Water and the Human Body Series:
Chapter 1 – Water Content in the
Human Body
Water is the largest single component of the human body, accounting for about
50–60% of total body mass. For a healthy lean young male with a body mass of 70
kg (154lbs), total body water will consist of about 42 liters. A healthy lean young
woman is about 50% of total body mass. This is due to a women typically having
less skeletal muscle and more body fat than males.
Water makes up between 45 and 75% of body weight, with the variability due
primarily to differences in body fat. While most tissues including muscle, skin,
and visceral organs are over 75% water, adipose tissue contains less than 10%
water.
Water is also contained inside organs, in gastrointestinal, cerebrospinal,
peritoneal, and ocular fluids.
The gender differences, from the teenager years onwards, are due to their
differing fat levels, as is the drop in the elderly who replace muscle mass with fat.
There is little difference with gender or age from childhood onwards, if allowance
is made for this fat content.
The estimation of body water will vary with factors such as
Type of population
Number of people sampled
Age of people sampled
Body fat percentage
Variation due to Age
Neonates contain more water than adults: 75-80% water with proportionately
more extracellular fluid (ECF) then adults. At birth, the amount of interstitial fluid
is proportionally three times larger than in an adult. By the age of 12 months, this
has decreased to 60% which is the adult value.
Total body water as a percentage of total body weight decreases progressively
with increasing age. By the age of 60 years, total body water (TBW) has
decreased to only 50% of total body weight in males mostly due to an increase in
adipose tissue.
For both men and women, the percent of body weight that is water decreases
with age:
Fetus – 90% of total weight
Infant – 74% of total weight
Child – 60% of total weight
Teenager
Male 59% of total weight
Female 56% of total weight
Adult
Male 59% of total weight
Female 50% of total weight
Adult over 50 years
Male 56% of total weight
Female 47% of total weight
Variation by Body Fat Percentage
Adipose (fat) tissue is the least hydrated tissue in the body (20% hydrated), even
bone contains more water than fat. In contrast, skeletal muscle contains 75%
water. So, the more muscles one has, the higher the total body water percentage
will be.
The so-called lean body mass, which means a body stripped of fat, contains 0.69
parts of water (69%) of the total body weight in all persons. – Such high values
are observed in the newborn and in extremely fit athletes with minimal body fat.
Babies have a tenfold higher water turnover per kg of body weight than adults do.
As an average females have a low body water percentage compared to males.
Such differences show sex dependency, but the important factor is the relative
content of body fat, since fat tissue contains significantly less water (only 10%)
than muscle and other tissues (70%). This is why the relative water content
depends upon the relative fat content.
The percentage of body weight that is water therefore varies inversely with body
fat. In the average lean adult male around 60% of the body weight is water. The
remaining body weight consists of 16-18% fat with 22-24% protein, carbohydrate
and other solids. In the female the percentage of body weight that is water is
lower due to a relatively greater amount of subcutaneous fat.
Variation between Tissues
Most tissues are water-rich and contain 70-80% water. The three major
exceptions to this are:
Plasma: 93% water (and 7% ‘plasma solids’)
Fat: 10-15% water
Bone: 20% water
Fluid Compartments of the Human Body
Fluid compartments in the human body broadly comprise two compartments,
each with several subdivisions:
The Intracellular Fluid (ICF)
The Extracellular Fluid (ECF)
The Intracellular Fluid (ICF)
The intracellular fluid (ICF) is about 40 % of body weight and is contained within
the various cells of the body. Intracellular fluid (ICF) makes up approximately
60-65% of total body water.
The ICF is the fluid that is confined within the cell membranes. Intracellular fluid
is found inside the two-layered plasma membrane of the body’s cells, and is the
matrix in which cellular organelles are suspended, and chemical reactions take
place. In humans, the intracellular compartment contains on average about 28
liters of fluid.
The Extracellular Fluid (ECF)
The extracellular fluid (ECF) makes up 35-40% of total body water.
Extracellular fluid (ECF) or extracellular fluid volume (ECFV) usually denotes all
body fluid outside of the cells. The volume of extracellular fluid is typically 15
liters where 12 liters is interstitial fluid and 3 liters is plasma).
The ECF is divided into several smaller compartments:
Plasma
Interstitial fluid
Fluid of bone and dense connective tissue and
Transcellular fluid
These compartments are distinguished by different locations and different kinetic
characteristics. The composition of ECF is high in sodium and chloride and low in
potassium and magnesium.
Plasma is the only major fluid compartment that exists as a real fluid collection all
in one location. It differs from ISF in its much higher protein content and its high
bulk flow (transport function). Blood contains suspended red and white cells so
plasma has been called the ‘interstitial fluid of the blood’.
Interstitial fluid (ISF) consists of all the bits of fluid which lie in the interstices of
all body tissues. This is also a ‘virtual’ fluid meaning that it exists in many
separate small bits but is spoken about as though it was a pool of fluid of uniform
composition in the one location.
The ISF bathes all the cells in the body and is the link between the ICF and the
intravascular compartment. Oxygen, nutrients, wastes and chemical messengers
all pass through the ISF.
Lymph is considered as a part of the ISF. The lymphatic system returns protein
and excess ISF to the circulation.
The fluid of bone and dense connective tissue is significant because it contains
about 15% of the total body water. This fluid is mobilized only very slowly and this
lessens its importance when considering the effects of acute fluid interventions.
Trans-cellular fluid is a small compartment that represents all those body fluids
which are formed from the transport activities of cells. It is contained within
epithelial lined spaces.
It includes cerebral-spinal fluid (CSF), gastrointestinal tract fluid (GIT), bladder
urine, aqueous humour and joint fluid. It is important because of the specialized
functions involved. The fluid fluxes involved with GIT fluids can be quite
significant.
Typical values for the size of the fluid compartments are listed in the table below.
Body Fluid Compartments (70 kg male)
Body Fluid
Compartments
ECF
Plasma
% of Body
Weight
27
4.5
% of Total
Body Water
45
7.5
Volume
(Liters)
19
3.2
ISF
Dense CT water
Bone water
Trans-cellular
12.0
4.5
4.5
1.5
20.0
7.5
7.5
2.5
8.4
3.2
3.2
1.0
ICF
TBW
33
60%
55
100%
23
42 liters
Future Chapters in the Series:
Chapter 2 – The Function of Water in the Human Body
Chapter 3 – Water Gain (Consumption) in the Human Body
Chapter 4 – Water Loss by the Human Body
Chapter 5 – Water Balance in the Human Body
Chapter 6 – Dehydration
Chapter 7 – Waters Effect on Neurological Health
Chapter 8 – Edema – Fluid Retention
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