Kidney filtration and solute handling

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Turn nephron transport into a mass-balance problem: what was filtered, what was reabsorbed, what was secreted, and what finally appeared in urine?

About 18 minutes, including filtered-load, excretion, and transport-saturation examples.

Start with the renal mass balance

The nephron handles a solute through four bookkeeping terms: filtration, reabsorption, secretion, and excretion. The relationship is:

Excretion = Filtration − Reabsorption + Secretion.

That equation is more reliable than memorizing whether every solute “goes up” or “goes down” in urine. If a solute is filtered and then almost completely reabsorbed, urinary excretion can be tiny even though filtration was large. If a solute is filtered and additionally secreted into the tubule, excretion can exceed the filtered amount.

Concept: filtration is not the same as excretion

Glomerular filtration transfers water and small dissolved substances from glomerular capillary plasma into Bowman’s space, subject to the filtration barrier. Cells and most large plasma proteins are retained under normal conditions. The filtered load of a freely filtered solute can be calculated as:

Filtered load = GFR × plasma concentration.

After filtration, tubular epithelial cells selectively reabsorb substances back to blood or secrete substances from blood into tubular fluid. What reaches the ureter is the result of all of those steps, not filtration alone.²

Worked application: calculate filtered and excreted loads

An original kidney model has a glomerular filtration rate of 120 mL/min. Plasma concentration of solute X is 2 mg/mL. Urine flow is 1.5 mL/min and urine concentration of X is 40 mg/mL. Assume X is freely filtered.

The filtered load is:

120 mL/min × 2 mg/mL = 240 mg/min.

The urinary excretion rate is:

1.5 mL/min × 40 mg/mL = 60 mg/min.

Therefore the kidney excretes 60 mg/min even though 240 mg/min was filtered. The difference—180 mg/min—represents net reabsorption if no additional information indicates simultaneous secretion.

If instead excretion had been 300 mg/min while filtered load remained 240 mg/min, the extra 60 mg/min would require net secretion into the tubule.

Pattern: transporters can saturate

Carrier-mediated tubular transport has finite capacity. For a reabsorbed solute such as glucose in a simplified model, filtered load rises linearly as plasma concentration rises. Reabsorption can rise with filtered load only until the relevant transporters approach their maximum transport rate. Beyond that point, additional filtered solute remains in tubular fluid and appears in urine.³

Once reabsorption plateaus at 220 mg/min, every additional milligram filtered appears in urine under this simplified model. The urinary threshold therefore reflects transporter capacity, not a sudden change in glomerular filtration physics.

Rule: clearance compares excretion with plasma concentration

Renal clearance is the virtual plasma volume from which a solute is completely removed per unit time:

Clearance = (urine concentration × urine flow) / plasma concentration.

For a freely filtered solute, clearance below GFR supports net reabsorption; clearance above GFR supports net secretion. A clearance close to GFR is consistent with little net tubular handling under the stated conditions. These interpretations depend on the assumptions that the solute is freely filtered and measured accurately.

Acid–base handling and nitrogen excretion use the same accounting mindset. Renal secretion of H⁺ and handling of bicarbonate affect systemic acid–base balance, while nitrogenous wastes such as urea are excreted after metabolism. Do not confuse “present in urine” with “made by the kidney.”

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Sources

1. AAMC. What’s on the MCAT Exam? Content Outline. Content Category 3B.
2. NCBI Bookshelf. Physiology, Renal.
3. NCBI Bookshelf. Physiology, Glucose Reabsorption.