Nephron
Steven C. Campbell, M.D., Ph.D.

# Overview
A nephron (from Greek  (nephros) meaning "kidney") is the basic structural and functional unit of the kidney. Its chief function is to regulate water and soluble substances like Sodium salts by filtering the blood, reabsorbing what is needed and excreting the rest as urine. Nephrons eliminate wastes from the body, regulate blood volume and pressure, control levels of electrolytes and metabolites, and regulate blood pH. Its functions are vital to life and are regulated by the endocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone.

# Types of nephrons
Two general classes of nephrons are cortical nephrons and juxtamedullary nephrons, both of which are classified according to the location of their associated renal corpuscle. Cortical nephrons have their renal corpuscle in the superficial renal cortex, while the renal corpuscles of juxtamedullary nephrons are located near the renal medulla. The nomenclature for cortical nephrons varies, with some sources distinguishing between superficial cortical nephrons and midcortical nephrons; other sources simply call all non-juxtamedullary nephrons superficial nephrons.
Functionally, cortical and juxtamedullary nephrons have distinct roles. Cortical nephrons (85% of all nephrons) mainly perform excretory and regulatory functions, while juxtamedullary nephrons (15% of nephrons) concentrate and dilute urine.

# Anatomy
Each nephron is composed of an initial filtering component (the "renal corpuscle") and a tubule specialized for reabsorption and secretion (the "renal tubule"). The renal corpuscle filters out large solutes from the blood, delivering water and small solutes to the renal tubule for modification.

## Renal corpuscle
Composed of a glomerulus and Bowman's capsule, the renal corpuscle (or "Malphigian corpuscle") is the beginning of the nephron. It is the nephron's initial filtering component.

## Renal tubule
The flow of the renal tubule is as follows:
After traveling the length of the distal convoluted tubule, only about 1% of water remains, and the remaining salt content is negligible.

## Collecting duct system
Each distal convoluted tubule delivers its filtrate to a system of collecting ducts, the first segment of which is the collecting tubule. The collecting duct system begins in the renal cortex and extends deep into the medulla. As the urine travels down the collecting duct system, it passes by the medullary interstitium which has a high sodium concentration as a result of the loop of Henle's countercurrent multiplier system.
Though the collecting duct is normally impermeable to water, it becomes permeable in the presence of antidiuretic hormone (ADH). As much as three-fourths of the water from urine can be reabsorbed as it leaves the collecting duct by osmosis. Thus the levels of ADH determine whether urine will be concentrated or diluted.  An increase in ADH is an indication of dehydration, while water sufficiency results in low ADH allowing for diluted urine.
Lower portions of the collecting duct are also permeable to urea, allowing some of it to enter the medulla of the kidney, thus maintaining its high ion concentration (which is very important for the nephron).
Urine leaves the medullary collecting ducts through the renal papilla, emptying into the renal calyces, the renal pelvis, and finally into the bladder via the ureter.
Because it has a different origin during the development of the urinary and reproductive organs than the rest of the nephron, the collecting duct is sometimes not considered a part of the nephron. Instead of originating from the metanephrogenic blastema, the collecting duct originates from the ureteric bud.

## Juxtaglomerular apparatus
The juxtaglomerular apparatus occurs near the site of contact between the thick ascending limb and the afferent arteriole. It contains three components:
Juxtaglomerular cells are the site of renin synthesis and secretion and thus play a critical role in the renin-angiotensin system.

# Clinical relevance
Because of its importance in body fluid regulation, the nephron is a common target of drugs that treat high blood pressure and edema. These drugs, called diuretics, inhibit the ability of the nephron to retain water, thereby increasing the amount of urine produced.