Water and electrolyte balance: core idea

· Water and electrolyte balance = maintaining enough body water and dissolved ions/electrolytes for normal physiological function.
· Balance is essential for health, exercise performance, thermoregulation, cardiovascular function, nerve function and muscle function.
· It is strongly influenced by the environment, especially heat, humidity and prolonged exercise conditions.
· Key exam link: explain how fluid loss, electrolyte loss and core temperature increase can impair performance.

This diagram shows why water is central to body function: many organs and tissues contain a high proportion of water. It helps students link hydration status to whole-body function rather than just thirst or drinking. Use it to support explanations of why dehydration can affect multiple systems during exercise. Source

Intake and loss of water and electrolytes

· Water and electrolyte intake occurs via the large intestine.
· Fluid and electrolyte loss occurs through:
· evaporation through the skin during sweating
· evaporation through the respiratory tract during breathing
· excretion via osmosis.
· During exercise, especially in hot conditions, sweating increases fluid and electrolyte loss.
· If intake does not match loss, the body moves away from homeostasis, affecting both health and performance.

This diagram shows the urinary system involved in fluid regulation and excretion. IB does not assess detailed kidney or nephron structure for this subtopic, but the image helps locate the organs involved in maintaining water balance. Use it only to support broad understanding of regulation, not detailed nephron anatomy. Source

Electrolytes: why they matter

· Electrolytes = charged ions dissolved in body fluids, including sodium and potassium.
· Sodium and potassium are especially important for water balance, muscle function and nerve function.
· Electrolytes affect osmotic gradients, helping determine where water moves in the body.
· Loss or dilution of electrolytes can disrupt muscle contraction, nerve signalling, fluid distribution and exercise performance.
· Do not just write “drink water”: high-quality answers consider both fluid replacement and electrolyte replacement.

Osmosis and osmolarity

· Osmosis = movement of water across a semi-permeable membrane from lower solute concentration to higher solute concentration.
· Osmolarity = concentration of dissolved solutes in a fluid.
· Higher urine osmolarity usually means urine is more concentrated, suggesting greater fluid conservation or possible dehydration.
· Lower urine osmolarity usually means urine is more diluted, suggesting greater water availability or overhydration.
· Exam phrase: water follows solutes, so electrolyte concentration influences fluid movement between compartments.

Three imbalance states

· Dehydration = insufficient body water, usually from fluid loss exceeding intake.
· Dehydration can reduce plasma volume, increase cardiovascular strain, increase core temperature and impair endurance performance.
· Hypernatremia = abnormally high sodium concentration, often linked to water loss exceeding sodium loss.
· Hyponatremia = abnormally low sodium concentration, often linked to excessive water intake or sodium dilution.
· All three states can affect health, cognitive function, muscle function, thermoregulation and performance.
· Exam warning: hyponatremia can occur when an athlete replaces sweat losses with too much plain water and not enough electrolytes.

Measuring hydration and electrolyte balance

· Body weight: compare before and after exercise to estimate fluid change.
· Urine colour: darker urine often suggests greater dehydration; pale urine usually suggests better hydration, but it can be affected by supplements, diet or medication.
· Osmolarity: gives a more objective indication of solute concentration in body fluids or urine.
· Best answers evaluate methods using validity, reliability, practicality and limitations.
· Thirst is useful but imperfect because it may not appear until dehydration has already started.

Regulation of water and electrolytes

· Electrolyte balance is regulated by the hypothalamus, pituitary gland and kidneys.
· The hypothalamus monitors internal conditions linked to water balance and helps coordinate responses.
· The pituitary gland releases antidiuretic hormone (ADH), which supports water retention.
· The kidneys adjust water and electrolyte loss in urine.
· IB exam limit: detailed nephron function and detailed kidney structure are not assessed for this subtopic.

This diagram shows hormonal control of water and sodium balance. For IB SEHS A.2.1, focus on the big picture: the hypothalamus, pituitary gland and kidneys help regulate water retention and electrolyte balance. Avoid overlearning detailed kidney/nephron mechanisms beyond the syllabus. Source

Cardiovascular drift

· Cardiovascular drift occurs during prolonged steady-state submaximal/aerobic exercise.
· It is caused by water loss from the body and/or an increase in core body temperature.
· It can occur in thermoneutral and hot environments, but is more problematic in heat.
· Typical pattern: heart rate increases while stroke volume decreases, even when exercise intensity stays constant.
· Performance effect: the cardiovascular system works harder to maintain cardiac output, oxygen delivery and heat dissipation.
· Strong exam link: dehydration → reduced fluid availability/plasma volume → increased cardiovascular strain → reduced performance.

Environment and performance links

· Hot or humid environments increase the challenge of maintaining water and electrolyte balance.
· High sweat rates increase loss of water and electrolytes, especially during prolonged activity.
· Dehydration reduces the body’s ability to support thermoregulation and cardiovascular stability.
· A suitable hydration strategy depends on activity duration, exercise intensity, environment and individual sweat rate.
· Exam answers should connect the environment to both physiological strain and performance outcome.

Exam application phrases

· “Water and electrolyte balance supports homeostasis during exercise.”
· “Sweat loss reduces body water and electrolytes, increasing risk of dehydration and impaired performance.”
· “Hyponatremia can result from excessive water intake without sufficient sodium replacement.”
· “Urine colour is practical but less objective than osmolarity.”
· “Cardiovascular drift is associated with fluid loss and/or increased core temperature during prolonged submaximal exercise.”