The nephron spans two main layers of the kidney. The outer cortex contains the filtering structures (the glomerulus and Bowman's capsule) alongside the convoluted tubules. The inner medulla houses the Loop of Henlé and the collecting duct. As blood flows through the kidney, the nephron uses three sequential processes to clean it: ultrafiltration (forcing small molecules out of the blood), selective reabsorption (taking back useful substances), and urine formation (collecting the remaining waste).
The fluid that leaves the blood and enters the nephron is called the glomerular filtrate. It travels along a highly specific pathway: Bowman's capsule Proximal Convoluted Tubule (PCT) Loop of Henlé Distal Convoluted Tubule (DCT) Collecting duct.
To force fluid out of the blood, the body creates high hydrostatic pressure. Blood enters the glomerulus through a wide afferent arteriole but must exit through a much narrower efferent arteriole. This "bottleneck" effect builds intense pressure, pushing fluid out of the capillaries. However, the fluid must pass through the basement membrane, a non-cellular layer that acts as a molecular sieve. It separates molecules entirely based on size.
| Substance | Present in Blood | Present in Filtrate | Reason for Presence / Absence in Filtrate |
|---|---|---|---|
| Red / White Blood Cells | Yes | No | Too large to pass through the basement membrane. |
| Large Proteins (e.g., antibodies) | Yes | No | Too large to pass through the basement membrane. |
| Glucose & Amino Acids | Yes | Yes | Small enough to be forced through the filter. |
| Water & Urea | Yes | Yes | Small enough to be forced through the filter. |
Exactly 100% of the filtered glucose is reabsorbed back into the blood from the PCT. Because the glucose must often move against its concentration gradient, this process requires active transport. Amino acids and some mineral ions are also reabsorbed here. As these solutes move back into the blood, they alter the osmotic gradient, causing roughly 85% of the filtered water to follow them passively via osmosis.
The PCT is highly adapted to maximise the efficiency of selective reabsorption. The epithelial cells lining the tubule feature tiny folds called microvilli, which vastly increase the surface area available for transport proteins. Furthermore, these cells contain many mitochondria to supply the ATP (energy) required to actively pump glucose against its concentration gradient.
The Loop of Henlé dips down into the medulla. The descending limb is permeable to water but not salts, allowing water to leave by osmosis. The ascending limb is impermeable to water but actively pumps sodium and chloride ions out into the surrounding tissue. This active transport creates a very high salt concentration (a low water potential) deep in the medulla, setting up a steep concentration gradient.
The final adjustment of water volume happens in the collecting duct, controlled by Antidiuretic Hormone (ADH).
Students often say substances 'diffuse' into the Bowman's capsule during ultrafiltration. This is incorrect; they are forced or pushed out by high hydrostatic pressure.
In 6-mark questions explaining glucose reabsorption, examiners expect you to state clearly that exactly 100% of glucose is reabsorbed, specifically in the PCT, and that this requires active transport.
When explaining osmoregulation, always explicitly link the sequence of events: ADH levels → Permeability of the collecting duct → Volume of water moving by osmosis → Final urine concentration.
Never mention glucose reabsorption when discussing the Loop of Henlé or the collecting duct; glucose is only reabsorbed in the proximal convoluted tubule.
Nephron
The microscopic functional unit of the kidney where blood filtration and selective reabsorption occur.
Cortex
The outer layer of the kidney containing the glomerulus, Bowman's capsule, and convoluted tubules.
Medulla
The inner region of the kidney that contains the Loop of Henlé and the collecting duct, where a high salt concentration is maintained.
Ultrafiltration
The process where small molecules are forced out of the blood in the glomerulus and into the Bowman's capsule under high hydrostatic pressure.
Selective reabsorption
The process of taking back specific useful substances, such as glucose and mineral ions, from the filtrate into the bloodstream.
Glomerular filtrate
The fluid that is forced into the Bowman's capsule during ultrafiltration, containing water, glucose, urea, amino acids, and ions.
Proximal Convoluted Tubule (PCT)
The first section of the nephron tubule where 100% of glucose is selectively reabsorbed into the blood via active transport.
Glomerulus
A tightly coiled knot of capillaries located inside the Bowman's capsule where blood is filtered.
Bowman's capsule
The cup-shaped structure at the very start of a nephron that collects the glomerular filtrate.
Afferent arteriole
The wider blood vessel that brings blood into the glomerulus.
Efferent arteriole
The narrower blood vessel that carries blood away from the glomerulus, creating a bottleneck that builds pressure.
Hydrostatic pressure
The physical pressure exerted by a fluid (like blood), which forces small molecules out of the capillaries during ultrafiltration.
Basement membrane
A selectively permeable, non-cellular layer between the glomerulus and Bowman's capsule that acts as a molecular sieve based on size.
Active transport
The movement of substances against a concentration gradient, which requires energy in the form of ATP.
Osmosis
The net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
Microvilli
Tiny, finger-like projections on the surface of epithelial cells in the PCT that increase the surface area for reabsorption.
Loop of Henlé
A U-shaped part of the nephron that dips into the medulla, responsible for creating a steep salt concentration gradient to aid water reabsorption.
Collecting duct
The final tube of the nephron where the ultimate volume of water reabsorbed into the blood is regulated by ADH.
Osmoregulation
The homeostatic maintenance of a constant internal balance of water and dissolved salts in the body fluids.
Antidiuretic Hormone (ADH)
A hormone released by the pituitary gland that increases the permeability of the collecting duct to water.
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Nephron
The microscopic functional unit of the kidney where blood filtration and selective reabsorption occur.
Cortex
The outer layer of the kidney containing the glomerulus, Bowman's capsule, and convoluted tubules.
Medulla
The inner region of the kidney that contains the Loop of Henlé and the collecting duct, where a high salt concentration is maintained.
Ultrafiltration
The process where small molecules are forced out of the blood in the glomerulus and into the Bowman's capsule under high hydrostatic pressure.
Selective reabsorption
The process of taking back specific useful substances, such as glucose and mineral ions, from the filtrate into the bloodstream.
Glomerular filtrate
The fluid that is forced into the Bowman's capsule during ultrafiltration, containing water, glucose, urea, amino acids, and ions.
Proximal Convoluted Tubule (PCT)
The first section of the nephron tubule where 100% of glucose is selectively reabsorbed into the blood via active transport.
Glomerulus
A tightly coiled knot of capillaries located inside the Bowman's capsule where blood is filtered.
Bowman's capsule
The cup-shaped structure at the very start of a nephron that collects the glomerular filtrate.
Afferent arteriole
The wider blood vessel that brings blood into the glomerulus.
Efferent arteriole
The narrower blood vessel that carries blood away from the glomerulus, creating a bottleneck that builds pressure.
Hydrostatic pressure
The physical pressure exerted by a fluid (like blood), which forces small molecules out of the capillaries during ultrafiltration.
Basement membrane
A selectively permeable, non-cellular layer between the glomerulus and Bowman's capsule that acts as a molecular sieve based on size.
Active transport
The movement of substances against a concentration gradient, which requires energy in the form of ATP.
Osmosis
The net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
Microvilli
Tiny, finger-like projections on the surface of epithelial cells in the PCT that increase the surface area for reabsorption.
Loop of Henlé
A U-shaped part of the nephron that dips into the medulla, responsible for creating a steep salt concentration gradient to aid water reabsorption.
Collecting duct
The final tube of the nephron where the ultimate volume of water reabsorbed into the blood is regulated by ADH.
Osmoregulation
The homeostatic maintenance of a constant internal balance of water and dissolved salts in the body fluids.
Antidiuretic Hormone (ADH)
A hormone released by the pituitary gland that increases the permeability of the collecting duct to water.