Glomerular Filtration
The first stage of urine formation takes place in the renal corpuscles. Here, the liquid part of the blood plasma passes through the filtration barrier (blood-air barrier) from the glomerular capillaries into the capsular lumen.
The main condition for filtration is high hydrostatic pressure in the glomerular capillaries, which reaches 50–60 mm Hg in the cortical system.
As a result, primary urine (filtrate) is formed. Only about 10% of the volume of blood flowing through the kidneys passes into it.
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, certain globulins).
Tubular reabsorption (back-diffusion)
Since a huge amount of valuable substances and water is lost with the primary urine (180 liters per day!), the body returns them back to the blood. This process is called reabsorption and occurs in the nephron tubules and collecting ducts.
In the proximal convoluted tubules, obligate (obligatory, hormone-independent) reabsorption takes place. Here, practically 100% of glucose, amino acids, proteins, and up to 85% of sodium ions are actively reabsorbed with energy expenditure. Following sodium, about 85% of water is reabsorbed passively (along the osmotic gradient). Proteins are captured by cells via pinocytosis, and low-molecular-weight substances are transported via symport with sodium ions.
In the distal tubules and collecting ducts, facultative reabsorption occurs. It is regulated by hormones depending on the body's current needs.
Tubular secretion
Concurrently with reabsorption, secretion takes place in the distal nephron and collecting ducts — the active transport of additional substances from the blood into the tubular lumen (into 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 $Na^+$ reabsorption 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 action of hormones:
- Aldosterone stimulates the production of transport proteins in the distal segments. It enhances the reabsorption of $Na^+$ and $Cl^-$ (retaining them in the body) and simultaneously stimulates the secretion of $K^+$ and $H^+$.
- Antidiuretic hormone (ADH, vasopressin) regulates the volume of excreted water. Normally, the walls of the distal tubules are impermeable to water. ADH makes them permeable by stimulating the insertion of water channels (aquaporins, specifically Aquaporin 2) into the membranes of the collecting duct cells, which leads to an increased return of water to the blood and a decrease in urine volume.
Briefly: answers to topical questions
What layers make up the renal filtration barrier?
The renal filtration barrier consists of three layers separating the lumen of the glomerular blood capillary and the capsular space.
- Capillary endothelium — the first layer, whose cells contain pores and thinnings (fenestrae) that ensure the permeability of the barrier.
- Basement membrane — a common three-layer structure located between endothelial cells and podocytes, performing a structural framework function.
- Visceral layer of the capsule — formed by large cells called podocytes, comprising cell bodies and processes (pedicels) between which filtration slits remain.
Which hormones regulate the process of facultative reabsorption in the nephron?
The process of facultative reabsorption in the distal tubules and collecting ducts is regulated by the hormones aldosterone and antidiuretic hormone (ADH).
- Aldosterone — induces the synthesis of transport proteins, leading to the stimulation of active reabsorption of electrolytes ($Na^+$ and $Cl^-$ ions) and secretion of $K^+$ and $H^+$ ions.
- Antidiuretic hormone (ADH) — stimulates passive water reabsorption. Under its influence, the distal tubules become permeable to water, and in the collecting ducts, the specific protein aquaporin 2 moves to the apical surface of the cells.
Which nephron structures form the renal corpuscle?
The renal corpuscle (corpusculum renale) includes two main functional components.
- Bowman's capsule — a double-walled epithelial structure consisting of an inner layer (podocytes) and an outer layer.
- Glomerulus — a vascular element formed by the branching of the afferent arteriole.
It is worth noting that in a strict histological sense, the capillary glomerulus belongs to the vascular system and is not part of the nephron itself, merely serving as a component of the functional renal corpuscle.
Class and exam questions
What is the difference between primary and final urine?
Primary urine (filtrate) 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 1.5 L/day; it lacks glucose and proteins, but has an increased concentration of metabolic waste products.
Where and how is glucose absorbed?
Glucose is completely reabsorbed in the proximal convoluted tubules. The process proceeds via secondary active transport (symport with sodium ions).
What are obligate and facultative reabsorption?
Obligate reabsorption occurs continuously in the proximal tubules and is not regulated by hormones (returning the bulk of substances). Facultative reabsorption takes place in the distal tubules and collecting ducts under the control of hormones (aldosterone, ADH) for fine-tuning.
What else to study on this topic
- Mechanisms of symport and ion pumps on tubular cell membranes
- Structure of the filtration barrier in detail
- Types and localization of aquaporins in the nephron
- Substances not subject to reabsorption in the proximal tubules
- Structure of the renal medulla and papilla
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