USMLE Step 1 - Renal Physiology, Acid-Base Balance & Glomerular Diseases

The GFR can be calculated using the formula: $GFR=\frac{U_{Inulin}\times V}{P_{Inulin}}$, where $U_{Inulin}$ is the urinary concentration of inulin, $V$ is the urine flow rate, and $P_{Inulin}$ is the plasma concentration of inulin.

The PCT is the primary site for reabsorption of most filtered substances, including $\approx 60-70\%$ of Na+, water, K+, Cl-, HCO3-, glucose, and amino acids. It also secretes organic acids/bases and ammonium.

The Na-K-2Cl cotransporter (NKCC2) is crucial. This segment is impermeable to water, leading to dilution of the tubular fluid and concentration of the medullary interstitium, essential for the countercurrent multiplier.

Principal cells reabsorb Na+ and water (under ADH influence) and secrete K+. Aldosterone increases Na+ reabsorption and K+ secretion by increasing ENaC channels and Na+/K+ ATPase activity.

The Henderson-Hasselbalch equation is $pH=p{K}_a+\log \frac{[HC{O}_3^{-}]}{0.03\times P_{aC{O}_2}}$. It describes the relationship between pH, bicarbonate concentration, and partial pressure of CO2, allowing for interpretation of acid-base disorders.

Winter's Formula is used to calculate the expected respiratory compensation in metabolic acidosis: $PaC{O}_2=1.5\times [HC{O}_3^{-}]+8\pm 2$. It helps determine if the respiratory compensation is appropriate, inadequate, or excessive.

The Anion Gap (AG) is the difference between the primary measured cations (Na+) and the primary measured anions (Cl- and HCO3-). Formula: $AG=[N{a}^{+}]-([C{l}^{-}]+[HC{O}_3^{-}])$. Normal range is typically 8-12 mEq/L.

MUDPILES: Methanol, Uremia, Diabetic Ketoacidosis, Paraldehyde, Iron/Isoniazid, Lactic Acidosis, Ethylene Glycol, Salicylates.