A male client is color blind. The nurse understands that this client has a problem with:
A. Rods.
B. Cones.
C. Lens.
D. Aqueous humor.
Correct Answer: B. Cones.
Cones provide daylight color vision, and their stimulation is interpreted as color. If one or more types of cones are absent or defective, color blindness occurs. Very few individuals are truly color blind, but instead, see a disrupted range of colors. The most common forms are protanopia and deuteranopia, conditions arising from loss of function of one of the cones, leading to dichromic vision.
Option A: Rods are sensitive to low levels of illumination but can’t discriminate color. Rods are the cells primarily responsible for scotopic vision, or low-light vision. Rods are the more abundant cell-type of the retina and reach their maximum density approximately 15 to 20 degrees from the fovea, a small depression in the retina of the eye where visual acuity is highest. There are approximately 90 million rod cells in the human retina.
Option C: The lens is responsible for focusing images. The lens is the adjustable component of the refractive system: its shape is altered by the contraction or relaxation of the ciliary muscle to focus on objects that are near or far.
Option D: Aqueous humor is a clear watery fluid and isn’t involved in color perception. Aqueous humor is a low viscosity fluid secreted from plasma components by the ciliary body into the posterior chamber of the eye. The humor then travels to the anterior chamber and proceeds to drain into the systemic cardiovascular circulation by an incompletely understood mechanism. Aqueous humor circulation forms the basis of intraocular pressure (IOP), which is associated with glaucoma; this is how the synthesis, circulation, and drainage of aqueous humor become clinically significant.
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