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An Example: If the total gas pressure of the atmosphere averages 760 mm Hg and oxygen makes up 20%, then the partial pressure of oxygen (P_O2 ) is 0.20 x 760 = 150 mm Hg.



An Example: At 20^o C fresh water can hold a maximum of 9.2 mg/L, if the water is heated to 25^o C, the solubility will decrease and since the concentration is still 9.2 mg/L the P_O2 will increase slightly and the water will lose O₂ to the atmosphere until the P_O2 declines to equilibrium with the atmosphere and the concentration in the water reaches 8.4 mg/L.


An Example: If you have a beaker of water at air saturation the P_O2 is approx. 150 mm Hg or 0.2 A, the same as in the air above it. Stirring or bubbling the water will not change this. But if you cover the beaker and enrich the O₂ concentration over it until it is say, 40% instead of the normal 20%, the P_O2 will also double as O₂ diffuses into the water and reaches equilibrium resulting in a P_O2 in the water of 300 mm Hg instead of the normal 150 mm Hg. Alternatively, you may increase the P_O2 of water by increasing the total gas pressure, say by sealing the top of the beaker and pressurizing the normal air mixture to 2A (1500 mm Hg), this would also lead to a P_O2 in the water of 300 mm Hg.


An Example: If a bubble is driven down, say, 30 m, then the partial pressures of oxygen and nitrogen are multiplied by 3 . Their solubilities remain the same so the water now contains 3 times the amount (mg/L) of the gases. When that pressure is released the amount the water holds in solution decreases and the gases effervesce, like opening a carbonated drink. At surface, S_O2,25C x 150 mm Hg = 9.2 mg/L so at 30 m S_O2,25C x 450 mm Hg = 27.6 mg/L that means an additional 18.4 mg/L may go into solution at 30 m and that same 18.4 mg/L will be released at the surface.


An Example: Some very pelagic fish (e.g. tuna , billfish and many, but not all, sharks) lack the ability to buccal pump and are obligatory ram ventilators and must swim to breathe.




An Example: Fish raised in crowded hatchery conditions nearly always show some hyperplasia of gill epithelial cells caused by degraded environmental conditions such as high ammonia concentrations. This is commonly termed "clubbed gills" because the lamellae appear clubbed under low magnification.



An Example: In the feet of birds, parallel blood vessels run in close contact with each other. Cold blood from feet flowing up is warmed by blood flowing down from the heart and body, this allows shore birds to wade in frigid waters with cold feet, but warm bodies. A similar heat exchanger is used in energy efficient houses to provide fresh air without losing heat to the outside.



An Example: Powers (1972) compared the hemoglobins of two sympatric species of suckers, Catostomus insignis and C. clarkii. He found that C. clarkii had 20% of the its hemoglobins lacking the Bohr effect, while all of C. insignis hemoglobin underwent the Bohr effect. Ecologically, C. clarkii lives in faster water (riffles and runs) and C. insignis lives in slower pools. He hypothesized that the 20% of Hb without the Bohr shift in C. clarkii constitutes a reserve that will continue to pick up oxygen even if extreme activity in fast water causes temporary acidic conditions at the gill. Science 177:360.




An Example : Primitive organisms are organisms that resemble their early ancestors in morphological and anatomical characteristics more than do the advanced ones. Trout and perch had a common ancestor which the trout resembles more than the perch, therefore trout are considered more primitive teleosts than perch. A primitive condition is a trait that strongly resembles one of an ancestor.




An Example : When I was in graduate school, I witnessed the bowfin's ability survive in extreme conditions aided by it's ability to breathe air. A bowfin and several bluegills were being held in a livewell of a boat. Due to the hot summer sun and a lack of water circulation into the livewell, the bluegills died and decomposed. When the livewell was opened after several days, there was a foul smell, the water temperature was 95^o F and had0.00 mg/L of dissolved oxygen, yet the bowfin was swimming about and quite lively.




An Example: Milliosmole/liter (mOsmol/l) is a measure of pressure based on a simple salt solution. If one millimole (mMol) of NaCl is dissolved in a liter of water, the Na⁺ and Cl^- ions each contribute to the osmotic pressure, so this solution would have 2 mOsmol/l of osmotic pressure resulting from a mMol/l of Na⁺ and a mMol/l of Cl^-. Not only salt, but any solute or colloid contributes to osmotic pressure, so blood or water has many particles contributing to its osmotic pressure, however the total pressure is expressed in mOsmol/l as if it were entirely a simple salt solution. In fact, in FW fish about 260 of the 300 mOsmol/l are due to NaCl.




An Example: A human adrift on a life raft will die of dehydration more quickly if he attempts to drink sea water. This is because a gut full of 1000 mOsmol water simply draws the water from the body through osmosis, because there is no mechanism to move the salt and thereby move the water into the blood. Animals that can successfully drink salt water such as SW teleosts or marine birds must have a gut salt pump to move water and a another salt pump (the gill in fish, the nasal gland in birds) to rid themselves of the excess salt.




An Example: In humans with uncontrolled diabetes, the blood sugar is so high that the proximal segment cannot reabsorb all of the glucose in the filtrate and sugar passes out in the urine. In a normal animal, very little glucose energy is wasted by loss in the urine.