Yes, it does, but it's not based on how full your bladder is.
Filtering the blood passes out loads of water, too much to be reasonable to lose. So the kidneys process the urine before it leaves them, and recover a variable amount of the water, which is returned to the blood stream. This is controlled by antidiurectic hormone (vasopressin). The hypothalamus generates vasopressin in response to how hydrated you are, the less hydrated, the more vasopressin it generates, which causes more water to be recovered from the urine and returned to the blood. This is a key part of the mechanism to regulate hydration and blood pressure. The pituitary (the organ which knows best when it's day or night) steals some vasopressin during the day, and releases it at night. This is so you generate less volume (more concentrated) urine at night, so you are less likely to need to get up to have a pee. (This mechanism can deteriorate as you get older.) However, it also means your blood pressure control is slightly disabled at night, and it's probably not a good idea to drink a lot of fluid just as you go to bed, because the vasopressin released by the pituitary could stop your kidneys from ejecting it overnight, raising your blood pressure.
When you wake in the morning, your urine is usually quite concentrated (and this is why that's the preferred sample to use for urine analysis). Often people have a second pee quite soon after the first. This is because the vasopressin released by the pituitary overnight has gone, and now the kidneys are ejecting more of the excess water which was temporarily held in the blood overnight.
Edited by member 25 Aug 2024 at 18:33
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