The kidney is made up of tiny things called nephrons. These are like little tubes, only about 14mm long and very skinny. There are two kinds ofrons: cortical ones, which are mostly in charge of getting rid of waste and keeping things in balance, and juxtamedullary ones, which help to make urine more concentrated or more watery. So, next time you think about your kidneys, remember that it's the nephrons doing all the hard work! And if you're interested in learning more about how they work, you can start by reading up on the different types of nephrons, like the cortical and juxtamedullary ones.

The structures that constitute the nephron

The nephron is like a tiny machine in your kidney, made up of different parts that all have different jobs. Let's take a closer look at them! First up is Bowman's capsule, which is like pocket. This surrounds bunch of tiny blood vessels called the glomerulus, and together they're known as the corpuscle. Next is the proximal convoluted tubule, which is a twisty section of the nephron that helps to absorb important things from the corpuscle. It cells that have hairs on them, calledilli, increase and make the corpuscle is called the glomerular filtrate, and it moves into the next part of the nephron, called the Loop of Henle. This is a long, U-shaped tube that goes deep into the kidney and then back out again. It's surrounded by blood vessels and helps to establish a gradient, which is important for making sure your body can balance the amount of water and salt in your urine. After that, there's the distal convoluted tubule, which is another twisty section of the nephron that helps to fine-tune the balance of fluids and salts in your urine. And finally, there's the collecting duct, which is like a drain that carries all the urine out of your kidney and into your bladder. So there you have it - a quick tour of the different parts of the nephron!

‍

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

‍

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, causing 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 fenestrations in the glomerular endothelium and podocytes. Fenestrations are gaps between the basement membrane of the glomerular endothelium that allow easy passage of fluids between cells, but are too small for large proteins, red and white blood cells, and platelets. Podocytes 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.

‍

Proximal convoluted tubule

After the filtration in the glomerulus, 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 adaptations include microvilli on their apical side, which increase the surface area for reabsorption from the lumen. Infoldings at the basal side increase 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 concentration gradientDuringabsorption in the proximallyoluted tubule, Na (sodium) + ions are actively transported out of the epithelial cells and into the interstitium by the Na-K pump. 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, including 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.

In addition, most water reabsorption occurs in the proximal convoluted tubule. Water molecules follow the solutes that are reabsorbed by the co-transporters and diffuse down their concentration gradient through aquaporins in the apical and basal sides of the epithelial cells. This process is called osmosis, and it allows the body to conserve water when needed. Overall, the proximal convoluted tubule plays a crucial role in the selective reabsorption of solutes and water from the filtrate, ensuring that the body retains what it needs to function properly.

The Loop of Henle

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

The loop of Henle consists of two limbs: a thin descending limb and a thick ascending limb. The descending limb is permeable to water, but not to electrolytes, while the ascending limb is impermeable to water, but highly permeable to electrolytes.

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

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

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 ATP. 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. 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. 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.

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. 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.

Overall, the loop of Henle plays a crucial role in maintaining the cortico-medullary gradient and producing concentrated urine.

The distally convoluted tubule

The distal convoluted tubule is responsible for making fine adjustments to the reabsorption of ions from the filtrate and regulating blood pH by controlling the excretion and reabsorption of H + and bicarbonate ions. The epithelium of the distal convoluted tubule has many mitochondria and microvilli to provide the ATP needed for the active transport of ions and increase the surface area for selective reabsorption and excretion. The collecting duct runs 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 distal convoluted tubule and collecting duct are under the control of hormones such as aldosterone and antidiuretic hormone (ADH), which regulate the excretion and reabsorption of water and ions depending on the body's needs. Aldosterone stimulates the reabsorption of Na + and the excretion of K +, while ADH increases the permeability of the collecting duct to water, allowing for the reabsorption of more water and the production of concentrated urine. Overall, the distal convoluted tubule and collecting duct play a vital role in the selective reabsorption and excretion of ions and the regulation of water balance and urine concentration.

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

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

What is the structure of the 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.

What's a nephron?

The nephron is the functional unit of the kidney. 

What are the 3 main functions of the nephron?

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. 

Where is the nephron located in the kidney?

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

What happens in the nephron?

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.

‍