We have noted that some tubule regions, such as the PCT, are highly permeable to water whereas others, such as the thick ascending limb of the loop of Henle, are impermeable to water. What causes these differences in water permeability in different regions of the nephron? *Aquaporins are a class of membrane proteins that form water channels. Regions of the nephron that are highly permeable to water have greater numbers of aquaporins. Thus aquaporins are abundant in kidney PCT cells and in descending limbs of the loops of Henle, but not in the ascending limbs of the loop of Henle.
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*connect the concepts As explained in Key Concept 6.3, water does not cross cell membranes easily, but aquaporins are protein channels that make cells permeable to water.
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As an interesting evolutionary note, aquaporins are also important in maintaining water balance in amphibians. Many amphibians can gain water from a moist substrate because they have aquaporins in the epithelial cells of their belly skin. Thus water can cross their skin into the interstitial fluid by osmosis.