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Urine Formation

Q: Which nephron structures form the renal corpuscle?

The renal corpuscle (corpusculum renale) includes two main functional components: Bowman's capsule — a double-walled epithelial cup consisting of an inner visceral layer (podocytes) and an outer parietal layer.Glomerulus — a tuft of capillaries formed by the branching of the afferent arteriole. Note that strictly speaking from a histological perspective, the capillary tuft belongs to the vascular system and is not part of the nephron proper, but acts as a functional component of the renal corpuscle.

For medical students2 min readUpdated 2026-10-10

Urine formation is a complex physiological process occurring in the nephrons of the kidneys, aimed at clearing the blood of metabolic waste products while retaining essential substances. It comprises three sequential steps: glomerular filtration, tubular reabsorption, and tubular secretion.

Blood volumeAbout 1,800 L of blood passes through the kidneys per day.

Primary urineAbout 180 L of ultrafiltrate (primary urine) is formed daily.

Final urineThe daily volume of final urine is about 1.5 L.

ReabsorptionOver 99% of water and all glucose are reabsorbed back into the blood.

Glomerular Filtration

The first step of urine formation takes place in the renal corpuscles. Here, the liquid part of the blood plasma passes through the filtration barrier (glomerular filtration barrier) from the glomerular capillaries into the capsular lumen.

The main driving force for filtration is the high hydrostatic pressure in the glomerular capillaries, which reaches 50–60 mm Hg in the cortical nephrons.

As a result, primary urine (the ultrafiltrate) is formed. Only about 10% of the blood volume flowing through the kidneys is filtered into the capsule.

Composition of primary urine:

Water and inorganic ions ($Na^+$, $K^+$, $Cl^-$).
Low-molecular-weight organics: glucose, amino acids.
Metabolic waste products: urea, creatinine, uric acid.
Small plasma proteins (albumins, some globulins).

Tubular Reabsorption

Since a huge volume of valuable substances and water is lost in the primary urine (180 L per day!), the body reclaims them back into the blood. This process is called reabsorption and occurs along the renal tubules and collecting ducts.

In the proximal convoluted tubules, obligate (mandatory, hormone-independent) reabsorption takes place. Here, virtually 100% of glucose, amino acids, proteins, and up to 85% of sodium ions are actively reabsorbed with energy expenditure. Following sodium passively (along an osmotic gradient), about 85% of water is also reabsorbed. Proteins are taken up by cells via pinocytosis, while low-molecular-weight substances are transported via secondary active transport coupled with sodium ions.

In the distal tubules and collecting ducts, facultative (optional) reabsorption occurs. It is regulated by hormones according to the current needs of the body.

Tubular Secretion

Concurrently with reabsorption, secretion takes place in the distal parts of the nephron and collecting ducts—this is the active transport of additional substances from the blood into the tubular lumen (forming the urine).

This process is closely linked to sodium reabsorption:

In the distal tubules, $K^+$ and $H^+$ ions are secreted into the urine in exchange for the reabsorption of $Na^+$ into the blood.
In the collecting ducts, in addition to hydrogen ions, ammonia is secreted (as $NH_4^+$).

Hormonal Regulation of Urine Formation

Fine-tuning of the volume and composition of the final urine occurs under the influence of hormones:

Aldosterone stimulates the synthesis of transport proteins in the distal segments. It enhances the reabsorption of $Na^+$ and $Cl^-$ (retaining them in the body) while simultaneously stimulating the secretion of $K^+$ and $H^+$.
Antidiuretic hormone (ADH, vasopressin) regulates the volume of water excreted. Normally, the walls of the distal tubules and collecting ducts are impermeable to water. ADH makes them permeable by stimulating the insertion of water channels (aquaporins, specifically Aquaporin-2) into the apical membranes of collecting duct cells, leading to enhanced water reabsorption into the blood and a decreased urine volume.

Frequently asked questions

What layers make up the renal filtration barrier?

The renal filtration barrier consists of three layers separating the lumen of the glomerular capillary from the capsular space:

Capillary endothelium — the first layer, containing pores (fenestrations) that ensure high permeability.
Basement membrane — a thick, trilaminar extracellular matrix located between endothelial cells and podocytes, serving as a structural scaffold.
Visceral layer of Bowman's capsule — formed by specialized epithelial cells called podocytes, featuring cell bodies and primary/secondary processes (pedicels or foot processes) between which filtration slits are located.

Which hormones regulate facultative reabsorption in the nephron?

Facultative reabsorption in the distal tubules and collecting ducts is regulated by aldosterone and antidiuretic hormone (ADH).

Aldosterone — induces the synthesis of transport proteins, stimulating active reabsorption of electrolytes ($Na^+$ and $Cl^-$) and secretion of $K^+$ and $H^+$ ions.
Antidiuretic hormone (ADH) — stimulates passive water reabsorption. Under its influence, the distal nephron becomes permeable to water, and Aquaporin-2 channels are translocated to the apical cell membrane in the collecting ducts.

Which nephron structures form the renal corpuscle?

The renal corpuscle (corpusculum renale) includes two main functional components:

Bowman's capsule — a double-walled epithelial cup consisting of an inner visceral layer (podocytes) and an outer parietal layer.
Glomerulus — a tuft of capillaries formed by the branching of the afferent arteriole.

Note that strictly speaking from a histological perspective, the capillary tuft belongs to the vascular system and is not part of the nephron proper, but acts as a functional component of the renal corpuscle.

What is the difference between primary and final urine?

Primary urine (ultrafiltrate) is formed in the glomeruli with a volume of about 180 L/day and contains useful substances (glucose, amino acids). Final urine is formed after reabsorption and secretion, with a volume of about 1.5 L/day; it lacks glucose and proteins, but has a higher concentration of metabolic waste products.

Where and how is glucose reabsorbed?

Glucose is completely reabsorbed in the proximal convoluted tubules. The process occurs via secondary active transport (cotransport/symport with sodium ions).

What are obligate and facultative reabsorption?

Obligate reabsorption occurs constantly in the proximal tubules and is not regulated by hormones (reclaiming the bulk of water and solutes). Facultative reabsorption takes place in the distal tubules and collecting ducts under hormonal control (aldosterone, ADH) for precise physiological tuning.

Go deeper

Mechanisms of symport and ion pumps in tubular cell membranes
Detailed ultrastructure of the filtration barrier
Types and localization of aquaporins along the nephron
Substances exempt from reabsorption in the proximal tubules
Architecture of the renal medulla and papilla