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Physical Processes of Respiratory Gas Exchange
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Adequate ATP production for nutrient molecule oxidation in cells requires oxygen from the environment.
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Oxidative metabolism (cellular respiration) in cells produces carbon dioxide as an end-product, which must
be lost to the environment to prevent toxic effects.
Air is a better respiratory medium than water
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It is easier to obtain O2 from air than from water.
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The O2 content in air is about 20 times higher than in water.
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O2 diffuses 8,000 times more rapidly in air.
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Breathing air, which is less dense and less viscous than water, requires less work for an animal.
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Slow molecular diffusion of O2 is a problem for both water and air breathers because O2 must diffuse through
the aqueous cytoplasm of the cell to reach the machinery for O2 use (mitochondria).
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Diffusion of O2 in water is so slow that even cells with low metabolism must be only 1–2 mm from the O2
source.
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Animals that have no internal transport of O2 are either severely limited in size or have evolved bodies that
are flat, hollow, or have specialized structures. (See Figure 48.1.)
High temperatures create respiratory problems for aquatic animals
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Most water breathers have body temperatures close to the water temperatures surrounding them (they are
ectothermic).
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As temperatures rise, so does the animal’s metabolism and the need for more O 2.
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However, warm water holds less dissolved O2 than cold water. (See Figure 48.2.)
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Therefore, more energy must be expended to get a decreasing O2 supply.
O2 availability decreases with altitude
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The decrease in O2 as altitude increases depends on barometric pressure.
Air has 20.9 percent O2.
At sea level, the partial pressure of O2 (PO2) is about 159 mm Hg.
At the top of Mt. Everest, the P O2 is only about 50 mm Hg.
The diffusion rate of O2 into cells under reduced PO2 is greatly reduced, and O2 uptake is constrained.
Humans venturing to great heights must breathe O2 from pressurized containers.
Carbon dioxide is lost by diffusion
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CO2 diffuses out of the body at about the same rate as O2 diffuses in.
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The partial pressures of the gases are not the same because the amount of CO 2 in the atmosphere is low (0.03
percent).
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This usually means there is a good partial pressure gradient for loss of CO 2 from air-breathing animals.
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For water-breathing animals, a surrounding environment high in decaying organic material (generating high
levels of CO2) may be unable to support life.
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For both types of breathers, transport of CO2 out of the cells and into the environment is a crucial limiting
factor.
Fick’s law applies to all systems of gas exchange
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Fick’s law of diffusion summarizes the rate at which a substance such as O 2 diffuses between two locations.
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Fick’s law can be expressed as an equation: Q = DA (P1  P2/L)
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Q is the rate at which a substance diffuses between two locations.
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D is the diffusion coefficient; it depends on the diffusing substance, the medium, and the temperature.
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A is the cross-sectional area over which the substance is diffusing.
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P1 and P2 are the partial pressures of the gas at two locations.
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L is the distance between these locations.
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Animals maximize the diffusion coefficient by using air rather than water for diffusion whenever possible.
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Other adaptations for maximizing respiratory gas exchange must influence the surface area for exchange (A)
or the partial pressure gradient across that surface area [(P1 – P2)/L].
Adaptations for Respiratory Gas Exchange
Respiratory organs have large surface areas
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Anatomical adaptations to maximize the surface area for diffusion (A in Fick’s law) include external and
internal gills and lungs. (See Figure 48.3.)
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External gills are branched and folded thin membranes on the body surface that provide a larger diffusion
area.
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Internal gills are similar to external gills but are better protected from damage by their location inside body
cavities.
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Lungs are internal cavities for gas exchange in air breathers.
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Lungs are highly divided to provide greater surface area and are elastic to permit inflation and deflation.
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Insects have a unique system for gas exchange called tracheae, which are branched tubules.
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The tracheae penetrate through fine terminal branches to tissues and cells, presenting an enormous surface
area for gas exchange.
Transporting gases to and from the exchange surfaces optimizes partial
pressure gradients
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Driving diffusion of gases across gas exchange membranes thus maximizing the partial pressure gradients (P1
– P2)/L is accomplished in several ways:
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Thin membranes shorten the diffusion path.
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Ventilation of the environmental side of the gas exchange area brings in fresh air with the highest possible
partial pressure of O2 and the lowest possible partial pressure of CO2.
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Perfusion of the internal side of the gas exchange area by the circulatory system helps maintain the lowest
possible partial pressure of O2 and the highest possible partial pressure of CO2.
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The animal’s gas exchange system is made up of its gas exchange surfaces and the mechanisms it uses to
ventilate and perfuse those surfaces.
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Insect tracheae:
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The tracheae of insects are a system that delivers air to the deep recesses of the body.
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This system of air tubes begins at openings on the outside of the body called spiracles, which admit air. (See
Figure 48.4.)
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By an ever-finer branching of air tubes, O2 is delivered to air capillaries not more than a few micrometers
away from cell mitochondria.
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Since O2 diffuses at a higher rate in air than in water, this system ensures an abundant supply for high
metabolism.
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However, small diameter and total length of these dead-end airways limits body size.
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Some insects that stay under water for extended periods carry a bubble of air with them.
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Even when the O2 lowers in the bubble, more diffuses in from the water, giving them nearly limitless time
underwater.
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Fish gills:
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Gills in fish are constructed to enable water to pass into the mouth, over the gills and out the opercular flaps.
(See Figure 48.5.)
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This constant water flow over the gills maximizes the PO2 on the external surfaces.
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Blood flow on the internal side minimizes the P O2 by sweeping O2 away as rapidly as it diffuses across the
gill.
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Highly divided, the gills present enormous surface area for gas exchange.
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The subunits of gill filaments and lamellae minimize the path length for diffusion.
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The perfusing blood flow on the inner surface of the lamellae is unidirectional.
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Afferent (to gills) and efferent (away from gills) blood vessels ensure a countercurrent (opposite direction)
flow to maximize the PO2 gradient. (See Figure 48.5.)
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Some fish, such as sharks and tuna, swim almost constantly with mouths open to ventilate their gills.
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Most fish use a two-pump mechanism activated by opening and closing the mouth to push water over the
gills.
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Bird lungs:
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Birds can sustain high activity levels much longer and at higher altitudes than mammals can.
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This ability is achieved through a unique structure allowing air to flow unidirectionally through the lungs
rather than in and out via the same airway, as in mammals. (See Figure 48.8 and Animated Tutorial 48.1.)
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Birds also have air sacs in the body and air spaces within the wings to hold air but not exchange it. (See
Figure 48.7.)
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In most air-breathing vertebrates, air enters the trachea, and then passes to the bronchi, to smaller
bronchioles, and finally to dead-end membranous air sacs where gases are exchanged.
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In birds, the trachea leads to air sacs, then to parabronchi connected by air capillaries in the lungs where gas
exchange occurs.
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Air next passes to bronchi and to other air sacs, which then vent back to the trachea, bypassing the lungs.
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This arrangement ensures a unidirectional flow of fresh air through the lungs.
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Tidal breathing in humans:
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In mammal lungs, ventilation is tidal: air flows in and out by the same route.
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Measurements of lung capacity:
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At rest, the amount of air that moves in and out is the tidal volume.
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The additional volume of air that we can take in by inhaling deeply is the inspiratory reserve volume.
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The additional volume we can exhale is the expiratory reserve volume.
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The total of these three volumes in the vital capacity.
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Even with forceful breathing, there is residual volume keeping the lungs from collapsing. Some of this exists
in what is called the anatomical dead space—airways in which gas exchange cannot occur.
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The total lung capacity is the sum of the residual volume and the vital capacity. (See Figure 48.9.)
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Tidal breathing severely limits the P O2 gradient because fresh air is not moving into the lungs during half of
the respiratory cycle.
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The incoming air also must mix with the stale air remaining in the lung.
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The volume of this stale air is the sum of the residual volume and, depending on how deeply one is breathing,
some or all of the expiratory reserve volume.
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Tidal breathing also reduces gas exchange efficiency by not permitting countercurrent gas exchange between
air and blood.
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To offset the inefficiencies of tidal breathing, mammalian lungs have an enormous surface area and a very
short path length for diffusion.
Gas Exchange in Human Lungs
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The air pathway in humans consists of the following components:
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An oral or nasal cavity, followed by the pharynx (an area for both food and air)
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The larynx (voice box), which leads to the trachea
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The trachea, which branches into two bronchi (both of these have cartilage support)
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The bronchi, which branch repeatedly into bronchioles
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The bronchioles, which terminate in the alveoli
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The alveoli, which are thin-walled air sacs and are the sites of gas exchange
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There are about 300 million alveoli in human lungs. (See Figure 48.10.)
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The proliferation of alveoli gives a gas exchange surface area one-fourth the size of a basketball court.
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Capillary blood vessels closely surround the air sacs, resulting in a diffusion path of less than 2 m, which is
less than the diameter of a red blood cell.
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(See Video 48.1.)
Respiratory tract secretions aid ventilation
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Two adaptations that aid the breathing process in mammals are mucus production and surfactant production.
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Mucus production:
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Cells lining the airways produce a sticky mucus that captures dirt and microbes that might be inhaled.
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This mucus is cleared by cilia beating upward toward the trachea and pharynx, where it is swallowed.
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This phenomenon has been called the mucus escalator, and it can be immobilized by smoking.
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In the genetic disease cystic fibrosis, a faulty chloride channel leads to mucus that is dehydrated, thick, and
difficult to clear, resulting in blockage and infection.
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Surfactant production:
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A surfactant is a chemical substance that reduces the surface tension of a liquid.
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The aqueous lining of the lung has surface tension that must be overcome to permit inflation.
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Cells in the alveoli produce surfactant molecules when they are stretched.
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Premature babies may develop respiratory stress syndrome if they are born before cells in the alveoli are
producing surfactant.
Lungs are ventilated by pressure changes in the thoracic cavity
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The human lungs are suspended in the thoracic cavity.
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This cavity is bounded by the shoulder girdle at the top, the rib cage on the sides, and the diaphragm muscle
on the bottom. (See Figure 48.10.)
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The lungs are positioned in separate, closed pleural cavities.
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Breathing involves changes in volume of the thoracic cavity.
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An increase in its volume creates negative pressure (suction) inside the pleural cavity.
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Even between breaths, there is a slight negative pressure inside the pleural cavity keeping the alveoli partially
inflated. If the thoracic cavity is punctured, air leaks into the pleural cavity, the lung collapses, and ventilation of the
alveoli in the lung ceases.
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With inhalation, the diaphragm muscle contracts downward to cause suction, and air flows into the lung. (See
Figure 48.11 and Animated Tutorial 48.2.)
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exhaled.
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When the diaphragm stops contracting and relaxes, pushing upward, exhalation occurs.
The diaphragm is not the only muscle that changes the volume of the thoracic cavity.
In the rib cage, intercostal muscles lift the ribs up and down to increase thoracic cavity volume.
External intercostal muscles expand the thoracic cavity and increase the volume of air inhaled.
Internal intercostal muscles decrease the volume of the thoracic cavity and increase the amount of air
This system is particularly important during strenuous exercise.
Blood Transport of Respiratory Gases
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Remember that ventilation and perfusion work together. Ventilation delivers O 2 to the environmental side of
the exchange surface, where it diffuses into the body and is swept away by perfusion; perfusion delivers CO2 to the
exchange surface, where it diffuses out and is swept away by ventilation.
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Perfusion of the lungs is one of the functions of the circulatory system, which transports respiratory gases
through a network of blood vessels powered by a pump (the heart).
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As O2 diffuses from the alveoli into the blood, it is swept away and delivered to the cells and tissues of the
body.
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Blood plasma (the liquid part of blood) carries only a small portion of O 2 in solution.
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It is the red blood cell with its oxygen-binding pigment, hemoglobin, that is the pack-horse for oxygen
transport.
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Hemoglobin has 60 times the capacity of plasma to transport O2.
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(See Video 48.2.)
Hemoglobin combines reversibly with oxygen
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Hemoglobin is a protein consisting of four polypeptide subunits. Each polypeptide surrounds a heme (ironcontaining) group. (See Figure 3.8.)
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Each heme group can reversibly bind a molecule of O2.
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As diffusion of O2 into the blood occurs, it binds to hemoglobin, increasing the PO2 gradient and driving O2
into red blood cells.
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When the PO2 of blood plasma is high, as in the lung capillaries, each hemoglobin complex can carry four
molecules of O2.
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As the red blood cell circulates to the body, the P O2 values drop, and the hemoglobin releases some of the O2
it is carrying.
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The relationship between saturation of the hemoglobin polypeptides and P O2 values is complicated, and
follows a sigmoidal curve. (See Figure 48.12.)
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The O2 binding by hemoglobin polypeptides is influenced by positive cooperativity; that is, binding the first
molecule makes the second binding easier, and so on.
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It takes a relatively greater PO2 to achieve 100 percent saturation of all four polypeptides.
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Carbon monoxide (CO) binds to hemoglobin with a much higher affinity than does O2.
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Therefore, CO is a deadly poison, as it destroys the ability of hemoglobin to transport and release O 2 to body
tissues.
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When blood circulates through the body, it releases, on average, only one of the four molecules of O2 it
carries.
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While this seems inefficient, it is really adaptive because the hemoglobin keeps a 75 percent O 2 reserve for
peak demands.
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If a tissue is starved for O2 and its local PO2 falls below 40 mm Hg, the hemoglobin will release the reserved
O2 to the starved tissue.
Myoglobin holds an O2 reserve
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Myoglobin in muscle cells is an oxygen-binding molecule that can take up one molecule of O2.
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It has a higher affinity for O2 than hemoglobin does and provides an oxygen reserve for high metabolic
demand or when blood flow is interrupted. (See Figure 48.13.)
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Diving mammals such as seals have high concentrations of myoglobin in their muscles, allowing them to stay
under water for long periods using their reserves.
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Even in nondiving animals, muscles called on for extended periods of work frequently have more myoglobin
than muscles used for short, intermittent periods do.
The affinity of hemoglobin for O2 is variable
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Various factors influence the oxygen-binding properties of hemoglobin, including the chemical composition
of the hemoglobin, pH, and the presence of 2,3 bisphosphoglyceric acid.
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Hemoglobin composition:
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Hemoglobin of normal human adults consists of two kinds of polypeptide chains: -globin and -globin.
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Before birth, human fetuses have two -globin chains and two -globin chains.
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The -globin chains result in a greater affinity for O2 in fetal hemoglobin. (See Figure 48.13.)
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This difference in O2 affinity facilitates transfer of O2 from the mother’s blood to fetal blood in the placenta.
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Mammals such as llamas, which live at high altitudes, have hemoglobin that becomes saturated with O 2 at
lower PO2 values than that of other animals.
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pH:
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The influence of pH on the function of hemoglobin is known as the Bohr effect.
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This effect holds when the pH of the blood falls and the oxygen-binding curve shifts to the right.
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The hemoglobin will then release more O2 to the tissues.
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2,3 Bisphosphoglyceric acid:
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Another regulator of hemoglobin function is 2,3 bisphosphoglyceric acid (BPG), a metabolite of glycolysis.
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Mammalian red blood cells have a high concentration of BPG, which combines with deoxygenated
hemoglobin and causes it to have a lower affinity for O2.
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The result is that the hemoglobin releases more of its bound O2 to tissues than usual.
CO2 is transported as bicarbonate ions in the blood
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Carbon dioxide (CO2), a metabolic waste, must be taken away from living tissues by the blood.
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CO2 is highly soluble, moving easily through cell membranes into the blood, where the partial pressure of
CO2 is lower.
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However, most CO2 is transported via blood to the lungs as bicarbonate ion (HCO3–). (See Figure 48.14.)
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The endothelial cells of capillaries and the red blood cells produce carbonic anhydrase enzyme, which speeds
the conversion of CO2 to H2CO3 (carbonic acid).
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The H2CO3 dissociates, and bicarbonate ions enter the plasma in exchange for chloride ions (Cl –).
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This conversion reduces the partial pressure of CO2 in these cells and in plasma, promoting the diffusion of
CO2 out of the tissue cells.
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Although most CO2 goes to the lung as bicarbonate, some CO2 is also carried in chemical combination with
hemoglobin.
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In the lung, the CO2 and bicarbonate reactions are reversed.
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Carbonic anhydrase speeds the conversion of bicarbonate to water and CO2.
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CO2 diffuses from the blood plasma into the alveolar air and is exhaled.
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As the partial pressure of CO2 in the blood falls, more bicarbonate is converted into CO2.
Regulation of Breathing
Breathing is controlled in the brain stem
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The autonomic nervous system maintains breathing and modifies depth and frequency to meet body demands.
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The brain stem generates and controls the breathing rhythm. (See Figure 48.15.)
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Groups of neurons within the medulla increase their firing rate just prior to inhalation.
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With increased firing, the diaphragm contracts and inhalation occurs.
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Suddenly, the firing stops, the diaphragm relaxes, and exhalation occurs.
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Exhalation is actually a passive elastic recoil of lung tissue.
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When breathing demands are high, such as during exercise, the motor neurons for the intercostal muscles are
fired to increase inhale/exhale volumes.
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Brain areas above the medulla modify breathing to allow speech, eating, coughing, and emotional states.
Regulating breathing requires feedback information
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Sensors measuring the PO2 and PCO2 in the blood provide feedback to regulate the breathing rhythm.
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In humans and other mammals, CO2 sensitivity is very high, but O2 sensitivity is remarkably low. (See Figure
42.16.)
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CO2 sensors are located on the medulla surface near the neurons that generate the breathing rhythm. (See
Figure 48.17.)
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O2 sensors also reside in tissue nodes on the aorta and carotid arteries.