Filtration & Reabsorption in the Nephron Revision Notes | A Level AQA Biology

1. Why the Nephron Matters

Your kidneys filter approximately 180 litres of filtrate per day, meaning your entire blood volume (roughly 5 L) is filtered about 36 times each day. The unit doing that work is the nephron — and understanding it is central to understanding homeostasis at A Level.

Each kidney contains around one million nephrons. Every nephron carries out the same sequence: filter the blood aggressively, then carefully reclaim everything the body needs. What remains becomes urine.

2. Structure of the Nephron

Picture the nephron as a long, coiled tube with distinct regions, each with a specific job. Blood arrives via the afferent arteriole, passes through the glomerulus (a knot of capillaries inside the Bowman's capsule), then the filtrate travels through the proximal convoluted tubule (PCT), the loop of Henle, the distal convoluted tubule (DCT), and finally the collecting duct.

The cortex and medulla are not just anatomical labels — they matter functionally. The PCT and DCT sit in the cortex, close to a rich blood supply. The loop of Henle dips deep into the medulla, where it creates the osmotic conditions needed for water reabsorption later in the collecting duct.

3. Formation of Glomerular Filtrate

Here is where filtration begins. Blood enters the glomerulus under high hydrostatic pressure — higher than in most capillary beds — because the efferent arteriole (leaving the glomerulus) is narrower than the afferent arteriole (entering it). This pressure difference forces fluid out of the capillaries.

Three layers act as the filtration barrier: the capillary endothelium (with pores called fenestrations), the basement membrane (a mesh of glycoproteins that blocks large molecules), and the podocytes of the Bowman's capsule (whose foot processes leave filtration slits). All three must be intact for normal filtration.

The resulting glomerular filtrate is essentially blood plasma minus the large proteins and cells. Its composition — water, glucose, urea, sodium ions, chloride ions, amino acids — reflects this. The filtrate is not yet urine; it still contains everything the body needs to keep.

4. Reabsorption in the Proximal Convoluted Tubule

The filtrate entering the PCT contains glucose — a molecule the body cannot afford to lose. The PCT is where the bulk of selective reabsorption happens, and its structure is exquisitely matched to this task.

The cells lining the PCT have microvilli on their luminal surface, forming a brush border that dramatically increases surface area. They also contain large numbers of mitochondria, which supply the ATP needed for active transport. The basal membrane faces the peritubular capillaries, allowing reabsorbed substances to pass directly into the blood.

4a. Glucose Reabsorption

Glucose reabsorption in the PCT uses a two-step mechanism. First, sodium ions are actively pumped out of the PCT cell into the blood by sodium-potassium ATPase pumps on the basal membrane. This lowers the sodium ion concentration inside the cell, creating a concentration gradient from the filtrate into the cell.

Glucose then enters the PCT cell from the filtrate via co-transporter proteins on the luminal membrane. These co-transporters couple glucose movement to the movement of sodium ions down their concentration gradient — so glucose is carried in alongside sodium without requiring direct ATP expenditure at this step. This is secondary active transport. Once inside the cell, glucose leaves via facilitated diffusion into the peritubular capillaries.

4b. Water Reabsorption in the PCT

As solutes such as glucose, amino acids, and sodium ions are reabsorbed from the filtrate, the water potential of the filtrate falls relative to the PCT cells and the surrounding tissue. Water follows by osmosis through aquaporin channels in the cell membranes, moving from the filtrate into the peritubular capillaries.

Approximately 65–70% of the filtered water is reabsorbed in the PCT. This is an obligatory process — it happens regardless of the body's hydration state, driven entirely by the osmotic gradient created by solute reabsorption. The PCT does not regulate water reabsorption; that fine-tuning happens later in the collecting duct under hormonal control.

The PCT therefore handles the bulk of reabsorption: all glucose and amino acids under normal conditions, around two-thirds of filtered water, and large quantities of sodium, chloride, and bicarbonate ions. What leaves the PCT is a reduced volume of fluid, still containing urea and some ions, heading into the loop of Henle.

5. The Loop of Henle and the Medullary Gradient

The loop of Henle is the nephron's most elegant piece of engineering. Its job is to build up a high concentration of sodium ions in the medulla — a gradient that makes water reabsorption in the collecting duct possible. Without this gradient, the body could not produce concentrated urine.

The loop has two limbs: the descending limb and the ascending limb. They run in opposite directions, and this countercurrent arrangement is the key to how the gradient is built. The descending limb is permeable to water but not to sodium ions. The ascending limb actively transports sodium and chloride ions out into the medulla but is impermeable to water.

Worked example: How the Loop of Henle Builds the Medullary Sodium Gradient

Trace the movement of sodium ions and water through the loop of Henle to explain how a sodium ion concentration gradient is established in the medulla.

1. Filtrate flows down the descending limb into the medulla. The descending limb is permeable to water but impermeable to sodium ions. — As filtrate descends into the increasingly concentrated medulla, water leaves by osmosis into the surrounding tissue and capillaries. The filtrate becomes progressively more concentrated in sodium ions as it descends.
2. At the base of the loop (deepest point in the medulla), the filtrate has its highest sodium ion concentration. — The longer the loop of Henle, the deeper it descends and the more concentrated the filtrate becomes at the bend. This is why animals in arid environments (e.g. desert rodents) have longer loops.
3. In the thick ascending limb, sodium and chloride ions are actively transported out of the filtrate into the medullary tissue. The ascending limb is impermeable to water. — Because water cannot follow the ions out, the medullary interstitial fluid accumulates a high sodium ion concentration. The filtrate inside the ascending limb becomes progressively more dilute as it rises.
4. Filtrate in the descending limb flows downward while filtrate in the ascending limb flows upward. Sodium ions pumped out of the ascending limb raise the concentration of the medulla, which draws more water out of the descending limb. — This countercurrent multiplier effect means the gradient is continuously reinforced. The sodium ion concentration in the medulla increases progressively from cortex to the deepest part of the medulla.
5. The collecting duct passes through the medulla, which now has a high sodium ion concentration (low water potential). When ADH is present, aquaporins open and water moves out of the collecting duct by osmosis. — Without the sodium gradient built by the loop of Henle, there would be no osmotic driving force to reabsorb water from the collecting duct, and the body could not concentrate urine.

Answer: The loop of Henle acts as a countercurrent multiplier: the ascending limb actively pumps sodium ions into the medulla (without water following), and the descending limb loses water by osmosis into this concentrated medulla. The result is a progressively increasing sodium ion concentration gradient from cortex to medulla, which drives water reabsorption from the collecting duct.

Notice the elegant logic here: the ascending limb creates the gradient, and the descending limb responds to it. Neither limb alone could achieve this — it is the countercurrent arrangement that multiplies the effect. This is why the loop of Henle is described as a countercurrent multiplier.

6. Comparing the Limbs: Structure Meets Function

A common exam task is to compare the two limbs of the loop of Henle. The key is to link permeability to function precisely.

This comparison table is worth memorising in its logic, not just its content. Every difference between the two limbs exists because the two limbs have opposite roles in building the gradient. The descending limb responds to the gradient; the ascending limb creates it.

7. Exam Technique and Common Mistakes

Questions on the nephron appear in every paper, and the marks are often lost not through lack of knowledge but through imprecise language. The following guidance targets the most common errors seen in mark schemes.

Extended-response questions on this topic often ask you to evaluate how structure relates to function. A strong answer links each structural feature (e.g. microvilli, mitochondria, aquaporins, impermeability of the ascending limb) to a specific functional consequence, using precise causal language: 'because', 'therefore', 'this means that'.

Every spec requirement in this lesson — filtrate formation, glucose and water reabsorption in the PCT, and the sodium ion gradient maintained by the loop of Henle — connects to the same overarching principle: the nephron filters aggressively and then selectively reclaims, using structural adaptations precisely matched to each transport mechanism.