Final answer:
Phenylephrine is a mydriatic used to dilate pupils for eye examinations by mimicking the effect of norepinephrine. Pilocarpine can reverse this dilation after exams, and phenylephrine can also increase blood pressure in specific medical conditions.
Step-by-step explanation:
Drugs that cause pupils to dilate for an internal examination of the eye are called mydriatics, and one common drug used is phenylephrine. The process of pupil dilation is known as mydriasis, and this physiological response occurs when the sympathetic system causes the radial fibers of the iris smooth muscle to dilate when norepinephrine binds to an adrenergic receptor. Mydriatics like phenylephrine mimic the action of norepinephrine by binding to the same receptor, thus inducing dilation when applied to the eye's surface in a doctor's office.
It's interesting to note that pilocarpine, a nonspecific muscarinic agonist, is typically used to reverse mydriasis after an eye exam. By contracting the smooth muscle of the iris, pilocarpine constricts the pupil and also encourages drainage of aqueous humor by contracting the ciliary muscle, thus reducing intraocular pressure which can be advantageous for glaucoma patients. Conversely, phenylephrine is occasionally used to increase blood pressure in medical situations where the cardiac function is compromised.
A take-home experiment that reflects this principle involves observing the dilation of pupils in different lighting conditions. Under normal light, the pupils have a smaller diameter, but in reduced light, the pupils dilate to allow more light in. This natural reaction can provide insight into how mydriatics like phenylephrine manually induce the dilation of pupils for medical examination purposes.