Saturday 21 January 2012
NEPHRONE
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 Malphigian 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]
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