The kidneys are the body’s central filtering apparatus: they cleanse the blood of waste products and ensure that harmful substances are excreted with the urine, while substances that are of use to the body are returned to the blood. The entire blood volume – approximately seven liters – is pumped through the kidneys 200 times per day.
The filtering function is performed by millions of tiny functional entities known as nephrons. They consist of renal corpuscles – tufts made up of tiny blood vessels (glomeruli), surrounded by a capsule – and very fine tubules. Primary urine first forms in the renal corpuscles. Substances that are eliminated with the urine (renally cleared substances) pass from the blood through the capillary walls (blood-urine barrier) together with water molecules, are collected and fed into the tubules. The kidneys produce 180 liters of primary urine per day. 99 percent of the fluid and substances that are important to the body are then returned to the blood from the tubules.
1.5 liters of fluid remain and are excreted as urine, containing only waste products and substances that would be toxic to the body at higher concentrations. As well as cleansing the blood, the kidneys are responsible for regulatory processes that are essential to life: they secrete hormones that maintain the equilibrium of the volume and concentration of the blood, the body’s fluid and salt balance and bone metabolism.
For example, the kidneys produce the hormone renin, which is involved in the regulation of blood pressure, and the hormone erythropoietin (EPO), which stimulates the production of red blood cells. The kidneys additionally produce calcitriol, which is important for absorption of calcium and excretion of phosphate and consequently affects bone metabolism.
The information in this chapter has been compiled from the following sources, among others. Further relevant information can also be found there.
National Kidney Foundation, http://www.kidney.org/kidneydisease/ aboutckd.cfm
Das Nierenportal, Deutsche Gesellschaft für Nephrologie (http://www.dgfn.eu/patienten/ basisinformationen-zur-nephrologie.html)
Deutsche Nierenstiftung (http://www.nierenstiftung.de/ hilfreiche-informationen/)
World Health Organization, The Diabetes Programme, About Diabetes, Complications of Diabetes, including Diabetic retinopathy (eye disease), Nephropathy (kidney disease), Neuropathy (nerve disease), Cardiovascular disease (http://www.who.int/diabetes/action_ online/basics/en/index3.html)