Nephron
Overview
The renal tubule is the portion of the nephron containing the tubular fluid
Tubular fluid
Tubular fluid is the fluid in the tubules of the kidney. It starts as a renal ultrafiltrate in the glomerulus, changes composition through the nephron, and ends up as urine leaving through the ureters.-Composition table:...

 filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system
Collecting duct system
The collecting duct system of the kidney consists of a series of tubules and ducts that connect the nephrons to the ureter. It participates in electrolyte and fluid balance through reabsorption and excretion, processes regulated by the hormones aldosterone and antidiuretic hormone.There are several...

, which is not part of the nephron.

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
    Loop of Henle
    In the kidney, the loop of Henle is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discoverer F. G. J...

     (hair-pin like i.e.
Encyclopedia
The renal tubule is the portion of the nephron containing the tubular fluid
Tubular fluid
Tubular fluid is the fluid in the tubules of the kidney. It starts as a renal ultrafiltrate in the glomerulus, changes composition through the nephron, and ends up as urine leaving through the ureters.-Composition table:...

 filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system
Collecting duct system
The collecting duct system of the kidney consists of a series of tubules and ducts that connect the nephrons to the ureter. It participates in electrolyte and fluid balance through reabsorption and excretion, processes regulated by the hormones aldosterone and antidiuretic hormone.There are several...

, which is not part of the nephron.

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
    Loop of Henle
    In the kidney, the loop of Henle is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discoverer F. G. J...

     (hair-pin like i.e. U-shaped and lies in medulla)
    • Descending limb of loop of Henle
      Descending limb of loop of Henle
      The descending limb of loop of Henle is the portion of the renal tubule constituting the first part of the loop of Henle.-Physiology:The permeability is as follows:...

    • 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
        Thin ascending limb of loop of Henle
        The thin ascending limb of loop of Henle is a sub-portion of the loop of Henle in the juxtamedullary nephron of the kidney. The thin ascending limb is impermeable to water, and is also permeable to ions in particular Na and Cl...

      • Thick ascending limb of loop of Henle
        Thick ascending limb of loop of Henle
        The thick ascending limb of loop of Henle also known as distal straight tubule, is a segment of the nephron in the kidney. It can be divided into two parts: that in the renal medulla, and that in the renal cortex.-Medullary thick ascending limb:...

         (enters cortex and becomes DCT-distal convoluted tubule.)
  • Distal convoluted tubule
    Distal convoluted tubule
    The distal convoluted tubule is a portion of kidney nephron between the loop of Henle and the collecting duct system.- Physiology :It is partly responsible for the regulation of potassium, sodium, calcium, and pH...


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 ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s (e.g., sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

), carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

s (e.g., glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

), and amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s (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.

The proximal tubule as a part of the nephron can be divided into an initial convoluted portion and a following straight (descending) portion. 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
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 solutes (primarily glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 and amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s).

The loop of Henle
Loop of Henle
In the kidney, the loop of Henle is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discoverer F. G. J...

, 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.

Considerable differences distinguish the descending and ascending limbs of the loop of Henle. The descending limb
Descending limb of loop of Henle
The descending limb of loop of Henle is the portion of the renal tubule constituting the first part of the loop of Henle.-Physiology:The permeability is as follows:...

 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
Osmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...

 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.

Unlike the descending limb, the ascending limb of Henle's loop is impermeable to water, a critical feature of the countercurrent exchange
Countercurrent exchange
Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some component, between two flowing bodies flowing in opposite directions to each other. The flowing bodies can be liquids, gases, or...

 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
Distal convoluted tubule
The distal convoluted tubule is a portion of kidney nephron between the loop of Henle and the collecting duct system.- Physiology :It is partly responsible for the regulation of potassium, sodium, calcium, and pH...

 in the renal cortex.

The distal convoluted tubule
Distal convoluted tubule
The distal convoluted tubule is a portion of kidney nephron between the loop of Henle and the collecting duct system.- Physiology :It is partly responsible for the regulation of potassium, sodium, calcium, and pH...

 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
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

) for active transport
Active transport
Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...

 to take place. Much of the ion transport taking place in the distal convoluted tubule is regulated by the endocrine system
Endocrine system
In physiology, the endocrine system is a system of glands, each of which secretes a type of hormone directly into the bloodstream to regulate the body. The endocrine system is in contrast to the exocrine system, which secretes its chemicals using ducts. It derives from the Greek words "endo"...

. In the presence of parathyroid hormone
Parathyroid hormone
Parathyroid hormone , parathormone or parathyrin, is secreted by the chief cells of the parathyroid glands as a polypeptide containing 84 amino acids...

, the distal convoluted tubule reabsorbs more calcium and excretes more phosphate. When aldosterone
Aldosterone
Aldosterone is a hormone that increases the reabsorption of sodium ions and water and the release of potassium in the collecting ducts and distal convoluted tubule of the kidneys' functional unit, the nephron. This increases blood volume and, therefore, increases blood pressure. Drugs that...

 is present, more sodium is reabsorbed and more potassium excreted. Atrial natriuretic peptide
Atrial natriuretic peptide
Atrial natriuretic peptide , atrial natriuretic factor , atrial natriuretic hormone , or atriopeptin, is a powerful vasodilator, and a protein hormone secreted by heart muscle cells. It is involved in the homeostatic control of body water, sodium, potassium and fat...

 causes the distal convoluted tubule to excrete more sodium. In addition, the tubule also secretes hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 and ammonium
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...

 to regulate pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

.

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
Collecting duct system
The collecting duct system of the kidney consists of a series of tubules and ducts that connect the nephrons to the ureter. It participates in electrolyte and fluid balance through reabsorption and excretion, processes regulated by the hormones aldosterone and antidiuretic hormone.There are several...

, 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). 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
Dehydration
In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...

, while water sufficiency results in low ADH allowing for diluted urine.

Lower portions of the collecting organ are also permeable to urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....

, allowing some of it to enter the medulla of the kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...

, thus maintaining its high concentration (which is very important for the nephron).

Urine leaves the medullary collecting ducts through the renal papilla
Renal papilla
In the kidney, the renal papilla is the location where the medullary pyramids empty urine into the minor calyx. Histologically it is marked by medullary collecting ducts converging to form a duct of Bellini to channel the fluid. Transitional epithelium begins to be seen.-Role in disease:Some...

e, emptying into the renal calyces
Minor calyx
The minor calyx, in the kidney, surrounds the apex of the renal pyramids. Urine formed in the kidney passes through a papilla at the apex into the minor calyx then into the major calyx....

, the renal pelvis
Renal pelvis
The renal pelvis or pyelum is the funnel-like dilated proximal part of the ureter in the kidney.In humans, the renal pelvis is the point of convergence of two or three major calyces...

, and finally into the urinary bladder
Urinary bladder
The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...

 via the ureter
Ureter
In human anatomy, the ureters are muscular tubes that propel urine from the kidneys to the urinary bladder. In the adult, the ureters are usually long and ~3-4 mm in diameter....

.

Because it has a different origin during the development of the urinary and reproductive organs
Development of the urinary and reproductive organs
The development of the urinary and reproductive organs as a part of the prenatal development, concerns the urinary system and sex organs. The latter is a part of the stages of sexual differentiation....

 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
Ureteric bud
The ureteric bud is a protrusion from the mesonephric duct during the development of the urinary and reproductive organs. It later develops into the adult kidney, except for the nephrons, which, in contrast, originate from the metanephric blastema....

.

Juxtaglomerular apparatus

The juxtaglomerular apparatus
Juxtaglomerular apparatus
The juxtaglomerular apparatus is a microscopic structure in the kidney, which regulates the function of each nephron. The juxtaglomerular apparatus is named for its proximity to the glomerulus: it is found between the vascular pole of the renal corpuscle and the returning distal convoluted tubule...

 is a specialized region of the nephron responsible for production and secretion of the enzyme renin
Renin
Renin , also known as an angiotensinogenase, is an enzyme that participates in the body's renin-angiotensin system -- also known as the Renin-Angiotensin-Aldosterone Axis -- that mediates extracellular volume , and arterial vasoconstriction...

, involved in the renin-angiotensin system
Renin-angiotensin system
The renin-angiotensin system or the renin-angiotensin-aldosterone system is a hormone system that regulates blood pressure and water balance....

. This apparatus occurs near the site of contact between the thick ascending limb and the afferent arteriole. It contains three components: the macula densa
Macula densa
In the kidney, the macula densa is an area of closely packed specialized cells lining the wall of the distal tubule at the point of return of the nephron to the vascular pole of its parent glomerulus, ....

, juxtaglomerular cell
Juxtaglomerular cell
The juxtaglomerular cells are cells in the kidney that synthesize, store, and secrete the enzyme renin. They are specialized smooth muscle cells in the wall of the afferent arteriole that delivers blood to the glomerulus...

s, and extraglomerular mesangial cell
Extraglomerular mesangial cell
Extraglomerular mesangial cells are light-staining cells in the kidney found outside the glomerulus, near the vascular pole and macula densa....

s.

Clinical relevance

Because of its importance in body fluid regulation, the nephron is a common target of drugs that treat high blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

 and edema
Edema
Edema or oedema ; both words from the Greek , oídēma "swelling"), formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin or in one or more cavities of the body that produces swelling...

. These drugs, called diuretic
Diuretic
A diuretic provides a means of forced diuresis which elevates the rate of urination. There are several categories of diuretics. All diuretics increase the excretion of water from bodies, although each class does so in a distinct way.- Medical uses :...

s, inhibit the ability of the nephron to retain water, thereby increasing the amount of urine produced.
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