Respiratory and Renal Systems: Acid-Base, Nephron and Lung Physiology on Step 1

Scope and time budget

Thirty-six questions is more than a full 30-minute block and a half, and the row has 20 subtopics, so it deserves about two weeks in a 12-week plan. Give renal physiology and acid-base the first week, since they underlie the electrolyte, diuretic and glomerular topics. Give the second week to lung mechanics, gas exchange and respiratory disease. Reference values are given in conventional units: arterial pH 7.35-7.45, PaCO2 33-45 mm Hg, PaO2 75-105 mm Hg, bicarbonate 22-28 mEq/L, sodium 136-145 mEq/L, potassium 3.5-5.0 mEq/L, creatinine 0.6-1.2 mg/dL. The exam shows its own reference list, so practice with it.

What is tested

• Respiratory physiology. Lung anatomy, mechanics and volumes, pulmonary function tests (obstructive versus restrictive), gas exchange, V/Q mismatch and the oxygen-hemoglobin dissociation curve.
• Respiratory disease. Asthma, COPD and respiratory drugs, interstitial lung disease and pneumoconioses, pulmonary embolism and pulmonary hypertension, pneumonia and tuberculosis, lung cancer, pleural disease and pneumothorax, and respiratory failure, ARDS and neonatal respiratory distress.
• Renal physiology. Anatomy, glomerular filtration and clearance, tubular transport and diuretics.
• Fluid, electrolyte and acid-base. Acid-base disorders and disorders of sodium, potassium, calcium and phosphate.
• Renal disease. Acute kidney injury, chronic kidney disease and renal replacement, nephritic and nephrotic syndromes, tubulointerstitial disease, stones, renal tubular acidosis, urinary tract infection, obstruction, congenital anomalies and tumors.
• Renal hormones. Renin-angiotensin-aldosterone, ADH, atrial natriuretic peptide and erythropoietin.

Anchor facts and US conventions

• Clearance. Clearance = (urine concentration x urine flow) / plasma concentration. Inulin clearance estimates GFR, and para-aminohippuric acid clearance estimates renal plasma flow.
• Anion gap. Anion gap = Na minus (Cl + HCO3), with a normal of about 8-12 mEq/L. High-gap acidosis causes follow MUDPILES: methanol, uremia, diabetic ketoacidosis, propylene glycol, isoniazid or iron, lactic acidosis, ethylene glycol, salicylates.
• Diuretics. Loop diuretics block the Na-K-2Cl cotransporter in the thick ascending limb. Thiazides block the Na-Cl cotransporter in the distal convoluted tubule and cause hypercalcemia. Spironolactone blocks aldosterone receptors and can cause hyperkalemia.
• Organisms. Streptococcus pneumoniae is the most common cause of community-acquired pneumonia. Escherichia coli is the most common cause of urinary tract infection.
• Pulmonary function. Obstructive disease reduces the FEV1/FVC ratio. Restrictive disease reduces lung volumes with a normal or high ratio.
• Acid-base logic. Check pH first, then the primary disturbance (PaCO2 or bicarbonate), then compensation, then the anion gap.
• US conventions. Blood gases in mm Hg, electrolytes in mEq/L, creatinine and BUN in mg/dL, and US generic names such as albuterol.

Three typical traps

1. Skipping the compensation step in acid-base. Many stems give both a gas and a chemistry panel. If you identify only the primary disorder you miss the mixed disorder that the question is built on. Always ask whether compensation is appropriate or whether a second process exists.
2. Mixing up where a diuretic acts and what it does. Each diuretic has a site, a transporter, an effect on urine and blood electrolytes and a signature toxicity (ototoxicity with loop diuretics, hypercalcemia with thiazides, hyperkalemia with potassium-sparing agents). Learn them as a single row.
3. Choosing by one lab value. In nephritic versus nephrotic syndrome, renal injury or hyponatremia, the diagnosis depends on a pattern: urine sediment, proteinuria level, serum values, volume status. Name the pattern before looking at the options.

How to revise this system

• Do acid-base by hand. Practice a dozen blood gases from scratch using a fixed routine, then use it in every block.
• Draw the nephron. At each segment, write the transport, the hormone that acts, the drug that blocks and the electrolyte effect.
• Use the flow-volume and PFT patterns. Know how the loop and the numbers look in obstructive and restrictive disease, and the causes of each.
• Table glomerular disease. Rows: nephritic and nephrotic causes. Columns: age, immunofluorescence or electron microscopy pattern, clinical clue.
• Link to cardiovascular and endocrine. Heart failure, hypertension and adrenal disease cross the renal system. See the cardiovascular page and the reproductive and endocrine page.

The study plan guide shows where this system fits.

What the questions look like

Expect a vignette with vital signs, a blood gas or electrolyte panel, a urinalysis or a spirometry result, and a final question about the diagnosis, the mechanism, the best next step or the effect of a drug. Some items show a chest image or a graph of flow or volume. Our original practice bank follows the same format with conventional units and five options, and it does not copy official or recalled items. The Step 1 hub lists all the systems and weights, and the high-yield topics guide collects the equations used here.

A daily acid-base drill

Take five blood gases a day for two weeks, solving each with the same four steps on paper. After the first few days, add the chemistry panel and ask what disease would produce the pattern. The habit takes about ten minutes and turns acid-base from a feared topic into a dependable source of marks.