Nephron as the structural and functional unit of the kidney
The nephron is the basic structural and functional unit of the kidney, ensuring blood filtration and urine formation. It consists of vascular and tubular components that are closely linked both anatomically and functionally.
The vascular part begins with the afferent arteriole, which branches into the glomerular capillaries (the primary capillary network). Blood leaves the glomerulus via the efferent arteriole, whose diameter is smaller than that of the afferent arteriole. This anatomical feature creates a high hydrostatic pressure within the glomerulus. Next, the secondary capillary network surrounding the tubules is formed, leading to venous outflow.
The tubular part includes the sequentially arranged segments:
- Bowman's capsule — a double-walled structure surrounding the glomerulus;
- Proximal convoluted tubule;
- Descending and ascending limbs of the loop of Henle;
- Distal convoluted tubule;
- Collecting duct, opening into the renal pelvis system.
Glomerular filtration and the filtration barrier
Glomerular filtration is the process of fluid and dissolved solutes passing from the glomerular capillaries into the capsular space of Bowman's capsule through the filtration barrier.
The filtration barrier consists of three layers:
- Glomerular capillary endothelium (fenestrated);
- Triple-layered basement membrane;
- Visceral layer of Bowman's capsule containing specialized cells called podocytes.
Normally, blood cells and most plasma proteins do not pass through the barrier, whereas low-molecular-weight compounds are freely filtered.
Effective filtration pressure
Effective filtration pressure (EFP) is the resultant force that drives fluid movement from the blood into the capsular space. It is determined by the balance between pressures directed into the capsular lumen and opposing forces.
- Capillary hydrostatic pressure in the glomerular capillaries (approx. 60–70 mm Hg) — the primary driving force for filtration;
- Hydrostatic pressure of the primary urine within the capsule (approx. 15 mm Hg) — opposes filtration;
- Plasma oncotic pressure (approx. 25 mm Hg), generated by proteins, retains water within the vessel;
- Oncotic pressure of the primary urine (approx. 0 mm Hg) — practically absent, as proteins are normally not filtered.
EFP ≈ Phydrost.blood − Phydrost.urine − Poncot.blood + Poncot.urine.
Reabsorption and its mechanisms
Around 150–180 liters of primary urine are produced daily, whereas final urine output is only 1.5–2 liters. This is accomplished via reabsorption — the transport of water, ions, and organic substances from the tubules back into the blood of the secondary capillary network.
Reabsorption can be obligate (mandatory, predominantly in the proximal tubules) and facultative (hormonally regulated, in the distal segments and collecting ducts).
In the proximal convoluted tubule, glucose, amino acids, low-molecular-weight proteins, vitamins, trace elements, and a significant portion of ions and water are reabsorbed. Glucose is classified as a threshold substance: when transport carrier capacity is exceeded (e.g., in diabetes mellitus), it appears in the final urine, leading to glucosuria.
The loop of Henle operates via a countercurrent multiplier mechanism: water leaves the tubule into the interstitium in the descending limb, concentrating the urine; sodium is actively reabsorbed in the ascending limb, diluting the urine. This establishes an osmotic gradient in the renal medulla, which is essential for subsequent urine concentration.
Hormonal regulation of reabsorption
Aldosterone is a mineralocorticoid produced by the adrenal cortex. Its synthesis is regulated by the renin–angiotensin system and ACTH. Aldosterone acts on the distal tubules, increasing the reabsorption of Na⁺ ions; water follows sodium passively down the osmotic gradient. The result is an increase in circulating blood volume and blood pressure.
Antidiuretic hormone (ADH, vasopressin) is synthesized in the hypothalamus and released from the neurohypophysis. In the collecting ducts, ADH binds to V2 receptors, stimulating the insertion of water channels—aquaporins—into the apical membrane. This dramatically increases the water permeability of the epithelium, allowing water to move into the interstitium along the osmotic gradient, thereby concentrating the urine.
Disorders of these systems underlie arterial hypertension, edema syndromes, and diabetes insipidus.
Tubular secretion and the formation of secondary urine
Secretion is the active transport of substances from the blood of the secondary capillary network into the tubular lumen. Substances secreted include H⁺ ions, K⁺ ions, ammonia, creatinine, foreign substances, and drug metabolites.
Of particular importance is ammonia secretion for the regulation of acid-base balance (ABB). In tubular cells, glutamine is converted into NH₃, which binds to a proton to form the ammonium ion (NH₄⁺) and is excreted in the urine. Simultaneously, carbonic acid (H₂CO₃) dissociates into H⁺ and HCO₃⁻: the proton is secreted, while bicarbonate is returned to the blood to support buffer systems.
Secondary urine is the result of filtration, reabsorption, and secretion. Unlike primary urine, it does not contain glucose, amino acids, proteins, and vitamins, but contains urea, creatinine, uric acid, ions, and secretion products. The creatinine level is used to calculate the glomerular filtration rate (GFR), a key clinical indicator of renal function that determines the dosing of renally eliminated drugs.
Key points
- The nephron is the structural and functional unit of the kidney, combining the glomerulus and the tubular system.
- The filtration barrier is formed by the capillary endothelium, the basement membrane, and the podocytes of Bowman's capsule.
- Effective filtration pressure is determined by the balance of hydrostatic and oncotic pressures of the blood and primary urine.
- Obligate reabsorption of glucose, amino acids, and most ions occurs in the proximal tubule.
- The countercurrent multiplier mechanism of the loop of Henle establishes the osmotic gradient in the renal medulla.
- Aldosterone enhances the reabsorption of Na⁺ and water; ADH increases the water permeability of the collecting ducts via aquaporins.
- Tubular secretion of H⁺ and NH₄⁺ contributes to the maintenance of acid-base balance.
- GFR, calculated using creatinine, is a critical clinical indicator of renal function.