Water and Electrolytes I - Lecture Notes PDF

Water and electrolytes I Dr Xikombiso Nkuna Chemical Pathology Department Aims and Objectives 1. Explain the basic concepts of pure water gain and loss. 2. Explain the basic concepts of water and salt loss. 3. Understand the basic concepts of water and electrolytes and their distribu...

Water and electrolytes I Dr Xikombiso Nkuna Chemical Pathology Department Aims and Objectives 1. Explain the basic concepts of pure water gain and loss. 2. Explain the basic concepts of water and salt loss. 3. Understand the basic concepts of water and electrolytes and their distribution across body fluids. 4. Discuss the different mechanisms that affect the movement of fluid and solute across different compartments. 5. Discuss how blood volume and RAAS plays a role in water and electrolyte balance Introduction Water is the most abundant molecule in the human body. The electrolyte compositions of the ECF and the ICF are different. The extracellular space is predominantly made up of sodium and the intracellular space of potassium. Na /K ATPase pump Na /K ATPase pump Most widespread and physiologically important active transporter in cells. Moves three Na+ ions out of the cell and two K + ions into the cell with each cycle of ATP hydrolysis. Responsible for generating the typical Na + and K+ gradients found across the cell membrane. Na /K ATPase pump Na+ gradient is used to power coupled transport of glucose and many other substances. It is estimated that in a body at rest, the activity of the Na +/K+-ATPase consumes about a third of all ATP. If the Na/K-ATPase stops working, the concentration gradients of Na+ and K+ on the inside and outside of the cell may be affected. This can interrupt cell signals. Water distribution Water moves from the intravascular space based on the differences in pressure. Pressure Effect Hydrostatic Drives fluid from vessels into interstitial space. Oncotic Driven by albumin and holds water in the intravascular compartment Osmotic Pulls water from low solute to high solute compartment Osmolality Osmolality is a physical property of a solution that is based on the concentration of osmotically active solutes. Normal ECF osmolality is in the range 275–295 mmol/kg water. Water loss from the ECF ↑ osmolality and result in movement of water from the ICF → ECF. Osmolality can be directly measured or calculated osmolality = 2x [Na+] + [urea] + [glucose] Regulation of hydration status Mechanism Source Stimulus Effect GFR Kidney Permits Na & water excretion Aldosterone Adrenal ↓ Renal perfusion Renal Na & water retention ADH Hypothalamus ↑ ECF tonicity ↓↓↓ blood Pure water retention volume ANF Cardiac atria ↑ Blood volume Renal Na & water excretion Osmolality vs tonicity The hypothalamic osmostat controls both ADH release and the sensation of thirst. The hypothalamic osmostat is acutely sensitive to small changes in plasma osmolality. The cell membrane is selectively permeable to a variety of solutes. Urea and alcohol is freely permeable. An increase in plasma osmolality due to sodium implies an increase in osmotic pressure across the cell membrane and withdraws water from the cell to equalize osmolalities. An increase in plasma osmolality due to urea does not have this effect because of the free permeability of urea between the ICF and ECF. Effective osmolality or tonicity under physiological conditions, is primarily dependent on plasma sodium concentration. Osmolality vs tonicity Changes in cell volume are particularly important in the case of the brain. Cerebral dehydration due to hypertonicity causes osmolar imbalance leading to EC movement of water and cerebral shrinkage→ rupture of vessels. Hypertonicity leads to IC movement of water and cerebral swelling (oedema)→ compression. The brain can adapt by altering the content of “osmolytes”. Blood volume Water depletion Severe water depletion causes cerebral dehydration→ cerebral bleeding through damage to blood vessels. In the short term: cerebral shrinkage is limited by movement of EC ions into cerebral cells→ osmotic shift of water. If dehydration persists : brain cells adapt by synthesizing osmotically active organic compounds (‘osmolytes’) Excessive fluid replacement may cause cerebral oedema because of rapid IC movement of water. Water depletion Osmolal gap Measured osmolality (in mmol/kg of water) and calculated osmolarity (in mmol/L of solution) are normally very similar. Normal gap is