The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [247]
Fluid balance
Definition
In the human homoeostatic state, the intake of fluids equals fluid excreted from the body, thereby maintaining optimal hydration. In nursing practice, this term refers to the procedure of measuring fluid input and output to determine fluid balance (Marieb and Hoehn 2010, Scales and Pilsworth 2008).
Anatomy and physiology
Body composition
The human body is made up of approximately 60% water (this varies with age, gender and percentage of fatty tissue) (Alexander et al. 2000, Scales and Pilsworth, 2008, Urden et al. 2002). Bodily water/fluid is essential to life and vital for:
controlling body temperature
the delivery of nutrients and gases to cells
the removal of waste
acid–base balance and
the maintenance of cellular shape (Baumberger-Henry 2008).
Total body water is distributed between two main compartments: intracellular fluid (ICF, within the cell) and extracelluar fluid(ECF, outside the cell). ECF is further divided into the intravascular space, within the blood vessels (known as plasma), the interstitial space which surrounds the cells and the transcellular space. The transcellular space contains specialized fluids such as cerebrospinal fluid but does not readily exchange fluid with other compartments so is rarely considered in fluid management (Bishop 2008, Haskal 2007).
Bodily fluid is a composition of water and a variety of dissolved solutes which Marieb and Hoehn (2010) classify as electrolytes and non-electrolytes. Non-electrolytes include glucose, lipids, creatine and urea and are molecules that do not dissociate in solution and have no electrical charge. Electrolytes such as potassium, sodium, chloride, magnesium and bicarbonate all dissociate in solution into charged ions that conduct electricity. Concentration of these solutes varies depending on the compartment in which they are contained; for example, ECF has a high sodium content (135–145 mmol/L) and, relatively low in potassium (3.5–4.5 mmol/L) and ICF is the reverse – high in potassium but lower in sodium. The movement and distribution of fluid and solutes between compartments are controlled by the semi-permeable phospholipid cellular membranes that separate them (Baumberger-Henry 2008).
Transport and movement of water and solutes
Water can readily and passively pass across the membrane and does so by osmosis (see Table 8.1) in response to changing solute concentrations. The amount of solute in solution determines the osmolarity – the higher the solute concentration, the higher the osmolarity; this is also referred to as the osmotic pressure (or pull). Electrolytes move across the membrane via the protein channels, some by diffusion (see Table 8.1) and also via a passive mode of transport where solutes move towards an area of low solute concentration. Sodium and potassium are an exception to this rule, as they are required to move against the concentration gradient in order to preserve higher intravascular sodium concentrations. Energy is utilized to pump sodium out of the cell via the protein channels and pump potassium back into the cell, known as the sodium/potassium pump (see Table 8.1).
Table 8.1 Molecule transport modes
Transport mode Description Diagram
Osmosis Movement of water from an area of low solute concentration to an area of high solute concentration
Diffusion Movement of solutes from an area of high concentration to an area of low concentration
Facilitated diffusion Movement of solutes from an area of high concentration to an area of low concentration, facilitated by a carrier molecule (e.g. glucose only enters the cell carried by insulin)
Active transport Movement of solutes against the concentration gradient from an area of low concentration to an area of high concentration. This transport requires