The reason that the steroid hormone aldosterone affects only a small number of cells in the body is that

Only small number of the cells in the body are the target cell for the aldosteron

Explanation:

Target cell for a certain hormone is a cell that have hormone receptor specific for that hormone. There are two main types of receptors on target cell:

Hormone binding to the receptor leads to the signal cascade within the cell, which results in cell response.

The aldosterone mineralocorticoid receptors are nuclear receptors mostly located in the kidneys.

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