USMLE Physiology Syllabus

Physiology syllabus — full chapter and topic list

Expand any chapter to see its topics and sub-topics. This is the whole examinable outline for Physiology in USMLE, not a summary of it.

1. 1

   Cardiovascular System

   3 topics

   • Cardiac cycle
   • Blood flow
   • Blood pressure regulation
2. 2

   Neurophysiology

   3 topics

   • Neuron action potential
   • Synaptic transmission
   • Sensory systems
3. 3

   Renal Physiology

   3 topics

   • Glomerular filtration
   • Tubular function
   • Acid-base balance

Physiology flashcards for USMLE

22 of 51 cards from the Physiology deck — real questions with worked answers.

1. What are the two main phases of the cardiac cycle, and which valves are open during each?

   Systole (ventricular contraction): AV valves (mitral/tricuspid) closed, semilunar valves (aortic/pulmonary) open during ejection. Diastole (ventricular relaxation): semilunar valves closed, AV valves open during filling.

2. During which phase of the cardiac cycle does isovolumetric contraction occur, and what defines it?

   Isovolumetric contraction occurs at the start of systole, after the AV valves close but before the semilunar valves open. All valves are closed, ventricular pressure rises rapidly, and ventricular volume stays constant.

3. What causes the first ($S_1$) and second ($S_2$) heart sounds?

   $S_1$ is caused by closure of the AV valves (mitral and tricuspid) at the start of systole. $S_2$ is caused by closure of the semilunar valves (aortic and pulmonary) at the start of diastole.

4. What does the 'a' wave of the jugular venous pulse (JVP) represent?

   The 'a' wave represents right atrial contraction. It is absent in atrial fibrillation and becomes a large 'cannon a wave' when the atrium contracts against a closed tricuspid valve (e.g., complete heart block).

5. Define stroke volume and state its formula in terms of ventricular volumes.

   Stroke volume (SV) is the volume of blood ejected per beat. $$SV = EDV - ESV$$ where EDV is end-diastolic volume and ESV is end-systolic volume.

6. Write the formula for cardiac output in terms of heart rate and stroke volume.

   $$CO = HR \times SV$$ where CO is cardiac output, HR is heart rate, and SV is stroke volume.

7. Define ejection fraction and give its formula and normal value.

   Ejection fraction (EF) is the fraction of end-diastolic volume ejected per beat. $$EF = \frac{SV}{EDV} = \frac{EDV - ESV}{EDV}$$ Normal EF is $\geq 55\%$.

8. State the Frank-Starling law of the heart.

   The Frank-Starling law states that stroke volume increases in response to an increase in end-diastolic volume (preload). Greater ventricular filling stretches cardiac muscle, increasing contractile force and stroke volume.

9. Define preload and afterload in cardiovascular physiology.

   Preload is the ventricular wall tension/stretch at end-diastole, approximated by end-diastolic volume (venous return). Afterload is the resistance the ventricle must overcome to eject blood, approximated by aortic/arterial pressure (mean arterial pressure).

10. Write the Fick principle equation used to calculate cardiac output.

    $$CO = \frac{VO_{2}}{Ca_{O_2} - Cv_{O_2}}$$ where $VO_2$ is oxygen consumption, and $Ca_{O_2}$ and $Cv_{O_2}$ are the arterial and venous oxygen contents.

11. State the relationship (Ohm's law analog) linking blood flow, pressure gradient, and resistance.

    $$Q = \frac{\Delta P}{R}$$ where $Q$ is flow, $\Delta P$ is the pressure gradient across the vessel, and $R$ is vascular resistance.

12. Write Poiseuille's equation and identify which variable has the greatest effect on flow.

    $$Q = \frac{\pi \Delta P\, r^{4}}{8 \eta L}$$ Flow is proportional to the fourth power of the radius ($r^4$), so vessel radius has by far the greatest effect on flow and resistance.

13. Write the formula for the Reynolds number and state what high values predict.

    $$Re = \frac{\rho v D}{\eta}$$ where $\rho$ is density, $v$ velocity, $D$ diameter, and $\eta$ viscosity. A high Reynolds number (roughly $> 2000$) predicts turbulent flow (e.g., producing murmurs/bruits).

14. In which part of the circulation is velocity of blood flow lowest, and why is this important?

    Velocity is lowest in the capillaries because they have the largest total cross-sectional area. Since flow $=$ velocity $\times$ area is constant, low velocity maximizes time for exchange of gases, nutrients, and wastes.

15. Where is the greatest drop in blood pressure (largest resistance) in the systemic circulation?

    The arterioles account for the greatest pressure drop and are the primary site of resistance. They are the main regulators of systemic vascular resistance and are richly innervated by sympathetic fibers.

16. Write the four Starling forces governing fluid movement across a capillary wall.

    $$J_{v} = K_{f}\left[(P_{c} - P_{i}) - \sigma(\pi_{c} - \pi_{i})\right]$$ Capillary hydrostatic pressure $P_c$ (pushes out), interstitial hydrostatic $P_i$, capillary oncotic $\pi_c$ (pulls in), and interstitial oncotic $\pi_i$.

17. Give the formula for mean arterial pressure (MAP) in terms of systolic and diastolic pressures.

    $$MAP \approx DBP + \frac{1}{3}(SBP - DBP)$$ or equivalently $MAP = \frac{SBP + 2\,DBP}{3}$, where the diastolic term is weighted because diastole occupies about two-thirds of the cardiac cycle.

18. Write MAP in terms of cardiac output and total peripheral resistance.

    $$MAP = CO \times TPR$$ where CO is cardiac output and TPR is total peripheral resistance. (More precisely, $MAP = CO \times TPR + CVP$, but central venous pressure is usually negligible.)

19. Describe the baroreceptor reflex response to a sudden drop in blood pressure.

    Decreased BP reduces stretch on carotid sinus/aortic arch baroreceptors, decreasing afferent firing. This increases sympathetic and decreases parasympathetic output, raising heart rate, contractility, and vasoconstriction (TPR), restoring BP.

20. Which cranial nerves carry afferent signals from the carotid sinus and aortic arch baroreceptors?

    The carotid sinus is innervated by the glossopharyngeal nerve (CN IX, via the nerve of Hering). The aortic arch is innervated by the vagus nerve (CN X).

21. Outline the renin-angiotensin-aldosterone system (RAAS) cascade.

    Low BP/low renal perfusion triggers renin from juxtaglomerular cells. Renin converts angiotensinogen to angiotensin I; ACE (lung) converts angiotensin I to angiotensin II. Angiotensin II causes vasoconstriction and stimulates aldosterone release, increasing $\ce{Na+}$/water retention.

22. What are the major actions of angiotensin II?

    Potent vasoconstriction (raises TPR), stimulates aldosterone (increases $\ce{Na+}$ reabsorption), stimulates ADH release and thirst, increases proximal tubule $\ce{Na+}$/$\ce{H2O}$ reabsorption, and constricts efferent arteriole to maintain GFR.

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