Came across this image of retina's cross section: Retina cross section 1) How do the cone cells directly underneath the blood vessel ('demarcated in the picture') receive a spectrally correct representation of the incident light? Wouldn't the blood absorb most of the spectrum, leaving only red behind. Is it because the blood vessel is too thin?

2) The fovea region seems to be void of any such capillaries, how do the cells then receive nutrition and oxygen?


1 Answer 1


1) The photoreceptors in fact don't receive all the light because some of it is absorbed by the blod vessels and nerve cells above the photoreceptors. The Wikipedia article on the retina states:

Although the overlying tissue is partly transparent, and the accompanying glial cells have been shown to act as fibre-optic channels to transport photons directly to the photoreceptors, light scattering does occur.

Here is the direct link to the article describing the fiber-optic effects.

The red blood cells in the capillaries do absorb some light but the brain corrects for the color distortion. The most notable effect of the capillaries is that you can actually see white blood cells moving across your retina when looking into the skye:

The white blood cells in the capillaries in front of the photoreceptors can be perceived as tiny bright moving dots when looking into blue light. This is known as the blue field entoptic phenomenon (or Scheerer's phenomenon).

However, our eyes are still extremely sensitive:

A rod cell is sensitive enough to respond to a single photon of light.

Furthermore, the inverted retina actually protects the light sensitive cells from overexposure and damage by UV light.

Interestingly, cephalopods (e.g. squid, octopus, nautilus) have their photoreceptors on top of the retina, making their eyes more sensitive and better adapted to the low light conditions of the deep sea.

2) While blood circulation is necessary to maintain nutrient concentrations in large tissues, small distances can be covered pretty well by diffusion of the nutrients through the tissue. The fovea is only 5.5 mm in diameter which is small enough for diffusive nutrient supply.


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