Water turnover is considered in terms of external balance and internal fluxes.
External balance refers to the comparison between the water input from and the water output to the external environment. Over any period of time, input equals output and the organism is in water balance.
Internal balance or flux refers to the movement of water across the capillaries of the body (including the secretion and absorption of the various transcellular fluids) and movement of water between interstitial and intracellular fluids.
Total body water volume is tightly controlled with sensitive mechanisms that respond to changes in osmolality or intravascular volume. Estimates of daily water requirements are based several factors but that based on metabolic rate is probably the most accurate.
Estimation of Daily Water Requirements in Unstressed Healthy Adults
In the presence of disease, these estimates of fluid requirements are unreliable. Fluid administration should always be based on clinical circumstances (eg blood loss, internal fluid loss in third spaces, abnormal haemodynamics requiring intravascular volume loading, oliguria with acute renal failure). These disorders will not be discussed here as the emphasis is on normal physiology.
Water is required to replace losses which normally consist of insensible losses (from the skin and respiratory tract), urine, sweating and faecal loss.
An obligatory urine loss occurs because of the need to remove various solutes from the body. Other losses (eg sweating and faecal losses) are quite small under normal conditions. Faecal water loss averages about 200 mls/day but diarrhoea can be associated with large fluid & electrolyte losses.
Daily water requirements can vary greatly. The minimum water required for urine is dependent on the daily solute excretory load and the maximum urinary concentration achievable. For example, a typical daily solute load of 600 mOsms in a patient with a maximum urinary concentrating ability of 1200 mOsm/kg will require a minimum urine volume of 500mls/day to excrete it. If urine volume was less than this amount, solutes would accumulate and renal failure would be present. Ill or elderly patients are typically not able to achieve urine osmolality of 1200 mOsm/kg so the obligatory minimum urine volume required for solute excretion can be much higher than 500 mls.
The minimal amount of fluid loss from the body that can occur is referred to as the obligatory water loss. This sets a figure for the minimal amount of fluid intake that is required to maintain total water balance.
Components of Daily Obligatory Water Loss
The typical values in this example total to 1,600 mls. In stressed individuals, this obligatory loss may be much higher. Obligatory urine volume is variable (eg. the solute load may be decreased or the maximum urine concentration may be much lower then 1200 mOsm/l). There is also an inter-relationship between these two factors: as daily solute load increases, the maximum urine osmolality decreases until, at high solute loads, it is the same as plasma osmolality. This occurs because the increased urine flows necessary to carry the increased solute washes out the medullary osmoles and the time spent in the tubules is decreased.
Fluid input is from 2 major sources:
Food is an important source of water as nearly all food was once living (i.e. cellular) and has a high water content. Some processed foods may have a very low water content.
Metabolic water is water produced during the oxidation of food. Carbohydrates are completely metabolised to carbon dioxide and water. Metabolic water is about 350 to 400 mls/day (ie 5 mls/kg). This offsets some of the obligatory water losses.
Water intake in excess of requirements is excreted as urine. The other routes of fluid loss are not under regulatory control for maintaining water balance. The kidney is effectively the major effector organ in excreting excess water.
Hospitalised patients may have other sources of fluid input or loss (eg. IV fluids, vomiting, third space losses, diarrhoea) and the volumes involved can be very significant.
The net movement of water between the intravascular and interstitial compartments across the capillary membrane depends on the balance of hydrostatic and oncotic pressures as described by Starling's hypothesis.
The major factor governing net movement of water between the ICF and the ISF is the osmotic forces.
Some examples of internal fluxes are:
The diffusional turnover is very much higher than the net fluid loss from the capillaries via filtration.