Step 1 Physiology Questions: Cardio, Renal and Respiratory

Physiology is the second pillar of Step 1. The USMLE Step 1 content outline lists it at 30 to 40 percent of the exam, second only to pathology. Because items are integrative, physiology is also hidden inside questions that are classified as pathology or pharmacology: a drug question often asks what happens to a physiological variable, and a disease question often asks why a finding appears.

Three systems carry a large share of that physiology: cardiovascular, renal and respiratory. They are tested heavily because each is built on a small set of relationships that an examiner can change one variable at a time. This guide shows how those questions are built, which relationships to know cold, and how to practise them with a qbank.

How physiology questions are built

Most physiology questions follow one pattern: a change happens, and you predict the direction of the result. The change might be a disease, a drug, exercise, altitude, haemorrhage or a change in posture. The lead-in then asks which variable increases, decreases or stays the same.

That means the key skill is not recall but reasoning:

1. Identify the primary change (what was done to the system).
2. Find the relationship that links it to the variable in the question.
3. Decide the direction of the change, then check for any compensation (reflexes, hormones or the kidney) that the question wants you to include.

Many wrong answers on physiology questions come from step 3: the student predicts the first effect correctly but forgets that the body responds, or includes a compensation the question did not ask about. Read the lead-in carefully for words such as "immediately", "initially" or "after several days".

Cardiovascular physiology

Cardiovascular physiology is built on a few relationships:

• Cardiac output = heart rate × stroke volume.
• Mean arterial pressure ≈ cardiac output × total peripheral resistance.
• Stroke volume depends on preload, afterload and contractility.

The Frank-Starling relationship says that, within the normal range, a greater end-diastolic volume (preload) produces a stronger contraction and a larger stroke volume. Afterload is the pressure the ventricle must work against; raising it reduces stroke volume. Contractility (inotropy) changes the force at any given preload, for example with sympathetic stimulation.

Pressure-volume loops are a favourite question format because they show all three at once. Learn what each change does to the loop: more preload widens the loop to the right, more afterload makes it taller and narrower, and more contractility raises the end-systolic pressure-volume line so the loop ends at a smaller end-systolic volume.

The baroreceptor reflex is the most common compensation. A fall in arterial pressure, for example after haemorrhage or standing quickly, reduces baroreceptor firing, which raises sympathetic output: heart rate, contractility and peripheral resistance increase. When a vignette describes acute blood loss with a fast pulse, that tachycardia is the reflex, not the primary problem.

A worked example (illustration). A patient is given a drug that is a pure arteriolar vasodilator. What happens first? Total peripheral resistance falls, so mean arterial pressure falls. The baroreceptor reflex then raises heart rate. If the question asks about heart rate, the answer is "increased", even though the drug has no direct effect on the heart. This is exactly how cardiovascular physiology overlaps with pharmacology, covered in our pharmacology practice questions guide.

For practice by system, the cardiovascular questions page shows how our cardio questions are organised.

Renal physiology

Renal physiology looks equation-heavy, but a few ideas cover most questions.

Filtration. Glomerular filtration rate (GFR) depends on the balance of Starling forces across the glomerular capillary. Constricting the afferent arteriole lowers glomerular capillary pressure and GFR. Constricting the efferent arteriole raises glomerular capillary pressure and tends to raise GFR while lowering renal plasma flow. Filtration fraction is GFR divided by renal plasma flow, so efferent constriction raises it.

Clearance. The clearance of a substance is the volume of plasma cleared of it per unit time: urine concentration × urine flow ÷ plasma concentration. A substance that is freely filtered and neither reabsorbed nor secreted, such as inulin, has a clearance equal to GFR. Clearance above GFR means net secretion; clearance below GFR means net reabsorption.

Nephron segments. Know what each segment reabsorbs and which transporter it uses, because diuretic questions are really segment questions. Loop diuretics block the Na-K-2Cl cotransporter in the thick ascending limb; thiazides block the Na-Cl cotransporter in the distal convoluted tubule; aldosterone acts on principal cells of the collecting duct to increase sodium reabsorption and potassium secretion.

Acid-base. Acid-base questions reward a fixed routine:

1. Look at the pH: acidaemia or alkalaemia?
2. Is the primary change in bicarbonate (metabolic) or in PCO2 (respiratory)?
3. For metabolic acidosis, calculate the anion gap: Na − (Cl + HCO3).
4. Check compensation. For metabolic acidosis, Winter's formula predicts the expected PCO2 as 1.5 × HCO3 + 8 (± 2). A PCO2 outside that range suggests a second, respiratory disorder.

The renal questions page groups our renal questions, including acid-base and diuretics, so you can drill this routine in one block.

Respiratory physiology

Respiratory physiology centres on getting oxygen into the blood and carbon dioxide out.

Ventilation-perfusion matching. In a shunt (perfusion without ventilation, for example a collapsed or fluid-filled region of lung), blood passes through without picking up oxygen, so giving supplemental oxygen corrects the hypoxaemia poorly. In dead space (ventilation without perfusion, for example a pulmonary embolus), air reaches alveoli that receive no blood. This difference between shunt and dead space is a common question.

The alveolar gas equation and the A-a gradient. The alveolar oxygen tension is estimated as PAO2 = FiO2 × (Patm − PH2O) − PaCO2 ÷ R. At sea level on room air, with a PaCO2 of 40 mm Hg and R of 0.8, that gives about 0.21 × (760 − 47) − 50, or roughly 100 mm Hg. Comparing this with the arterial value gives the A-a gradient. Hypoventilation lowers arterial oxygen with a normal A-a gradient; V/Q mismatch, shunt and diffusion problems widen it. Asking "is the gradient normal?" sorts the causes of hypoxaemia quickly.

The oxygen-haemoglobin curve. A right shift means haemoglobin gives up oxygen more easily. Increased CO2, increased H+ (lower pH), increased temperature and increased 2,3-BPG shift the curve right, which helps unload oxygen in exercising tissue. Fetal haemoglobin and carbon monoxide shift it left.

Obstructive versus restrictive patterns. In obstructive disease, FEV1 falls more than FVC, so the FEV1/FVC ratio is reduced. In restrictive disease both fall and the ratio is normal or increased. This links directly to the pathology of asthma, COPD and interstitial lung disease, covered in our pathology questions guide. The respiratory questions page collects our respiratory practice.

Why these three systems connect

Cardio, renal and respiratory physiology are rarely tested in isolation. A patient with heart failure has low effective circulating volume, which activates renin-angiotensin-aldosterone, which changes sodium and potassium handling in the kidney, which may produce a metabolic alkalosis from diuretics. A patient with COPD retains CO2, and the kidney compensates by retaining bicarbonate over days.

When you review a question, ask which other system responded. That habit turns three separate topics into one network and makes integrative questions much easier.

Using a qbank for physiology

• Predict before you look. Read the stem and lead-in, write the direction of change (up, down, no change), then look at the options. Physiology options are often designed so that every direction is offered.
• Draw it. Sketch a pressure-volume loop, a nephron or an oxygen-haemoglobin curve when you miss a question. Drawing exposes the step you skipped.
• Separate primary change from compensation. For every miss, write what changed first and what responded.
• Review every option. In USMLE Step 1 QBank, each answer option has its own explanation, so a wrong "decreased" option teaches you which variable actually decreases.
• Mix systems later. Start with system blocks, then move to mixed timed blocks to check that you can recognise the system from the vignette alone.

The NBME-style practice questions guide explains how vignettes and lead-ins are structured, which helps with the "initially versus after several days" wording.

Common physiology mistakes

• Memorising equations without directions. Know what each variable depends on, not just the formula.
• Forgetting the reflex. Blood pressure and volume changes almost always trigger the baroreceptor reflex or hormones.
• Including compensation that has not happened yet. Renal compensation for a respiratory disorder takes days.
• Mixing up shunt and dead space. Ask whether supplemental oxygen would fix the problem.
• Treating physiology as separate from pathology. Most disease findings are physiology in motion.

Fitting physiology into your schedule

Physiology underpins pathology and pharmacology, so review it at the start of each system rather than in a separate block at the end. Our Step 1 dedicated study schedule guide shows sample 4, 6 and 8 week plans that combine system review with question blocks. If you are still choosing a question bank, our checklist on how to choose the best USMLE Step 1 QBank covers what to look for in explanations and system organisation.

Practise physiology by system

Pick one system, do a block, and for every miss write the primary change and the compensation. Start the free trial for 7 days or 30 questions with no card, then see the packages if you want full access to every system.