UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 19/05/2023 [SEARCH]

 Homeostasis: 4. Water balance - the body's control of water input and out

Doc Brown's Biology exam study revision notes

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There are various sections to work through, after 1 they can be read and studied in any order.

INDEX of biology notes on homeostasis: Kidney structure and function - osmoregulation

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(4) More on controlling water content - water input and output

"You can survive for several weeks without food, but only a few days without water"!

Water is extremely important for a healthy life for the following reasons ....

  1. 65% of our body is water, its in every cell and makes up most of the blood.

  2. It is a versatile solvent - nutrients like glucose, salts, amino acids are all soluble in blood, carbon dioxide and oxygen are exchanged via blood and other waste products like urea are dissolved and removed in urine.

  3. It is essential for enzyme action - water is the medium for their substrates and products.

  4. Water has a specific high heat capacity, that is important for temperature regulation - thermoregulation.

Typical daily human input and output of volumes of water - which must be constantly kept in balance

Volume taken in cm3: drink 1050; in food 600;  from respiration 250; total 1900 cm3

Volume given out cm3: urine 1000;  exhaled air 350;  skin 400;  faeces 150; total 1900 cm3

There is considerable variation in these values from person to person and textbook to textbook!

If the water or ion content of the body is wrong, too much water may move into or out of the cells and damage them.

If your blood becomes too concentrated in e.g. ions (salts), water is drawn out of the cells and tissues by osmotic action and they become dehydrated. Under these circumstances enzymes cease to work efficiently and so do the cells.

If the concentration of e.g. sodium ions in the blood increases, the concentration of water in the blood decreases too. The brain will detect the blood needs more water and the pituitary gland releases more ADH, so more water will be reabsorbed. from the collecting ducts in the kidney, so the water content in the blood will rise.

See ADH negative feedback mechanism for water balance

If the blood becomes too dilute the reverse osmotic action happens. Water will collect in the tissues and the cells swell up - again a situation of imbalance.

The water content of the blood is continually influenced by the temperature around us (how much we sweat), our diet and the amount of water we lose (see below).

The water content of the body our skin keeps us waterproof but water leaves the body via ...

(i) the lungs when we breathe out (you see the condensed water from our breath in the winter),

(ii) the skin when we sweat to cool us down,

(iii) excess water is lost via the kidneys in the urine,

(iv) and in our faeces.

and this 'output' is balanced by the 'input' of water we take in from food and drinks,

BUT, we can't control how much we lose in the ways described above, so we need a balancing system between the amount of water we consume and the amount of water removed by the kidneys in urine.

We need to, and are continually taking in water via drinks and food.

Any loss needs to be replaced, more so in the summer when we sweat more than in the winter.

Changing conditions

On cold days you sweat less and pass more pale dilute urine, and on warm days you sweat more and pass more darker coloured more concentrated urine (assuming a similar fluid intake each day), either way the water balance is maintained.

After vigorous exercise (keeping fit or sport) you will have lost water and ions through sweat as well as burning up more glucose than usual.

Sports drinks contain water, ions and sugar to replace those lost but there are many products on the market competing for this lucrative revenue stream and each claiming to be just the right drink to take!

I gather from a TV program using a cheap, 'to dilute' fruit juice drink with a spoonful of sugar dissolved in it, is just as effective - but my doctorate is in chemistry!

The control of the body's water balance via the ADH hormone negative feedback mechanism is described in the next section.


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