1. BODY WATER
Lecture summary
• Volumes and distribution
• Turnover
• Functions of water
• Dehydration and overhydration
Volumes and distribution
Total water content of the body varies according to
• Age
• Sex
• Body fat %
Variations in body fat content depends on body fat content e.g.
• 70% for a baby
• 60% for a healthy man
• 50% for an obese man
• 50% for a healthy woman
• 40% for an obese woman
Water content as percentage of total body mass by age and sex
Age Male Female
Infant 65% 65%
17-39 61% 51%
60+ 52% 46%
Body fluid components
,• The body fluids are in two components: the extracellular fluid (ECF) and the
intracellular fluid (ICF)
• The ECF and ICF are in osmotic equilibrium but have very different chemical
composition
• Intracellular fluid is 2⁄3 of the total body water volume, material moving in and out
of the ICF must cross the cell membrane
• Extracellular fluid includes all fluid outside the cells, 1⁄3 of total body fluid volume
• ECF consists of interstitial fluid (IF) lies between the circulatory system and the
cells, is 75% of the ECF volume, and plasma, the liquid matrix of blood, is 25% of
the ECF volume
• Substances moving between the plasma and interstitial fluid must cross the leaky
exchange epithelium of the capillary wall
Example
Terminology
• Intracellular fluid: water inside cell
• Extracellular fluid: water in all spaces outside cell. Consists of:
a. Interstitial fluid: intercellular
b. Blood plasma
c. Trans-cellular fluid: inside hollow organs (lymph, cerebrospinal fluid, fluid in GI
tract, intra-ocular fluids etc.)
d. Intravascular fluid: inside blood vessels
e. Extravascular fluid: outside blood vessels
Water turnover
• The volume of water in the body is controlled by a delicate balance between
intake, excretion, and reabsorption
• Water gain is 2.2 l/day (2.2 l/day from food and drink, 0.3 l/day from metabolism
• Water loss is 2.5 l/day (insensible water loss 0.9 l/day from skin and lungs, 1.5
l/day from urine, and 0.1 l/day from faeces)
• Thus intake (2.2 l/day) + metabolic production (0.3 l/day) – output (2.5 l/day) = 0
,Water balance in the GI tract
• Fluid input into digestive system (9.0 l total input into lumen)
• Ingestion is 2.0 l food and drink
• Secretion is 1.5 l saliva (salivary glands), 0.5 l bile liver, 2.0 l gastric secretions,
1.5 l pancreatic secretions, and 1.5 l intestinal secretions
• Fluid removed from digestive system (9.0 l removed from lumen)
• Absorption is 7.5 l from small intestine, 1.4 l from large intestine
• Excretion is 0.1 l in faeces
, Functions of water
• Solvent and transport medium: soluble substances (ions) and insoluble
substances (lipids)
• Absorption: digestive system
• Excretion: skin and kidneys
• Medium for chemical reactions and reagent in some chemical reactions
• Ionization of atoms: determine electrical activity of cells
• Lubrication: joints and digestive system
• Stabilizing body temperature (thermo-homeostasis)
• Properties of water that makes it suitable are great heat capacity, good heat
conduction, and high latent temperature of evaporation
Dehydration and overhydration
Water loss
• The body has no water reserves
• 5% water loss causes thirst, 10% water loss makes you feel sick, and 20% water
loss causes death
• No water intake 4-6 days causes death
• Minimum intake is 0.5-1 l/day. Compulsory urine loss is 300-500 ml/day
• Anuria: no urine passed
Dehydration
• Dehydration: a state of excessive loss of body water
• Causes: persistent vomiting, diarrhoea, excessive sweating, diuresis, and
insufficient water intake
• Treatment: water per mouth, stomach tube, saline infusion
• Manifestation: thirst, dry mouth and throat, difficulty speaking and swallowing,
loss of mass, depression of the fontanelle in infants, increased pulse rate,
decreased blood pressure, increased haematocrit, acidosis, disorientation, and
collapse
