Why does countercurrent exchange optimize gas exchange across some gills?

a. It sustains an optimal thermal environment for gas exchange.
b. It optimizes perfusion to the respiratory surface.
c. It sustains an optimal respiratory gas concentration gradient, allowing diffusion to occur
across the entire respiratory surface.
d. It optimizes delivery of the respiratory medium to the respiratory surface.
e. It minimizes diffusional area.


C

Biology & Microbiology

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Defective _____ cause color blindness

a. hair cells c. cone cells b. rod cells d. neuroglia

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The plants and animals now present on acreage from which the trees were removed ten years earlier

represent a. primary succession. b. a climax forest. c. pioneer species. d. secondary succession. e. species introductions.

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What is the age of the twig in the accompanying figure?


a. 0 years
b. 1 year
c. 2 years
d. 3 years
e. 5 years

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