Week 6 Fluid and Electrolyte Imbalance Fluid movement through the capillary membrane is due to pressure placed on the membrane by the blood (hydrostatic pressure) - The hydrostatic pressure pushes the fluid out the capillary, whereas osmotic pressure pulls fluid into the capillary There are several mechanisms that create the balance between fluid intake and output: ð Thirst ð Antidiuretic hormone (ADH) ð RAAS ð Arterial naturetic peptide (ANP) Hypovolemia The thirst mechanism: - Thirst centre is activated when the blood volume drops due to water loss or when serum osmolality increases. This mechanism is highly effective in regulating extracellular sodium levels - Increased sodium levels in the extracellular fluid, increase serum osmolality and therefore stimulates the thirst centre - An increase of 2-3% in plasma osmolality triggers the thirst centre of the hypothalamus - Secondarily, a 10-15% drop in blood volume also triggers thirst - This is a significantly weaker stimulus ADH - Regulator of the water excretion by the kidneys - Osmoreceptors located in the hypothalamus respond to an increase in serum osmolality and the associated decrease in the blood volume; this activates ADH production and release - With increased water absorption thanks to the ADH, the urine output falls, blood volume is restored, and serum osmolality drops as the water dilutes bodily fluid RAAS - Works to maintain the balance of intravascular fluid and blood pressure - When there is a decrease to the blood flow and pressure to the kidneys, special receptors are stimulated in the nephrons to produce the renin enzyme - Renin converts angiotensin to angiotensin I - ANG I then travels through the bloodstream into the lungs, where its converted into ANG II