Is there somewhere in the body that can store fluid, apart from the bladder? Most nights I wake to empty my bladder after exactly 4 hours鈥� sleep, but on occasional nights where I haven鈥檛 been to the loo all day previously I have to get up three or four times instead of only once. It鈥檚 obvious that the urine has been building up during the day, but it wasn鈥檛 there in my bladder before I went to bed. So where was it?
Is there somewhere in the body that can store fluid, apart from the bladder? There is, and it鈥檚 simple when you think about it 鈥� it鈥檚 the bloodstream. Faced with a bladder which is full and hasn鈥檛 been emptied all day, the kidneys cut down on urine production. When the bladder is emptied at last, they work to restore the blood鈥檚 correct electrolyte balance, and the bladder soon fills again.
Kidney activity is to some extent also controlled by the body鈥檚 clock, and less urine is produced at night. Most people urinate several times during the day but will not have to get up during their sleep. If you reverse the cycle by flying halfway round the world you find yourself not needing to pee all day then getting up all night. This is undoubtedly a trained response and your questioner seems to have trained her body into the opposite cycle 鈥� making more urine at night 鈥� which may suit her lifestyle or job.
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鈥淢ost people urinate several times during a day but will not have to get up during the night鈥�
Guy Cox, Cell Biologist University of Sydney, Australia
One answer is that the fluid is found in your legs and feet. Patients with any degree of heart failure or kidney disease know this well. Fluid pools in the legs during the day, and as soon as the body is horizontal the fluid re-enters circulation, straight through the kidneys and into the bladder.
You don鈥檛 have to be sick to see this. If you鈥檝e ever been on a long flight, consider why you always have to find a toilet while standing in the queue for customs. Prolonged inactivity causes extracellular fluid to pool in your lower half. This is why it is hard to tie the laces on your shoes when it鈥檚 time to disembark. Once your leg muscles start pumping the extra fluid back into the general pool, you soon need to go to the toilet.
Michael Carrette, Cairns, Queensland, Australia
A 70-kilogram man is 65 per cent water. That amounts to approximately 45 litres. Most of this is contained inside the trillions of body cells. A lesser amount surrounds these cells, while an even smaller amount circulates as plasma. Fluctuations in body weight over hours, days or a month are a feature of the body鈥檚 ability to vary this volume. Variable tightness of finger rings, end-of-the-day aching legs and tight shoes are testimony to this movement of fluid.
To answer the question 鈥� the water is everywhere in the body and there is a lot of it. Urine is just the fluid kept in the bladder as a result of renal activity. The need to urinate is the result of only 200 to 300 millilitres of urine in the bladder.
Consuming a lot of salt encourages retention of fluid and can increase blood pressure, while several complex hormones such as antidiuretic hormone and aldosterone play a significant part in this process of fluid balance, as does oestrogen, leading to weight gain during some parts of the menstrual cycle.
Renal function slows during the course of the night allowing most of us (but not the author of this question!) to get a good night鈥檚 sleep. Bladder tone also decreases at this time, enabling it to hold more fluid.
John Curran, Beverley, East Yorkshire, UK