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PathophysiologyBody fluids are continually in motion, moving in and out of the blood and lymph vessels, through the spaces surrounding the cells, and through the bodies of the cells themselves. Fluid within the cell is considered to be in one compartment (intracellular) and fluid outside the cell in another (extracellular) (Figure 3-1). The three types of extracellular fluid (ECF) and body fluid distribution are shown in Box 3-1. Excretion of the body's fluid is mainly achieved through the kidney. Control of fluid balance is managed by:• Osmoreceptors in the hypothalamus sense the internal environment and promote the intake of fluid (thirst mechanism) when needed.• Antidiuretic hormone (ADH), released by the posterior pituitary, controls how much fluid leaves the body in the urine and causes reabsorption of water from the kidney tubules.• Aldosteroneand atrial natriuretic peptide (ANP)regulate the reabsorption of water andsodium ions from the kidney tubules.• Baroreceptors in the carotid sinus and aortic arch detect pressure changes that indicate an increase or decrease in blood volume and stimulate the sympathetic or parasympathetic nervous system to return the pressure to normal.FIGURE 3-1 Factors that influence body fluid distribution.Box 3-1Body Fluid DistributionExtracellular Fluid (Outside of Cells)• Approximately one third of total body water• Transports water, nutrients, oxygen, waste, etc., to and from the cells• Regulated by renal, metabolic, and neurologic factors• High in sodium (Na+) contentIntravascular Fluid• Fluid within the blood vessels• Consists of plasma and fluid within blood cells
• Contains large amounts of protein and electrolytesInterstitial Fluid• Fluid in the spaces surrounding the cells• High in Na+contentTranscellularFluid• Includes aqueous humor; saliva; cerebrospinal, pleural, peritoneal, synovial, and pericardial fluids; gastrointestinal secretions; and fluid in the urinary system and lymphaticsIntracellular Fluid (Within Cells)• About two thirds of total body fluid• Fluid contained within the cell walls; most cell walls are permeable to water• High in potassium (K+) contentModified from deWit SC, O'Neill P: Fundamental concepts and skills for nursing, ed. 4, Philadelphia, 2013, Saunders.31Pain, nausea, and stress can also cause the release of ADH by the pituitary. When the ECF volume is low, or when sodium concentration is elevated, the adrenal cortex releases aldosterone, which causes reabsorption of sodium from the renal tubules. The renin-angiotensin-aldosterone system regulates the release of aldosterone. Renin is released when there is decreased blood flow to the kidney. Baroreceptors in the atrium of the heart detect fluid overload and stimulate the myocardium to release ANP. ANP helps protect the body from fluid overload by increasing sodium excretion. Where