Editing Nephron (section)

{{Short description|Microscopic structural and functional unit of the kidney}}
{{About|the unit of the kidney|the journal|Nephron (journal)|the drug with trade name Nephron|Epinephrine}}
{{Infobox anatomy
| Name = Nephron
| Image = Kidney Nephron.svg
| Caption = Diagram (left) of a long ''juxtamedullary nephron'' and (right) of a short ''cortical nephron''. The left nephron is labelled with six named nephron segments. Collecting duct is the last part of the nephron.
| Width = 240
| Image2 =
| Caption2 =
| Precursor = [[Metanephric blastema]] ([[intermediate mesoderm]])
| System = [[Urinary system]]
| Artery =
| Vein =
| Nerve =
| Lymph =
}}

The '''nephron''' is the minute or microscopic structural and functional unit of the [[kidney]]. It is composed of a [[renal corpuscle]] and a [[Nephron#Renal tubule|renal tubule]]. The renal corpuscle consists of a tuft of [[capillary|capillaries]] called a [[glomerulus (kidney)|glomerulus]] and a cup-shaped structure called [[Bowman's capsule]]. The renal tubule extends from the capsule. The capsule and tubule are connected and are composed of [[epithelial cells]] with a [[Lumen (anatomy)|lumen]]. A healthy adult has 1 to 1.5 million nephrons in each kidney.<ref name="lote">{{cite book | vauthors = Lote CJ |title= Principles of Renal Physiology | edition = 5th |year=2012|publisher=Springer}}</ref>{{rp|22}} Blood is filtered as it passes through three layers: the [[endothelial cells]] of the capillary wall, its [[basement membrane]], and between the [[podocyte foot processes]] of the lining of the capsule. The tubule has adjacent [[peritubular capillaries]] that run between the descending and ascending portions of the tubule.<ref name="2024-Kumaran">{{cite journal |vauthors=Kumaran GK, Hanukoglu I |title=Mapping the cytoskeletal architecture of renal tubules and surrounding peritubular capillaries in the kidney |journal=Cytoskeleton (Hoboken) |volume=81 |issue=4-5 |pages=227–237 |date=2024 |pmid=37937511 |doi=10.1002/cm.21809 |url=}}</ref> As the fluid from the capsule flows down into the tubule, it is processed by the epithelial cells lining the tubule: water is reabsorbed and substances are exchanged (some are added, others are removed); first with the [[interstitial fluid]] outside the tubules, and then into the plasma in the adjacent peritubular capillaries through the endothelial cells lining that capillary. This process regulates the volume of body fluid as well as levels of many body substances. At the end of the tubule, the remaining fluid—[[urine]]—exits: it is composed of water, [[metabolic waste]], and [[toxins]].

The interior of Bowman's capsule, called Bowman's space, collects the filtrate from the filtering capillaries of the [[glomerular tuft]], which also contains [[mesangial cell]]s supporting these capillaries. These components function as the [[Renal physiology#Filtration|filtration]] unit and make up the [[renal corpuscle]]. The filtering structure (glomerular filtration barrier) has three layers composed of [[endothelial cell]]s, a [[basement membrane]], and podocyte foot processes. The tubule has five anatomically and functionally different parts: the [[proximal tubule]], which has a convoluted section called the [[proximal convoluted tubule]] followed by a straight section (proximal straight tubule); the [[loop of Henle]], which has two parts, the descending loop of Henle ("descending loop") and the [[ascending loop of Henle]] ("ascending loop"); the [[distal convoluted tubule]] ("distal loop"); the [[connecting tubule]], and the last part of nephron the [[collecting duct]]s. Nephrons have two lengths with different urine-concentrating capacities: long juxtamedullary nephrons and short cortical nephrons.

The four mechanisms used to create and process the filtrate (the result of which is to convert blood to urine) are [[filtration]], [[reabsorption]], [[secretion]] and [[excretion]]. Filtration or [[Ultrafiltration (kidney)|ultrafiltration]] occurs in the glomerulus and is largely passive: it is dependent on the intracapillary blood pressure. About one-fifth of the plasma is filtered as the blood passes through the glomerular capillaries; four-fifths continues into the peritubular capillaries. Normally the only components of the blood that are not filtered into Bowman's capsule are [[blood proteins]], [[red blood cell]]s, [[white blood cell]]s and [[platelet]]s. Over 150 liters of fluid enter the glomeruli of an adult every day: 99% of the water in that filtrate is reabsorbed. Reabsorption occurs in the [[renal tubule]]s and is either passive, due to [[diffusion]], or active, due to pumping against a concentration gradient. Secretion also occurs in the tubules and collecting duct and is active. Substances reabsorbed include: [[water]], [[sodium chloride]], [[glucose]], [[amino acid]]s, [[Lactic acid|lactate]], [[magnesium]], [[calcium phosphate]], [[uric acid]], and [[bicarbonate]]. Substances secreted include [[urea]], [[creatinine]], [[potassium]], [[hydrogen]], and [[uric acid]]. Some of the [[hormones]] which signal the tubules to alter the reabsorption or secretion rate, and thereby maintain homeostasis, include (along with the substance affected) [[vasopressin|antidiuretic hormone]] (water), [[aldosterone]] (sodium, potassium), [[parathyroid hormone]] (calcium, phosphate), [[atrial natriuretic peptide]] (sodium) and [[brain natriuretic peptide]] (sodium). A countercurrent system in the [[renal medulla]] provides the mechanism for generating a hypertonic interstitium, which allows the recovery of solute-free water from within the nephron and returning it to the venous vasculature when appropriate.

Some diseases of the nephron predominantly affect either the glomeruli or the tubules. Glomerular diseases include [[diabetic nephropathy]], [[glomerulonephritis]] and [[IgA nephropathy]]; renal tubular diseases include [[acute tubular necrosis]] and [[polycystic kidney disease]].