Physiology — High-Yield — USMLE Step 1 Notes
Free, high-yield revision notes for USMLE Step 1. Read here, or drill the same material as questions and flashcards in the app.
These are high-yield revision notes written for first-order recall. A revision aid, not editorially reviewed and not clinical advice — verify against a primary source before relying on anything clinically.
Cardiovascular
- Cardiac output = stroke volume × heart rate; SV set by preload, afterload, contractility
- Frank-Starling: increased preload (venous return) increases stroke volume
- Pulse pressure ∝ stroke volume; widens in aortic regurgitation, narrows in tamponade/failure
- Baroreceptor reflex: ↑BP → carotid sinus stretch → ↓sympathetic → ↓HR
Respiratory
- Oxygen-dissociation curve: right shift (unload O2) with ↑CO2, ↑temp, ↑2,3-BPG, ↓pH (Bohr)
- Left shift: CO, fetal haemoglobin, methaemoglobin, alkalosis, cold
- A-a gradient: normal in hypoventilation and low FiO2; raised in V/Q mismatch, shunt, diffusion limit
- Shunt does not correct with 100% oxygen; V/Q mismatch does
Renal & acid-base
- Anion gap = Na − (Cl + HCO3); high-gap acidosis = MUDPILES
- Winter's formula: expected PaCO2 = 1.5×HCO3 + 8 (±2)
- Aldosterone: retains sodium and water, excretes potassium and hydrogen
- ADH: inserts aquaporins in the collecting duct (water reabsorption)
Endocrine signalling
- cAMP hormones (FLAT ChAMP): FSH, LH, ACTH, TSH, CRH, hCG, ADH(V2), MSH, PTH, calcitonin, GHRH, glucagon
- Steroid/thyroid hormones act on intracellular receptors (lipophilic)
- Insulin and growth factors use receptor tyrosine kinase (MAP kinase pathway)
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