The capsule is cup-shaped, with an outer parietal wall and an inner visceral wall. In the renal corpuscle, blood is driven through the capillaries at high pressure. Renal capsules are usually separated from nephric tubules by a narrow neck segment. The proximal tubule returns sugars, amino acids, vitamins and salts to the bloodstream. The distal tubule acidifies the filtrate and removes sodium and chloride ions.
Water is returned to the body by both tubules.
Details of tubule function are varied and complicated, and related to environmental factors influencing the organism. Short proximal and distal tubules to return solutes. Nitrogen released as ammonia or urea. Special cells in gills excrete salt. Little or no distal tubules.