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Nephron

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Biology

The nephron is the functional unit of the kidney. It consists of a 14mm tube with a very narrow radius closed at both ends.

There are two types of nephrons in the kidney: cortical (mainly in charge of excretory and regulatory functions) and juxtamedullary (concentrate and dilute urine) nephrons.

The structures that constitute the nephron

The nephron consists of different regions, each with different functions. These structures include:

  • Bowman's capsule: the beginning of the nephron, which surrounds a dense network of blood capillaries called the glomerulus. The inner layer of Bowman's capsule is lined with specialised cells called podocytes that prevent the passage of large particles such as cells from the blood into the nephron. The Bowman's capsule and the glomerulus are called the corpuscle.
  • Proximal convoluted tubule: the continuation of the nephron from the Bowman's capsule. This region contains highly twisted tubules surrounded by blood capillaries. Furthermore, the epithelial cells lining the proximally convoluted tubules have microvilli to enhance the reabsorption of substances from the glomerular filtrate.

Microvilli (singular form: microvillus) are microscopic protrusions of the cell membrane that expand the surface area to enhance the rate of absorption with very little increase in cell volume.

The glomerular filtrate is the fluid found in the lumen of the Bowman's capsule, produced as a result of filtration of the plasma in the glomerular capillaries.

  • Loop of Henle: a long U-shaped loop that extends from the cortex deep into the medulla and back into the cortex again. This loop is surrounded by blood capillaries and plays an essential part in establishing the corticomedullary gradient.
  • Distal convoluted tubule: the continuation of the loop of Henle lined with epithelial cells. Fewer capillaries surround the tubules in this region than the proximal convoluted tubules.
  • Collecting duct: a tube into which multiple distal convoluted tubules drain. The collecting duct carries urine and eventually drains into the renal pelvis.

Figure 1. The general structure of a nephron and its constituting regions.

Various blood vessels are associated with different regions of the nephron. The table below shows the name and description of these blood vessels.

Blood vessels

Description

Afferent arteriole

This is a small artery arising from the renal artery. The afferent arteriole enters the Bowman's capsule and forms the glomerulus.

Glomerulus

A very dense network of capillaries arising from the afferent arteriole where fluid from the blood is filtered into the Bowman's capsule. The glomerular capillaries merge to form the efferent arteriole.

Efferent arteriole

The recombination of glomerular capillaries forms a small artery. The narrow diameter of the efferent arteriole increases the blood pressure in the glomerular capillaries allowing more fluids to be filtered. The efferent arteriole gives off many branches forming the blood capillaries.

Blood capillaries

These blood capillaries originate from the efferent arteriole and surround the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. These capillaries allow the reabsorption of substances from the nephron back into the blood and the excretion of waste products into the nephron.

Table 1. The blood vessels associated with different regions of a nephron.

The function of different parts of the nephron

Let’s study the different parts of a nephron.

Bowman’s capsule

The afferent arteriole that brings blood to the kidney branches into a dense network of capillaries, called the glomerulus. The Bowman's capsule surrounds the glomerular capillaries. The capillaries merge to form the efferent arteriole.

The afferent arteriole has a larger diameter than the efferent arteriole. This causes increased hydrostatic pressure inside which in turn, causes the glomerulus to push fluids out of the glomerulus into the Bowman's capsule. This event is called ultrafiltration, and the fluid created is called the glomerular filtrate. The filtrate is water, glucose, amino acids, urea, and inorganic ions. It does not contain large proteins or cells since they are too large to pass through the glomerular endothelium.

The glomerulus and the Bowman's capsule have specific adaptations to facilitate ultrafiltration and reduce its resistance. These include:

  1. Fenestrations in the glomerular endothelium: the glomerular endothelium has gaps between its basement membrane that allow easy passage of fluids between cells. However, these spaces are too small for large proteins, red and white blood cells, and platelets.
  2. Podocytes: the inner layer of the Bowman's capsule is lined with podocytes. These are specialised cells with tiny pedicels that wrap around the glomerular capillaries. There are spaces between podocytes and their processes that allow fluids to pass through them quickly. Podocytes are also selective and prevent the entry of proteins and blood cells into the filtrate.

The filtrate contains water, glucose, and electrolyte, which are very useful to the body and need to be reabsorbed. This process happens in the next part of the nephron.

Figure 2. Structures within the Bowman's capsule.

Proximal convoluted tubule

The majority of the content in the filtrate are useful substances that the body needs to reabsorb. The bulk of this selective reabsorption occurs in the proximal convoluted tubule, where 85% of the filtrate is reabsorbed.

The epithelial cells lining the proximally convoluted tubule possess adaptations for efficient reabsorption. These include:

  • Microvilli on their apical side increase the surface area for reabsorption from the lumen.
  • Infoldings at the basal side, increasing the rate of solute transfer from the epithelial cells into the interstitium and then into the blood.
  • Many co-transporters in the luminal membrane allow for the transport of specific solutes such as glucose and amino acids.
  • A high number of mitochondria generating ATP is needed to reabsorb solutes against their concentration gradient.

Na (sodium) + ions are actively transported out of the epithelial cells and into the interstitium by the Na-K pump during reabsorption in the proximally convoluted tubule. This process causes the Na concentration inside the cells to be lower than in the filtrate. As a result, Na ions diffuse down their concentration gradient from the lumen into the epithelial cells via specific carrier proteins. These carrier proteins co-transport specific substances with Na as well. These include amino acids and glucose. Subsequently, these particles move out of the epithelial cells at the basal side of their concentration gradient and return into the blood.

Furthermore, most water reabsorption occurs in the proximal convoluted tubule as well.

The Loop of Henle

The loop of Henle is a hairpin structure extending from the cortex into the medulla. The primary role of this loop is to maintain the cortico-medullary water osmolarity gradient that allows for producing very concentrated urine.

The loop of Henle has two limbs:

  1. A thin descending limb that is permeable to water but not to electrolytes.
  2. A thick ascending limb that is impermeable to water but highly permeable to electrolytes.

The flow of content in these two regions is in opposite directions, meaning it is a counter-current flow, similar to the one seen in the fish gills. This characteristic maintains the cortico-medullary osmolarity gradient. Therefore, the loop of Henle acts as a counter-current multiplier.

The mechanism of this counter-current multiplier is as follows:

  1. In the ascending limb, electrolytes (especially Na) are actively transported out of the lumen and into the interstitial space. This process is energy-dependent and requires ATP.
  2. This lowers the water potential at the interstitial space level, but water molecules cannot escape from the filtrate since the ascending limb is impermeable to water.
  3. Water passively diffuses out of the lumen by osmosis at the same level but in the descending limb. This water that has moved out does not change the water potential in the interstitial space since it gets picked up by the blood capillaries and is carried away.
  4. These events progressively occur at every level along the loop of Henle. As a result, the filtrate loses water as it goes through the descending limb, and its water content gets to its lowest point when it reaches the turning point of the loop.
  5. As the filtrate goes through the ascending limb, it is low in water and high in electrolytes. The ascending limb is permeable to electrolytes such as Na, but it does not allow water to escape. Therefore, the filtrate loses its electrolyte content from the medulla to the cortex since the ions are actively pumped out into the interstitium.
  6. As a result of this counter-current flow, the interstitial space at the cortex and medulla is in a water potential gradient. The cortex has the highest water potential (lowest concentration of electrolytes), while the medulla has the lowest water potential (highest concentration of electrolytes). This is called the cortico-medullary gradient.

The distally convoluted tubule

The primary role of the distal convoluted tubule is to make more fine adjustments to the reabsorption of ions from the filtrate. Furthermore, this region helps regulate the blood pH by controlling the excretion and reabsorption of H + and bicarbonate ions. Similar to its proximal counterpart, the epithelium of the distal convoluted tubule has many mitochondria and microvilli. This is to provide the ATP needed for the active transport of ions and to increase the surface area for selective reabsorption and excretion.

The collecting duct

The collecting duct goes from the cortex (high water potential) towards the medulla (low water potential) and eventually drains into the calyces and the renal pelvis. This duct is permeable to water, and it loses more and more water as it goes through the cortico-medullary gradient. The blood capillaries absorb the water that enters the interstitial space, so it does not affect this gradient. This results in urine being highly concentrated.

The permeability of the collecting duct's epithelium is adjusted by the endocrine hormones, allowing for fine controlling of the body water content.

Figure 3. A summary of reabsorptions and secretions along the nephron.

Nephron - Key takeaways

  • A nephron is a functional unit of a kidney.
  • The convoluted tubule of the nephron possesses adaptations for efficient reabsorption: microvilli, infolding of the basal membrane, a high number of mitochondria and the presence of lots of co-transporter proteins.
  • The nephron consists of different regions. These include:
    • Bowman's capsule
    • Proximal convoluted tubule
    • Loop Henle
    • Distally convoluted tubule
    • Collecting duct
  • The blood vessels associated with the nephron are:
    • Afferent arteriole
    • Glomerulus
    • Efferent arteriole
    • Blood capillaries

Nephron

The nephron is composed of Bowman’s capsule and a renal tube. The renal tube is comprised of the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and the collecting duct.

The nephron is the functional unit of the kidney.

The kidney actually has more than three functions. Some of these include: Regulating the body’s water content, regulating the blood’s pH, excretion of waste products, and endocrine secretion of EPO hormone.

The majority of the nephron is located in the cortex but the loop of Henle and the collecting extend down into the medulla.

The nephron first filtrates the blood in the glomerulus. This process is called ultrafiltration. The filtrate then travels through the renal tube where useful substances, such as glucose and water, are reabsorbed and waste substances, such as urea, are removed.

Final Nephron Quiz

Question

What is the functional unit of the kidney?

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Answer

The nephron.

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Question

What are the two parts of the nephron?

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Answer

The Bowman’s capsule and the renal tube.

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Question

What happens in the glomerulus?


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Answer

Ultrafiltration. The blood plasma is filtered into the Bowman’s capsule.

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Question

What is the main function of the proximal convoluted tubule?


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Answer

Reabsorption of water and useful substance such as glucose and amino acids from the filtrate.

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Question

What does the loop of Henle maintain?


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Answer

The cortico-medullary gradient.

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Question

Where is the collecting duct located?


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Answer

The collecting duct starts in the cortex and goes towards the medulla.

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Question

________ arteriole branches to form the glomerulus. 


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Answer

Afferent

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Question

The glomerulus branches recombine to form the _______ arteriole.


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Answer

Efferent

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Question

What are the adaptations of the proximal convoluted tubule epithelium for efficient reabsorption?


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Answer

  • Microvilli on their apical side increase the surface area for reabsorption from the lumen.
  • Infoldings at the basal side, increasing the rate of solute transfer from the epithelial cells into the interstitium and then into the blood. 
  • Many co-transporters in the luminal membrane allow for the transport of specific solutes such as glucose and amino acids. 
  • A high number of mitochondria generating ATP is needed for the active reabsorption of solutes against their concentration gradient. 

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Question

How are substances selectively reabsorbed in the proximal convoluted tubule?


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Answer

The epithelium of the proximal convoluted tubule actively pumps Na ions out into the interstitial space. This allows maintaining the intracellular concentration of Na at a lower level than the filtrate. Specific co-transporter proteins on the apical side of the epithelium use the Na concentration gradient to co-transport glucose or amino acids with Na into the epithelial cells. These substances diffuse out of the epithelial cells at the basal side into the interstitium via channel proteins and transporters. They are then picked up by the blood capillaries and returned to the bloodstream.

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Question

What are podocytes and what role do they have?

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Answer

The inner layer of the Bowman's capsule is lined with podocytes. These are specialized cells with tiny pedicels that wrap around the glomerular capillaries. There are spaces between podocytes and their pedicels that allow fluids to pass through them easily. Podocytes are also selective and prevent the entry of proteins and blood cells into the filtrate. 

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Question

True or False: The thin descending limb of the loop of Henle is permeable to electrolytes. 

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Answer

False

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Question

True or False: The thick ascending limb of the loop of Henle is permeable to water. 

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Answer

False

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Question

What is the glomerular filtrate?

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Answer

The glomerular filtrate is the fluid found in the lumen of the Bowman's capsule that is produced as a result of filtration of the plasma in the glomerular capillaries. 

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Question

What are microvilli? 

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Answer

Microvilli (singular form: microvillus) are microscopic protrusions of the cell membrane that expand the surface area to enhance the rate of absorption with very little increase in cell volume.  

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