Everything about Nephron totally explained
A
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
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.
Types of nephrons
Two general classes of nephrons are cortical nephrons and
juxtamedullary nephrons, both of which are classified according to the location of their associated
renal corpuscle. Cortical nephrons have their renal corpuscle in the superficial
renal cortex, while the renal corpuscles of juxtamedullary nephrons are located near the
renal medulla. The nomenclature for cortical nephrons varies, with some sources distinguishing between
superficial cortical nephrons and
midcortical nephrons; other sources simply call all non-juxtamedullary nephrons
superficial nephrons.
Functionally, cortical and juxtamedullary nephrons have distinct roles. Cortical nephrons (85% of all nephrons in humans) mainly perform excretory and regulatory functions, while juxtamedullary nephrons (15% of nephrons in humans) concentrate and dilute urine. After passing the tubule, the filtrate continues to the
collecting duct system.
The components of the kidney tubule are:
Tubule component functions
The following table describes each component of a tubule in detail.
| Name |
Description |
| Proximal tubule |
convoluted |
S1 |
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). |
S2 |
| straight |
S3 |
| Loop of Henle |
The loop of Henle (sometimes known as the nephron loop) is a tube, it's often u-shaped in diagrams for simplicity but in reality it looks more like one loop of a coil (hence, 'loop'). It extends from the proximal tube and 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, 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. |
|
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 thin segment
thick segment
|
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 isn't 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 secernates hydrogen and ammonium to regulate pH. |
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, 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-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. An increase in ADH is an indication of
dehydration, 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 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.
Juxtaglomerular apparatus
The
juxtaglomerular apparatus occurs near the site of contact between the thick ascending limb and the afferent arteriole. It contains three components:
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.
Further Information
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