In healthy people, protein molecules are normally too large to pass out of the capillaries into the interstitial fluid. Osmotic pressure created by dissolved electrolytes in solution is called crystalloid oncotic pressure (Scales and Pilsworth, 2008). When generated by the presence of protein molecules in solution it is called colloid oncotic pressure. Osmotic pressure is generated by the molecules in a solution (Day et al, 2009).Hydrostatic pressure is created by the pumping action of the heart, and the effect of gravity on the blood within the blood vessels (Scales and Pilsworth, 2008).Formation of tissue fluidĭistribution and movement of water between the intracellular and interstitial spaces is determined by hydrostatic and osmotic pressures (Day et al, 2009): This is normally a passive process but it can be facilitated by a carrier molecule, usually a specialist protein (Davies, 2010).įluid also moves by osmosis, defined by Montague et al (2005) as “the flow of water across a semipermeable membrane from a dilute solution to a more concentrated solution until stability is reached”. Movement continues until the concentration is equally distributed” (Casey, 2004). This is “the random movement of particles from regions where they are highly concentrated to areas of low concentration. Movement of fluidsįluid circulates between compartments by diffusion. It is important to remember that, although these fluid compartments are classed as separate areas, water and electrolytes continually circulate between them (Timby, 2008).
There is also a third space, known as “transcellular fluid”, which is contained in body cavities, such as cerebral spinal fluid and synovial, peritoneal and pleural fluids (Day et al, 2009). Two-thirds of total body fluid is intracellular, and the remaining third is extracellular fluid, which is divided into plasma and interstitial fluid (Docherty and McIntyre, 2002) (Fig 1). To make a competent assessment of fluid balance, nurses need to understand the fluid compartments within the body and how fluid moves between these compartments (Davies, 2010). Too much or too little of these electrolytes can cause cardiac arrhythmias (Docherty, 2006). Plasma electrolytes are balanced as it is important to have the correct concentration of ions in the blood, especially sodium, potassium and magnesium. For example, sodium chloride (NaCl) dissolves in solution to form an equal number of positively charged sodium (Na +) ions, and negatively charged chlorine (Cl -) ions (Waugh, 2007). These compounds disassociate into particles which carryĪn electrical charge these particles in solutions are called electrolytes. This consists of water and molecules containing, for example, sodium, chloride and potassium (Mooney, 2007). What is fluid balance?įluid balance is a term used to describe the balance of the input and output of fluids in the body to allow metabolic processes to function correctly (Welch, 2010).Īround 52% of total body weight in women and 60% in men is fluid. It also discusses the importance of measuring fluid balance accurately, and the health implications of dehydration and overhydration. This article provides an overview of fluid balance, including what fluid balance is, and how and why it is measured. The report suggests fluids are being left out of reach, or are not being given at all for long periods. However, according to a recent report from the Care Quality Commission (2011), some hospital patients are not being given enough water to drink. Water is essential for life, and maintaining the correct balance of fluid in the body is crucial to health (Welch, 2010).
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