CLINICAL systems of life Homeostasis part 4: fluid balance fiG 1. HoRmoNal CoNtRol of tHe kiDNey By NeGative feeDBaCk looPs Authors Brendan Docherty, msc, PGCe, RN, l Chemical and bioelectrical distributions within and is patient access manager, Executive Director’s between cells and tissues; Unit, Prince of Wales Hospital, Sydney, Australia; l Heat, hormone and nutrient distribution around Colette foudy, RN, GradDip, is clinical care the tissues; coordinator, intensive care unit, St George Private l Carriage of waste products to the appropriate Hospital, Sydney, Australia. organ for excretion – generally through the liver and renal system. In this article, the last in a four-part series, we explore fluid maintenance with particular reference to Homeostatic control disorders of the pituitary gland. Fluid management When fluid volume decreases, the concentration techniques and related nursing care are also covered. of sodium in the blood will increase (increased Body fluids are mainly water and electrolytes, and osmolarity, the amount of solute per unit volume), the three main organs that regulate fluid balance which in turn stimulates the hypothalamus (Tortora are the brain, the adrenal glands and the kidneys and Grabowski, 2002). The hypothalamus is an (Tortora and Grabowski, 2002). One-third of the total osmoreceptor – a sensory end organ that reacts to is circulatory fluid, sometimes known as extracellular changes in osmotic pressure and has an effect on fluid (ECF); the remainder is intracellular fluid (ICF) the pituitary gland. contained within cells (Docherty and McIntyre, 2002; In response, the posterior pituitary gland releases Edwards 2001). The anatomy and physiology of antidiuretic hormone (ADH, sometimes called homeostasis are covered in part one of this series. vasopressin) into the bloodstream, resulting in the Most tissues contain a lot of water (bones and kidneys retaining water. This in turn results in more adipose tissue are the two main exceptions). It has concentrated urine and an increase in water returned many functions, including (Tortora and Grabowski, to the ECF, thus correcting the volume depletion 2002; Adam and Osborne, 1997): (Tortora and Grabowski, 2002; Edwards, 2001) (Fig 1). l Oxygen transport and carbon dioxide regulation in When sodium concentration in the blood decreases the blood; the adrenal cortex is stimulated into secreting the 22 NT 25 April 2006 Vol 102 No 17 www.nursingtimes.net keywoRDs n Fluid balance n Haemodynamic assessment n Hormone control hormone aldosterone, which instructs the distal 0.6–1.0mg/dl (Metheny, 1996). These figures may ReFeReNces nephrons of the kidney to retain more sodium. differ slightly according to local policies. The average Normal levels of sodium in the ECF will attract and fluid input per day is 2,500ml (water as food 1,000ml, Adam, S., Osborne, S. (1997) maintain the optimum amount of water (Tortora and water as liquid 1,200ml, water from catabolism Critical Care Nursing: Science Grabowski, 2002; Edwards, 2001). 300ml) and output is 2,500ml. Daily input and output and Practice. Oxford: Oxford ADH release is also influenced in the following is summarised in Table 1. University Press. circumstances. Sensors detect stretching of the atria In some cases patients will need fluid and Docherty, B. (2002) of the heart indicating excessive returning volume of electrolyte replacement therapy, which nurses are Cardiorespiratory physical ECF (increased venous return). They stop ADH responsible for delivering and monitoring. In general assessment for the acutely ill: secretion, which leads to increased excretion of water crystalloid fluids (for example saline 0.9% solution or part 2. British Journal of Nursing; through renal filtration. The aorta and carotid arteries Hartmann’s solution) are recommended as they 11: 12, 800–807. also have receptors that are sensitive to a reduction remain in the ECF longer and are isotonic – that is, in blood pressure (related to the pressure in the left they match blood tonicity (Docherty and McIntyre, Docherty B., McIntyre L. (2002) ventricle). These receptors trigger ADH release, thus 2002; RCUK, 2000). Colloid solutions are not generally Nursing considerations for fluid conserving water at the kidneys (Tortora and indicated for most patients for ECF replacement management in hypovolaemia. Grabowski, 2002; Metheny, 1996). (RCUK, 2005; Nolan, 2001; Smith, 2000) because they Professional Nurse; 17: 9, 545–549. In addition to regulating total volume, the move easily into the ICF making them less effective Edwards, S. (2001) Regulation of osmolarity of bodily fluids is closely monitored and and creating other clinical issues (for example water, sodium and potassium: regulated because variations may cause damage to pulmonary oedema and hypotension). implications for practice. Nursing cellular structure (swelling or shrinking), disrupting Albumin levels are also important in fluid balance, Standard; 15: 22, 36–42. normal cellular function (Edwards, 2001; Metheny, and should be monitored closely. The protein assists 1996). Regulation of ECF osmolarity is achieved by in maintaining colloid osmotic pressure in the Kokko, J.P., Tannen, R.L. (1996) balancing the intake and output of sodium with that circulation (approximately 70–80% of osmotic Fluids and Electrolytes. of water. pressure is created by albumin). A reduction in Philadelphia, PA: WB Saunders Co. albumin due to loss, for example in sepsis where Fluid balance there is a higher rate of albumin loss into the tissues, Metheny, N.M. (1996) Fluid and Electrolyte Balance: Nursing To attain the correct balance of ECF and ICF a may result in hypotension and hypovolaemia (Kokko Considerations. Philadelphia, PA: patient must also take in the correct amount of and Tannen, 1996). Albumin replacement colloid Lippincott Publishers. fluid (Docherty and McIntyre, 2002). In addition, therapy is indicated for hypoalbuminaemia volume- ‘electrolyte balance’ is essential, in other words depleted patients. The greatest effect is in patients Nolan, J. (2001) Fluid resuscitation the correct concentration of various ions in the body, with temporary albumin loss, for example trauma, for the trauma patient. namely sodium, potassium and magnesium. If there surgery or burns (Kokko and Tannen, 1996). Resuscitation; 48: 1, 57–69. is too much or too little of any of these electrolytes this can cause problems. For example, cardiac Nursing care Resuscitation Council UK (2000) arrhythmias are triggered by low potassium and low Other issues that nurses should consider when Advanced Life Support Course magnesium levels (Docherty, 2002; Smith, 2000). nursing patients with fluid balance problems include: Manual. London: RCUK. The serum sodium level defined as ‘normal’ the accurate measurement and monitoring of IV fluids www.resus.org.uk. is 135–145mg/dl; the normal potassium level is over a particular 24-hour period, including correct Resuscitation Council UK (2005) 3.5–4.5mg/dl; and the normal magnesium level is documentation and prescription of fluids and fluid A Systematic Approach to types; being aware of electrolyte levels and the the Acutely Unwell Patient. taBle 1. aveRaGe ReCommeNDeD correct administration of replacement elements as London: RCUK. Daily iNPut aND outPut of fluiD prescribed; the accurate measurement of oral fluid input and urine output, working in Smith, G. (2000) Acute Life- water gain = water loss partnership with the patient where possible. threatening Events Recognition Daily morning weights are useful to establish a and Treatment Manual. Intake: 2,500ml Output: 2,500ml trend in overall fluid balance and useful in chronic Portsmouth: Open Learning, fluid management conditions such as renal failure University of Portsmouth. GI tract (100ml/day) Metabolic (RCUK, 2005; Docherty and McIntyre, 2002; Adam (200ml/day) Tortora, G.J., Grabowski, S.R. Lungs and Osborne, 1997). (2002) Principles of Anatomy (300ml/day) In seriously ill patients urinary catheterisation is Ingested and Physiology. Chichester: recommended to assist with accurate fluid balance moist foods Skin John Wiley & Sons. (800ml/day) (600ml/day) measurement, as is regular vital sign monitoring including pulse, blood pressure (remember that the blood pressure may be normal initially as the This article has been double-blind peer-reviewed. Ingested liquids Kidneys peripheral vessels compensate), respiratory rate, (1,500ml/day) (1,500ml/day) pulse oximetry oxygen saturation and central venous For related articles on this subject pressure if available (RCUK, 2005; Docherty and and links to relevant websites see McIntyre, 2002; Smith, 2000). n www.nursingtimes.net NT 25 April 2006 Vol 102 No 17 www.nursingtimes.net 23.