Study guide
This chapter covers the two organ systems that most directly determine oxygen delivery to tissue and are tested heavily throughout Step 1 as fused physiology-pathology-pharmacology vignettes. Cardiovascular and respiratory content together are consistently among the highest-yield organ-system categories on the exam, spanning normal hemodynamics and gas exchange through ischemic, valvular, and obstructive/restrictive disease. Expect single-best-answer items that require you to trace a mechanism from a bedside finding (a murmur, a blood gas, a hemodynamic tracing) back to the underlying physiology, not just recall a diagnosis.
Cardiac Physiology and the Baroreceptor Reflex
The heart's output is set by preload, afterload, contractility, and heart rate, and short-term arterial pressure is defended primarily by the baroreceptor reflex. Stretch receptors in the carotid sinus and aortic arch fire in proportion to arterial pressure; their afferents travel via cranial nerves IX and X to the nucleus tractus solitarius in the medulla. A fall in pressure (as with standing) reduces vessel wall stretch and lowers baroreceptor firing rate, which disinhibits sympathetic outflow and withdraws vagal tone, producing tachycardia and peripheral vasoconstriction within seconds. This is a fast neural loop, distinct from the slower hormonal responses (renin-angiotensin-aldosterone, ADH, catecholamines) that stabilize pressure over minutes to hours. Step 1 vignettes frequently invert the direction of this reflex as a distractor, so always work from the stimulus (increased or decreased stretch) to the resulting change in afferent firing, then to the appropriate autonomic output. Related physiology to master alongside this reflex includes the determinants of cardiac output (CO = SV x HR), the pressure-volume loop, Frank-Starling mechanics, and how the reflex is blunted in autonomic neuropathy (e.g., diabetic orthostatic hypotension).
Heart Failure and Cardiogenic Pulmonary Edema
Left ventricular systolic or diastolic dysfunction raises left ventricular end-diastolic pressure, which is transmitted retrograde to the left atrium and pulmonary veins, elevating pulmonary capillary hydrostatic pressure. By Starling forces, this drives net fluid filtration out of the capillary into the interstitium and, once lymphatic clearance is overwhelmed, into the alveoli, producing cardiogenic pulmonary edema with dyspnea and bilateral crackles. This hydrostatic mechanism must be distinguished from other causes of edema: decreased plasma oncotic pressure (nephrotic syndrome, liver failure), increased capillary permeability (ARDS, inflammatory injury), and lymphatic obstruction (post-surgical or malignant). Right heart failure, by contrast, causes systemic venous congestion — jugular venous distension, hepatomegaly, and peripheral edema — rather than pulmonary edema. Compensatory neurohormonal responses to falling cardiac output (RAAS activation, sympathetic stimulation, ADH release) initially support pressure but chronically worsen remodeling and volume overload, which underlies the rationale for ACE inhibitors, beta-blockers, and diuretics in heart failure management. Know the classic exam distinctions between systolic (reduced ejection fraction) and diastolic (preserved ejection fraction) heart failure, and the auscultatory/imaging findings (S3 gallop, Kerley B lines, cardiomegaly) that support cardiogenic over non-cardiogenic edema.
Oxygen Transport and the Hemoglobin Dissociation Curve
Oxygen delivery depends on hemoglobin's cooperative, sigmoidal binding curve, which shifts to optimize loading in the lung and unloading in tissue. In metabolically active tissue, increased CO2, increased H+ (lower pH), increased temperature, and increased 2,3-bisphosphoglycerate all shift the curve rightward, decreasing hemoglobin's oxygen affinity and favoring unloading — the Bohr effect specifically describes the pH/CO2-driven shift. The reverse conditions (alkalosis, hypothermia, low 2,3-BPG, fetal hemoglobin) shift the curve leftward, increasing affinity and impairing peripheral unloading, which is why stored blood (low 2,3-BPG) and fetal hemoglobin behave differently. Carbon monoxide poisoning both directly reduces oxygen-carrying capacity by occupying binding sites and shifts the curve leftward, worsening tissue hypoxia at any given PaO2 — a classic exam trap because pulse oximetry can appear falsely reassuring. You should be able to read a dissociation curve and identify a shift direction from a described clinical scenario, and connect the curve to P50 (the PO2 at 50% saturation) as a quantitative marker of affinity.
Mechanisms of Hypoxemia: Shunt, V/Q Mismatch, Diffusion Limitation, and Hypoventilation
Four mechanisms produce hypoxemia, and distinguishing them by response to supplemental oxygen and the alveolar-arterial (A-a) gradient is a recurring Step 1 theme. Hypoventilation raises PaCO2 with a normal A-a gradient and corrects with supplemental oxygen. Diffusion limitation (interstitial lung disease, early pulmonary edema) widens the A-a gradient but improves substantially with 100% oxygen because the increased alveolar oxygen gradient drives transfer across a thickened but still-ventilated membrane. V/Q mismatch (the most common cause of hypoxemia clinically, as in COPD or pulmonary embolism) also widens the A-a gradient and generally responds well to supplemental oxygen. Shunt — blood perfusing alveoli that receive no ventilation at all, as in lobar pneumonia with consolidation, ARDS, or an intracardiac right-to-left shunt — widens the A-a gradient and classically fails to correct with 100% oxygen, because the shunted blood never contacts the oxygen-rich alveolar gas. This oxygen-refractory hypoxemia is the single most testable discriminating feature of shunt physiology. Pair this concept with dead space physiology (high V/Q, wasted ventilation as in pulmonary embolism), which impairs CO2 elimination rather than oxygenation.
Obstructive and Restrictive Lung Disease and Pulmonary Function Testing
Pulmonary function tests separate lung disease into obstructive patterns (reduced FEV1/FVC ratio, from asthma, COPD/emphysema, or bronchiectasis) and restrictive patterns (normal or increased FEV1/FVC with reduced total lung capacity, from interstitial fibrosis, neuromuscular weakness, or chest wall disease). Obstructive disease increases airway resistance and traps air, raising residual volume and total lung capacity in emphysema specifically, while restrictive disease reduces lung compliance or the ability to expand the chest wall, lowering all lung volumes proportionally. The diffusion capacity for carbon monoxide (DLCO) further discriminates within these categories: it is reduced in emphysema (destroyed alveolar-capillary surface area) and in interstitial fibrosis (thickened membrane), but normal or increased in asthma and in restrictive disease caused purely by chest wall/neuromuscular limitation (since the alveolar membrane itself is intact). Step 1 expects you to read a flow-volume loop or a table of spirometry values and assign the correct category and likely diagnosis, and to connect chronic hypoxemia/hypercapnia in COPD to secondary polycythemia and cor pulmonale (right heart strain from pulmonary hypertension).
Ischemic Heart Disease and Valvular Pathology
Atherosclerotic coronary artery disease produces a spectrum from stable angina (demand exceeds fixed-stenosis supply, relieved by rest/nitrates) to unstable angina and myocardial infarction (plaque rupture with thrombosis). Cardiac biomarkers (troponin rising within hours and staying elevated 7-10 days) and the evolution of ECG changes (peaked T waves, ST elevation, Q wave formation) are frequently tested alongside the time course of gross and microscopic infarct pathology (coagulative necrosis peaking at 1-3 days, neutrophilic infiltration, and eventual fibrous scar). Valvular disease is tested through the mechanics of murmurs: know which lesions (aortic stenosis, mitral regurgitation, mitral valve prolapse, aortic regurgitation) produce systolic versus diastolic murmurs and how maneuvers (Valsalva, handgrip, squatting) change murmur intensity by altering preload and afterload. Rheumatic heart disease (post-streptococcal, molecular mimicry) and infective endocarditis (vegetations, septic emboli, Janeway lesions/Osler nodes) are classic multisystem tie-ins connecting cardiovascular pathology to immunology and microbiology.
Key terms
- Baroreceptor reflex
- — A rapid autonomic feedback loop in which carotid sinus and aortic arch stretch receptors adjust sympathetic and parasympathetic outflow to buffer acute changes in arterial pressure.
- Frank-Starling mechanism
- — The principle that increased ventricular preload (stretch) increases stroke volume by optimizing sarcomere length, up to a physiologic limit.
- Bohr effect
- — The rightward shift of the oxygen-hemoglobin dissociation curve caused by increased CO2/H+, which lowers hemoglobin's oxygen affinity and promotes tissue unloading.
- A-a gradient
- — The difference between calculated alveolar and measured arterial oxygen tension; it is normal in pure hypoventilation and widened in V/Q mismatch, diffusion limitation, and shunt.
- Shunt physiology
- — Perfusion of unventilated alveoli (e.g., consolidation, ARDS) producing hypoxemia that fails to correct fully with 100% supplemental oxygen.
- DLCO
- — Diffusion capacity of the lung for carbon monoxide; reduced in emphysema and interstitial fibrosis, normal in asthma and pure chest-wall restriction.
- Cor pulmonale
- — Right ventricular hypertrophy and failure secondary to pulmonary hypertension, classically from chronic hypoxic lung disease such as COPD.
- 2,3-Bisphosphoglycerate (2,3-BPG)
- — An erythrocyte metabolite that decreases hemoglobin oxygen affinity (right shift); increased in chronic hypoxia and exercise, decreased in stored blood.
Exam tips
- When a vignette gives a physiologic perturbation (standing, hemorrhage, exercise), work forward step-by-step from stretch/receptor change to afferent firing to autonomic output — do not jump straight to the answer choice that 'sounds autonomic.'
- If hypoxemia does not correct with 100% oxygen, default to shunt (consolidation, ARDS, right-to-left cardiac shunt) rather than V/Q mismatch or diffusion limitation.
- Distinguish loop diuretic (Na-K-2Cl, thick ascending limb), thiazide (Na-Cl, distal convoluted tubule), and carbonic anhydrase inhibitor (proximal tubule) targets — these are frequently cross-tested with cardiovascular volume-overload vignettes.
- For murmur questions, map the maneuver to its hemodynamic effect (Valsalva/standing decrease preload, squatting/handgrip increase afterload/preload) before matching it to a lesion.
- Remember carbon monoxide poisoning both reduces oxygen content and left-shifts the dissociation curve — pulse oximetry can be falsely normal, a classic trap answer.
- When given spirometry or a flow-volume loop, first classify obstructive vs. restrictive by the FEV1/FVC ratio, then use DLCO to narrow the specific diagnosis.