Ultrafiltration and Selective Reabsorption

Urine formation begins with two key processes: ultrafiltration in the glomerulus, which indiscriminately drives small molecules out of the blood, and selective reabsorption in the proximal convoluted tubule, which rescues almost all the useful substances from the filtrate before the loop of Henle. Together, they ensure that the kidney can handle enormous volumes of blood quickly while still preserving the body's stores of glucose, amino acids, ions and water. This lesson examines both processes at the molecular level, matching OCR A-Level Biology A specification module 5.1.2(f)–(g).

Key Definitions:

• Ultrafiltration — the high-pressure filtration of blood plasma through a specialised basement membrane, producing glomerular filtrate.
• Glomerular filtrate — fluid entering the Bowman's capsule after ultrafiltration. It contains water, glucose, amino acids, ions and urea, but not red blood cells or large proteins.
• Selective reabsorption — the return of specific substances from the filtrate back into the blood, particularly in the proximal convoluted tubule.
• Podocyte — a specialised cell of Bowman's capsule whose finger-like processes form filtration slits around the glomerular capillaries.

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Ultrafiltration in the Glomerulus

Ultrafiltration is a passive process, but it requires very high hydrostatic pressure in the glomerulus — much higher than in ordinary capillaries. Three structural features combine to create this pressure and to filter selectively based on size.

Why the Glomerulus Has Such a High Pressure

• The afferent arteriole supplying the glomerulus is wider than the efferent arteriole that drains it. Blood therefore arrives quickly but leaves slowly, causing a back-up of blood that raises hydrostatic pressure in the glomerular capillaries to ~55 mmHg (about three times a normal capillary).
• The glomerulus is a tight capillary knot, offering a large surface area for filtration.
• The renal arteries are short and wide, so blood arrives with little loss of pressure from the aorta.

The resulting high pressure forces water and small solutes out through the filtration barrier into the Bowman's capsule.

The Three-Layer Filtration Barrier

The barrier between the glomerular blood and the capsular space has three layers:

1. Fenestrated capillary endothelium. The endothelial cells of the glomerulus have pores (fenestrations) about 70–100 nm across. These allow plasma fluid and solutes to leave readily but block red blood cells, white blood cells and platelets.
2. Basement membrane. A delicate mesh of collagen and glycoproteins (notably laminin and heparan sulphate) stretches between the capillary endothelium and the podocytes. It acts as the main sieve, blocking molecules larger than about 69 kDa (the size of albumin) and negatively charged proteins by electrostatic repulsion. This is the key selective barrier.
3. Podocytes. The inner layer of Bowman's capsule is formed by podocytes, cells with long processes (pedicels or "foot processes") that interdigitate over the surface of the glomerular capillaries. Between adjacent pedicels are narrow filtration slits about 25 nm wide, covered by a thin slit diaphragm that adds a final sieve.

flowchart LR
    A[Blood plasma in<br/>glomerular capillary] --> B[Fenestrated endothelium<br/>~70-100 nm pores]
    B --> C[Basement membrane<br/>main sieve]
    C --> D[Podocyte filtration slits<br/>~25 nm]
    D --> E[Capsular space<br/>glomerular filtrate]

What Crosses and What Doesn't

• Crosses freely: water, glucose, amino acids, urea, inorganic ions (Na⁺, K⁺, Cl⁻, HCO₃⁻), creatinine, small hormones.
• Does not cross: red blood cells, white blood cells, platelets, plasma proteins (albumin, globulins, fibrinogen), any molecule bound tightly to these proteins (some hormones, free fatty acids).

The glomerular filtrate therefore has the same composition as plasma except that it contains no cells and almost no protein.

The Glomerular Filtration Rate (GFR)

In a human, the GFR is about 125 cm³ min⁻¹ (180 L day⁻¹). This is a colossal volume — roughly 60 times the circulating plasma volume per day. Clearly, almost all of it must be reabsorbed: only about 1.5 L is excreted as urine each day.

Net Filtration Pressure

Not all of the hydrostatic pressure drives filtration. Opposing forces reduce it:

• Hydrostatic pressure of the blood in the glomerulus: ~55 mmHg, pushes fluid out.
• Hydrostatic pressure of filtrate in the capsule: ~15 mmHg, pushes fluid back.
• Water potential of blood (made negative by plasma proteins): ~−30 mmHg (equivalent to osmotic pressure), pulls water back.

The net filtration pressure is therefore:

$55 - 15 - 30 = +10 \text{ mmHg}$55−15−30=+10 mmHg

This modest net pressure, across the enormous surface area of the glomerular capillaries, produces the 125 cm³ min⁻¹ GFR.

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Selective Reabsorption in the Proximal Convoluted Tubule

The filtrate leaving the Bowman's capsule is essentially deproteinised plasma — it contains too much of everything useful. The proximal convoluted tubule (PCT) must now rescue:

• ~100 % of the filtered glucose.
• ~100 % of the filtered amino acids.
• ~85 % of the filtered Na⁺, Cl⁻ and water.
• Most of the filtered HCO₃⁻ (for pH balance).
• Some of the filtered urea.

This massive reabsorptive task requires highly specialised cells.

PCT Cell Structure

The PCT is lined by cuboidal epithelial cells adapted for active transport: