Ingredients of glomerular filtrate

The first step in the formation of urine is to filter large amounts of fluid through the glomerular capillaries in the Bowman's compartment - about 180 litres of fluid per day is filtered through the kidneys, but only about 1 litre of fluid is excreted. Most of this fluid is reabsorbed by the kidneys depending on the fluid. A high level of glomerular filtration requires a high rate of flow through the kidney, as well as the special properties of the membrane filter. In this chapter, we discuss factors that determine the glomerular filtration rate (GFR) and the regulation of GFR and renal blood flow.

Like most capillaries, glomerular capillaries are impermeable proteins, so the filtered fluid (called glomerular filtrate) has very little free protein and no cells, including red blood cells. Glomerular filtrate consists mainly of salts and organic molecules, similar to serum. With some exceptions, low molecular weight molecules such as calcium and fatty acids are not freely filtered because they are partially bound to plasma proteins. For example, near plasma calcium and most fatty acids are protein-bound and these attachments are not leached through the glomerular capillaries.