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44
Gas Exchange
Lecture Outline
I.
Respiratory structures are adapted for gas exchange in air or water
A. Gills are adapted for respiration in water
1. Gills are energetically expensive
B. Trachea and lungs are adapted for terrestrial respiration
1. Oxygen is more concentrated in air than water
C. Gas exchange takes place across a moist surface
1. Terrestrial organisms lose water during respiration
II.
Many types of surfaces have evolved for gas exchange
A. The body surface may be adapted for gas exchange
1. Small animals respire by diffusion through the surface of the body
2. Annelids, shell-less mollusks, and amphibians respire via the body surface
B. Tracheal tube systems of arthropods deliver air directly to the cells
1. Insects and a few other arthropods have tracheal tubes
2. Air may be brought in by actively pumping air in through the spiracles
3. Branching of the tracheal tubes extends throughout the body
C. Gills of aquatic animals are respiratory surfaces
1. Gills are evaginations of the body surface and are particularly efficient in
water
2. Dermal gills are found in echinoderms
3. Mollusk gills are highly folded
a) Bivalve gills are ciliated and adapted for filter feeding
4. Chordate gills are typically internally located
a) Bony fish have an operculum covering the gills
b) The highly vascularized gill filaments are particularly efficient because
of the countercurrent flow of blood and water
D. Terrestrial vertebrates exchange gases through lungs
1. Lungs are invaginations from the body surface or an internal cavity (the
book lungs in arachnids)
2. Early lobe-finned fishes had lungs that were adaptive for the droughts
during the Devonian period
a) Most modern fish have swim bladders, which aid in buoyancy control
3. Many amphibians rely on cutaneous respiration in addition to lungs
4. Reptilian lungs are relatively simple, with some inner folds
5. Birds have air sacs, which increase respiratory efficiency
a) Air flow is one-way and flow of air is opposite of that of the flow of
blood (countercurrent flow)
III.
b) The parabronchi are the site of gas exchange
The mammalian respiratory system consists of an airway and lungs
E. The airway conducts air into the lungs
1. The nasal cavities filter, warm, and moisten inhaled air
2. Mucus traps particles in inspired air, and cilia move it back towards the
throat
3. The nasal and oral cavities connect via the pharynx
4. The pharynx leads to the larynx, a cartilaginous structure adapted for
sound production
5. The epiglottis covers the entrance to the larynx during swallowing
6. Air passes from the larynx to the trachea, which is supported by
cartilaginous rings
7. The trachea branches into two bronchi
8. The trachea and bronchi are lined with a ciliated mucus membrane
F. Gas exchange occurs in the alveoli of the lungs
1. The two lungs are divided into lobes
2. The lungs are covered by pleural membranes, which form a pleural cavity
a) The pleural fluid provides lubrication during breathing
3. Bronchi branch many times, ultimately forming the millions of microscopic
bronchioles
4. The bronchioles lead to alveoli, which are composed of a single layer of
epithelial cells
5. The alveoli are highly vascularized
G. Ventilation is accomplished by breathing
1. Breathing involves inspiration and expiration
2. Inspiration is accomplished primarily by contraction of the diaphragm,
which increases the volume of the thoracic cavity
a) Forced inspiration or expiration is aided by muscles of the chest wall
3. Expiration occurs when the diaphragm and most of the rib muscles relax,
decreasing the volume of the thoracic cavity
4. The work of breathing is reduced by pulmonary surfactant, a phospholipid
mixture secreted by epithelial cells that line the alveoli
H. The quantity of respired air can be measured
1. The tidal volume is the amount of air moved in or out in a normal
inspiration or expiration
2. The vital capacity is the maximum exhalation following a maximal
inhalation
3. The residual capacity is the amount of air remaining in the lungs after a
normal expiration
I. Gas exchange takes place in the alveoli
1. Oxygen and carbon dioxide move by diffusion from areas of higher
concentration to lower concentration
2.
Dalton’s law of partial pressures describes the pressure that a single gas
exerts when in combination with other gases
3. Fick’s law describes the diffusion of oxygen or carbon dioxide based on the
differences in partial pressure across a membrane
J. Gas exchange takes place in the tissues
1. The partial pressure of arterial blood is about 100 mm Hg
2. Because blood passes rapidly through the capillaries, the partial pressure
of oxygen in venous blood is about 40 mm Hg
K. Respiratory pigments increase capacity for oxygen transport
1. Hemoglobin in vertebrate blood greatly increases the amount of oxygen,
which can be transported by the blood
2. Hemoglobin is also found in many invertebrates, and may be located in
cells or dissolved in the plasma
3. Hemoglobin is a group of compounds that have an iron-porphyrin (heme)
group bound to a protein (globin)
a) The globin molecule varies among different animals
4. Hemocyanins are copper-containing respiratory pigments
a) Hemocyanins are dissolved in the plasma of some mollusks and
arthropods
5. Oxygen is transported in combination with hemoglobin
a) Plasma alone cannot supply sufficient oxygen in a human
b) Hemoglobin (Hb) forms a reversible bond with oxygen
(1) In the lung, Hb and oxygen form oxyhemoglobin (HbO2)
c) Oxygen-hemoglobin dissociation curves show that as oxygen
concentrations rise, the concentration of HbO2 increases
d) HbO2 dissociates more readily in the acidic environment of the
capillaries (due to carbon dioxide) or in active muscles (due to lactic
acid), called the Bohr effect
e) Some carbon dioxide is transported by hemoglobin
L. Carbon dioxide is transported mainly as bicarbonate ions
1. 7% of the carbon dioxide is dissolved in the plasma; 20% is carried by
hemoglobin
2. Carbon dioxide combines with water to form carbonic acid, catalyzed by
carbonic anhydrase, an enzyme in red blood cells
3. Carbonic acid dissociates, forming hydrogen ions and bicarbonate ions
M.
Breathing is regulated by respiratory centers in the brain
1. The medulla controls the basic rhythm of breathing
2. The pons controls the transition for inspiration to expiration
3. Chemoreceptors in the medulla, aorta, and carotid arteries sense the carbon
dioxide concentration (pH)
4. CPR can restore respiratory and cardiac arrest
a) The ABCs of CPR are: clear Airway, restore Breathing, and restore
Circulation
N.
1.
Hyperventilation reduces carbon dioxide concentration
Hyperventilation reduces the impulse to breathe
O. High flying or deep diving can disrupt homeostasis
1. Hypoxia (oxygen deficiency) results in drowsiness, fatigue, or headache
2.
Rapid decreases in pressure cause decompression sickness
a) Bubbles of nitrogen in the circulatory system may block capillaries,
cause pain, and even cause death
3. Decompression sickness is more common in scuba diving than in flying
because the changes in pressure are more rapid
P. Some mammals are adapted for diving
1. Diving mammals do not take in extra air before they dive, but exhale to
make their lungs less buoyant and facilitates gliding downward
2. Seals have about twice the volume of blood as nondiving mammals
3. Diving mammals also have high concentrations of myoglobin, which stores
oxygen in muscles
4. At the limit of a dive, the diving reflex is activated
a) Metabolic rate decreases
b) Heart rate slows
c) Blood is redistributed from organs that can survive with less oxygen
IV.
Breathing polluted air damages the respiratory system
Q. Bronchial constriction is a normal reaction against inhaling particulates
1. Bronchial constriction also results in a lesser inspiratory volume
R. The smallest bronchioles and alveoli lack mucus or cilia to expel inhaled
particulates
S. COPD (chronic obstructive pulmonary disease), including chronic bronchitis
and pulmonary emphysema, is linked to smoking and breathing polluted air
Research and Discussion Topics

Fish evolved in freshwater habitats, and had functional lungs. Describe the
development of the lung into the swim bladder, and the retention of the lung in the
lungfish and lobefins (which shared an ancestor with the amphibians).

Diving seals and whales have a variety of respiratory adaptations that allow them to
dive deeply. Discuss these anatomic and physiologic adaptations.

Discuss the problems seen in scuba divers, including nitrogen narcosis and
decompression sickness. Why do human divers experience these potential
problems, but diving whales and seals do not?