Maintaining water balance is essential for homeostasis. The kidneys are central to this process, regulating water through filtration, reabsorption, secretion, and excretion. Proper regulation ensures the body retains necessary water while eliminating excess, maintaining the osmolarity of bodily fluids.
Concepts in Water Balance
1. Filtration
- Occurs in the glomerulus of the nephron.
- Blood is filtered through the glomerular membrane:
- Water & small solutes → Bowman’s capsule
- Proteins & blood cells → remain in bloodstream
2. Reabsorption
- Majority of water filtered is reabsorbed in the renal tubules, primarily the proximal convoluted tubule.
- Influenced by:
- Osmotic gradients
- Aquaporins (water channels) in tubule cells
- Antidiuretic Hormone (ADH) regulates water reabsorption in distal tubule and collecting duct:
- High ADH → concentrated urine, reduced volume
- Low ADH → dilute urine, increased water excretion
3. Secretion
- Active secretion of excess ions and waste occurs in the distal tubule and collecting duct.
- Fine-tunes filtrate composition → determines final urine volume and concentration
4. Excretion
- Urine passes from kidneys → ureters → bladder → urethra.
- Volume and concentration depend on hydration status and balance between water intake and loss.
Hormonal Regulation of Water Balance
| Hormone | Source | Function & Effect |
|---|---|---|
| Antidiuretic Hormone (ADH) | Hypothalamus (posterior pituitary) | Increases water reabsorption; high levels → concentrated urine, water retention |
| Aldosterone | Adrenal cortex | Promotes sodium reabsorption in distal tubule & collecting duct; water follows sodium osmotically |
| Atrial Natriuretic Peptide (ANP) | Heart atria | Reduces sodium & water reabsorption → promotes diuresis, lowers blood volume & pressure |
| Renin-Angiotensin-Aldosterone System (RAAS) | Kidneys & liver | Activated by low blood pressure/volume; renin → angiotensin II → aldosterone release → increased sodium & water reabsorption; vasoconstriction raises blood pressure |
Osmoreceptors and Volume Receptors
Osmoreceptors
- Location: Hypothalamus (supraoptic & paraventricular nuclei)
- Function: Detect changes in plasma osmolality
- Mechanism:
- Plasma osmolality ↑ → osmoreceptors shrink → stimulate ADH release
- ADH ↑ → collecting ducts more permeable → water reabsorbed → urine concentrated → plasma osmolality normalizes
- Thirst Mechanism: High osmolality triggers thirst → promotes fluid intake
Volume Receptors (Baroreceptors)
- Location: Atria of heart, aortic arch, carotid sinuses
- Function: Detect blood volume & pressure changes
- Mechanism:
- Blood volume ↓ → atrial stretch ↓ → receptor firing ↓ → ANP secretion ↓
- Stimulates RAAS → increases sodium & water reabsorption → blood volume restored
- ADH Secretion: Volume depletion stimulates ADH release, promoting water retention
- Sympathetic Response: Vasoconstriction, ↑ heart rate, ↑ renal sodium & water reabsorption
Integration of Mechanisms
Water balance is maintained through a complex interplay of renal processes, hormonal signals, and neural mechanisms:
- Kidneys adjust water excretion according to needs
- ADH, aldosterone, ANP, and RAAS coordinate reabsorption and excretion
- Osmoreceptors and volume receptors detect plasma osmolality and blood volume changes
- Sympathetic nervous system supports rapid compensation during dehydration or blood loss
By integrating these mechanisms, the kidneys ensure homeostasis, keeping the internal environment stable for optimal physiological function.
