ABSTRACT
This page describes human physiology from the perspective of scuba
diving. Most affected by scuba diving is the cardiovascular
(or circulatory) system and respiratory (or pulmonary) system, the ears
and sinuses. This page gives an introduction in scuba diving physiology
and describes these systems. Special attention is paid to the heart, the
fossa ovalis, patent foramen ovale (pfo), the lungs, breathing, alveoli,
ears, sinuses and equalizing.
The cardiovascular (or circulatory) system and respiratory system are two systems in the human body that are extremely vital: they continuously provide the body with a flow of nutrients (from the digestive system) and dissolved gases and take care of removal of waste products of the body. Disfunctioning of these systems result in permanent damage of the body within minutes. Of all bodily systems these two systems are the most influenced by the underwater environment when diving.
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Every part of the human body constantly needs Oxygen. Oxygen is needed for what is called the oxidative metabolism: the oxidation of nutrients in order to supply energy needed for living. All body cells are involved and need Oxygen. Especially the brain is extremely sensitive to Oxygen deficit. After 2-6 minutes of no Oxygen the brain is permanently damaged. Carbon Dioxide originating as waste product from the metabolism has to be removed. The cardiovascular system takes care of the circulation of blood and transport of nutrients, Oxygen and Carbon Dioxide. It mainly consists of the heart, arteries, veins and capillaries. |
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The heart is a hollow muscular organ that is situated in the mediastinum, the space in the chest between the lungs. The heart lies behind the sternum (breast bone). Two third of the heart lies at the left side of the sternum, one third to the right side. The heart is a pump that beats about 70 times per minute in the average adult. At this rate the heart pumps 6 liters blood per minute at rest. This amount doubles or even triples depending on the amount of work its owner performs. Actually the heart is made up of two pumps: the right side and the left side of the heart. The left side pumps Oxygen rich blood from the lungs (pulmonary circuit) into the body (systemic circuit). The right side pumps Oxygen poor blood from the body into the lungs. Each pump consists of two chambers: the atrium at which the blood enters the heart and the ventricle from which the blood leaves the heart. |
Figure 1: The heart |
Figure 2: Opened left ventricle: the bicuspid valve or mitral valve |
Blood can flow from the atrium into the ventricle through the one-way atrioventricular valve. The valve is made up of flaps called leaflets or cusps. The right side of the heart contains a valve that has three cusps and hence it is named tricuspid valve. The left side of the heart contains a valve that has two cusps. This one is called the bicuspid valve. Since its shape resembles that of a bishop's hat, the bicuspid valve is also called mitral valve. Each atrium has a flap called auricle. The auricle acts as reservoir and expands when the atrium is filled with blood. (Usually, there is a bit of confusion about the words 'atrium' and 'auricle'. Often the atrium is called auricle. However, they are different parts of the heart). |
Figure 3: Opened right ventricle: the tricuspid valve |
Figure 4: Opened left side of the heart. |
In a fetus the lungs are not functioning. Most of the blood bypasses the lungs by a natural opening, the foramen ovale, between the left and right atrium. At birth, the pressure in the left atrium becomes slightly larger causing a flap valve closing over the opening. In the first year the opening is closed completely. The remnant structure of the valve is called the fossa ovalis. In about 30% of the population the foramen ovale is not completely closed. This is called Patent Foramen Ovale (PFO). However, because the pressure in the left atrium is slightly larger than the pressure in the right atrium, the flap valve remains closed, preventing blood flow. Under certain circumstances (forced respiration, the Valsalva maneuver, coughing, breath-holding) the pressure in the right atrium might increase, causing some blood flow from the right to the left atrium. Under normal circumstances this does not give rise to problems. In divers however, there is indication that silent gas bubbles flow from the right atrium into the left atrium, bypassing the lungs. This blood is deprived from gas exchange, increasing the risk for Decompression Sickness. The influence of PFO on the risk for Decompression Sickness is estimated to be not very large. Exact numbers are not given, however. With respect to other risk factors like deco-diving or diving below 30 m we can neglect the influence of PFO. |
Figure 5: Opened right atrium |
The heart is surrounded by a thin connective tissue sac, the pericardium. The pericardium is made up of two portions: the outer portion, the fibrous pericardium, is fairly tough and rigid. The inner portion, the serous pericardium, consists of two layers (the parietal and the visceral layer) enclosing the pericardial cavity. This cavity is filled with a lubricating film of fluid (pericaridal fluid). Due to this the heart can move independent from the surrounding tissue without friction.
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Oxygen-rich blood coming from the respiratory system (lungs) enters the heart on the left side (left atrium). The heart pumps this blood (via the left ventricle) into the aorta, the main artery in the body. Through a system of branches, the vascular tree, the blood reaches the capillaries everywhere in the body. Here transfer of nutrients and gases to bodily tissue (cells) takes place. Waste products and Carbon Dioxide coming from the cells is taken into the bloodstream. Capillaries gather into small veins, the venules. The venules branch together into larger veins, which transport the Oxygen-poor blood, eventually reaching the right side (right atrium) of the heart. This side pumps the blood (via the right ventricle) into the pulmonary trunk. This trunk branches into the pulmonary arteries that lead to the lungs. The lungs supply the blood with Oxygen again. From the lungs the Oxygen-enriched blood flows back to the left side (left atrium) of the heart through the pulmonary veins for another cycle. |
Figure 6: schematic circulatory system |
The respiratory system takes care of bringing Oxygen (O2) into the body and removing Carbon Dioxide (CO2) from the body. It mainly consists of the lungs and a series of air ducts.
When the reflex respiratory centers in the brain detect high Carbon Dioxide levels in the blood, they stimulate breathing. At rest the breathing rate is 10-20 breaths per minute. Due to increased Carbon Dioxide levels this rate increases, until the level returns to normal. Though peripheral chemoreceptors monitor Oxygen levels, it is mainly the (high) Carbon Dioxide level that triggers breathing. If Oxygen levels are to low but the respiratory centers do not detect over abundance of Carbon Dioxide it is possible that breathing is not stimulated. This is what happens in case of shallow water black out.
The stimulus from the reflex respiratory centers reaches the diaphragm. This is a large muscle between the chest cavity and the abdominal cavity. The muscle flexes downwards, increasing the chest cavity volume (i.e. lung volume). This decreases the pressure in the lungs, letting air flow in (thus equalizing the pressure in the lung and the ambient pressure). The air enters the body through the mouth and nose. It flows along the sinuses into the pharynx (common passage for air and food) until it reaches the epiglottis (valve between the trachea leading to the lungs and the esophagus leading to the stomach). From here the air flows through the larynx (voice box) into the trachea (windpipe). Under the sternum (breastbone) the trachea branches into the left and right main stem bronchus, leading to the left resp. right lung. The trachea and bronchi consist of a number of semicircular cartilaginous rings, which prevent the trachea and bronchi form collapsing. The right main stem bronchus is wider and shorter than the left one. In the lungs the bronchi keep branching into smaller air passages, the bronchioles, which finally branch into alveoli.
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Alveoli (alveolus comes from the Latin alveus, which mean cavity) are small air spaces (which shape resembling a bunch of grapes), surrounded by a net of small capillaries. These capillaries are separated from the alveoli only by thin gas-permeable membrane. This one cell thick membrane is called the alveolar-capillary membrane. Gases (Oxygen, Carbon Dioxide, Nitrogen) are exchanged through this membrane between the alveoli and the blood by a process called diffusion. Diffusion is flow from high concentration to low concentration. Diffusion stops if gas partial pressure on both sides are equal. |
Figure 8: electron microscopic view of the alveoli |
The branching in the lungs into numerous alveoli leads to a large contact area (typically 100 m2) between the alveoli (which are typically 100 micrometer in diameter) and the capillaries. However, only 10% of the Oxygen breathed in is used, resulting in 90% of the Oxygen leaving the body on breathing out.
The inner surface of the alveoli is covered with a substance called surfactant. This surfactant increases the surface tension of the alveoli, thereby reducing their tendency to collapse. If the surfactant were to dissolve, as may occur in near-drowning, alveoli will collapse. This may lead to less effective surface for gas-exchange and, in case of near-drowning, to secondary drowning.
Figure 9: electron microscopic view of the cilia |
The inner surface of the trachea and bronchi are lined with cells that have tiny hair-like bodies, called cilia. The cilia are covered with mucus produced by glands. The cilia act as an air filter. Foreign particles, like dust, are trapped. The mucus and particles are pushed upward by movement of the cilia. This mucus and particles end up in the pharynx where they are subsequently swallowed. Cilia are damaged or deactivated when exposed to nicotine. This result in 'smokers cough' (common to heavy smokers) in order to get rid of the mucus and foreign particles. Mucus buildup in the lungs may lead to gas trapping in the divers lungs. When diving, this may give rise to lung baro trauma. |
The lungs are composed of elastic tissue. Each lung is surrounded by two membranes that are called the pleura. The inner membrane covers the lung, the outer membrane covers the chest cavity. The space between the membranes is called the pleural cavity and is filled with a thin layer of fluid. Like the pericardial fluid in the heart, this pleural fluid acts as a lubricant allowing frictionless movement of the lungs during breathing.
The amount of air that is inhaled and exhaled during normal breathing is called tidal volume. The maximum volume that can be inhaled after complete exhalation is called vital volume. The amount of air that is left in the lungs after exhaling completely is called residual volume. Dead air space is the part of the tidal volume that does not take part in the exchange of gases: i.e. the air that does not reach the lungs (stays in the mouth, nose, sinuses, trachea and bronchi). On inhaling the first air that reaches the alveoli is exhaled air that remained in the dead air spaces at the end of previous exhalation. This is air with lower Oxygen level and higher Carbon Dioxide level. When a diver uses a regulator the dead air space is enlarged by the volume of the regulator second stage.
One of the parts of the body that are influenced by diving is the ear. Most of the diving injuries concern this organ. So let's have a closer look.
Figure 10: The ear |
Ear componentsFigure 10 shows the ear. The outer or external ear consists of the auricle (pinna) and the auditory canal leading to the tympanic membrane or eardrum. The air filled space behind the eardrum is called the middle ear. The eardrum forms a air and water tight barrier between the outer ear and the middle ear. In the middle ear three ossicles or ear bones are located: the malleus (hammer), the incus (anvil) and the stapes (stirrup). The inner ear consists of the cochlea and semicircular canals (labyrinth or vestibular endorgans). How the ear worksSound waves (in air or water) are captured by the outer ear, channeling them through the auditory canal to the eardrum. This vibration is amplified and transferred by the ear bones to the oval window of the cochlea. The oval window flexes inward and outward. The cochlea is filled with fluid called perilymph. The inner side of the cochlea is lined with hair-like bodies (hair cells) that move by the vibrating fluid. This movement is transferred into electrical signals that follow the auditory nerve into the brain. Exposure to loud (industrial) noise for long periods damages the hair-like bodies, leading to deafness. |
The inward/outward movement of the oval window is compensated by movement of the round window. Since fluid is incompressible a flexing inward of the oval window must be compensated by a flexing outward of the round window.
The semicircular canals are connected to the cochlea through the perilymphatic system. The canals do not play a role in the hearing process. However, the canals control balance and the sense of orientation. There are three canals, each of them is perpendicular (makes a right angle) to the other two. In this way each canal is sensitive to motion in one of the 3D directions. When moving the head fluid flows through one or more canals due to inertia of the fluid. This fluid movement is translated by the brain to sense of movement and orientation.
In both the Valsalva technique and Frenzel technique the diver blows air against a pinched nose. This relaxes the tissue surrounding the Eustachian tube and forces air through the tube simultaneously. In the Valsalva technique the diver uses his or her diaphragm, trying to breathe out against the blocked nose. In the Frenzel technique the diver uses the throat muscles to compress air against the blocked nose. The Frenzel method minimizes probability of round window rupture. Beginning divers usually start of by learning the Valsalva technique, because the Frenzel technique takes more time to master. Experienced divers however, begin using the Frenzel technique most unconsciously.
Though I assume the diver knows how to equalize, problems with these techniques may originate from the following causes:
Problems with equalizing may result in middle ear squeezes.
On ascending expanding air in the middle ear opens the Eustachian tube and flows into the nasal cavity. Usually this produces soft rapid 'popping' sounds. Usually the diver has to take no action with respect to equalizing the ears on ascending.
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The sinuses are air filled cavities in the head, located in the forehead and under the cheek bones. There are four sets of sinuses: maxillary, ethmoid, frontal and sphenoid sinuses. Sinuses are lined with tissue called mucosa. Glands in this tissue produce mucus. Sinuses are equalized together with the middle ear. Problems may occur if sinuses are congested as may occur in a head cold or an allergy. Inflammation of the sinuses is called sinusitis. |
Figure 12: The sinuses |
Figure 13: The sinuses |
