[ordu]

> The retina is one of the body’s most energetically expensive tissues.

I never knew, but it explains why when you close to fainting you lose your vision. Or when you are working at high heart rate close to your maximum. It works as a kind of a warning sign, than you are probably shouldn't try it that hard.

> The lack of blood vessels could also offer birds the advantage of better vision.

Now they are ready to reintroduce blood vessels back, but this time behind the retina.

[WalterBright]

> Or when you are working at high heart rate close to your maximum.

When I was in college, running on the track, I decided to see how fast I could run by ignoring the stress and pain. My vision began rolling and surging in a weird way impossible to describe. I stopped and laid down on the ground, unable to do anything but pant.

I realized that what I had done was extremely stupid and never did it again.

[yieldcrv]

How does it explain either of those things?

[ordu]

When you have not enough oxygen it breaks. And parts using a lot of it fail first. Vision fails first, not memory, or thinking. Thinking is impaired but still works.

I'm not sure how fainting works, but fainting looks to me like an energy crisis, so kinda not surprising the results are the same.

[KineticLensman]

> Vision fails first, not memory, or thinking

I've had two complete blackouts due to Ventricular Fibrillation, and one near blackout where the VF stopped after nine seconds (as reported by my ICD). In my experience, vision and thinking seem to stop at the same time, with increased dizziness being the first functional effect (after perhaps 7-8 seconds). My ICD is set to fire at 14 seconds, by which time I'm guaranteed unconscious and won't feel the painful shock. It takes 2-3 seconds to recognise the warning signs (painless fluttering sensation in my chest), so there are 4-5 seconds of normal consciousness when I can try to make sure I fail safe. Like sitting down.

This is why I don't drive anymore.

[ordu]

Hmm... Maybe the dynamics is different? When you push yourself physically your body don't stop getting oxygen suddenly, just the rate of oxygen absorption is less then you are consuming, and it is getting worse, when excesses of lactic acid reach your liver and it starts to suck out oxygen from blood to metabolize it. But the heart pumps blood at its maximum, some limited amount of oxygen is available.

I'm not sure how VF works, but maybe the deficit of oxygen develops much faster, so it leads to complete blackout in seconds?

> there are 4-5 seconds of normal consciousness when I can try to make sure I fail safe. Like sitting down.

Fainting and workout take more time. Definitely more. When I fainted it took tens few minutes from the moment I broke my leg. When I ride my bike up to a hill with all my might, it takes a couple of minutes to see darkness in my eyes.

[Sharlin]

You can get "tunnel vision" and blurred, black-and-white vision from anything that even slightly hinders blood flow into the head, including everyday things like going from lying down to standing up too fast, or stretching positions that make your neck muscles press on the carotid arteries. Never mind things like fighter pilots doing high-gee maneuvers.