Editing Assessment of kidney function (section)

==Description ==
{{main|Renal physiology}}
The functions of the kidney include maintenance of [[acid-base balance]]; regulation of [[fluid balance]]; regulation of [[sodium]], [[potassium]], and other [[electrolyte]]s; [[clearance (medicine)|clearance]] of [[toxin]]s; absorption of [[glucose]], [[amino acid]]s, and other small molecules; regulation of [[blood pressure]]; production of various [[hormone]]s, such as [[erythropoietin]]; and activation of [[vitamin D]].

The [[Glomerular filtration rate]] (GFR) is regarded as the best overall measure of the kidney's ability to carry out these numerous functions. An estimate of the GFR is used clinically to determine the degree of kidney impairment and to track the progression of the disease. The GFR, however, does not reveal the source of the kidney disease. This is accomplished by urinalysis, measurement of urine protein excretion, kidney imaging, and, if necessary, kidney biopsy.<ref>{{cite web |title=UpToDate |url=https://www.uptodate.com/contents/assessment-of-kidney-function |website=www.uptodate.com}}</ref>

Much of renal physiology is studied at the level of the [[nephron]] {{ndash}} the smallest functional unit of the kidney. Each nephron begins with a [[#Filtration|filtration]] component that filters the [[blood]] entering the kidney. This filtrate then flows along the length of the nephron, which is a tubular structure lined by a single layer of specialized [[cell (biology)|cells]] and surrounded by [[capillary|capillaries]]. The major functions of these lining cells are the [[reabsorption]] of water and small molecules from the filtrate into the blood, and the [[#Secretion|secretion]] of wastes from the blood into the [[urine]].{{citation needed|date=November 2021}}

Proper function of the kidney requires that it receives and adequately filters blood. This is performed at the microscopic level by many hundreds of thousands of filtration units called [[renal corpuscle]]s, each of which is composed of a [[Glomerulus (kidney)|glomerulus]] and a [[Bowman's capsule]]. A global assessment of renal function is often ascertained by estimating the rate of filtration, called the [[glomerular filtration rate]] (GFR).