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From Guyton and Hall Textbook of Medical Physiology. The paragraph is about how blood supply affects the intensity of salivary secretion by salivary glands. Salivary secretion can be induced by nerve signals, which is mentioned in the second sentence, and blood supply is an additional factor that also promotes it.

A secondary factor that also affects salivary secretion is the blood supply to the glands because secretion always requires adequate nutrients from the blood. The parasympathetic nerve signals that induce copious salivation also moderately dilate the blood vessels. In addition, salivation directly dilates the blood vessels, thus providing increased salivatory gland nutrition as needed by the secreting cells.

What would the last sentence mean if "needed" were used instead of "as needed"? What is the difference between the two in this case?

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The sentence wouldn't make sense without the as, unless you added the after providing.

...thus providing the increased salivary gland nutrition [which is] needed...

The sense of the clause as it stands is

...thus providing increased salivary gland nutrition as [it is] needed...

In other words, the cells only get this extra nutrition when they need it.

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  • Had to downvote because "providing increased salivary gland nutrition needed by..." sounds perfectly fine to me. Like: "Food provides essential nutrition needed by the body." I agree, though, that without "as" there's just the bare statement that the cells need the increased nutrition and salivation provides it; with "as," on the other hand, there's not as strong of an implication that the cells necessarily need the increased nutrition - rather, that if/when they do, the salivation provides it.
    – cruthers
    Commented Jul 16, 2022 at 16:18

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