The kidneys are the principal organs regulating electrolyte balance, supported by the hypothalamus, posterior pituitary gland, adrenal glands and heart. Their hormones adjust water and ion retention or excretion to maintain homeostasis.
The Regulation Mechanism
The kidneys filter blood and selectively reabsorb or excrete electrolytes such as sodium, potassium and chloride. Because water follows dissolved particles by osmosis, controlling sodium reabsorption also influences blood volume, blood pressure and hydration.
Osmoreceptors in the hypothalamus detect changes in blood osmolarity. If osmolarity rises, indicating dehydration, the hypothalamus stimulates thirst and causes the posterior pituitary to release antidiuretic hormone (ADH). ADH increases the permeability of kidney collecting ducts, so more water is reabsorbed and a smaller volume of concentrated urine is produced.
| Organ or structure | Hormone or mechanism | Main effect |
|---|---|---|
| Kidneys | Renin | Initiates the renin-angiotensin-aldosterone system when blood pressure or sodium delivery is low |
| Adrenal cortex | Aldosterone | Increases sodium reabsorption and potassium excretion by the kidneys; water follows sodium |
| Hypothalamus and posterior pituitary | ADH | Increases renal water reabsorption in response to increased blood osmolarity |
| Heart atria | Atrial natriuretic peptide (ANP) | Promotes sodium and water excretion when blood volume is high |
During exercise, sweating removes water and electrolytes, especially sodium and chloride. Reduced plasma volume activates renin release, while increased osmolarity promotes ADH release and thirst. Together, these responses conserve fluid and help preserve cardiovascular and neuromuscular function.
A common misconception is that ADH directly increases sodium reabsorption. ADH primarily regulates water balance, whereas aldosterone directly increases sodium reabsorption and potassium secretion.
Exam Technique
For IB SEHS A.2.1, explain the stimulus, hormone, target organ and response. Distinguish ADH from aldosterone and link electrolyte regulation to exercise-induced sweating, plasma volume and maintenance of homeostasis.
