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48
Physical Processes of Respiratory Gas Exchange
• The respiratory gases are oxygen (O2 to make
ATP) and carbon dioxide (CO2).
• Diffusion is the only means to exchange these
gases.
• The O2 content in air is about 20 times higher
than in water.
• O2 diffuses 8,000 times more rapidly in air.
• Animals that have no internal transport of O2 are
either severely limited in size or have evolved
bodies that are flattened or built around a central
cavity.
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Physical Processes of Respiratory Gas Exchange
• Fick’s law of diffusion:
Q = DA (P1 - P2/L)
 Q is the rate at which a substance diffuses between two
locations.
 D is the diffusion coefficient.
 A is the cross-sectional area over which the substance is
diffusing.
 P1 and P2 are the partial pressures of the gas at two
locations.
 L is the distance between these locations.
• Diffusion depends on Partial pressure (p) of the gases, Area, and
Diffusion length
• In Atmosphere - pOxygen (21%)> pCarbon dioxide (.03%)
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Physical Processes of Respiratory Gas Exchange
• Animals maximize the diffusion coefficient by
using air rather than water for diffusion whenever
possible.
• 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].
Figure 48.3 Gas Exchange Systems
Anatomical adaptations to maximize the surface area for gas diffusion
(A in Fick’s law) include external and internal gills and lungs
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Adaptations for Respiratory Gas Exchange
• Driving diffusion of gases across gas exchange
membranes (i.e., maximizing the partial pressure
gradients—(P1 – P2)/L in Fick’s law) is
accomplished in several ways:
• Thin membranes shorten the diffusion path (L).
• Ventilation brings in fresh air with the high PO2
and the low PCO2.
• Perfusion by the circulatory system helps
maintain the low PO2 and the high PCO2 on the
inside of exchange surfaces.
Figure 48.5 Fish Gills (Part 1)
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Adaptations for Respiratory Gas Exchange
• The perfusing blood flow on the inner surface of
the lamellae is unidirectional.
• Afferent (to gills) and efferent (away from gills)
blood vessels ensure a countercurrent flow to
maximize the PO2 gradient.
Figure 48.6 Countercurrent Exchange Is More Efficient than Concurrent Exchange
Figure 48.7 The Respiratory System of a Bird (Part 1)
Air flows unidirectionaly
Figure 48.8 The Path of Air Flow through Bird Lungs (Part 1)
Figure 48.8 The Path of Air Flow through Bird Lungs (Part 2)
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Adaptations for Respiratory Gas Exchange
• In mammal lungs, ventilation is tidal: Air flows in
and out by the same route.
• At rest, the amount of air exchanged is the tidal
volume.
• The additional volume of air taken in by inhaling
deeply is the inspiratory reserve volume.
• The additional volume we can exhale is the
expiratory reserve volume.
• The total of these three volumes in the vital
capacity.
Figure 48.9 Measuring Lung Ventilation
Figure 48.10 The Human Respiratory System (Part 1)
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Gas Exchange in Human Lungs
• The Bronchioles end in the alveoli which are thinwalled air sacs and are the sites of gas exchange.
• Capillary blood vessels closely surround the
alveoli, resulting in a diffusion path of less than 2
mm, which is less than the diameter of a red blood
cell.
Figure 48.10 The Human Respiratory System (Part 2)
Figure 48.10 The Human Respiratory System (Part 3)
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Gas Exchange in Human Lungs
• Two adaptations that aid the breathing process in
mammals are mucus and surfactants.
• Cells lining the airways produce a sticky mucus
that captures dirt and microbes.
• This mucus is cleared by cilia beating upward
toward the trachea and pharynx, where it is
swallowed.
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Gas Exchange in Human Lungs
• A surfactant is a chemical substance that reduces
the surface tension of a liquid.
• The aqueous lining of the lung has surface
tension that must be overcome to permit inflation.
• Cells in the alveoli produce surfactant molecules
when they are stretched.
• Premature babies may develop respiratory stress
syndrome if they are born before cells in the
alveoli are producing surfactant.
Figure 48.11 Into the Lungs and Out Again
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Blood Transport of Respiratory Gases
• Ventilation and perfusion work together.
Ventilation delivers O2 to the environmental side
of the exchange surface; perfusion delivers CO2
to the exchange surface, where it diffuses out and
is swept away by ventilation.
• As O2 diffuses from the alveoli into the blood, it is
swept away and delivered to the cells and tissues
of the body.
• Most O2 is carried by the oxygen-binding pigment,
hemoglobin, in red blood cells.
• Hemoglobin has 60 times the capacity of plasma
to transport O2.
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Figure 48.12 The Binding of O2 to Hemoglobin Depends On PO2
Figure 48.13 Oxygen-Binding Adaptations (Part 1)
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Blood Transport of Respiratory Gases
• The influence of pH on the
function of hemoglobin is
known as the Bohr effect.
• This effect occurs when the pH
of the blood falls and the H+
ions bind to hemoglobin and
decrease its affinity for O2.
• The oxygen-binding curve shifts
to the right.
• The hemoglobin will then
release more O2 to the tissues
where pH is low.
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Blood Transport of Respiratory Gases
• Another regulator of hemoglobin function is 2,3
bisphosphoglyceric acid (BPG).
• In red blood cells BPG combines with
deoxygenated hemoglobin and causes it to have
a lower affinity for O2.
• The result is that the hemoglobin releases more of
its bound O2 to tissues than usual.
• If a person goes to a high altitude or starts
exercising, the level of BPG goes up, and
hemoglobin releases more O2 where it is needed.
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Regulation of Breathing
• Breathing is controlled by the autonomic nervous system.
• The brain stem generates and controls the breathing rhythm.
• Groups of neurons within the medulla increase their firing rate just prior
to inhalation.
• With increased firing, the diaphragm contracts and inhalation occurs.
• When the firing stops, the diaphragm relaxes, and exhalation occurs.
• Exhalation is actually a passive elastic recoil of lung tissue. When
breathing demands are high, as during exercise, the motor neurons for
the intercostal muscles are fired to increase inhalation and exhalation
volumes.
• Brain areas above the medulla (Pons) modify breathing to allow speech,
eating, coughing, and emotional states.
Figure 48.15 Breathing is Generated in the Brain Stem
Figure 48.16 Carbon Dioxide Affects Breathing Rate
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Regulation of Breathing
• CO2 sensors (monitor pH high CO2-Low pH) are
located on the medulla surface near the neurons
that generate the breathing rhythm.
• However O2 sensors are also in tissue nodes on
the aorta and carotid arteries called carotid and
aortic bodies.
• If PO2 of blood drops, or if blood pressure drops,
chemoreceptors in the bodies send nerve
impulses to the brain breathing center.
Figure 48.17 Feedback Information Controls Breathing