Nephron

The nephron is the minute or microscopic structural and functional unit of the

The interior of Bowman's capsule, called Bowman's space, collects the filtrate from the filtering capillaries of the

The four mechanisms used to create and process the filtrate (the result of which is to convert blood to urine) are

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.

Structure

The nephron is the functional unit of the kidney.^[2] This means that each separate nephron is where the main work of the kidney is performed.

A nephron is made of two parts:

• a renal corpuscle, which is the initial filtering component, and
• a
  renal tubule that processes and carries away the filtered fluid.^[3]
  ^: 1024

The renal corpuscle is the site of the filtration of blood plasma. The renal corpuscle consists of the glomerulus, and the glomerular capsule or Bowman's capsule.^[3]: 1027

The renal corpuscle has two poles: a vascular pole and a tubular pole.^[4]: 397 The arterioles from the renal circulation enter and leave the glomerulus at the vascular pole. The glomerular filtrate leaves the Bowman's capsule at the renal tubule at the urinary pole.

Glomerulus

The glomerulus is the network known as a tuft, of filtering capillaries located at the vascular pole of the renal corpuscle in Bowman's capsule. Each glomerulus 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 plasma, and into the interior of Bowman's capsule, called Bowman's space.

Only about a fifth of the plasma is filtered in the glomerulus. The rest passes into an efferent arteriole. The diameter of the efferent arteriole is smaller than that of the afferent, and this difference increases the hydrostatic pressure in the glomerulus.

Bowman's capsule

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 simple squamous epithelium. Fluids from blood in the glomerulus are ultrafiltered through several layers, resulting in what is known as the filtrate.

The filtrate next moves to the renal tubule, where it is further processed to form urine. The different stages of this fluid are collectively known as the tubular fluid.

Renal tubule

The renal tubule is long pipe like structure containing the tubular fluid filtered through the glomerulus.^[5] After passing through the renal tubule, the filtrate continues to the collecting duct system.^[6]

The components of the renal tubule are:

• Proximal convoluted tubule (lies in cortex and lined by simple cuboidal epithelium with brush 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 the distal convoluted tubule.)
• Distal convoluted tubule
• Collecting tubule

The epithelial cells that form these nephron segments can be distinguished by the shapes of their actin cytoskeleton.^[7]

Blood from the efferent arteriole, containing everything that was not filtered out in the glomerulus, moves into the peritubular capillaries, tiny blood vessels that surround the loop of Henle and the proximal and distal tubules, where the tubular fluid flows. Substances then reabsorb from the latter back to the blood stream.

The peritubular capillaries then recombine to form an efferent venule, which combines with efferent venules from other nephrons into the renal vein, and rejoins the main bloodstream.

Length difference

Cortical nephrons (the majority of nephrons) start high in the cortex and have a short loop of Henle which does not penetrate deeply into the medulla. Cortical nephrons can be subdivided into superficial cortical nephrons and midcortical nephrons.^[8]

Juxtamedullary nephrons^[

Juxtamedullary nephrons are found only in birds and mammals, and have a specific location: medullary refers to the

cortical nephron

.

The juxtamedullary nephrons comprise only about 15% of the nephrons in the human kidney.[1]^: 24 However, it is this type of nephron which is most often depicted in illustrations of nephrons.

In humans, cortical nephrons have their renal corpuscles in the outer two thirds of the cortex, whereas juxtamedullary nephrons have their corpuscles in the inner third of the cortex.^[1]: 24

Functions

The nephron uses four mechanisms to convert blood into urine: filtration, reabsorption, secretion, and excretion.^{[4]: 395–396} These apply to numerous substances. The structure and function of the epithelial cells lining the lumen change during the course of the nephron, and have segments named by their location and which reflects their different functions.

Proximal tubule

The proximal tubule as a part of the nephron can be divided into an initial convoluted portion and a following straight (descending) portion.^[11] Fluid in the filtrate entering the proximal convoluted tubule is reabsorbed into the peritubular capillaries, including 80% of glucose, more than half of the filtered salt, water and all filtered organic solutes (primarily glucose and amino acids).^{[4]: 400–401}

Loop of Henle

The loop of Henle is a U-shaped tube that extends from the proximal tubule. It consists of a descending limb and an 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 enable an organism to produce concentrated urine, not by increasing the tubular concentration, but by rendering the interstitial fluid hypertonic.^[1]: 67

Considerable differences aid in distinguishing the descending and ascending limbs of the loop of Henle. 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. The hypertonicity of the medulla (and therefore concentration of urine) is determined in part by the size of the loops of Henle.^[1]

^: 76

Unlike the descending limb, the

hypotonic since it has lost much of its sodium content. This hypotonic filtrate is passed to the distal convoluted tubule in the renal cortex.^[1]

^: 72

Distal convoluted tubule

The

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 secretes more phosphate. When aldosterone is present, more sodium is reabsorbed and more potassium secreted. Ammonia is also absorbed during the selective reabsorption. Atrial natriuretic peptide

causes the distal convoluted tubule to secrete more sodium.

Connecting tubule

A part of Distal nephron. This is the final segment of the tubule before it enters the collecting duct system. Water, some salts and nitrogenous waste like urea and creatinine are passed out to collecting tubule.

Collecting duct system

Main article: Collecting duct system

Each distal convoluted tubule delivers its filtrate to a

connecting 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.^[1]

^: 67

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.^[12]

^: 50–51

Though the collecting duct is normally impermeable to water, it becomes permeable in the presence 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 a decrease in ADH allowing for diluted urine.^[4]

^: 406

Lower portions of the collecting organ are also permeable to urea, allowing some of it to enter the medulla, thus maintaining its high concentration (which is very important for the nephron).^{[1]: 73–74}

Urine leaves the medullary collecting ducts through the

urinary bladder via the ureter.^[4]

^{: 406–407}

Juxtaglomerular apparatus

Main article: Juxtaglomerular apparatus

The

angiotensinogen and results in the ten amino acid substance angiotensin-1 (A-1). A-1 is then converted to angiotensin-2, a potent vasoconstrictor, by removing two amino acids: this is accomplished by angiotensin converting enzyme (ACE). This sequence of events is referred to as the renin–angiotensin system (RAS) or renin-angiotensin-aldosterone system (RAAS). The JGA is located between the thick ascending limb and the afferent arteriole. It contains three components: the macula densa, juxtaglomerular cells, and extraglomerular mesangial cells.^[4]

^: 404

Clinical significance

Patients in early stages of chronic kidney disease show an approximate 50% reduction in the number of nephrons, comparable to the nephron loss that occurs with aging (between ages 18–29 and 70–75).^[13]

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, renal tubular acidosis, and polycystic kidney disease.

Additional images

• 
  Distribution of blood vessels in cortex of kidney. (Although the figure labels the efferent vessel as a vein, it is actually an arteriole.)
• 
  Glomerulus is red; Bowman's capsule is white.
• 
  Kidney tissue
• 
  Glomerulus
• 
  This image shows the types of cells present in the glomerulus part of a kidney nephron. Podocytes, Endothelial cells, and Glomerular mesangial cell are present.

See also

• Nephrology
• Urology

References

1. ^ ^{a b c d e f g h} Lote CJ (2012). Principles of Renal Physiology (5th ed.). Springer.
2. ISBN 978-0-19-856878-0
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3. ^
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4. ^
   ISBN 978-0-07-184268-6
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5. ^ "The Kidney Tubule I: Urine Production". Ecology & Evolutionary Biology - University of Colorado at Boulder. Archived from the original on October 2, 2007. Retrieved March 6, 2007.
6. ISBN 0-88167-885-6
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7. PMID 31605441
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8. ^ Nosek TM. "Section 7/7ch03/7ch03p16". Essentials of Human Physiology. Archived from the original on 2016-03-24.
9. ISBN 978-0-07-166339-7
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10. ^ "Regulation of Urine Concentration". Anatomy & Physiology. CliffsNotes. Archived from the original on 25 October 2012. Retrieved 27 November 2012.
11. ISBN 978-1-4160-2328-9
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12. ^ Mitchell B, Sharma R (2009). Embriology (2nd ed.). Churchill Livingstone Elsevier.
13. S2CID 53872296
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