You may not know this, but your body is actually a huge sack of water!
In general, your body consists 50-60% of water. So, if your weight is, let’s say, 80 kg, then the total amount of water in your body is 45 litres (±13 gallons)!
Most of this water is located inside all the cells (= intracellular water; about 66%). A smaller amount is located between the cells (= interstitial fluid; about 30%) and the smallest amount is in your blood vessels (= plasma; about 3 litres).
Every day, this water needs to be refreshed, the blood needs to be filtered, the waste removed etc. This also implies that, every day, we need to drink water.
As shown in the diagram, every day, we need to take, on average, about 2.4 litres/day (although this can vary a lot between individuals).
Most of this intake we do by drinking (water, coffee, coke, etc). But some 30% is taken through our food.
In order not to swell, we then also need to remove a similar amount of water everyday!
Most of the water (some 60%) is excreted by the kidney into urine.
But we also lose water with our respiration. Every time we breathe air inside our lungs, this air is wetted (‘humidified’). When we then exhale, this ‘wetted’ air flows out of our body and its water content is ‘lost’.
Some water is also lost though our skin. This is sometimes called “perspiratio insensibilis” (=invisible perspiration).
And, finally, we can also lose some water through real perspiration (sweating) and with our faeces.
But please note that all these values are highly variable. For example, in diarrhoea, you would lose much more water through the faeces!