Donating blood is stress for the body, naturally, not a single cell of your body understands the "social significance" of donation (if we are talking about it ...), therefore it experiences some sharp discomfort of "loss of system stability". The mechanism of "unexpected crisis" does not affect the body if the blood loss is less than 120-150 ml. blood / day. This is roughly comparable to menstrual bleeding.
If we are talking about the procedure for a clinical blood test, then there are some tips for those who are going through hard:
Have something to eat with you (like banana or sweet juice)
Warn nursing staff.
Try to provide yourself with a light breakfast (for example, a savory roll) before donating blood
After donating blood, sit in place for 5-7 minutes in your place, get up without sudden movements, go out into the air.
If you need to recover immediately after donating blood, then you need, firstly, to replenish the volume of fluid that the body has lost. Immediately after, tea is really suitable, and sweet, in order to replenish the glucose reserves. In the long term, you need to consume more fluids.
It is also good advice to normalize your diet in order to accelerate the recovery of blood cells.
Drink strong sweet tea or eat something sweet like cookies or candy. Sit quietly for a while (15-30 minutes), preferably in the fresh air. Profit!