(3.4) 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; produced from respiration 250

total input 1900 cm³

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

total output 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!

When the atmosphere is very humid (high water concentration), the body struggles to cool itself because sweat cannot evaporate efficiently into the moisture laden air. This causes your body temperatures to rise, leading to fatigue, heavy sweating, dehydration, and potential heat-related illnesses like heat exhaustion or heat stroke. The cardiovascular system works harder, causing a faster heart rate.

The control of the body's water balance via the ADH hormone negative feedback mechanism is described in homeostasis Part 5

Hormone controlled water concentration of the blood and urine - ADH negative feedback mechanism for water balance

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Key points Source of information is based on textbooks and syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points about water control in the body

Osmoregulation and Water Balance

What Is Osmoregulation?

Osmoregulation is the process by which the body maintains the correct balance of water and mineral salts (ions) in the blood and body fluids. It is a key part of homeostasis.

If this balance is disrupted:

• Too much water → cells may swell and burst.

• Too little water → cells may shrink and become dehydrated.

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Why Is Water Balance Important?

• Cells need a stable environment to function properly.

• Water is essential for:

  • Transport of substances in blood.

  • Enzyme activity (which depends on correct concentration).

  • Temperature regulation (via sweating).

• Imbalance can lead to cell damage, organ failure, or death.

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Water Input and Output

Water Is Gained From:

• Food and drink

• Aerobic respiration (produces water as a by-product)

Water Is Lost Through:

Only urine output can be regulated to control water balance.

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The Role of the Kidneys and ADH

• Kidneys filter the blood and adjust water content.

• Controlled by ADH (Anti-Diuretic Hormone) from the pituitary gland.

This is a negative feedback loop - the body detects a change and acts to reverse it.

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Sweating and Dehydration

Sweating

• Occurs when body temperature rises (e.g. during exercise or in hot weather).

• Sweat glands release sweat onto the skin.

• Evaporation of sweat cools the body.

• Sweat contains water, ions, and urea.

Dehydration

• Happens when water loss exceeds intake.

• Common causes: excessive sweating, vomiting, diarrhoea, not drinking enough.

• Symptoms:

  • Dry mouth

  • Dizziness

  • Dark urine

  • Fatigue

• Severe dehydration can lead to organ failure.

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Why This Is Important in Human Biology

• Demonstrates how the body maintains homeostasis.

• Shows the interaction between the nervous system, endocrine system, and excretory system.

• Helps explain the effects of exercise, climate, and illness on the body.

• Links to real-life issues like heatstroke, kidney failure, and sports hydration.

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Exam Tips

• Use key terms: osmoregulation, ADH, homeostasis, urine, dehydration, negative feedback.

• Be able to interpret graphs showing ADH levels or urine concentration.

• Understand how the body responds to changing conditions like heat or exercise.

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Keywords, phrases and learning objectives for this part on water balance in the body

In terms of homeostasis be able to describe the importance of water balance in your body'.

Know where the water input comes from (drinks, food, respiration) and where the water output goes (urine, sweat, breath).

Describe when water is needed if conditions change e.g. increase in temperature causing sweating or dehydration.

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