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Nephron

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

                              Nephron
   Nephron of the kidney without juxtaglomerular apparatus
   Nephron. Diagram is labeled in Polish, but flow can still be
   identified.
    Gray's   subject #253 1221
   Precursor Metanephric blastema
     MeSH    Nephrons

   A nephron is the basic structural and functional unit of the kidney.
   Its chief function is to regulate water and soluble substances 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.

   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.

Anatomy and function

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.
   glomerulus 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, basically the blood
   plasma, not filtered into the glomerulus passes into the narrower
   efferent arteriole. It then moves into the vasa recta, which are
   collecting capillaries intertwined with the convoluted tubules through
   the interstitial space, and 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.
   Bowman's capsule Bowman's capsule (also called the glomerular capsule)
   surrounds the glomerulus and is composed of visceral (simple squamous
   epithelial cells) (inner) and parietal (simple squamous epithelial
   cells) (outer), layers. Fluids from blood in the glomerulus are
   collected in the Bowman's capsule (i.e., glomerular filtrate) and
   further processed along the nephron to form urine.

Renal tubule

   The flow of the renal tubule is as follows:
   Name Description
   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. 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).
   loop of Henle The loop of Henle (sometimes known as the nephron loop)
   is a U-shaped tube that consists of a descending limb and ascending
   limb. It begins in the cortex, receiving filtrate from the proximal
   convoluted tubule, extends into the medulla, and then returns to the
   cortex to empty into the distal convoluted tubule. Its primary role is
   to concentrate the salt in the interstitium, the tissue surrounding the
   loop.
   It is divided into descending and ascending limbs:
   * descending limb Its descending limb is permeable to water but
   completely 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.
   * ascending limb Unlike the descending limb, the ascending limb of
   Henle's loop 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.
   Distal convoluted tubule The distal convoluted tubule is not similar to
   the proximal convoluted tubule in structure and function. 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.

   After traveling the length of the distal convoluted tubule, only 3% 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 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.

   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. Dehydration results in
   an increase in ADH, 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 embryonic origin than the rest of the
   nephron (the collecting duct is from endoderm whereas the nephron is
   from mesoderm), the collecting duct is usually not considered a part of
   the full nephron.

Juxtaglomerular apparatus

   The juxtaglomerular apparatus occurs near the site of contact between
   the thick ascending limb and the afferent arteriole. It contains three
   components:
   the macula densa a tightly-packed area of cells in the thick ascending
   limb
   juxtaglomerular cells specialized smooth muscle cells in the wall of
   the afferent arteriole
   extraglomerular mesangial cells couple to arterioles

   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.

Additional images

   Distribution of blood vessels in cortex of kidney.

   Glomerulus is red; Bowman's capsule is white.

   Kidney tissue

   Glomerulus is red; Bowman's capsule is green.

   Glomerulus

   Retrieved from " http://en.wikipedia.org/wiki/Nephron"
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   with only minor checks and changes (see www.wikipedia.org for details
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