SN6 SHORT NOTE 5M ★ High-yield

Describe the properties of water and the distribution of body water.

Biochemistry · 1st Professional — Pre-Clinical

Definition

Water is the medium of all biological reactions and the most abundant constituent of the body — about 60% of body weight in an adult male, 50–55% in a female, and up to 75% in a newborn.

Structure and Resulting Properties

A polar molecule with a bent shape; oxygen is electronegative, giving a partial negative charge, and the hydrogens a partial positive
Each molecule can form four hydrogen bonds
This hydrogen bonding accounts for nearly every unusual property water has

Property	Value or feature	Biological importance
High specific heat	1 cal/g/°C	Buffers body temperature against metabolic heat
High latent heat of vaporisation	540 cal/g	Sweating is an efficient cooling mechanism
High dielectric constant	80	Excellent solvent for ionic and polar compounds
High surface tension	—	Alveolar stability; countered by surfactant
Maximum density at 4 °C	—	Ice floats, insulating water below
Slight ionisation	Kw = 10−14	The basis of pH and of acid–base balance

Distribution of Body Water

Compartment	Percentage of body weight	Volume (70 kg adult)
Total body water	60%	42 L
Intracellular	40% (two-thirds of TBW)	28 L
Extracellular	20% (one-third)	14 L
— Interstitial	15%	10.5 L
— Plasma	5%	3.5 L
— Transcellular	1–2%	CSF, synovial, pleural, gastrointestinal secretions

Sodium is the chief extracellular cation; potassium the chief intracellular one
Measured by dilution techniques — deuterium oxide for total body water, inulin for extracellular fluid, Evans blue for plasma volume

Water Balance

Intake (mL/day)	Output (mL/day)
Drinking — 1200	Urine — 1500
Food — 1000	Skin (insensible) — 500
Metabolic water — 300	Lungs — 400
Total 2500	Faeces — 100; total 2500

Metabolic (oxidation) water is produced by the respiratory chain — fat yields the most (1.07 mL/g), which is how hibernating animals and the camel survive without drinking
Obligatory urine volume is about 500 mL, the minimum needed to excrete the daily solute load

Regulation

Thirst — hypothalamic osmoreceptors respond to a rise in osmolality of as little as 1–2%
Antidiuretic hormone (vasopressin) — released on a rise in osmolality or a fall in volume; acts on aquaporin-2 in the collecting duct
Renin–angiotensin–aldosterone system — conserves sodium and therefore water

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