Calculation of reabsorption and secretion rates

- The reabsorption or secretion rate is the difference between the amount filtered across the glomerular capillaries and the amount excreted in urine. It is calculated with the following equations:

Filtered load = GFR x [plasma] Excretion rate = V x [urine] Reabsorption rate = Filtered load - Excretion rate Secretion rate = Excretion rate - Filtered load

- If the filtered load is greater than the excretion rate, then net reabsorption of the substance has occurred. If the filtered load is less than the excretion rate, then net secretion of the substance has occurred.

Table 5-3. Effect of Changes in Starling Forces on GFR, RPF, and Filtration Fraction

Effect on GFR

Effect on RPF

Effect on Filtration Fraction

Constriction of afferent arteriole

(caused by I PGC)


No change

Constriction of efferent arteriole

(caused by T PGC)



Increased plasma [protein]

(caused by T tt-qc)

No change

(4- GFR/unchanged RPF)

Ureteral stone

(caused by T PBS)

No change

(1 GFR/unchanged RPF)

GFR = glomerular filtration rate; RPF = renal plasma flow.

GFR = glomerular filtration rate; RPF = renal plasma flow.



Figure 5-4. Processes of filtration, reabsorption, and secretion. The sum of the three processes is excretion.

- Example: A woman with untreated diabetes mellitus has a GFR of 120 ml/min, a plasma glucose concentration of 400 mg/dl, a urine glucose concentration of 2500 mg/dl, and a urine flow rate of 4 ml/min. What is the reabsorption rate of glucose?

Filtered load = GFR x Plasma [glucose] = 120 ml/min x 400 mg/dl = 480 mg/min Excretion = V x Urine [glucose]

= 4 ml/min x 2500 mg/dl = 100 mg/min Reabsorption = 480 mg/min - 100 mg/min = 380 mg/min

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