Tubular Functions & Renal Clearance

Vocabulary practice flashcards covering tubular transport mechanisms, renal clearance equations, inulin and creatinine as GFR markers, PAH clearance for renal plasma flow, and the dynamics of glucose transport including transport maximum and splay.

Tubular Reabsorption

The tubular transport process involving the movement of substances from the tubular fluid back into the blood.

Tubular Secretion

The tubular transport process involving the movement of substances from the peritubular capillary blood into the tubular fluid.

Filtered Load

The amount of a substance filtered at the glomerulus per minute, calculated for a freely filtered substance as: $\text{Filtered Load} = \text{GFR} \times P_{\text{sub}}$

Excretion Rate

The total amount of a substance excreted in the final urine per unit of time, calculated as: $\text{Excretion Rate} = \dot{V} \times U_{\text{sub}}$ where $\dot{V}$ is the urine flow rate and $U_{\text{sub}}$ is the urine concentration of the substance.

Net Tubular Transport

The difference between the filtered load and the excretion rate ($\text{Filtered Load} - \text{Excretion rate}$); a positive value indicates net reabsorption, zero indicates neither reabsorption nor secretion, and a negative value indicates net secretion.

Renal Clearance

The theoretical volume of plasma completely cleared of a substance by the kidneys per unit time, calculated as: $C_{\text{sub}} = \frac{U_{\text{sub}} \times \dot{V}}{P_{\text{sub}}}$

Inulin

An exogenous, physiologically inert plant alkaloid that is freely filtered and neither reabsorbed, secreted, synthesized, nor metabolized by the kidneys, making its clearance the gold standard marker for estimating glomerular filtration rate ($\text{GFR}$).

Clearance Ratio (Fractional Excretion)

The ratio of the renal clearance of a specific substance to that of inulin ($C_x : C_{\text{in}}$), equivalent to the ratio of excretion rate to filtered load ($\text{ER} / \text{FL}$).

Creatinine

An endogenous byproduct released from skeletal muscle at a constant rate proportional to muscle mass that is freely filtered and not reabsorbed, but slightly secreted in the proximal tubule, causing its clearance to slightly overestimate $\text{GFR}$.

Effective Renal Plasma Flow (ERPF)

The renal plasma flow estimated by the clearance of para-aminohippuric acid ($C_{\text{PAH}}$), which underestimates true renal plasma flow by about $10\%$ because roughly $10\%$ of plasma perfuses non-transporting kidney tissues such as the renal capsule and perinephric fat.

PAH Extraction Ratio ($E_{\text{PAH}}$)

The fraction of para-aminohippuric acid removed from renal arterial plasma during a single passage through the kidneys (normally about $0.9$ or $90\%$), calculated as: $E_{\text{PAH}} = \frac{[RA]_{\text{PAH}} - [RV]_{\text{PAH}}}{[RA]_{\text{PAH}}}$

Renal Blood Flow (RBF)

The total volume of whole blood flowing through the kidneys per unit time, calculated from renal plasma flow and hematocrit as: $\text{Renal blood flow} = \frac{\text{RPF}}{1 - \text{Hematocrit}}$

SGLT1 and SGLT2

Sodium-dependent glucose cotransporters located on the luminal (apical) membrane of proximal tubule cells that mediate the secondary active transport of glucose from tubular fluid into the cell.

GLUT1 and GLUT2

Glucose transporter proteins located on the basolateral membrane of proximal tubule cells that mediate the reabsorption of glucose into peritubular capillary blood via facilitated diffusion.

Transport Maximum ($T_m$) of Glucose

The maximal possible rate at which glucose can be reabsorbed by the nephrons (averaging about $375\,\text{mg/min}$), occurring when all available luminal glucose carrier proteins become fully saturated.

Theoretical Glucose Threshold

The theoretical plasma glucose concentration of approximately $300\,\text{mg/dL}$ ($3\,\text{mg/mL}$) at which glucose would begin spilling into urine if every nephron possessed identical reabsorptive capacities and reached transport maximum ($T_m$) simultaneously.

Renal Threshold for Glucose

The actual plasma glucose concentration of approximately $200\,\text{mg/dL}$ at which glucose begins to appear in the urine and the splay phenomenon begins.

Splay

The rounded deviation in the glucose titration reabsorption curve where glucose starts spilling into the urine before the absolute $T_m$ is reached, caused by nephron heterogeneity in length, size, and reabsorptive capacity.

Glucosuria

The appearance of detectable levels of glucose in the urine, occurring in diabetes mellitus, pregnancy (due to a $40\text{--}50\%$ increase in GFR lowering the threshold to $130\text{--}150\,\text{mg/dL}$), or through pharmacological inhibition of SGLT2.

Phlorizin

A naturally occurring plant compound that competitively inhibits glucose binding to both $SGLT1$ and $SGLT2$ cotransporters.

Empagliflozin

A selective $SGLT2$ inhibitor drug used in the treatment of Type 2 diabetes mellitus that induces glucosuria at plasma glucose levels lower than the normal renal threshold of $200\,\text{mg/dL}$.