Renal Physiology and Acid-Base Balance

Renal questions on Step 1 reward mechanism over memorization. If you can walk the nephron segment by segment, reason through the renin-angiotensin-aldosterone system (RAAS), and apply a simple stepwise approach to any arterial blood gas, most "hard" renal items become predictable. The concepts below tie filtration, hormones, diuretics, and acid-base into one integrated picture.

Core Idea

• The nephron filters, then selectively reclaims what the body needs. The proximal tubule reabsorbs the bulk load; distal segments do the fine, hormonally controlled tuning of sodium, potassium, and acid.
• Volume and acid-base are regulated by the same few hormones. Angiotensin II, aldosterone, and antidiuretic hormone (ADH) coordinate sodium, water, and potassium — and potassium status directly shifts acid-base balance.
• Every acid-base disorder is a two-step read: identify the primary process from pH and the driver (CO2 vs bicarbonate), then check whether compensation is appropriate.

Nephron Segments: Who Does What

• Proximal convoluted tubule (PCT): reabsorbs ~65% of filtered sodium and water, essentially all glucose and amino acids, and most bicarbonate (via carbonic anhydrase). It is the site of ammoniagenesis and is where the body generates new bicarbonate. Isotonic reabsorption keeps tubular fluid iso-osmotic here.
• Thick ascending limb of the loop of Henle: reabsorbs sodium, potassium, and chloride via the Na-K-2Cl cotransporter. It is impermeable to water, so it dilutes the tubular fluid and builds the medullary concentration gradient.
• Distal convoluted tubule (DCT): reabsorbs sodium and chloride via the Na-Cl cotransporter; site of parathyroid-hormone-driven calcium reabsorption. Also water-impermeable, further diluting fluid.
• Collecting duct: principal cells reabsorb sodium and secrete potassium under aldosterone; alpha-intercalated cells secrete hydrogen ions (making new bicarbonate). ADH inserts aquaporins here for final water reabsorption.

GFR, Clearance, and Filtration Fraction

• GFR is estimated by the clearance of a marker that is freely filtered but neither reabsorbed nor secreted (inulin; creatinine approximates it clinically but is slightly secreted, overestimating GFR).
• Clearance = (urine concentration x urine flow) / plasma concentration. If a substance's clearance exceeds GFR, it is net secreted; if less, it is net reabsorbed.
• Filtration fraction (FF) = GFR / renal plasma flow (RPF). RPF is estimated by para-aminohippurate (PAH) clearance. Constricting the efferent arteriole raises GFR and FF; constricting the afferent arteriole lowers both. Angiotensin II preferentially constricts the efferent arteriole, defending GFR when volume is low.

RAAS, ADH, and Volume Control

• Renin is released by juxtaglomerular cells in response to low renal perfusion, low distal sodium delivery, and sympathetic (beta-1) tone. Renin converts angiotensinogen to angiotensin I; ACE (mainly in lung) makes angiotensin II.
• Angiotensin II constricts vasculature, preferentially constricts the efferent arteriole, stimulates proximal sodium reabsorption, triggers aldosterone and ADH release, and stimulates thirst.
• Aldosterone acts on principal cells to reabsorb sodium and water while secreting potassium and (indirectly) hydrogen — expanding volume and promoting a metabolic alkalosis when in excess.
• ADH responds to high plasma osmolarity or severe volume loss, inserting aquaporins in the collecting duct to reabsorb free water and concentrate urine.

Diuretics by Site (Conceptual)

• Carbonic anhydrase inhibitors: act at the PCT, blocking bicarbonate reabsorption — cause a metabolic acidosis.
• Loop diuretics: block Na-K-2Cl in the thick ascending limb; most powerful, but waste potassium and can cause a metabolic alkalosis.
• Thiazides: block Na-Cl in the DCT; waste potassium, retain calcium.
• Potassium-sparing diuretics: act at the collecting duct (aldosterone antagonists or ENaC blockers); retain potassium and can cause a metabolic acidosis.

Acid-Base and Potassium

• Metabolic acidosis: low bicarbonate. Split by anion gap (Na - [Cl + HCO3]). High-gap causes include ketoacidosis, lactic acidosis, uremia, and toxins; normal-gap (hyperchloremic) causes include diarrhea and renal tubular acidosis. Expected respiratory compensation follows Winter's formula (PCO2 falls in proportion to bicarbonate).
• Metabolic alkalosis: high bicarbonate, often from vomiting or diuretics; compensated by hypoventilation.
• Respiratory disorders: high CO2 (hypoventilation) causes acidosis; low CO2 (hyperventilation) causes alkalosis. Renal compensation (adjusting bicarbonate) is slow, distinguishing acute from chronic.
• Potassium link: acidosis drives potassium out of cells (hyperkalemia) as hydrogen enters; alkalosis drives potassium in (hypokalemia). Hypokalemia in turn promotes intracellular acidosis and worsens metabolic alkalosis.

High-Yield Exam Patterns

• Efferent vs afferent constriction on GFR/FF is a favorite: efferent constriction raises FF, afferent lowers it.
• ACE inhibitors drop GFR in bilateral renal artery stenosis by dilating the efferent arteriole — a classic vignette.
• Match each diuretic to its segment and acid-base side effect; loops and thiazides cause alkalosis, acetazolamide and potassium-sparers cause acidosis.
• For any gas: read pH first, then decide CO2 or bicarbonate is the primary driver, then verify compensation — a "corrected" pH means a mixed disorder.
• Always calculate the anion gap in acidosis before choosing an answer; a normal gap points to GI or renal loss.
• Use the potassium-pH relationship to predict shifts: expect hyperkalemia with acidosis, hypokalemia with alkalosis.

Common Traps to Avoid

• Assuming compensation ever fully normalizes the pH — it never overcorrects; a normal pH with abnormal gases means a mixed disorder.
• Forgetting that creatinine slightly overestimates GFR because it is secreted.
• Confusing which arteriole angiotensin II acts on — it is predominantly the efferent.
• Mislabeling the loop's water permeability; the thick ascending limb is water-impermeable, which is why it dilutes urine.
• Treating potassium and acid-base as independent — they move together and each drives the other.