Kidney Function: Reabsorption and Secretion

Tubular Reabsorption and Secretion

• Modes of molecular transport, nephron tubule functions, and cellular features predicting nephron function must be understood
• Ability to describe urine concentration/dilution, and predict hormonal effects on tubular function/urine output is needed

Filtration, Reabsorption, Secretion of Solutes

• Glucose is entirely reabsorbed, with none excreted, achieving 100% reabsorption
• Bicarbonate is significantly reabsorbed with minimal excretion, achieving >99.9% reabsorption
• Sodium has high reabsorption, a small amount is excreted, and 99.4% reabsorption is achieved
• Chloride is highly reabsorbed, some is excreted, and 99.1% reabsorbed
• Half of the filtered urea is reabsorbed, and half is excreted
• Creatinine is not reabsorbed and is totally excreted, resulting in 0% reabsorption

Molecular Movement Across Membranes

• Molecules move across membranes via simple diffusion, channel-mediated facilitated diffusion, and carrier-mediated facilitated diffusion

Primary Active Transport

• Examples of primary active transport include: Na-K ATPase, H ATPase, H-K ATPase, and Ca ATPase

Secondary Active Transport

• Examples include: Na-Glucose, Na-Ca, Na-H, and Na-Amino acid.

Movement Across Membranes

• Molecules traverse membranes through transcellular (through cells) and paracellular (between cells) routes

Sodium Movement and Ion Diffusion

• Sodium reabsorption is directly linked to the diffusion of many other ions
• Increased lumen negativity enhances passive chloride reabsorption
• Water reabsorption increases luminal chloride and urea concentrations
• Increased luminal concentrations of chloride and urea contribute to passive reabsorption

Proximal Convoluted Tubule (PCT) Reabsorption

• PCT epithelial cells are highly metabolic, have an extensive brush border, and are tightly packed with membrane proteins
• The PCT reabsorbs 65% of filtered sodium and water
• Sodium movement occurs via cotransport with amino acids and glucose, counter-transport with hydrogen, while net sodium concentration remains unchanged
• The PCT is involved in secreting numerous organic acids and bases

Loop of Henle

• Thin descending and ascending limbs have thin epithelial membranes, no brush borders, few mitochondria, and low metabolic activity
• The descending limb is highly permeable to water and moderately permeable to solutes, resulting in 20% water reabsorption
• The thin ascending limb is impermeable to water
• Thick ascending limb features impermeable to water, thick epithelia cells with high metabolic activity, active reabsorption of ions including sodium/potassium/chloride, and a Na-Cl-K cotransporter coupled with Na-K ATPase.

Distal Convoluted Tubule

• The DCT contains the juxtaglomerular apparatus
• Overall, it is similar to the thick ascending limb due to it's high ion reabsorption
• Principal cells in the later portion enable potassium secretion
• Intercalated cells in the later portion enable hydrogen secretion

Collecting Duct

• Similar to the late portion of the DCT
• Sodium is reabsorbed
• Both potassium and hydrogen are secreted
• Late portion permeability to water is controlled by vasopressin (ADH)
• ADH induces water channel (aquaporin) expression

Countercurrent Mechanism

• Filtrate flows through the Loop of Henle in the opposite direction of blood flow in the vasa recta
• The descending limb is H₂O permeable
• The ascending limb is H₂O impermeable

Conclusions

• Reabsorption in tubules occurs through both active and passive mechanisms.
• PCTs absorb ions, nutrients, and water.
• DCTs mainly absorb ions and have secretory functions.
• The Loop of Henle concentrates and dilutes urine via a countercurrent mechanism.
• Hormone levels and blood flow regulate reabsorption