Function of Tubules — Revision Notes

⚡ 30-Second Revision

PCT: — Bulk reabsorption (65-70% water, $Na^+$, $Cl^-$, 100% glucose, amino acids). Obligatory water reabsorption. Secretion of organic acids/bases.
Loop of Henle: — Creates medullary osmotic gradient.

- Descending Limb: Permeable to water, impermeable to solutes. Filtrate concentrates. - Ascending Limb: Impermeable to water, actively transports $Na^+$, $K^+$, $Cl^-$ out. Filtrate dilutes.

DCT: — Selective reabsorption ($Na^+$, $Ca^{2+}$, water). Facultative water reabsorption (ADH). Secretion of $K^+$, $H^+$.
Collecting Duct: — Final water reabsorption (ADH). Urea recycling. $Na^+$ reabsorption, $K^+$ secretion (Aldosterone).
Hormones:

- ADH: Increases water permeability in DCT/Collecting Duct. - Aldosterone: Increases $Na^+$ reabsorption, $K^+$ secretion in DCT/Collecting Duct. - PTH: Increases $Ca^{2+}$ reabsorption in DCT.

2-Minute Revision

The renal tubules are vital for converting glomerular filtrate into urine. The Proximal Convoluted Tubule (PCT) is the primary site for bulk reabsorption, reclaiming about 70% of water, sodium, and chloride, and nearly all glucose and amino acids.

This is largely an obligatory process. Next, the Loop of Henle establishes the crucial medullary osmotic gradient. Its descending limb is permeable to water, allowing water to exit and concentrate the filtrate, while the ascending limb is impermeable to water but actively pumps out salts, diluting the filtrate and making the medulla hypertonic.

The Distal Convoluted Tubule (DCT) and Collecting Duct perform selective reabsorption and secretion, fine-tuning the urine composition. Water reabsorption here is facultative, regulated by Antidiuretic Hormone (ADH), which increases water permeability.

Aldosterone acts on these segments to increase sodium reabsorption and potassium secretion. Tubular secretion, occurring mainly in the PCT and DCT, removes additional wastes and excess ions, ensuring precise fluid, electrolyte, and acid-base balance.

5-Minute Revision

The journey of filtrate through the renal tubules is a meticulously controlled process of reabsorption and secretion, transforming initial filtrate into final urine. The Proximal Convoluted Tubule (PCT) initiates this by reabsorbing the majority of essential substances: approximately 65-70% of filtered water, sodium, and chloride, and virtually 100% of glucose and amino acids.

This is an energy-intensive process involving active transport (e.g., $Na^+/K^+$ ATPase) and co-transport mechanisms. Water reabsorption here is obligatory, following solutes osmotically. The PCT also secretes organic acids and bases.

The Loop of Henle is critical for establishing the medullary osmotic gradient. The descending limb is highly permeable to water but impermeable to solutes, allowing water to move out into the hypertonic medulla, concentrating the filtrate.

The ascending limb is impermeable to water but actively transports $Na^+$, $K^+$, and $Cl^-$ out into the medulla, diluting the filtrate and making the medulla hypertonic. This countercurrent multiplier system is fundamental for the kidney's ability to produce concentrated urine.

The Distal Convoluted Tubule (DCT) and Collecting Duct are the sites for fine-tuning. Their functions are heavily regulated by hormones. Antidiuretic Hormone (ADH) increases water permeability in these segments by inserting aquaporin-2 channels, leading to facultative water reabsorption and concentrated urine.

Aldosterone promotes $Na^+$ reabsorption and $K^+$ secretion in the DCT and collecting duct, influencing blood volume and electrolyte balance. Parathyroid Hormone (PTH) enhances $Ca^{2+}$ reabsorption in the DCT.

These segments also secrete $H^+$ ions, crucial for acid-base homeostasis. The collecting duct also participates in urea recycling in the inner medulla, contributing to the medullary gradient. Understanding these segment-specific functions and their hormonal control is key to mastering renal physiology.

Prelims Revision Notes

Nephron Structure: — Renal corpuscle (glomerulus + Bowman's capsule) and Renal tubule (PCT, Loop of Henle, DCT, Collecting Duct).
PCT (Proximal Convoluted Tubule):

- Location: Renal cortex. - Structure: Brush border, abundant mitochondria. - Reabsorption: Bulk reabsorption. ~65-70% water, $Na^+$, $Cl^-$. ~100% glucose, amino acids. ~80-90% $HCO_3^-$. Significant $K^+$. - Mechanism: Active transport ($Na^+/K^+$ ATPase), secondary active transport (glucose, amino acids), osmosis (water). - Water Reabsorption: Obligatory. - Secretion: Organic acids (uric acid, creatinine), organic bases, some $H^+$.

Loop of Henle:

- Location: Dips into renal medulla. - Function: Establishes medullary osmotic gradient (countercurrent multiplier). - Descending Limb: Permeable to water (aquaporin-1), impermeable to solutes. Filtrate concentrates (osmolarity increases). - Ascending Limb: Impermeable to water, actively transports $Na^+$, $K^+$, $2Cl^-$ out (NKCC2 co-transporter). Filtrate dilutes (osmolarity decreases).

DCT (Distal Convoluted Tubule):

- Location: Renal cortex. - Reabsorption: Selective. $Na^+$, $Cl^-$, $Ca^{2+}$, water. - Water Reabsorption: Facultative (regulated by ADH). - Secretion: $K^+$, $H^+$. - Hormonal Control: ADH (water), Aldosterone ($Na^+$, $K^+$), PTH ($Ca^{2+}$).

Collecting Duct:

- Location: Extends through cortex and medulla. - Reabsorption: Final water reabsorption (ADH). Urea reabsorption (inner medulla). $Na^+$. - Secretion: $K^+$, $H^+$. - Hormonal Control: ADH (water), Aldosterone ($Na^+$, $K^+$).

Countercurrent Mechanism:

- Multiplier: Loop of Henle (creates gradient). - Exchanger: Vasa Recta (maintains gradient).

Hormones & Actions:

- ADH (Vasopressin): Increases water permeability in DCT/Collecting Duct (aquaporin-2 insertion) $ightarrow$ Concentrated urine. - Aldosterone: Increases $Na^+$ reabsorption and $K^+$ secretion in DCT/Collecting Duct $ightarrow$ Regulates blood volume/pressure. - PTH: Increases $Ca^{2+}$ reabsorption in DCT. - ANP: Inhibits $Na^+$ reabsorption, ADH/Aldosterone release $ightarrow$ Increased $Na^+$/water excretion.

Vyyuha Quick Recall

Please Let Drinks Cool: PCT, Loop of Henle, DCT, Collecting Duct.

PCT: Great Absorption of Glucose, Amino acids, Water, Salts. (G-A-W-S) Loop of Henle: Creates Gradient. Descending Water Out, Ascending Salts Out. (CG DWO ASO) DCT/CD: Fine-tuning Hormonal Regulation. ADH for Water, Aldosterone for Salts. (FHR AWS)