How does the diameter of the afferent and efferent arterioles affect nephron function?

The afferent arteriole is wider than the efferent arteriole, creating high hydrostatic pressure in the glomerulus. This pressure is the driving force for ultrafiltration, pushing small molecules out of the blood and into the Bowman's capsule.

What is the fundamental difference between the ascending and descending limbs of the loop of Henle?

The descending limb is permeable to water but impermeable to ions, allowing water to leave by osmosis. Conversely, the ascending limb is impermeable to water but actively pumps out sodium and chloride ions to create the medullary gradient.

How does selective reabsorption differ from ultrafiltration in terms of energy and selectivity?

Ultrafiltration is a passive, non-selective process driven by blood pressure that filters molecules based on size. Selective reabsorption is an active, highly specific process that uses ATP and carrier proteins to reclaim useful molecules like glucose.

What is a common misconception regarding the transport of glucose in the PCT?

Many students believe glucose is moved by simple diffusion or direct active transport. In reality, it is moved via co-transport (secondary active transport) coupled with sodium ions, which requires a pre-existing sodium gradient.

Why is it incorrect to say that the loop of Henle produces concentrated urine?

The loop of Henle does not produce the final concentrated urine; instead, it creates the salt gradient in the medulla. The actual concentration of urine occurs in the collecting duct, where water is reabsorbed based on that gradient.

What error occurs if one assumes proteins are found in the glomerular filtrate?

Proteins are too large to pass through the basement membrane of the filtration barrier. Finding proteins in the filtrate or urine usually indicates damage to the glomerular capillaries or the basement membrane itself.

Define 'Podocytes' and their role in the nephron.

Podocytes are specialized epithelial cells in the Bowman's capsule that have finger-like projections called foot processes. They wrap around the glomerular capillaries, leaving narrow filtration slits that allow small molecules to pass into the nephron.

What are 'Aquaporins' and when are they utilized?

Aquaporins are specialized channel proteins that facilitate the rapid movement of water across cell membranes. They are inserted into the membranes of the collecting duct in response to ADH to increase water reabsorption.

Define 'Glycogenesis' in the context of renal glucose management.

While glycogenesis (conversion of glucose to glycogen) primarily occurs in the liver, it is the process stimulated by insulin to lower blood glucose. In the nephron, all filtered glucose must be reabsorbed to prevent its loss in urine.

Why does the PCT have a high concentration of mitochondria?

The PCT requires significant amounts of ATP to power the sodium-potassium pumps in the basal membrane. These pumps maintain the sodium gradient necessary for the secondary active transport of glucose and amino acids.

Nephron Function | AQA A-Level Biology

1. Mechanisms of Ultrafiltration

Hydrostatic Pressure Generation: Ultrafiltration is driven by high blood pressure in the glomerulus, which is created because the afferent arteriole (supplying blood) is significantly wider than the efferent arteriole (carrying blood away). This diameter mismatch creates a bottleneck effect that forces fluid and small solutes out of the capillary and into the Bowman's capsule.
The Filtration Barrier: The blood is filtered through three distinct layers: the fenestrated capillary endothelium, the basement membrane, and the podocytes of the Bowman's capsule. The basement membrane acts as the primary molecular sieve, preventing large proteins and blood cells from entering the nephron while allowing water, glucose, ions, and urea to pass through.
Glomerular Filtrate Composition: The resulting fluid, known as glomerular filtrate, contains all the small-molecule components of blood plasma. Because this process is based purely on size rather than biological utility, essential nutrients like glucose must be recovered in later stages of the nephron.

Diagram showing the filtration barrier layers: capillary endothelium, basement membrane, and podocyte processes.

2. Selective Reabsorption in the PCT

Secondary Active Transport: In the Proximal Convoluted Tubule (PCT), glucose and amino acids are reabsorbed via co-transport with sodium ions. Sodium-potassium pumps actively move $Na^+$ out of the epithelial cells into the blood, creating a low concentration gradient that pulls $Na^+$ (and coupled nutrients) from the filtrate into the cell.
Structural Adaptations: The epithelial cells of the PCT are highly specialized, featuring microvilli on the luminal surface to maximize the surface area for absorption. Additionally, these cells contain a high density of mitochondria to provide the ATP required for the active transport of ions against their concentration gradients.
Osmotic Recovery: As solutes like glucose and salts are moved back into the blood, the water potential of the blood decreases. This creates an osmotic gradient that causes water to follow the solutes out of the tubule and back into the peritubular capillaries.

3. The Loop of Henle and Medullary Gradient

Counter-Current Multiplier: The Loop of Henle functions to create a high salt concentration in the renal medulla, which is necessary for water conservation. The ascending limb actively pumps out sodium and chloride ions but is impermeable to water, effectively lowering the water potential of the surrounding interstitial fluid.
Water Extraction: The descending limb is permeable to water but impermeable to ions. As the filtrate moves down this limb into the increasingly salty medulla, water leaves the tubule by osmosis and is reclaimed by the blood, concentrating the remaining filtrate.
Gradient Maintenance: This mechanism ensures that the deepest part of the medulla has the lowest water potential. This steep gradient allows for the maximum possible reabsorption of water from the collecting duct when the body needs to conserve fluid.

4. Hormonal Regulation of Water Potential

ADH and Aquaporins: Antidiuretic Hormone (ADH) regulates the permeability of the distal convoluted tubule and the collecting duct. When blood water potential is low, ADH is released and triggers the insertion of aquaporins (water channels) into the cell membranes, allowing more water to be reabsorbed into the blood.
Urine Concentration: By increasing the number of aquaporins, the collecting duct becomes highly permeable to water. Water moves out of the duct into the concentrated medulla by osmosis, resulting in a small volume of highly concentrated urine.
Negative Feedback Loop: This system is a classic example of homeostasis. Once blood water potential returns to normal, osmoreceptors in the hypothalamus signal the pituitary gland to reduce ADH secretion, thereby decreasing the permeability of the ducts and preventing over-hydration.

5. Key Distinctions

Feature	Ultrafiltration	Selective Reabsorption
Location	Glomerulus / Bowman's Capsule	Proximal Convoluted Tubule
Driving Force	Hydrostatic Blood Pressure	Active Transport / ATP
Selectivity	Non-selective (based on size)	Highly selective (based on transporters)
Substances	Water, Glucose, Urea, Ions	Glucose, Amino Acids, $Na^+$

Ascending vs. Descending Limb: It is critical to remember that the ascending limb is the 'engine' that drives the system by actively moving salts, while the descending limb is the 'passive responder' that allows water to exit. The ascending limb's impermeability to water is the unique feature that prevents the gradient from being washed away.

6. Exam Strategy & Tips

Link Structure to Function: When describing the PCT, always mention microvilli for surface area and mitochondria for ATP. Marks are often lost for failing to connect the biological structure to the specific transport process it supports.
Terminology Precision: Use the term 'co-transport' specifically for glucose and amino acids in the PCT. Avoid saying glucose is 'filtered' back into the blood; it is 'reabsorbed'.
The ADH Mechanism: When explaining ADH action, follow the sequence: ADH release → binding to receptors → aquaporin insertion → increased permeability → osmosis into medulla. Missing any step in this chain often results in partial credit.
Sanity Check: If a question asks about the effect of a drug that inhibits active transport in the loop of Henle, the answer will always involve the production of large volumes of dilute urine because the medullary gradient cannot be established.

Nephron Function

Summary

1. Mechanisms of Ultrafiltration

2. Selective Reabsorption in the PCT

3. The Loop of Henle and Medullary Gradient

4. Hormonal Regulation of Water Potential

5. Key Distinctions

6. Exam Strategy & Tips

Nephron Function

Summary

1. Mechanisms of Ultrafiltration

2. Selective Reabsorption in the PCT

3. The Loop of Henle and Medullary Gradient

4. Hormonal Regulation of Water Potential

5. Key Distinctions

6. Exam Strategy & Tips