Body fluids

📌 Definition & Nature

🖇 Total Body Water (TBW): The total amount of water in the human body.

💧 Percentage of body weight

Young male: 60%

Young female: 51%

Newborn: 80%

Elderly / Obese: 45%

📉 TBW is inversely proportional to body fat.

🟣 Water content of fat-free tissue is constant: 0.72 L/kg.

🧪 Example: In a 70 kg male → TBW ≈ 42 L.

📌 Body Fluid Compartments

🟠 ICF (Intracellular Fluid): Inside the cells = 2/3 of TBW (~28 L).

🟣 ECF (Extracellular Fluid): Outside the cells = 1/3 of TBW (~14–15 L).

Plasma: 1/4 of ECF (~3–3.5 L)

Interstitial Fluid: 3/4 of ECF (~10.5 L)

Transcellular Fluid: ~1–2 L

📌 Composition of Body Fluids

💡 Plasma and interstitial fluid are similar (capillary permeability is high).

📊 Difference: Protein concentration is much higher in plasma (low capillary permeability to proteins).