Explain the Process of Filtration - Formation of Urine

The kidneys form urine in three phases: simple filtration, selective re-absorption and secretion. Let us begin with the process of filtration.

Simple filtration: The process of filtration takes place in the nephron where approximately 10% of the blood which the kidney receives gets filtered under pressure through the walls of the glomerular capillaries and Bowman's capsule. The filtrate is composed of water, ions (e.g., sodium, potassium and chloride), glucose and small proteins. The rate of filtration is approximately 125 ml/minute or 180 L each day. Also, the amount of any substance that gets filtered is the product of the concentration of that substance in the blood and the rate of filtration.

So, higher the concentration, greater the filtration rate, the more substance gets filtered. A filter separates large and small particles by retaining the larger particles and allowing the smaller particles to pass through. The semi-permeable wall of the glomerulus acts as a filter. It allows the blood constituents having a molecular weight less than 68,000 (such as water, food substances like glucose and amino acids, inorganic salts, waste products like urea, uric acid and creatinine etc.) to pass through. It retains the larger molecules (red blood cells, white blood cells, platelets and plasma proteins etc.). These high molecular weight substances are unable to pass through the semi-permeable membrane of the glomerulus. The volume of filtrate produced per minute is termed as the glomerular filtration rate. Let us get to know more about the GFR.