Section 1 introduction to renal physiology and water balance. What do kidneys do: regulate electrolytes, acid/ base control, blood volume control, regulation of blood pressure. Focus: maintain stability of extracellular fluid (ecf, electrolyte composition, osmolarity. Transcellular fluid = water in epithelial lined spaces. Barriers = limit movement of water and solutes between compartments. Capillaries = free exchange of water and plasma (no proteins) Due to no passive movement between each no equilibrium from diffusion. Does water travel through the plasma membrane, does it require a transporter? (s1p5) Ecf volume directly influences blood pressure from changing plasma volume. Increase ecf volume = increase plasma volume = increase arterial blood pressure. Sodium and anions (chloride) make up 90% of ecf solutes. Salt crossing the membrane = water follows due to osmosis: controlling salt levels = controls ecf volume. Dependent on dietary salt poor salt regulation.