fluid and electrolites

8/7/2019 Fluid and Electrolites

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Fluid and Electrolites

Composition and volumes of body fluid compartments

Fluid content of the body

Varies with age, sex, adipose tissue

Females 45-50% TBW

Males 50-60% TBW

Infants 77% TBW

RULE OF THIRDS

Intracellular: 2/3 (40% TBW)

Extracellular: 1/3 (20% TBW)

a)Interstitial + Lymph: 2/3 (15% TBW)b)Intravascular: 1/3 (5% T

Total body wateraccounts for60 per cent of the body weight and is contained within two majorcompartments.The ECF compartment represents water outside of cells and accounts for 20% of the bodyweight. The ICF compartment represents the water within cells and accounts for 40% of body weight.

The ECF is subdivided into several compartments. The largest is theinterstitial fluid (ISF), which is fluidsurrounding the cells in the various tissues of the body (includes lymph, bone) and accounts for 75 %of the ECF. The remaining 25 per cent of ECF isplasma localized within the circulatory system. So in a

70kg person: total body water!42 L, ECF !14 L, ICF !28 L, ISF !10.5 L and plasma volume!3.5 L.


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Figure 2 illustrates some of the major differences in composition between the main body fluidcompartments. The dominant cation in the ICF is potassium, while the dominant cation in the

ECF is sodium. Phosphates and negatively charged proteins constitute the major intracellularanions, while chloride and, to a lesser extent, bicarbonate dominate the ECF anions. An

important difference between the plasma and interstitial compartments of the ECF is that only

plasma contains significant concentrations of protein.

Figure 2Normal distribution of body water and electrolytes. Schematic representation of volume (l = litres) and composition(dominant ionic species only shown) of the intracellular fluid (ICF) and extracellular fluid (ECF) in a 70 kg male. The main

difference in composition between the plasma and interstitial fluid (ISF) is the presence of appreciable concentrations of proteinin the plasma (not shown) but not the ISF.

The major force maintaining the difference in cation concentration between the ICF and ECF isthe activity of the sodium-potassium pump (Na,K-activated ATPase) integral to all cell

membranes. Maintenance of the cation gradients across cell membranes is essential for many cellprocesses, including the excitability of conducting tissues such as nerve and muscle. The

difference in protein content between the plasma and the interstitial fluid compartment ismaintained by the protein permeability barrier at the capillary wall. This protein concentration

gradient contributes to the balance of forces across the capillary wall favouring fluid retentionwithin the capillaries (the colloid osmotic, or oncotic, pressure of the plasma), thus maintaining

an adequate circulating plasma volume.Distribution of different types of Replacement fluids

Figure below shows the relative effects on the compartments of the addition of identical volumes of

water, saline and colloid solutions. Thus, 1 L of water given intravenously as 5% dextrose is distributed

equally into all compartments, whereas the same amount of 0.9% saline remains in the extracellular

compartment. The latter is thus the correct treatment for extracellular water depletion sodium

keeping the water in this compartment. The addition of 1 L of colloid with its high oncotic pressure stays

in the vascular compartment and is the treatment for hypovolaemia.


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Sites involved in fluid loss

Each day, the body gains and loses fluid through several different processes. This illustration

shows the primary sites of fluid losses and gains as well as their average amounts. Gastric,intestinal, pancreatic, and biliary secretions are almost completely reabsorbed and aren't usually

counted in daily fluid losses and gains.


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fluid and electrolites

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