These in turn are controlled by intracellular osmoreceptors, principally in the hypothalamus, to some extent by volume receptors in capacitance vessels close to the heart, and via the renin-angiotensin system. Regulation of extracellular volume in health andīody water homeostasis is effected by thirst and the urine concentrating and diluting functions of the kidney. Prostaglandins 12 and E2 are also generated within the kidney in response to angiotensin II, acting to maintain glomerular filtration rate and sodium and water excretion, modulating the sodiumretaining effect of this hormone In addition sodium concentration in the distal tubule and sympathetic nerve activity alter renin release from the juxtaglomerular cells.
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Receptors in the walls of the afferent glomerular arterioles respond, via the juxtaglomerular apparatus, to changes in renal perfusion, and control the activity of the renin-angiotensin-aldosterone system. In addition volume receptors in the cardiac atria control the release of a powerful natriuretic hormone-atrial natriuretic peptide (ANP)- from granules located in the atrial walls. These volume receptors respond to a slight reduction in effective circulating volume and result in increased sympathetic nerve activity and a rise in catecholamines. These are located in the vascular tree in the left atrium and major thoracic veins and also in the sinus body and aortic arch. These ‘volume’ receptors can be divided into extrarenal and intrarenal baroreceptors.ĮXTRARENAL. This is achieved by activation of receptors which respond to extracellular volume rather than the change in sodium concentration. Concentration only implies the relative amounts of sodium to water and does not suggest the absolute amounts or volumes of either. Control of the body’s sodium is exerted by tight control over renal excretion. It is essential to distinguish between the concentration of sodium in the ECF (in practice the plasma sodium) and the amount of sodium. Sodium is ‘diluted’ by water to keep it at the right concentration. The addition of 1 litre of colloid with its high oncotic pressure stays in the vascular compartment and is the treatment for hypovolaemia. Regulation of extracellular volume The extracellular volume is controlled by the total body content of sodium. The latter is thus the correct treatment for extracellular water depletion-sodium keeping the water in this compartment. Thus, 1 litre of water given intravenously as 5% dextrose is distributed equally into all compartments, whilst 1 litre of 0.9% saline remains in the extracellular compartment. The relative effect of the addition of identical volumes of water, saline and colloid solutions on the different compartments. The composition of intracellular and extracellular fluids.ĭistribution of water between the vascular and extravascular The distribution of extracellular water between the vascular and extravascular (interstitial space) is determined by the equilibrium between hydrostatic and oncotic pressures (Fig.
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For instance, plasma alcohol or ethylene glycol concentration can be estimated by subtracting calculated from measured osmolality. The major determinant of plasma osmolality is sodium concentration: plasma osmolality can be approximated as (2 x + , unless there is an unmeasured osmotically active substance present. NaCl) have twice the osmolality of undissociated particles (urea, glucose). Fully ionized (dissociated) molecules (e.g. The major intracellular solute is potassium, balanced largely by phosphate and ionic protein, whilst the major extracellular solute is sodium chloride. Osmolality is determined by the concentration of osmotically active particles. There is no barrier to the passage of water between the extracellular and intracellular compartments as the distribution of water is governed by osmotic forces and all compartments have the same osmolality (280- 290 mosmol kg”). The remainder is extracellular fluid (ECF), comprising the interstitial fluid (10.5 litres) and the vascular compartment (3.5 litres). Two-thirds (28 litres) is intracellular fluid (I CF) This percentage varies with age, sex and body build, being less in women and the obese, as fat has a lower water content. Changes in total body water from one day to the next is, in an individual, reflected by changes in body weight. In a healthy, 70-kg male, total body water is approximately 42 litres, distributed between several compartments.
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Water accounts for 50–60% of total body weight.