Renal Function: Tubular Processing

Study Notes

Renal Function: Tubular Processing

Renal function involves tubular processing of the glomerular filtrate, including reabsorption and secretion.

Learning Objectives

Define renal processes: tubular reabsorption and secretion.
Describe modes of reabsorption for substances like Na+, K+, Cl-, glucose, urea, and water.
Describe modes of secretion for substances like K+, H+, and organic ions.
Define transport maximum (Tm), renal plasma threshold, and splay.

Basic Renal Processes

Renal processes include filtration, reabsorption, secretion, and excretion.
Excretion equals filtration minus reabsorption plus secretion.

Tubular Reabsorption

Definition: Highly selective and quantitatively large.
180 L of plasma is filtered daily, with 178.5 L reabsorbed.
Nutritional substances are completely reabsorbed.
Electrolytes are reabsorbed based on body needs.
Waste products are poorly reabsorbed.
Tubule is one-cell-layer thick.
Close proximity to peritubular capillaries.
Cells are joined by tight junctions.

Cell Transport

Passive transport: No energy required, occurs down electrochemical or osmotic gradients.
Active transport: Requires energy, occurs against electrochemical or osmotic gradients. Includes primary (direct use of ATP) and secondary (indirect use of ATP) active transport, cotransport, and countertransport.
Uniport: Transports one substance in one direction.
Symport: Transports two substances in the same direction.
Antiport: Transports two substances in opposite directions.

Tubular Reabsorption Details

Diagram shows peritubular capillaries, tubular cells, lumen, and bulk flow.
Reabsorption occurs via active and passive mechanisms (diffusion, osmosis).

Na+ Reabsorption

Primarily by primary active transport (Na+-K+-ATPase).
Reabsorption varies across different tubular regions.
65% in the proximal tubule.
No reabsorption in the distal loop of Henle (DLH).
25% in the ascending loop of Henle.
Distal portions (8%) are under hormonal control (aldosterone, atrial natriuretic peptide (ANP)).

Water Reabsorption

Primarily passive (via osmosis) coupled to Na+ reabsorption.
65% in the proximal tubule (highly permeable to water with aquaporins always open).
15% in the descending loop of Henle, and a further 20% in distal segments under ADH control.

Glucose and Amino Acid Reabsorption

100% reabsorbed in the proximal tubule via Na+-dependent secondary active transport.

Chloride and Urea Reabsorption

Urea is moderately permeable in the proximal tubule (50% reabsorption).
Water reabsorption increases urea concentration, facilitating passive urea reabsorption.
Chloride is passively reabsorbed (paracellular).
Chloride reabsorption is directly related to Na+ reabsorption.

Urea Reabsorption

Urea is passively reabsorbed down its concentration gradient.
Reabsorption happens in the proximal tubule.

Chloride and Urea Reabsorption (Summary)

Na+ reabsorption → H₂O reabsorption → increased luminal Cl- concentration → passive Cl- reabsorption.
Na+ reabsorption → H₂O reabsorption → increased luminal urea concentration → passive urea reabsorption.

Sodium, Chloride, and Potassium Transport in the TALH

Sodium, chloride, and potassium are transported in the thick ascending limb of Henle's loop (TALH).
Mechanisms include active and passive processes.
Loop diuretics affect this transport.

Potassium Reabsorption

65% reabsorbed in the proximal tubule (paracellular).
20% in the ascending loop of Henle.
Distal and collecting tubules can either reabsorb or secrete potassium as needed.

Tubular Secretion

Function is to actively move substances from blood to the tubular lumen.

Potassium Secretion

Most K+ is reabsorbed in the proximal tubule (paracellular).
Late distal and collecting tubules secrete K+ (principal cells).
K+ secretion is affected by plasma K+ levels and aldosterone.

H+ Secretion

Crucial for acid-base balance.
Occurs via Na+-H+ countertransport in the proximal tubule (secondary active).
Direct H+ pumping by H+-ATPase in the distal tubule and collecting duct (primary active).

Organic Ion Secretion

Two types of carriers in the proximal tubule for organic anions and cations.
This mechanism eliminates many drugs.

Filtration, Reabsorption, and Excretion of Substances

Table shows amounts filtered, reabsorbed, excreted, and percentage reabsorbed for various substances.

Filtration Load

Filtered load is the amount of a substance filtered into Bowman's space per unit time (minutes).
Filtered load = plasma concentration of the substance × GFR.

Transport Maximum (Tm)

Tm is the maximum amount of a substance that tubular cells can actively transport per unit time. This is limited by saturation of specific transport systems.
Tm for glucose is approximately 375 mg/min (for males, 225-500 mg/min).

Renal Threshold

Glucose starts appearing in urine when plasma concentration reaches 180 mg/100 mL (renal threshold).
Most nephrons do not have the same Tm.

Renal Threshold Glucose Graph

Graph shows filtered load, excretion, reabsorption, transport maximum, and threshold.

Different Kinds of Diuretics

Diuretics are substances that increase urine volume and can be categorized into different types with mechanisms of action affecting different parts of the nephron (proximal tubule, ascending loop of Henle, distal tubule, collecting duct).

Summary

Define processes of tubular reabsorption and secretion.
Detail various transport mechanisms across the tubular epithelium
Describe reabsorption and secretion of Na+, H₂O, Cl-, glucose, urea, and potassium.
Identify key substances secreted by renal tubules.
Define transport maximum, renal plasma threshold, and splay.