When the body is in a balanced and healthy state, or what is known as homeostasis, the body is not designed to store water. However, if the body is in a diseased state, the body has means to store water. (See Chapters on Dehydration, Hyponatremia and Edema -Fluid retention)
Water is being lost by the body on a constant basis, so the water content of the body is constantly changing.
Water loss from the human body is classified as either sensible loss or insensible loss. Sensible” loss is loss that can be perceived by the senses and can be measured. Insensible losses can neither be perceived nor measured directly. You’ve lost it, but you don’t know that you’ve lost it.
- Through the kidneys (urine excretion)
- Through the gastro-intestinal tract (feces)
- Trans-epidermal diffusion: water that passes through the skin and is lost by evaporation (perspiration and sweating)
- Evaporative water loss from the respiratory tract (by breathing)
The quantity of water loss varies with the lifestyle and environmental conditions of the individual, such as, gender, body size, weather, clothing worn, activity levels and a whole range of other factors. However, on average, a typical adult loses about 2.1 to 2.7 liters (L) of water per day, broken down as follows:
- lungs 400–500 mL (18.5%)
- skin 400–500 mL (18.5%)
- stool 80–100 mL (3%)
- urine 1–1.6 L (60%)
Figure 1: Average Intake and Output of water per day
An individual who engages in physical exercise or is in a hot environment will loss additional water via sweat. The amount of additional water loss via sweat may be up to several liters per day.
The environmental factors that effect water loss for each bodily organ and system, include:
Skin and lungs – If the air is dry and hot, water loss is increased
Urine – Urine water loss is dependent on the volume of the fluid consumed and total losses by other routes. Urine water loss is also dependent on sodium chloride content in the diet as well as protein consumption. The more sodium chloride and protein consumed, the more water loss is decreased due to the limited capacity of the kidneys to concentrate the urine. If water intake is restricted, the kidneys will conserve water by producing a more concentrated urine.
To maintain a healthy homeostatic state, the intake of water should be more than the loss via skin, lungs and feces and the any surplus is excreted by the kidneys. Typically urinary volume largely depends on intake of water, which should exceed the average output, taking into consideration external factors such as exercise, diet and environment.
Previously published Chapters in the Series
Future Chapters in the Series:
Chapter 5 – Water Balance in the Human Body
Chapter 6 – Dehydration
Chapter 7 – Waters Effect on Neurological Health
Chapter 8 – Edema – Fluid Retention