Water and the Human Body Series: Chapter 1 – Water Content in the Human Body

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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

WaterChangewithAge

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)

WaterBodyCompartments

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.

WaterBodyPercentages

Typical values for the size of the fluid compartments are listed in the table below.

Body Fluid Compartments (70 kg male)

Body Fluid Compartments

% of Body
Weight

% of Total
Body Water

Volume
(Liters)

ECF

27

45

19

Plasma

4.5

7.5

3.2

ISF

12.0

20.0

8.4

Dense CT water

4.5

7.5

3.2

Bone water

4.5

7.5

3.2

Trans-cellular

1.5

2.5

1.0

ICF

33

55

23

TBW

60%

100%

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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