MRCS Part A Physiology Flashcards

24 sample cards from the Physiology deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. What causes the first and second heart sounds?

   S1 ('lub') is closure of the atrioventricular (mitral and tricuspid) valves at the start of systole; S2 ('dub') is closure of the semilunar (aortic and pulmonary) valves at the start of diastole.

2. Describe the normal cardiac conduction pathway.

   SA node $\rightarrow$ atrial myocardium $\rightarrow$ AV node (delay) $\rightarrow$ bundle of His $\rightarrow$ left and right bundle branches $\rightarrow$ Purkinje fibres $\rightarrow$ ventricular myocardium.

3. What do the P wave, QRS complex and T wave represent on an ECG?

   P wave = atrial depolarization; QRS complex = ventricular depolarization (atrial repolarization is masked); T wave = ventricular repolarization.

4. Which ion is responsible for the plateau phase (phase 2) of the ventricular action potential?

   $\ce{Ca^2+}$ influx through L-type calcium channels balances $\ce{K+}$ efflux, producing the prolonged plateau that gives cardiac muscle its long refractory period.

5. What is the pacemaker potential of the SA node due to?

   A slow spontaneous depolarization (phase 4) driven mainly by the 'funny' current $I_f$ ($\ce{Na+}$ influx) plus $\ce{Ca^2+}$ influx, giving the SA node automaticity (intrinsic rate $\sim100/\text{min}$).

6. State the equation relating flow, pressure and resistance (Ohm's law for the circulation).

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

7. State the Poiseuille equation and the key implication of the radius term.

   $$Q = \frac{\pi \Delta P\, r^{4}}{8 \eta l}$$ Flow is proportional to $r^{4}$, so halving vessel radius reduces flow $16$-fold — radius is the dominant determinant of resistance.

8. How is mean arterial pressure (MAP) calculated?

   $$MAP \approx DBP + \frac{1}{3}(SBP - DBP)$$ i.e. diastolic pressure plus one-third of the pulse pressure. Also $MAP = CO \times SVR$.

9. Trace the pathway of air from the trachea to the alveoli.

   Trachea $\rightarrow$ main (primary) bronchi $\rightarrow$ lobar and segmental bronchi $\rightarrow$ bronchioles $\rightarrow$ terminal bronchioles $\rightarrow$ respiratory bronchioles $\rightarrow$ alveolar ducts $\rightarrow$ alveoli.

10. At what vertebral level does the trachea bifurcate, and why is the right main bronchus clinically important?

    At the carina, around $T4/T5$ (sternal angle). The right main bronchus is wider, shorter and more vertical, so inhaled foreign bodies preferentially lodge on the right.

11. Which muscles are responsible for quiet inspiration and expiration?

    Quiet inspiration is active — mainly the diaphragm (plus external intercostals). Quiet expiration is passive, driven by elastic recoil of the lungs and chest wall.

12. What is the role of pulmonary surfactant?

    Produced by type II pneumocytes, surfactant lowers alveolar surface tension, increasing compliance, preventing alveolar collapse, and stabilizing alveoli of different sizes (per Laplace's law, $P=\frac{2T}{r}$).

13. Define tidal volume, vital capacity and functional residual capacity.

    Tidal volume ($\approx500\,\text{mL}$) = normal breath. Vital capacity = maximal expiration after maximal inspiration ($IRV+TV+ERV$). FRC = volume remaining after normal expiration ($ERV+RV$).

14. State the alveolar gas equation for alveolar $P_{O_2}$.

    $$P_{A}O_{2} = P_{I}O_{2} - \frac{P_{a}CO_{2}}{R}$$ where $P_{I}O_{2}=F_{I}O_{2}(P_{atm}-P_{H_2O})$ and $R\approx0.8$ is the respiratory quotient.

15. Describe Fick's law of diffusion as it applies to gas exchange.

    $$V_{gas} \propto \frac{A \cdot D \cdot (P_1 - P_2)}{T}$$ Diffusion rate is proportional to surface area $A$, the partition/diffusion coefficient $D$, and the partial-pressure gradient, and inversely proportional to membrane thickness $T$.

16. How is the majority of carbon dioxide transported in blood?

    About $70\%$ as bicarbonate (via carbonic anhydrase: $\ce{CO2 + H2O <=> H2CO3 <=> H+ + HCO3-}$), $\sim23\%$ as carbamino compounds bound to haemoglobin, and $\sim7\%$ dissolved.

17. What does a rightward shift of the oxygen–haemoglobin dissociation curve signify, and what causes it?

    Decreased $\ce{O2}$ affinity (easier unloading to tissues). Caused by increased $\ce{CO2}$/$H^{+}$ (Bohr effect), raised temperature, and raised 2,3-BPG.

18. Where are the central and peripheral chemoreceptors and what do they sense?

    Central chemoreceptors (medulla) respond mainly to $\ce{CO2}$ via CSF $H^{+}$; peripheral chemoreceptors (carotid and aortic bodies) respond to low $P_aO_2$, and also to raised $\ce{CO2}$ and $H^{+}$.

19. Which brainstem centres control the basic rhythm of breathing?

    The medullary respiratory centre (dorsal and ventral respiratory groups) sets the basic rhythm; the pontine centres (pneumotaxic and apneustic) fine-tune the rate and depth.

20. Name the meninges from outermost to innermost.

    Dura mater, arachnoid mater, then pia mater. CSF circulates in the subarachnoid space between arachnoid and pia.

21. Where is cerebrospinal fluid produced and how does it circulate?

    Produced by the choroid plexus in the ventricles. Flow: lateral ventricles $\rightarrow$ (interventricular foramina) third ventricle $\rightarrow$ (cerebral aqueduct) fourth ventricle $\rightarrow$ subarachnoid space $\rightarrow$ absorbed by arachnoid granulations into venous sinuses.

22. Distinguish the roles of myelin and the nodes of Ranvier in nerve conduction.

    Myelin (Schwann cells in PNS, oligodendrocytes in CNS) insulates the axon; action potentials jump between nodes of Ranvier (saltatory conduction), greatly increasing conduction velocity.

23. Compare sympathetic and parasympathetic nervous systems by outflow and main neurotransmitters.

    Sympathetic: thoracolumbar ($T1$–$L2$) outflow, short pre-/long postganglionic fibres; postganglionic transmitter is noradrenaline. Parasympathetic: craniosacral outflow, long pre-/short postganglionic; transmitter is acetylcholine (also at all ganglia).

24. Describe the somatosensory pathway for fine touch and proprioception (DCML).

    Dorsal column–medial lemniscus: first-order neuron ascends ipsilaterally to the gracile/cuneate nuclei in the medulla, decussates as internal arcuate fibres, ascends as the medial lemniscus to the VPL of the thalamus, then to the primary somatosensory cortex.

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