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The lower blood pressure I can explain by having a lower volume of blood. But the glucose and cholesterol have to go somewhere. Where do they go? Are they filtered in the process after blood letting?

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Let's just pretend it's accurate and say your body burns 2000 kcal a day. If you have less glucose in your blood overall after giving blood, even if the ratio should theoretically remain the same, your body is still going to burn 2000 kcal anyway, and maybe the blood glucose equilibrium reaches a lower level.

If it's in the blood, doesn't it just leave the body together with the blood?

Yes, but the relative amount per volume blood should be the same. I think that's why he's asking.

If so, the answer is that the body replenishes plasma in a day and red cells in six weeks (redcrossblood.org FAQ). The relative amount does change quickly.

There is very little glucose in circulating blood at any given time. Unless you have severe uncontrolled diabetes (or a similar condition) your body regulates blood glucose level within a low, narrow range. Most glucose is stored in the muscles and liver as glycogen.

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