Functional Principles of Excretory Organs
Before studying about the excretory organs, it is important to learn about the basic concepts of osmolarity and membrane permeability. The presence of dissolved solute confers on a solution the property of osmotic pressure. Like other colligative properties, osmotic pressure or osmotic concentration f a solution depends on the P number of dissolved particles present per unit volume. While the chemical concentration of a solution is expressed in molarity (molesilitre), osmotic concentration of a solution is expressed in dsmolarity (osmoleditre). For ideal non-electrolytes (e.g., sucrose), a one molar solution is one osmolar.
An electrolyte solution, on the other hand, has a higher osmolarity than its molaiity. For example, Nacl in the solution dissociates into Na+ and Cl-. Thus, for everymolecule of NaCl, one gets two ionic particles in solution, one of Na+ and one of Cl-. Hence, one molar NACl solution is early two osmolar. In the same way, one molar CaCl2 solution is nearly three osmolar. An osrnole is defined as that amount of a solute which when dissolved in one litre of water has the same osmotic pressure as one mole of an ideal non-electrolyte in one litre of water. If two solutions (solution A and solution B) have the same osmotic concentration, they are said to be isopathic to each other. If solution A has a higher osmolarity than solution B, A is hyperosmotic to B or B is hypoosmotic to A.