Every cell in the body must obtain energy to maintain its life, growth, and function.
Cells obtain their energy from oxidation, which is a slow, controlled burning
of food materials. Oxidation requires fuel and oxygen. Respiration is the process
of exchanging oxygen and carbon dioxide during oxidation and releasing energy
and water.
Few body cells are close enough to the surface to have any
chance of obtaining oxygen and expelling carbon dioxide by direct air diffusion.
Instead, the gas exchange takes place via the circulating blood. The blood is
exposed to air over a large diffusing surface as it passes through the lungs. When
the blood reaches the tissues, the small capillary vessels provide another large
surface where the blood and tissue fluids are in close contact. Gases diffuse readily
at both ends of the circuit and the blood has the remarkable ability to carry both
oxygen and carbon dioxide. This system normally works so well that even the
deepest cells of the body can obtain oxygen and get rid of excess carbon dioxide
almost as readily as if they were completely surrounded by air.
If the membrane surface in the lung, where blood and air come close together,
were just an exposed sheet of tissue like the skin, natural air currents would keep
fresh air in contact with it. Actually, this lung membrane surface is many times
larger than the skin area and is folded and compressed into the small space of the
lungs that are protected inside the bony cage of the chest. This makes it necessary
to continually move air in and out of the space. The process of breathing and the
exchange of gases in the lungs is referred to as ventilation and pulmonary gas
exchange, respectively.
Respiration Phases. The complete process of respiration includes six important
phases:
- Ventilation of the lungs with fresh air
- Exchange of gases between blood and air in lungs
- Transport of gases by blood
- Exchange of gases between blood and tissue fluids
- Exchange of gases between the tissue fluids and cells
- Use and production of gases by cells
If any one of the processes stops or is seriously hindered, the affected cells cannot
function normally or survive for any length of time. Brain tissue cells, for
example, stop working almost immediately and will either die or be permanently
injured in a few minutes if their oxygen supply is completely cut off.
The respiratory system is a complex of organs and structures that performs the
pulmonary ventilation of the body and the exchange of oxygen and carbon dioxide
between the ambient air and the blood circulating through the lungs. It also warms
the air passing into the body and assists in speech production by providing air to
the larynx and the vocal chords. The respiratory tract is divided into upper and
lower tracts.
The upper respiratory tract consists of the
nose, nasal cavity, frontal sinuses, maxillary sinuses, larynx, and trachea. The
upper respiratory tract carries air to and from the lungs and filters, moistens and
warms air during each inhalation.
The lower respiratory tract consists of the left and right bronchi and the lungs,
where the exchange of oxygen and carbon dioxide occurs during the respiratory
cycle. The bronchi divide into smaller bronchioles in the lungs, the bronchioles
divide into alveolar ducts, the ducts into alveolar sacs, and the sacs into alveoli.
The alveolar sacs and the alveoli present about 850 square feet of space for the
exchange of oxygen and carbon dioxide that occurs between the internal alveolar
surface and the tiny capillaries surrounding the external alveolar wall.
The mechanics of taking fresh air into the lungs
(inspiration or inhalation) and expelling used air from the lungs (expiration or
exhalation) is diagrammed in Figure 3-3. By elevating the ribs and lowering the
diaphragm, the volume of the lung is increased. Thus, according to Boyle’s Law, a
lower pressure is created within the lungs and fresh air rushes in to equalize this
lowered pressure. When the ribs are lowered again and the diaphragm rises to its
original position, a higher pressure is created within the lungs, expelling the used
air.
figure3-3 Inspiration Process. Inspiration involves both raising the rib cage (left panel) and lowering the
diaphragm (right panel). Both movements enlarge the volume of the thoracic cavity and draw air into the lung.
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The chest cavity does not have space between the outer lung
surfaces and the surrounding chest wall and diaphragm. Both surfaces are covered
by membranes; the visceral pleura covers the lung and the parietal pleura lines the
chest wall. These pleurae are separated from each other by a small amount of fluid
that acts as a lubricant to allow the membranes to slide freely over themselves as
the lungs expand and contract during respiration.
The lungs are a pair of light, spongy organs in the chest and are the
main component of the respiratory system (see Figure 3-4). The highly elastic
lungs are the main mechanism in the body for inspiring air from which oxygen is
extracted for the arterial blood system and for exhaling carbon dioxide dispersed
from the venous system. The lungs are composed of lobes that are smooth and
shiny on their surface. The lungs contain millions of small expandable air sacs
(alveoli) connected to air passages. These passages branch and rebranch like the
twigs of a tree. Air entering the main airways of the lungs gains access to the
entire surface of these alveoli. Each alveolus is lined with a thin membrane and is
surrounded by a network of very small vessels that make up the capillary bed of
the lungs. Most of the lung membrane has air on one side of it and blood on the
other; diffusion of gases takes place freely in either direction.
figure3-4 Inspiration Process. Inspiration involves both raising the rib cage (left panel) and lowering the
diaphragm (right panel). Both movements enlarge the volume of the thoracic cavity and draw air into the lung.
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Ventilation of the respiratory system
establishes the proper composition of gases in the alveoli for exchange with the
blood. The following definitions help in understanding respiration (Figure 3-5).
figure3-5 Inspiration Process. Inspiration involves both raising the rib cage (left panel) and lowering the
diaphragm (right panel). Both movements enlarge the volume of the thoracic cavity and draw air into the lung.
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The respiratory cycle is one complete breath consisting of an
inspiration and exhalation, including any pause between the movements.
Respiratory dead space refers to the part of the respiratory
system that has no alveoli, and in which little or no exchange of gas between
air and blood takes place. It normally amounts to less than 0.2 liter. Air occupying
the dead space at the end of expiration is rebreathed in the following inspiration.
Parts of a diver’s breathing apparatus can add to the volume of the dead space and
thus reduce the proportion of the tidal volume that serves the purpose of respiration.
To compensate, the diver must increase his tidal volume. The problem can
best be visualized by using a breathing tube as an example. If the tube contains one
liter of air, a normal exhalation of about one liter will leave the tube filled with
used air from the lungs. At inhalation, the used air will be drawn right back into
the lungs. The tidal volume must be increased by more than a liter to draw in the
needed fresh supply, because any fresh air is diluted by the air in the dead space.
Thus, the air that is taken into the lungs (inspired air) is a mixture of fresh and
dead space gases.
The number of complete respiratory cycles that take place in 1
minute is the respiratory rate. An adult at rest normally has a respiratory rate of
approximately 12 to 16 breaths per minute.
The total lung capacity (TLC) is the total volume of air that
the lungs can hold when filled to capacity. TLC is normally between five and six
liters.
Vital capacity is the volume of air that can be expelled from the
lungs after a full inspiration. The average vital capacity is between four and five
liters.
Tidal volume is the volume of air moved in or out of the lungs
during a single normal respiratory cycle. The tidal volume generally averages
about one-half liter for an adult at rest. Tidal volume increases considerably during
physical exertion, and cannot exceed the vital capacity.
The respiratory minute volume (RMV) is the total
amount of air moved in or out of the lungs in a minute. The respiratory minute
volume is calculated by multiplying the tidal volume by the rate. RMV varies
greatly with the body’s activity. It is about 6 to 10 liters per minute at complete rest
and may be over 100 liters per minute during severe work.
The maximal
breathing capacity (MBC) and maximum ventilatory volume (MVV) are the
greatest respiratory minute volumes that a person can produce during a short
period of extremely forceful breathing. In a healthy young man, they may average
as much as 180 liters per minute (the range is 140 to 240 liters per minute).
The maximum
inspiratory flow rate (MIFR) and maximum expiratory flow rate (MEFR)
are the fastest rates at which the body can move gases in and out of the lungs.
These rates are important in designing breathing equipment and computing gas
use under various workloads. Flow rates are usually expressed in liters per second.
Respiratory quotient (RQ) is the ratio of the amount of
carbon dioxide produced to the amount of oxygen consumed during cellular
processes per unit time. This value ranges from 0.7 to 1.0 depending on diet and
physical exertion and is usually assumed to be 0.9 for calculations. This ratio is
significant when calculating the amount of carbon dioxide produced as oxygen is
used at various workloads while using a closed-circuit breathing apparatus. The
duration of the carbon dioxide absorbent canister can then be compared to the
duration of the oxygen supply.
Within the alveolar air spaces, the composition
of the air (alveolar air) is changed by the elimination of carbon dioxide from the
blood, the absorption of oxygen by the blood, and the addition of water vapor. The
air that is exhaled is a mixture of alveolar air and the inspired air that remained in
the dead space.
The blood in the capillary bed of the lungs is exposed to the gas pressures of alveolar
air through the thin membranes of the air sacs and the capillary walls. With
this exposure taking place over a vast surface area, the gas pressure of the blood
leaving the lungs is approximately equal to that present in alveolar air.
When arterial blood passes through the capillary network surrounding the cells in
the body tissues it is exposed to and equalizes with the gas pressure of the tissues.
Some of the blood’s oxygen is absorbed by the cells and carbon dioxide is picked
up from these cells. When the blood returns to the pulmonary capillaries and is
exposed to the alveolar air, the partial pressures of gases between the blood and
the alveolar air is again equalized.
Carbon dioxide diffuses from the blood into the alveolar air, lowering its pressure,
and oxygen is absorbed by the blood from the alveolar air, increasing its pressure.
With each complete round of circulation, the blood is the medium through which
this process of gas exchange occurs. Each cycle normally requires approximately
20 seconds.
The amount of oxygen consumed and carbon dioxide
produced increases markedly when a diver is working. The amount of blood
pumped through the tissues and the lungs per minute increases in proportion to the
rate at which these gases must be transported. As a result, more oxygen is taken up
from the alveolar air and more carbon dioxide is delivered to the lungs for
disposal. To maintain proper blood levels, the respiratory minute volume must
also change in proportion to oxygen consumption and carbon dioxide output.
Changes in the partial pressure (concentration) of oxygen and carbon dioxide
(ppO2 and ppCO2) in the arterial circulation activate central and peripheral
chemoreceptors. These chemoreceptors are attached to important arteries. The
most important are the carotid bodies in the neck and aortic bodies near the heart.
The chemoreceptor in the carotid artery is activated by the ppCO2 in the blood and
signals the respiratory center in the brain stem to increase or decrease respiration.
The chemoreceptor in the aorta causes the aortic body reflex. This is a normal
chemical reflex initiated by decreased oxygen concentration and increased carbon
dioxide concentration in the blood. These changes result in nerve impulses that
increase respiratory activity. Low oxygen tension alone does not increase
breathing markedly until dangerous levels are reached. The part played by
chemoreceptors is evident in normal processes such as breathholding.
As a result of the regulatory process and the adjustments they cause, the blood
leaving the lungs usually has about the same oxygen and carbon dioxide levels
during work that it did at rest. The maximum pumping capacity of the heart (blood
circulation) and respiratory system (ventilation) largely determines the amount of
work a person can do.
A diver’s oxygen consumption is an important factor
when determining how long breathing gas will last, the ventilation rates required
to maintain proper helmet oxygen level, and the length of time a canister will
absorb carbon dioxide. Oxygen consumption is a measure of energy expenditure
and is closely linked to the respiratory processes of ventilation and carbon dioxide
production.
Oxygen consumption is measured in liters per minute (l/min) at Standard Temperature
(0°C, 32°F) and Pressure (14.7 psia, 1 ata), Dry Gas (STPD). These rates of
oxygen consumption are not depth dependent. This means that a fully charged MK
16 oxygen bottle containing 360 standard liters (3.96 scf) of usable gas will last
225 minutes at an oxygen consumption rate of 1.6 liters per minute at any depth,
provided no gas leaks from the rig.
Minute ventilation, or respiratory minute volume (RMV), is measured at BTPS
(body temperature 37°C/98.6°F, ambient barometric pressure, saturated with water
vapor at body temperature) and varies depending on a person’s activity level, as
shown in Figure 3-6. Surface RMV can be approximated by multiplying the
oxygen consumption rate by 25. Although this 25:1 ratio decreases with increasing
gas density and high inhaled oxygen concentrations, it is a good rule-of-thumb
approximation for computing how long the breathing gas will last.
Unlike oxygen consumption, the amount of gas exhaled by the lungs is depth
dependent. At the surface, a diver swimming at 0.5 knot exhales 20 l/min of gas. A
scuba cylinder containing 71.2 standard cubic feet (scf) of air (approximately
2,000 standard liters) lasts approximately 100 minutes. At 33 fsw, the diver still
exhales 20 l/min at BTPS, but the gas is twice as dense; thus, the exhalation would
be approximately 40 standard l/min and the cylinder would last only half as long,
or 50 minutes. At three atmospheres, the same cylinder would last only one-third
as long as at the surface.
Carbon dioxide production depends only on the level of exertion and can be
assumed to be independent of depth. Carbon dioxide production and RQ are used
to compute ventilation rates for chambers and free-flow diving helmets. These
factors may also be used to determine whether the oxygen supply or the duration
of the CO2 absorbent will limit a diver’s time in a closed or semi-closed system.
figure3-6 Oxygen Consumption and RMV at Different Work Rates.
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