Water balance and its regulation

📌 Water Balance

⚖️ Water gain = Water loss ≈ 2300 ml/day

🟢 Water Gain:

Fluids (drinking): 1400 ml

Food: 700 ml

Metabolic water: 200 ml

🔴 Water Loss:

Urine (kidneys): ~1400 ml (500 ml obligatory, rest facultative)

Feces: ~100 ml

Insensible water loss (~700 ml):

Respiratory tract: ~350 ml

Skin (diffusion & evaporation): ~350 ml

Sweat: ~100 ml (can reach 1–2 L/h in hot weather or heavy exercise).

📌 Control of Water Balance

🟠 Control of Water Gain (Thirst):

Central Mechanism:

↑ ECF osmolality → water leaves thirst neurons → cell shrinkage → thirst.

↓ ECF volume → ↓ blood pressure → ↓ baroreceptor inhibition → thirst.

Peripheral Mechanism:

Dryness of mouth/pharynx due to ↓ salivary secretion.

🟣 Control of Water Loss (ADH):

💉 Secreted from posterior pituitary.

🏃‍♂️ Increases water reabsorption in collecting ducts → water retention + concentrated urine.

Stimuli for ADH secretion:

↑ ECF osmolality

↓ ECF volume

📌 Regulatory Scenarios

💧 If body water increases:

↑ Water loss (↑ urine volume)

↓ Water gain (↓ thirst)

🏜️ If body water decreases:

↓ Water loss (↓ urine via ADH)

↑ Water gain (↑ thirst)

🚨 Disturbances of Water Balance

🟠 Dehydration (Loss > Gain):

1. Iso-osmotic: Water and NaCl lost equally (e.g., hemorrhage, vomiting, diarrhea, burns).

2. Hyperosmotic: Water loss > NaCl loss (e.g., diabetes insipidus, diabetes mellitus, excessive sweating).

3. Hypo-osmotic: NaCl loss > water loss (e.g., Addison’s disease).

🟣 Overhydration (Gain > Loss):

1. Iso-osmotic overhydration

2. Hypo-osmotic overhydration