Physical Processes of Respiratory Gas Exchange

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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

• Ficks 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)gt 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.

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Figure 48.3 Gas Exchange Systems
Anatomical adaptations to maximize the surface
area for gas diffusion (A in Ficks 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 Ficks 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.

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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.

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Figure 48.6 Countercurrent Exchange Is More
Efficient than Concurrent Exchange
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Figure 48.7 The Respiratory System of a Bird
(Part 1)
Air flows unidirectionaly
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Figure 48.8 The Path of Air Flow through Bird
Lungs (Part 1)
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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.

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Figure 48.9 Measuring Lung Ventilation
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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
  thin-walled 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.

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Figure 48.10 The Human Respiratory System (Part
2)
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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.

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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
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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.

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Figure 48.15 Breathing is Generated in the Brain
Stem
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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.

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Figure 48.17 Feedback Information Controls
Breathing