Nephrons (Leaving Cert Biology): Revision Notes

Nephrons

Location of nephrons

Each kidney contains over one million tiny functional units called nephrons. These microscopic structures are approximately 3 centimetres long and are found throughout both the outer cortex and inner medulla regions of the kidney. Nephrons serve as the kidney's primary filtration and processing units, working continuously to produce urine and maintain the body's fluid balance.

Structure of a nephron

A nephron consists of several interconnected parts, each with a specific role in filtering blood and forming urine:

Bowman's capsule - A cup-shaped structure that surrounds a cluster of tiny blood vessels called the glomerulus. This forms the beginning of the nephron where initial filtration occurs.

Proximal convoluted tubule - A twisted tube that connects to Bowman's capsule. This region performs most of the reabsorption work, recovering valuable substances from the filtered liquid.

Loop of Henle - A hairpin-shaped portion consisting of:

• Descending limb - Allows water to pass through its walls
• Ascending limb - Permits salt movement but prevents water from crossing

Distal convoluted tubule - Another twisted section that fine-tunes the composition of the forming urine through selective reabsorption.

Collecting duct - The final pathway where multiple nephrons converge, allowing final adjustments to water and salt balance before urine formation is complete.

Blood supply to the nephron

The nephron receives its blood supply through a sophisticated network of vessels. Blood enters each kidney via the renal artery, which branches repeatedly to form smaller arterioles. Each nephron is supplied by an afferent arteriole that brings blood into the glomerulus.

Within Bowman's capsule, the afferent arteriole splits into a tight cluster of capillaries called the glomerulus. This arrangement creates the perfect conditions for filtration to occur. Blood then leaves the glomerulus through the efferent arteriole, which forms additional capillaries that surround the rest of the nephron tubules before eventually joining to form renal veins.

Formation of urine in nephrons

Urine production involves two main processes working together: filtration and reabsorption.

Filtration

Filtration takes place in the glomerulus within Bowman's capsule. Several factors make this process highly efficient:

• High blood pressure in the glomerular capillaries forces liquid and small molecules out of the blood
• Large surface area of the glomerular capillaries provides extensive filtering space
• Thin, porous capillary walls allow small molecules like water, glucose, amino acids, vitamins, salts, and waste products to pass through
• Selective barrier prevents large molecules such as blood cells, platelets, antibodies, and proteins from being filtered out

The liquid that emerges from this filtration process is called glomerular filtrate. Remarkably, about 180 litres of this filtrate are produced every 24 hours - that's roughly five times the total fluid content of the human body!

Reabsorption

Since the body cannot afford to lose 180 litres of fluid daily, an efficient reabsorption system recovers approximately 99% of the glomerular filtrate. This process occurs throughout different regions of the nephron:

In the proximal convoluted tubule:

• Most water is reabsorbed through osmosis
• Useful substances like glucose, amino acids, and vitamins are completely recovered through active transport and diffusion
• Most salts are reabsorbed through active transport or diffusion
• The tubule is specially adapted with a thin wall, extended length, numerous internal folds called microvilli, and abundant mitochondria to provide energy for active transport

In the descending limb of the loop of Henle:

• Cells allow water to pass through
• Small amounts of water are reabsorbed by osmosis

In the ascending limb of the loop of Henle:

• Cells prevent water from crossing
• Cells are permeable to salts, which move out initially by diffusion but are actively pumped out at the top of the ascending limb
• This salt removal concentrates the surrounding fluid, helping the kidney conserve water

In the distal convoluted tubule:

• Precise control of water, salt, and blood pH occurs
• Some water and salts are reabsorbed through a combination of osmosis and active transport

In the collecting duct:

• Final adjustments to water and salt balance take place
• Cells are permeable to water, and some reabsorption occurs due to the high salt concentration in the surrounding medulla
• Some salts are also reabsorbed

Summary of reabsorption processes

The following table summarises what happens in each region of the nephron:

Region	Substances reabsorbed	Amount of water reabsorbed
Proximal convoluted tubule	Most salts, glucose, amino acids, and vitamins	Most
Loop of Henle (descending limb)	None	A little
Loop of Henle (ascending limb)	Some salts	None
Distal convoluted tubule	Some salts	Some
Collecting duct	Some salts	Some

Glomerular filtrate vs. urine

The transformation from glomerular filtrate to urine involves significant changes in composition:

Glomerular filtrate characteristics:

• Contains much more water (making it more dilute than urine)
• Contains many useful molecules such as glucose and amino acids that are normally absent from urine

Final urine characteristics:

• More concentrated due to water reabsorption
• Contains primarily waste products and excess substances the body needs to eliminate
• Useful substances like glucose and amino acids have been removed and returned to the blood

This comparison highlights the kidney's remarkable efficiency in conserving valuable substances while eliminating waste products.

Remember!