Biotransport - Principles of Biomedical Engineering - page 23

3. The transcellular fl uid, which includes the fl uids formed during transport
activities (i.e., not inside cells but separated from plasma and interstitial
fl uid by cellular barriers). Examples include fl uid in the bladder, fl uid in
gastrointestinal tract, cerebrospinal fl uid, and sweat.

A general distribution of fluid compartments for a 70-kg male is shown in Fig-
ure 2.2(a). The total amount of intracellular fluid is the largest fraction of the fluid
volume in the body, contributing to nearly 40% of the body weight. In relation
to the body weight, water content decreases from infancy (~75% of body weight)
to old age, with the greatest decrease occurring within the first 10 years of life. In
adult males, water accounts for nearly 60% of body weight, whereas adult females
have less water content (nearly 50% body weight) in their body. This is due to more
subcutaneous (under the skin) adipose (fat) tissue, which contains less water.
The human body temperature is maintained at 37.4°C and alterations in body
temperature are regulated by many mechanisms including sweating during exercise.
Proper redistribution and replenishment of lost water due to various physiological
processes such as the metabolism is essential to normal health. An imbalance in
day-to-day water distribution can cause abnormalities in body function. Accumu-
lation of water in various parts of the body causes a number of life threatening
diseases including lung edema and cerebral edema.
As the water in sweat evaporates from the skin, evaporative cooling takes with
it some of the body heat, helping to cool the body. Along with water, the sweat

Capillary
membrane

Extracellular
fluid

Intracellular
fluid

Extracellular
fluid (14L)

Intake

Na + Na + 142 mEq/L 142 mEq/L 10 mEq/L 10 mEq/L

K + K +

4 mEq/L 4 mEq/L 140 mEq/L 140 mEq/L

Ca 2+ Ca 2+ 2.4 mEq/L 2.4 mEq/L 0.0001 mEq/L 0.0001 mEq/L

Lymphatics Mg 2+ Mg 2+ 1.2 mEq/L 1.2 mEq/L 58 mEq/L 58 mEq/L

Cl Cl

−

103 mEq/L 103 mEq/L 4 mEq/L 4 mEq/L

Plasma (3L)

Cell membrane

HCO 3 HCO 3 − 28 mEq/L 28 mEq/L 10 mEq/L 10 mEq/L

Phosphates Phosphates 4 mEq/L 4 mEq/L 75 mEq/L 75 mEq/L

Output
Kidney
Lungs
Feces
Sweat
Skin

SO 4 −2 SO 4 2 1 mEq/L 1 mEq/L 2 mEq/L 2 mEq/L

Glucose Glucose 90 mg/dl 90 mg/dl 0 to 20 mg/dl 0 to 20 mg/dl

Amino acids Amino acids 30 mg/dl 30 mg/dl 200 mg/dl 200 mg/dl

Cholesterol Cholesterol

Phospholipids
Neutral fat
Proteins

Phospholipids
Neutral fat
Proteins

0.5 (g/dl) 0.5 (g/dl) 2 to 95 mg/dl 2 to 95 mg/dl

2 g/dl (5 mEq/L) 2 g/dl (5 mEq/L) 16 g/dl (40 mEq/L) 16 g/dl (40 mEq/L)

P CO2 P CO2 46 mm Hg 46 mm Hg 50 mm Hg 50 mm Hg

P O2 P O2 35 mm Hg 35 mm Hg 20 mm Hg 20 mm Hg

pH pH 7.4 7.4 7.0 7.0

(a) (b)

Figure 2.2 (a) Distribution of fl uid in the body. (b) The composition of extracellular and intracel-
lular fl uids.