The pores in the membrane are extremely fine, so much so that only substances with a molecular mass of 68,000 or less can pass through them. Plasma proteins such as fibrinogen, albumin, etc. are therefore not filtered. Even so, about 180 litres of fluid is filtered every 24 hours under normal circumstances. This is more than 20 times the volume of blood. There are waste products present in the filtrate, such as urea, uric acid, creatine and others.
Useful products are also present there, such as amino acids, glucose, mineral salts and water.
Re-absorption About 1.5 litres of urine is excreted per day on average. While the filtrate is passing along the renal tubules, some of the substances in it are selectively reabsorbed by the epitheleal cells which line these tubules. These substances then pass into the surrounding blood capillaries. The substances which will be reabsorbed depend on the current needs of the body. The mechanism by which these substances are reabsorbed is different in the different sections of the renal tubule.