NEPHRON

Nephron (from Greek νεφρός - nephros, meaning "kidney") is the basic structural
and functional unit of the kidney. Its chief function is to regulate the concentration of water and soluble substances like sodium salts by filtering the blood, reabsorbing what is needed and excreting the rest as urine. A nephron eliminates wastes from the body, regulates blood volume and blood pressure, controls levels of electrolytes and metabolites, and regulates 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.[1] In humans, a normal kidney contains 800,000 to 1.5 million nephrons.[2] Types of nephrons Two general classes of nephrons are
cortical nephrons and juxtamedullary nephrons, both of which are classified according to the length of their associated Loop of Henle and location of their renal corpuscle. All nephrons have their renal corpuscles in the cortex. Cortical nephrons
have their Loop of Henle in the renal medulla near its junction with the renal cortex, while the Loop of Henle of juxtamedullary nephrons is located deep in
the renal medulla; they are called
juxtamedullary because their renal
corpuscle is located near the medulla (but
still in the cortex). The nomenclature for
cortical nephrons varies, with some sources distinguishing between superficial
cortical nephrons and midcortical nephrons,
depending on where their corpuscle is located within the cortex.[3] The majority of nephrons are cortical.
Cortical nephrons have a shorter loop of Henle compared to juxtamedullary nephrons. The longer loop of Henle in
juxtamedullary nephrons create a
hyperosmolar gradient that allows for the creation of concentrated urine.[4] 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.[citation needed] Renal corpuscle Composed of a glomerulus and the Bowman's capsule, the renal corpuscle (or Malpighian corpuscle) is the beginning of the nephron. It is the nephron's initial filtering component.[citation needed] The glomerulus is a capillary tuft that receives its blood supply from an afferent arteriole of the renal circulation. The glomerular blood pressure provides the
driving force for water and solutes to be
filtered out of the blood and into the space
made by Bowman's capsule. The remainder of the blood (only approximately 1/5 of all
plasma passing through the kidney is
filtered through the glomerular wall into
the Bowman's capsule) passes into the
efferent arteriole.The diameter of efferent
arteriole is comparatively less than that of afferent arteriole. It then moves into the
vasa recta, which are only found in
juxtamedullary nephrons and not cortical
nephrons. The vasa recta are collecting
capillaries intertwined with the
convoluted tubules through the interstitial space, in which the reabsorbed substances
will also enter. This then combines with
efferent venules from other nephrons into
the renal vein, and rejoins the main bloodstream.[citation needed] The Bowman's capsule, also called the glomerular capsule, surrounds the
glomerulus. It is composed of a visceral
inner layer formed by specialized cells
called podocytes, and a parietal outer layer composed of a single layer of flat cells
called simple squamous epithelium. Fluids from blood in the glomerulus are filtered
through the visceral layer of podocytes,
and the resulting glomerular filtrate is further processed along the nephron to form urine.[citation needed] Renal tubule Renal tubule Latin tubulus renalis Gray's subject #253 1223 The renal tubule is the portion of the
nephron containing the tubular fluid filtered through the glomerulus.[5] After passing through the renal tubule, the
filtrate continues to the collecting duct system, which is not part of the nephron.[citation needed] The components of the renal tubule are: Proximal convoluted tubule (lies in cortex and lined by simple cuboidal
epithelium with brushed borders which
help to increase the area of absorption
greatly.) Loop of Henle (hair-pin like i.e. U-shaped and lies in medulla) Descending limb of loop of Henle Ascending limb of loop of Henle The ascending limb of loop of
Henle is divided into 2 segments:
Lower end of ascending limb is
very thin and is lined by simple
squamous epithelium. The distal
portion of ascending limb is thick and is lined by simple cuboidal
epithelium. Thin ascending limb of loop of
Henle Thick ascending limb of loop of
Henle (enters cortex and becomes DCT-distal convoluted tubule.) Distal convoluted tubule Functions The nephron carries out nearly all of the
kidney's functions. Most of these functions
concern the reabsorption and secretion of various solutes such as ions (e.g., sodium), carbohydrates (e.g., glucose), and amino acids (e.g., glutamate). Properties of the cells that line the nephron change
dramatically along its length; consequently,
each segment of the nephron has highly specialized functions.[citation needed] The proximal tubule as a part of the
nephron can be divided into an initial
convoluted portion and a following straight (descending) portion.[6] Fluid in the filtrate entering the proximal convoluted tubule is
reabsorbed into the peritubular capillaries,
including approximately two-thirds of the
filtered salt and water and all filtered organic solutes (primarily glucose and amino acids).[citation needed] The loop of Henle, also called the nephron loop, is a U-shaped tube that extends from
the proximal tubule. It consists of a
descending limb and ascending limb. It
begins in the cortex, receiving filtrate from
the proximal convoluted tubule, extends
into the medulla as the descending limb, and then returns to the cortex as the
ascending limb to empty into the distal
convoluted tubule. The primary role of the
loop of Henle is to concentrate the salt in
the interstitium, the tissue surrounding the loop.[citation needed] Considerable differences distinguish the
descending and ascending limbs of the loop
of Henle. The descending limb is permeable to water and noticeably less impermeable
to salt, and thus only indirectly contributes
to the concentration of the interstitium. As
the filtrate descends deeper into the hypertonic interstitium of the renal medulla, water flows freely out of the
descending limb by osmosis until the tonicity of the filtrate and interstitium
equilibrate. Longer descending limbs allow
more time for water to flow out of the
filtrate, so longer limbs make the filtrate
more hypertonic than shorter limbs.[citation needed] Unlike the descending limb, the ascending limb of Henle's loop[disambiguation needed] is impermeable to water, a critical feature of
the countercurrent exchange mechanism employed by the loop. The ascending limb
actively pumps sodium out of the filtrate,
generating the hypertonic interstitium that
drives countercurrent exchange. In passing
through the ascending limb, the filtrate
grows hypotonic since it has lost much of its sodium content. This hypotonic filtrate
is passed to the distal convoluted tubule in the renal cortex.[citation needed] The distal convoluted tubule has a different structure and function to that of the
proximal convoluted tubule. Cells lining the
tubule have numerous mitochondria to produce enough energy (ATP) for active transport to take place. Much of the ion transport taking place in the distal
convoluted tubule is regulated by the endocrine system. In the presence of parathyroid hormone, the distal convoluted tubule reabsorbs more calcium and
excretes more phosphate. When aldosterone is present, more sodium is reabsorbed and more potassium excreted. Atrial natriuretic peptide causes the distal convoluted tubule to excrete more sodium.
In addition, the tubule also secretes hydrogen and ammonium to regulate pH.[citation needed] After traveling the length of the distal
convoluted tubule, only about 1% of water
remains, and the remaining salt content is negligible.[citation needed] Collecting duct system Main article: 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.[citation needed] Though the collecting duct is normally
impermeable to water, it becomes
permeable in the presence of antidiuretic hormone (ADH). ADH affects the function of aquaporins, resulting in the reabsorption of water molecules as it passes through the
collecting duct. Aquaporins are membrane
proteins that selectively conduct water
molecules while preventing the passage of
ions and other solutes. As much as three-
quarters 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.[citation needed] Lower portions of the collecting organ are
also permeable to urea, allowing some of it to enter the medulla of the kidney, thus maintaining its high concentration (which
is very important for the nephron).[citation needed] Urine leaves the medullary collecting ducts
through the renal papillae, emptying into the renal calyces, the renal pelvis, and finally into the urinary bladder via the ureter.[citation needed] 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.[citation needed] Juxtaglomerular apparatus Main article: Juxtaglomerular apparatus The juxtaglomerular apparatus is a specialized region of the nephron
responsible for production and secretion of
the hormone renin, involved in the renin- angiotensin system. This apparatus occurs near the site of contact between the thick
ascending limb and the afferent arteriole.
It contains three components: the macula densa, juxtaglomerular cells, and extraglomerular mesangial cells.[citation needed] 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.