Nephron

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

The structures that constitute the nephron

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

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

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.

Fig. 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 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.

Fig. 3 - A summary of reabsorptions and secretions along the nephron