As blood flows through the glomerulus, blood pressure forces materials through the glomerular wall and through the wall of the glomerular capsule into the nephron. The ?uid that enters the nephron, the glomerular ?ltrate, consists mainly of water, electrolytes, soluble wastes, nutrients, and toxins. The main waste material is urea, the nitrogenous (nitrogen-containing) byproduct of protein metabolism. The filtrate should not contain any cells or proteins such as albumin. The waste material and the toxins must be eliminated, but most of the water, electrolytes, and nutrients must be returned to the blood or we would rapidly starve and dehydrate. This return process, termed tubular reabsorption, occurs through the peritubular capillaries that surround the nephron. As the filtrate flows through the nephron, other processes further regulate its composition and pH. The concentration of the filtrate is also adjusted under the effects of the pituitary hormone antidiuretic hormone (ADH). Finally, the filtrate, now called urine, flows into the collecting tubules to be eliminated.

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