Why do we lose 40% water from our brains and only 10% water from our bodies when we age?

Question

Why do we lose 40% water from our brains and only 10% water from our bodies when we age?

Answer 1

There may be two reasons behind this sort of water loss from brain ( as you have stated 40%) at aged person or adults as they grow.

The first reason may be the hormone corticosteroids. The level of this hormone in a growing individual ( as he becomes older) tends to increase. I have read in an medical research article that as the level of corticosteroids increases with age, the blood-brain barrier permeability decreases, and it affects in a greater water loss in older or aged person than in young and small children.

It was suggested that the water content of brain is about 70% in infants, teenager and young but with age brain starts to lose more water than body surface( skin, muscles and so on). The proportion is about 2-3 times less than brain.

Why we lose less water from our body when we age?  To answer this question you need to  remember that in childhood, young age or even when we are middle aged, we are active. There is a lot of muscle function (work) and metabolism rate is higher. Thus, the water loss is naturally higher from body surface in young age than in old age.

Take the instance how it changes with age and also particularly with older people. They have less body metabolism rate(BMR) and also function of the muscles is neglegible. Body is much more covered with fat and for that reason water loos is decreased.

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Read this research work for understanding blood brain barrier permeability in brain. The experiment was carried out with lambs at different age.

   http://ajpregu.physiology.org/cgi/conten...