Nephron Physiology: Structure and Function of Renal Tubules

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Nephron Physiology

Overview of Nephron Structure and Function

The nephron is the functional unit of the kidney, responsible for filtering blood, reabsorbing essential substances, and excreting waste products in urine. Each nephron consists of several segments, each with distinct physiological roles in the process of urine formation.

Renal Corpuscle: Site of blood filtration, producing filtrate that enters the tubules.
Proximal Convoluted Tubule (PCT): Major site for reabsorption of water, ions, and nutrients.
Nephron Loop (Loop of Henle): Establishes osmotic gradient for water conservation.
Distal Convoluted Tubule (DCT): Fine-tunes reabsorption and secretion, regulated by hormones.
Collecting System: Final adjustments to urine composition, regulated by aldosterone and ADH.

Table of renal structures and their functions Diagram of nephron structure and tubular fluid flow

Proximal Convoluted Tubule (PCT)

Functions and Mechanisms

The PCT is responsible for the reabsorption of the majority of filtered water, ions, and nutrients. It uses both active and passive transport mechanisms to reclaim substances from the filtrate.

Reabsorption: Glucose, amino acids, sodium, chloride, bicarbonate, and water are reabsorbed efficiently.
Secretion: Hydrogen ions, ammonium ions, and certain drugs are secreted into the tubular fluid.
Mechanisms: Carrier-mediated transport, cotransport, and countertransport with sodium ions.
Example: Glucose reabsorption occurs via sodium-glucose cotransporters.

Diagram of PCT transport mechanisms

Glucose Reabsorption in the PCT

Glucose is reabsorbed in the PCT through sodium-glucose cotransporters. This process is essential for preventing glucose loss in urine and maintaining blood glucose levels.

Key Point: Nearly all filtered glucose is reabsorbed in the PCT under normal conditions.
Clinical Relevance: Glucosuria (glucose in urine) occurs when transporters are saturated, as in diabetes mellitus.

Diagram of PCT transport mechanisms

Nephron Loop (Loop of Henle)

Structure and Function

The Loop of Henle consists of descending and ascending limbs, each with unique permeability characteristics. It is crucial for establishing the osmotic gradient in the renal medulla, which enables water conservation and concentration of urine.

Descending Limb: Permeable to water, allowing water reabsorption.
Ascending Limb: Permeable to sodium and chloride, but not water; active transport of ions creates osmotic gradient.
Mechanisms: Countercurrent multiplication and active transport of Na+ and Cl-.
Example: The osmotic gradient established by the loop is essential for water reabsorption in the collecting duct.

Diagram of nephron loop and transport mechanisms Diagram of nephron loop permeability and osmotic concentration

Distal Convoluted Tubule (DCT)

Functions and Regulation

The DCT is involved in the selective reabsorption and secretion of ions, regulated by hormones such as aldosterone and antidiuretic hormone (ADH). It plays a key role in acid-base balance and electrolyte homeostasis.

Reabsorption: Sodium and chloride ions, calcium ions (under parathyroid hormone stimulation).
Secretion: Hydrogen ions, potassium ions, ammonium ions.
Regulation: Aldosterone increases sodium reabsorption and potassium secretion; ADH increases water reabsorption.

Diagram of DCT sodium and chloride reabsorption Diagram of aldosterone-regulated sodium and potassium exchange Diagram of hydrogen ion secretion and bicarbonate reabsorption in DCT

Collecting System/Duct

Regulatory Factors: Aldosterone and ADH

The collecting duct is the final site for regulation of water and solute reabsorption. Aldosterone and ADH are the primary hormones controlling its function.

Aldosterone: Stimulates sodium reabsorption and potassium secretion.
ADH (Antidiuretic Hormone): Increases water reabsorption by making the duct more permeable to water.
Example: In the presence of ADH, urine becomes concentrated; in its absence, urine is dilute.

Table of DCT and collecting system functions

No ADH vs. With ADH

The presence or absence of ADH dramatically affects the volume and concentration of urine produced by the kidneys.

No ADH: Collecting duct is impermeable to water, resulting in large volume of dilute urine.
With ADH: Collecting duct becomes permeable to water, resulting in small volume of concentrated urine.

Diagram of urine formation in absence of ADH

Summary Table: Renal Structures and Their Functions

The following table summarizes the main functions and mechanisms of each nephron segment:

Segment	General Functions	Specific Functions	Mechanisms
Renal corpuscle	Filtration of plasma; generates filtrate	Filtration of water, inorganic and organic solutes from plasma; retention of plasma proteins and blood cells	Glomerular hydrostatic/blood pressure; selective permeability; filtration slits
Proximal convoluted tubule (PCT)	Reabsorption of 60–70% of filtrate	Active reabsorption of glucose, amino acids, ions; passive reabsorption of water; secretion of hydrogen ions, ammonium ions, drugs	Carrier-mediated transport; cotransport; countertransport with sodium ions
Nephron loop	Reabsorption of 25% of filtrate	Reabsorption of sodium and chloride ions; water	Active transport for Na+/Cl-; osmosis
Distal convoluted tubule (DCT)	Reabsorption of variable amount of water and ions	Reabsorption of sodium, chloride, calcium ions; secretion of hydrogen, potassium, ammonium ions	Cotransport; carrier-mediated transport; regulation by aldosterone and ADH
Collecting system	Reabsorption of variable amount of water and ions	Reabsorption of sodium, chloride, water; secretion of potassium, hydrogen, ammonium ions	Carrier-mediated transport; regulation by aldosterone and ADH