Since oxygen can disrupt many life processes and reactions, organisms have had to adapt to the rise of oxygen as an atmospheric gas.  Its presence as ozone in the upper atmosphere protects organisms form ultraviolet radiation.  Oxygen is essential in aerobic life as an electron acceptor in the cell respiratory process.  Oxygen and carbon dioxide are exchanged in the respiratory system and transported in the circulatory system.

GAS EXCHANGE SURFACES

• Gas exchange in animals requires extensive, thin, moist membranes, such as those seen in the gill and lung.  Such membranes are called the respiratory interface.

• In complex sponges, gas exchange across the body interface works well because of the extensive canal system.  It also works in coelenterates because of the extremely thin (two cell layered) body wall.  Denser flatworms increase the respiratory interface through the highly branched gastrovascular cavity, but this severely restricts their size.  The surface of the echinoderm is increased by the presence of dermal brancheae.
• Earthworms remain in moist soil most of the time, and the salamander is nocturnal, so they can safely exchange gases through the skin interface. Further, both have an efficient closed circulatory system and blood containing hemoglobin.

Expanding the Interface: Tracheae

• Most terrestrial animals have internalized respiratory interfaces that help resist desiccation. Insects have a tracheal system, which includes body openings called spiracles, highly branched tubes called tracheae and tracheoles, and in some , thin walled air sacs. Gas exchange occurs in the fine branches, which are often fluid filled. In aquatic insects the tracheae terminate external tracheal gills.

Complex Interface Gills

• The gill is a similar in invertebrates and vertebrates, with the respiratory interface intimately associated with the circulatory system. Feathery structures contain  extensive capillary beds where blood and surrounding water are separated by a single membrane .
• Through the use of cilia clams draw water in through an incurrent siphon. Water crosses the vascular gill exchange surface and exits through a excurrent siphon. Oxygen transport is aided by the copper containing protein hemocyanin.
• The lobster gills lie in chambers below a protective carapace. Water is drawn across the gills by an appendage called a bailer.  The circulatory system is open in most of the body, but blood vessels occur in the gills.
• The fish gill includes supporting gill arches with rows of gill filaments, each containing many capillary beds in plate like lamellae.  Blood enters the filament from afferent vessels, crosses the lamellar capillaries and exits the filament efferent vessels.  During its transit, CO₂ is exchanged for O₂ across the thin walls.  Water flow across the lamellae opposes blood flow, setting up an efficient counter current exchange.
• Water holds less oxygen than air and diffusion is slower; thus aquatic animals must expend energy to move a large volume of water across the gills (or to move themselves through the water).

Complex Interfaces: Lungs

• The earliest fishes gulped air at the surface, using a vascularized pharynx for exchange.  This system, and example of preadaptation, underwent modification into the lung in the first terrestrial vertebrates.  In addition, internal nares, openings between the nasal cavity and mouth, evolved in the early terrestrial animals.  In mammals, the palate came to separate the pharynx into mouth and nasal cavities.  In fishes, the primitive lung evolved into the swim bladder, a flotation device.

• The vertebrate lung is highly branched inpocketing that, in many species, contain numerous air sacs, intimately associated with capillaries, and inflated and deflated by a bellows action.
• The amphibian lung is a simple paired saclike affair, inflated and deflated by a pumping action in muscular mouth floor.  Check valves in the nostrils and glottis help coordinate the filling and emptying cycles.  Amphibians also use the skin as an exchange surface.

• Reptile lungs are spongy and complex and, aside from the vascular cloaca of aquatic turtles, represent the only respiratory interface.  Breathing occurs through a bellows like action of the entire body wall.

• In birds, air passes through the lung rather than in and out. Air moving through the trachea bypasses the lung to enter posterior air sacs and from there enters the lung.  The passage of air through the lung is crosscurrent to the flow of blood through the lung capillaries. The bird respiratory system represents a specific adaptation to flight at higher altitudes where oxygen is limited.

THE HUMAN RESPIRATORY SYSTEM

• Major structures of the human respiratory system include the mouth, nasal passages, pharynx, larynx, trachea, bronchi, bronchioles, alveoli and lungs.

• The palate separates the mouth from the nasal passages which moisten, warm and filter entering the lungs.  Goblet cells secrete dust trapping mucus in the nasal passage which moisten, warm, and filter air entering the lungs.  Goblet cells secrete dust trapping mucus in the nasal passages and cilia move the mucus film toward the pharynx for swallowing.  Warming of incoming air is sided by a countercurrent blood flow.
• Incoming air moves through the larynx, into the trachea and primary bronchi, through branching bronchioles, and finally into the alveoli.  The larynx contains the voice box and vocal cords.
• Much of the respiratory tree contains a mucus secreting and ciliated epithelium that traps dust particles and sweeps them upward, out of the system.
• The lungs lie in the pleural cavity, enclosed by double membranes, the pleurae.  The dome shaped diaphragm forms the lower part of the cavity.

The Breathing Movements

• In inspiration, intercostal muscles contract, elevating the rib cage, and the diaphragm flattens, creating a partial vacuum in the pleural cavity.  The lungs passively expand in response to the inward movement of air.  In exhalation the rib cage drops, the diaphragm resumes its relaxed dome shape, and the elastic lungs resume their former shape, forcing air out.
• While the minimal (resting) exchange of air is about 0.5 liters, the maximum, or vital capacity, is about 4.0 to 6.0 liters.  Some residual air always remains in the lungs.

The Exchange of Gases

• While all gases of the atmosphere contribute to total atmospheric pressure, each gas exerts a partial pressure - Pg.  Under standard conditions (total pressure 760 mm Hg) Po2 = 160 mm Hg and Pco2 = 0.3 mm Hg.  Pg decreases with altitude.
• The diffusion of a gas follows its partial pressure gradient.
• In blood arriving at the alveolar capillaries, Po2 = 40 mm Hg, while Pco2 = 45 mm Hg.
• In alveoli air, the Po2 = 100 mm Hg and Pco2 = 40 mm Hg. Thus CO2 leaves the blood and O2 enters the blood.
• In blood leaving the alveoli, Po2 = 96 mm Hg and Pco2 = 40 mm Hg.
• In very metabolically active tissues, these values can change to 25 and 46 mm Hg respectively.

Oxygen Transport

• Hemoglobin can carry 60 times as much oxygen as water.  Its four polypeptide chains contain four heme groups  and each deoxyhemoglobin can reversibly associate with one O2, forming oxyhemoglobin.

• The association and subsequent dissociation of hemoglobin and oxygen is complex, depending on pH, Pco2, and temperature.
• Hemoglobin has a built-in safety factor.   At sea level the blood leaving the lungs is nearly saturated with oxygen, and increasing the Po2 has little effect.  But at half the sea level Po2 (high altitude), the blood will be 80% saturated.  In addition, when a consistently low Po2 occurs in the blood, the hormone erythropoietin is released, stimulating new red cell formation, a slower, long-term adjustment.

Carbon Dioxide Transport

• Because of the Bohr effect, the affinity of hemoglobin for oxygen is inversely proportional to the partial pressure of carbon dioxide.  Thus, in blood passing metabolically active tissues with a high CO2 concentration, substantially more O2 will dissociate.
• In the transport of CO2,
• about 8% of the CO2 goes into solution in plasma;
• the remainder enters the red cells.  Some associates with amino acids in hemoglobin, forming carbaminohemoglobin, and the rest forms carbonic acid with water in the red cell;
• the speed of dissociation reactions depends on the action of the enzyme carbonic anhydrase;
• within the red cell, carbonic acid dissociates into bicarbonate and hydrogen ions (acid).
• some hydrogen ions are buffered by hemoglobin while the carbonate ions diffuse out to the plasma, where they are buffered by sodium ions, forming sodium bicarbonate - a part of the body's acid-base buffering system;
• all reactions are reversible, according to the mass action law.
• The reactions of carbon dioxide reverse in the lungs where the Pco2 in the alveolus is low.  The restoration of ions into CO2 is greatly speeded by the action of carbonic anhydrase.

The Control of Respiration

• Respiration is under the control of the autonomic nervous system and is ultimately involuntary.
• Respiration is strongly influenced by the Pco2 and the pH of the blood.  A respiratory control center in the medulla reacts to changes in these factors.  Other sensors are located in the aortic and carotid bodies, and fourth ventricle of the brain.