Gas Exchange Strategies Oxygen Anaerobic life has been in existence for the past 3.5 billion years. Oxygen only because available because of the photolysis of water from ferrous rocks (as well as oxygen-producing cyanobacteria, which were the result of prokaryotic organisms acquiring chlorophyll). Between 2.3 and 2 billion years ago, the full ocean and mineral sinks saw an increase in oxygen. Oxygen is somewhat of a toxic compound. Small amounts can cause large problems with free radicals, such as those formed by superoxide (OO-) and hydrogen peroxide, which both are poisonous and damage cells. This is known as oxidative stress. Modern organisms have mechanisms in place to reduce the threat from oxidative stress. They may contains enzymes such as superoxide dismutase, peroxidase and catalase, which all break down harmful oxygenic compounds. They may also contain within their cells anti-oxidants derived from food sources, such as vitamins E and C and !- carotene. However, oxygen is also very useful. Readily available molecular O2 forms the protective ozone layer, can be used for the synthesis of biological macromolecules such as collegen and facilitated the evolution of metazioan life, which all respire aerobically via internal respiration and the exchange of gases. The availability of oxygen declines the further away from sea level, not because it is less concentrated but because the PO2 becomes lower at higher altitudes. Oxygen exchange Occurs at the exchange site, which will always have a large surface area and thin epithelial layers. In large, more active animals, this is then taken up by the ventilatory system and the circulatory system, which transfer dissolved oxygen into tissues. At a cellular level, oxygen exchange occurs by diffusion according to partial pressure gradients. In large cell aggregates, diffusion alone is not enough, meaning convection methods are coupled with diffusion in the respiratory apparatus. Diffusion The passive movement of a gas from high concentration to low concentration (partial pressures). The diffusion in the gas
D=1/(density)0.5.
phase depends on Graham's law: The diffusion across a blood-gas barrier, however, depends on Fick's law. The fick equation related tissue O2 to cardiac output and arterial venous return.
Gas transport in vertebrates Environmental oxygen (from water or air) is pumped by either the buccal cavity or the chest wall into the airways, where it diffuses into the gas exchange area (gill or lung). This is then convected to the heart, diffuses into the intermedium and then into the cell. Returning CO2 diffuses from the cell into the intermedium and then is convected using vasculature to the gas exchange area, which it diffuses into, and is then convected again in the upper or lower airways and back into the environment.
Respiratory Pigments Once oxygen diffuses across, it combines with a respiratory pigment, which are complexes of protein and metallic ion. They tend to have a characteristic colour. Pigment
Colour
Function
Location
Taxa
Hemoglobin
Red
Binds oxygen. Without, total oxygen saturation is around
0.3mL, with - around 20mL. Four haem molecules and iron.
Blood
Invertebrates and vertebrates, although some fish lack it (antarctic tooth fish)
Myoglobin
Red
Equivalent to one hemiglobin molecule
Muscle
Vertebrates
Brachiopoda, annelida
Hemerythrin
Violet
iron (non-heme) and protein
Blood (cells), coelomic fluid and muscles
Chlorocrucrin
Green (dilute), red (concentrated)
Iron and protein, related to hemeglobin
Plasma
Annelida
Hemocyanin
Copper
Similar function to hemeglobin. Second most abundant pigment, consists of copper and protein.
Hemolymph
Mollusca, arthropoda (horseshoe crabs) - two types with a separate evolutionary lineage.
Respiratory Structures Gills
Lungs
Gas exchangers Gas exchanges vary as in general they have different
1. organisation of respiratory surfaces
2. Blood supply
3. Arrangement of ambient fluid
4. pO2 gradients
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