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

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.

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