteries and to the pH level of the blood. The CO2 can be brought back to the lungs in three different ways; dissolved in plasma, as carboxyhaemoglobin, or as carbonic acid. This particular form of acid is for the most part broken down immediately by carbonic hydrase into bicarbonate and hydrogen ions. The Medulla Oblongata reacts to both CO2 and pH levels in the blood, which triggers the breathing process so that more oxygen can enter the body to replace the oxygen that has been utilized. The Medulla Oblongata sends neural impulses down through the spinal chord and into the diaphragm. The impulse contracts the diaphragm down to the floor of the chest cavity, and at the same time there is a message sent to the chest muscles to expand, causing a partial vacuum to be formed in the lungs. The partial vacuum will draw air into the lungs.

When there is an oxygen debt (lack of oxygen reaching the muscles), lactic acid is produced, which lowers the pH level in the blood. The Medulla Oblongata would then be stimulated, producing the urge to breathe. If the pH rises, the body begins a process known as the Bohr shift. The Bohr shift occurs when there are extremely high oxygen and carbon dioxide pressures present in the body. These high pressures make it difficult for oxygen and carbon dioxide to attach to hemoglobin. Moreover, when the body is exposed to high altitudes, oxygen will not attach to the hemoglobin properly, causing oxygen levels to drop. This in turn results in dizziness or even black out. This Bohr shift theory also applies to divers who go to great depths, resulting in large partial pressures of oxygen that may even become poisonous. Another trigger for breathing occurs when the major arteries in the body, called the aortic and carotid bodies, sense a build-up of carbon dioxide. Once carbon dioxide levels reach a certain threshold in the blood, the Medulla triggers a breathing response. Therefore, the need to breathe is not stimulated by the depletion of oxygen but the accumulation of carbon dioxide in the blood. The implications of this phenonenon are of vital importance to the understanding of shallow water blackout - the most common cause of death among freedivers.

Basic Physical Laws

The behaviour of all gases is affected by three factors: the temperature of the gas, the pressure of the gas, and the volume of the gas. The relationships among these three factors have been defined in what are called the Gas Laws. Five of these, Dalton's Law, Boyle's Law, Charles' Law, Henry's Law, and the General Gas Law, are of special importance to all divers.

In the following equations, P, V, and T denotes absolute pressure, volume, and absolute temperature respectively. Subscript indexes ( 1, 2, etc. ) are used to distinguish values at different moments such as initial, final, etc. Other special symbols are defined as required.

Dalton's Law
The total pressure exerted by a mixture of gases is equal to the sum of the pressures that would be exerted by each of the gases if it alone were present and occupied the total volume.

PTotal= Pp1+ Pp2+ ... + Ppn

Pp denotes the partial pressure of the particular gas component.
In a gas mixture, the portion of the total pressure contributed by a single gas is called the partial pressure of that gas.

Boyle's Law
At constant temperature, the volume of a gas varies inversely with absolute pressure, while the density of a gas varies directly with absolute pressure.

P1V1 = P2V2 = constant (at constant T)

Boyle's Law is important to divers because it relates changes in the volume of a gas to changes in pressure (depth) and defines the relationship between pressure and volume in the lungs of a diver.

Charles' Law
At a constant pressure, the volume of a gas varies directly with absolute temperature. For any gas at a constant volume, the pressure of a gas varies directly with absolute temperature.

P1V1 = P2V2 at constant volume and constant pressure

T1 T2

Temperature significantly affects the pressure and volume of a gas; it is therefore essential to have a method of including this effect in calculations of pressure and volume. To a diver, knowing the effect of temperature is essential, because the temperature of the water deep in the oceans or in lakes is often significantly different from the temperature of the air at the surface.

Henry's Law
The amount of any given gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of the gas that is in contact with the liquid and the solubility coefficient of the gas in the particular liquid.

Vg = alphaP1


Vg = volume of the gas dissolved at STP (standard T and P)
VL =volume of the liquid
alpha = Bunson solubility coefficient at specified temperatures
P1 =partial pressure in atmospheres of the gas above the liquid

This law simply states that, because a large percentage of the human body is water, more gas will dissolve into the blood and body tissues as depth increases, until the point of saturation is reached. Depending on the gas, saturation takes from 8 to 24 hours or longer. As long as the pressure is maintained, and regardless of the quantity of gas that has dissolved into the diver's tissues, the gas will remain in solution.

A simple example of the way in which Henry's Law works can be seen when a bottle of carbonated soda is opened. Opening the container releases the pressure suddenly, causing the gases in solution to come out of solution and to form bubbles. This is similar to what happens in a diver's tissues if the prescribed ascent rate is exceeded. The significance of this phenomenon for divers is developed in more detail in the discussion of decompression.

Decompression and Nitrogen Narcosis

The chief hazard in deep diving is the compression of air in the lungs. This compression results in a higher gas partial pressures (similar to the concept of concentration) in the alveoli and, hence, in the blood. Unfortunately, at higher pressures, most gases become toxic. In addition, if the diver returns too quickly to the surface, the dissolved gases in the blood come out of solution and are released as bubbles. The formation of these bubbles in the tissues and blood causes numerous reactions and tissue injury, which leads to decompression sickness or "the bends", which can be fatal.

One of the main gases related to decompression sickness is nitrogen (N2). Air consists of 78% nitrogen, 21% oxygen and 1% argon. The main problem with nitrogen is that when you dive, the tissues become saturated with nitrogen, because under the increase pressure more nitrogen dissolves into the tissue. If the diver returns too rapidly to the surface, the nitrogen is released from solution as small bubbles within many tissues and the blood stream. This is somewhat similar to the effect you get when you open a "pop" bottle. When you release the pressure, carbon dioxide comes out of solution in the form of bubbles. Bubble formation in tissues causes great pain and quite often this bubble development takes place in the fluid of the joints. Furthermore, formation of gas bubbles in the capillaries could cause an obstruction resulting in an embolism. Nitrogen has an anaesthetic effect under pressure (high nitrogen pressure effects nerve conduction). For scuba divers breathing compressed air, at depths greater than 120 feet, nitrogen narcosis begins. Some divers experience no narcotic effect at depths up to 140 feet, whereas others feel some effect at around 100 feet. One thing is certain: once begun, the narcotic effect increases with increasing depth. Divers experiencing nitrogen narcosis generally have impaired motor ability, i.e., movement of arms and legs and so forth. Impairment of judgement also begins and you essentially behave like a drunk. At 300 feet, there is complete incapacitation (i.e., you are senseless drunk).

The phenomenon of decompression sickness or nitrogen narcosis is more pronounced in scuba diving than in deep breath-hold diving. Pressure increases at the rate of one atmosphere for every 33 feet (~10 meters) of depth. The deeper one goes, the longer the decompression time required after a deep dive. That is, the gases that dissolve in the blood require time to dissolve. However, you also need time for these gases to come out of solution in to the lung upon decompression. Essentially, the more time you spend at a given depth, the more gases you will dissolve in the blood and the more decompression time you require. Therefore, since scuba divers spend a greater time at depth than breath-hold divers the effects of decompression sickness are more pronounced for scuba divers. Moreover, scuba divers are breathing a higher concentration (partial pressure) of nitrogen at depth than breath-hold divers. There is also some evidence that some divers can become partially acclimated to the effects of excess nitrogen; the more frequently they dive the less each subsequent dive appears to affect them. It is important to note that breath-hold divers are not immune to the effects of decompression sickness and thus should be aware of the symptoms and prevention of nitrogen narcosis. For most casual free divers that don't dive beyond 30 to 40 feet decompression related problems are not an issue.

Cavities of the Body (Barotrauma)

The Ears
The ears are organs of hearing and equilibrium. Both functions might be disturbed under water because of the inability of the diver to equalize the pressure. The ear is divided into external, middle and internal parts. The external ear includes the auricle (the outer flap of the ear) and the external auditory canal, which leads to the eardrum (or tympanic membrane). The middle ear is located on the inner part of the eardrum and is connected to the back of the nose and throat (nasopharynx) by the eustachian tube. It contains three bones - the hammer handle, the anvil and the stapes. The internal ear, or labyrinth, is composed of the cochlea ("snail shell"), the vestibule and three canals.

Physiology and Hydrostatic Pressure
1 to 2 meters (3-7 feet) no earaches
3 meters (10 feet) a feeling of weight in the ears
4 meters (13 feet) pain
6 to 7 meters (20-23 feet) the pain spreads out to the jaws, face and the whole head
8 to 9 meters (26-30 feet) strong and unbearable pain which persuades the person to ascend if he does not know how to equalize

Under water, divers often experience pain in their ears. This is due to the raised water pressure that causes the eardrum to bend. In order to equalize the pressure out of and in the middle ear, it is necessary that air enters from the mouth through the eustachian tube to the middle air. The valve of the eustachian tube is usually closed and it can be opened only through contractions of the nasopharyngeal muscles.
This can be done through swallowing, yawning, gently blowing with closed nostrils, moving the tongue and other ways. You may also wish to visit the Training for a description on the different methods of equalization. These methods should start from the surface and be repeated every 1-2 meters. If the pressure cannot be equalized at once, divers should go up 1-2 meters and try again. If the diver persists going underwater without proper equalization, the difference between the ambient pressure and that of the body's air-containing cavities may cause injury by damaging the involved tissues. This injury is called barotrauma.

The Sinuses
Sinuses are air-filled cavities located in the head and cheekbones. These cavities are connected to the nasal cavity by means of large openings through which aid can pass with no difficulty. Problems occur with divers suffering from sinusitis or cold. In these cases, the openings become so narrow that air cannot pass freely. As a result, strong pains are felt in the sinuses because of the impossibility of air to either enter of leave and of pressure to be equalized.

The Mouth Cavity
Under water, pains might be felt in cavities in a rotten tooth or under fillings and crowns. During ascent, air that has entered any hollow places in the tooth cannot come out because of pressure. This leads to breaking the tooth or removing fillings or crowns.


The large difference between the ambient pressure and that of the body's air-containing cavities may cause injury by damaging the involved tissues. This injury is called barotrauma.

Barotitis Media
This is inflammation of the middle ear due to insufficient pressure equalization. Another name for barotitis media that is more popular among divers is middle ear squeeze. If this infection does not worsen, the diver will feel much better in a week. Symptoms and Treatment Barotitis media is characterized by earaches, a feeling of fullness and reddened eardrum. It is cured with warm applications to the ear and antiseptic medicines.

Middle Ear Squeeze or "Ear-Deafening"
The feeling of "deafened" ears disappears after drying them. Middle ear squeeze affects the ear - plugged eustachian tube, damaged inner ear's mucous membrane.

Eardrum Rupture, Labyrinth Crisis
If the diver does not equalize the pressure but ignores the pain and continues to dive deeper, raised pressure exceeds the strength of the eardrum membrane and tears it. As a result, water enters the middle ear and cools the surrounding tissues and the inner ear. Most affected are the cells of the vestibular system which start transmitting chaotic information for 1-2 minutes. The diver loses orientation, is dizzy and nauseous. This state is known as labyrinth crisis. Prevention In a case of labyrinth crisis and disorientation, it is recommended to let some air bubbles and follow them to the surface. Avoid wearing ear-stoppers if you do not know how to use them. Otherwise, instead of isolating the external ear from contact with water, ear-stoppers might even cause eardrum rupture because of the impossibility to equalize.
Inner-Ear Decompression Sickness
It occurs as a result of rapid decompression. In the fluid of the inner ear (or labyrinth) are formed nitrogen bubbles which irritate the vestibular system and cause inner-ear decompression sickness. Its symptoms are similar to those of the labyrinth crisis.


Many people think hyperventilating gives the body more oxygen in order to hold ones breath longer. This is not entirely true. Over-hyperventilating only tricks the brain by making it think the body has more oxygen than in really does. The diver, feeling good, might decide to stay under water longer and might potentially suffer a sudden blackout. It is standard for most free divers to hyperventilate somewhat before a dive, however a good understanding of the physics and physiology of Free-Diving and Shallow Water Blackout is essential in order to practice safe diving.

Breathing is a process in which oxygen (O2) is inhaled and carbon dioxide (CO2) is exhaled. In a state of apnea, the release of carbon dioxide temporarily stops which results in the accumulation of carbon dioxide in the cells, blood and lungs. Simultaneously, carbon dioxide starts irritating the respiratory center, stimulating the need to breath. When carbon dioxide levels reach a certain threshold in the blood, the irritation becomes so unbearable that the person is not able to hold their breath anymore and discontinues the apnea. This irresistible will to exhale is called an impulse of breathing. The concentration of carbon dioxide in the blood, which forces the impulse of breathing is called the critical line. The critical line for each person cannot be strictly determined because of individual physiological differences. With training, practice, control and familiarity, the "critical line" or uncontrollable impulse to breath becomes longer as the body adapts to the new state of apnea. In summary, the impulse to breath is not due to the depletion of oxygen in the blood but the accumulation of carbon dioxide.

Hyperventilation is defined by excessive, rapid, and deep breathing. During hyperventilation, the body acquires large amounts of oxygen, which it can't immediately consume, while simultaneously decreasing the carbon dioxide levels in the blood. This high level of oxygen and low level of carbon dioxide is abnormal and might cause disturbances such as dizziness, nausea or convulsions. Most freedivers want to hyperventilate in order to raise their critical line - resulting in a longer "down" time. However, hyperventilation hides the potential danger of running out of oxygen and blacking out unsuspectingly underwater.

Swimming or moving actively under water increases the release of oxygen, which adds up to quick exhaustion of oxygen in the blood. At the same time, vigorous hyperventilation has led to a very low level of carbon dioxide in the blood. Therefore, as oxygen levels decrease to dangerously low levels in the blood, the build up of carbon dioxide will not reach the critical line and therefore not send any signals to the respiratory center warning the diver of the immanent danger of a blackout. In this case, the diver loses consciousness under water before he has any need to breathe (i.e. he cannot feel the decrease of oxygen in his blood). Such cases of drowning are even common among trained divers