## Fluid Compartments

Example: 70-kg man (1 L = 1 kg)

### Total Body Water:

42,000 mL (60% of BW)

• Intracellular: 28,000 mL (40% of BW)
• Extracellular: 14,000 mL (20% of BW)
• Plasma: 3500 mL (5% of BW)
• Interstitial: 10,500 mL (15% of BW in a 70-kg man)

## Total Blood Volume

Total blood volume = 5600 mL (8% of BW in a 70-kg man)

## Red Blood Cell Mass

• Man, 20–36 mL/kg (1.15–1.21 L/m2) • Woman, 19–31 mL/kg (0.95–1.0 L/m2)

## Water Balance

• 70-kg man

The minimum obligate water requirement to maintain homeostasis (if temperature and renal-concentrating ability are normal and solute [urea, salt] excretion is minimal) is about 800 mL/d, which would yield 500 mL of urine.

### “Normal” Intake:

2500 mL/d (about 35 mL/kg/d baseline)

• Oral liquids: 1500 mL
• Oral solids: 700 mL
• Metabolic (endogenous): 300 mL

### “Normal” Output:

1400–2300 mL/d

• Urine: 800–1500 mL
• Stool: 250 mL
• Insensible loss: 600–900 mL (lungs and skin). (With fever, each degree above 98.6°F [37°C] adds 2.5 mL/kg/d to insensible losses; insensible losses are decreased if a patient is undergoing mechanical ventilation; free water gain can occur from humidified ventilation.)

## Baseline Fluid Requirement

35 mL/kg/24 h

### If Not a 70-kg Adult:

Calculate the water requirement according to the following kg method:

• For the first 10 kg of body weight: 100 mL/kg/d plus
• For the second 10 kg of body weight: 50 mL/kg/d plus
• For the weight above 20 kg: 20 mL/kg/d

## Electrolyte Requirements

• 70-kg adult, unless otherwise specified

### Sodium (as NaCl):

80–120 mEq/d (children, 3–4 mEq/kg/24 h)

### Chloride:

80–120 mEq/d as NaCl

### Potassium:

50–100 mEq/d (children, 2–3 mEq/kg/24 h). In the absence of hypokalemia and with normal renal function, most of this K is excreted in the urine. Of the total amount of K, 98% is intracellular, and 2% is extracellular.

If the serum K level is normal, about 4.5 mEq/L, the total extracellular pool of K+ = 4.5 × 14 L = 63 mEq. K is easily interchanged between intracellular and extracellular stores under conditions such as acidemia or alkalemia. K demands increase with diuresis and building of new body tissues (anabolic states).

### Calcium:

1–3 g/d, most of which is secreted by the GI tract. Routine administration is not needed in the absence of specific ...

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