The body gains water from drinks, food and cellular respiration, while electrolytes come mainly from food and fluids. It loses both through urine, sweat and faeces; water is also lost through exhaled air.
Routes of Gain and Loss
| Route | Water | Electrolytes |
|---|---|---|
| Drinks and food | Main source of water intake | Supplies ions such as sodium, potassium, chloride and calcium |
| Metabolism | Metabolic water is produced during cellular respiration | Does not provide a significant electrolyte source |
| Urine | Usually the largest adjustable water loss | Removes variable quantities of electrolytes |
| Sweat | Loss increases during exercise and in hot conditions | Mainly loses sodium and chloride, with smaller amounts of other ions |
| Faeces | Normally a relatively small loss | Contains some electrolytes |
| Exhaled air | Water vapour is lost during breathing | No significant electrolyte loss |
Regulation of Balance
Water balance depends on matching water intake with water output. When blood water content falls, its solute concentration rises. Osmoreceptors in the hypothalamus detect this change, stimulating thirst and the release of antidiuretic hormone (ADH) from the posterior pituitary gland.
ADH increases water reabsorption in the kidney collecting ducts. This produces a smaller volume of more concentrated urine. When the body contains excess water, less ADH is released, producing a larger volume of dilute urine.
Electrolyte balance is regulated mainly by the kidneys. Aldosterone, released by the adrenal cortex, increases sodium reabsorption and potassium excretion. Water follows reabsorbed sodium by osmosis, helping maintain blood volume and pressure.
A common misconception is that sweat contains only water. Sweat also contains electrolytes, especially sodium and chloride, so prolonged exercise can require replacement of both fluid and electrolytes. However, replacing sweat with highly concentrated salt solutions may worsen fluid imbalance.
Exam Technique
For an IB SEHS response, distinguish clearly between routes of gain, routes of loss and hormonal regulation. Link exercise to increased sweating, reduced blood volume and the roles of thirst, ADH, aldosterone and the kidneys rather than simply stating that athletes “become dehydrated.”
