Urine Formation by Excretory Organs

Urine formation by the excretory organs involves

1. The movement of water (solvent)' and dissolved molecules of small size (solutes) from the  lumen of the excretory organ into the interstitial fluid and blood and
2. Movement back across the plasma membranes of the intervening cells (cells of the renal organs and blood capillaries).

While a variety of renal organs are encountered in the animal kingdom, the principles of their function-are basically similar. In all cases, renal function involves the initial -appearance in the tubular lumen of a fluid called primary urine which is isosmotic to blood or body fluid from which it is derived. The composition and volume of the primary urine are later altered by the reabsorption of useful substances (e.g., glucose) into the blood and by the secretion of unwanted substances (e.g., urea) from the blood into the tubular fluid. Both reabsorption and secretion involve active transport. These modifications in the composition and volume of primary urine result in the formation of urine which is the fluid finally excreted from the body.