Overview and Clinical Importance
Cardiovascular physiology forms the foundation for understanding heart disease, shock, anesthesia monitoring, and fluid therapy across all veterinary species. This guide covers four essential areas: the cardiac cycle and heart sounds (correlating mechanical and electrical events), cardiac output regulation (Frank-Starling mechanism and determinants of performance), blood pressure regulation (autonomic and hormonal control), and hemostasis and the coagulation cascade (primary, secondary, and tertiary hemostasis). Mastery of these concepts is essential for clinical decision-making in emergency medicine, surgery, anesthesia, and internal medicine.
High-YieldThe BCSE frequently tests the integration of cardiovascular physiology with pharmacology. Know how drugs affect preload, afterload, contractility, and heart rate. Common tested scenarios include anesthetic drug effects on cardiac output and antiarrhythmic mechanisms.
| Phase |
Valve Status |
Key Events |
| Atrial Systole |
AV valves open; Semilunar valves closed |
P wave on ECG. Atrial contraction contributes 10-30% of ventricular filling (atrial kick). More important at faster heart rates. |
| Isovolumetric Contraction |
All valves closed |
QRS complex on ECG. Ventricular pressure rises rapidly. S1 heart sound produced (AV valve closure). Volume constant. |
| Rapid Ejection |
Semilunar valves open; AV valves closed |
Ventricular pressure exceeds aortic/pulmonary pressure. Maximum aortic flow velocity. Two-thirds of stroke volume ejected. |
| Reduced Ejection |
Semilunar valves open; AV valves closed |
T wave on ECG. Ventricular repolarization. Remaining one-third of stroke volume ejected. |
| Isovolumetric Relaxation |
All valves closed |
Ventricular pressure falls rapidly. S2 heart sound produced (semilunar valve closure). End of T wave. |
| Rapid Ventricular Filling |
AV valves open; Semilunar valves closed |
Ventricular pressure falls below atrial pressure. 70-80% of ventricular filling occurs passively. S3 may be heard. |
| Slow Ventricular Filling (Diastasis) |
AV valves open; Semilunar valves closed |
Minimal pressure gradient between atria and ventricles. Atria and ventricles fill together from venous return. |
| Sound |
Timing |
Cause |
Clinical Significance |
| S1 (Lub) |
Beginning of systole; coincides with QRS |
Closure of AV valves (mitral and tricuspid). Mitral component louder due to higher left-sided pressures. |
Loudest at apex (mitral area). Duration approximately 0.15 seconds at 25-45 Hz. |
| S2 (Dub) |
End of systole; follows T wave |
Closure of semilunar valves (aortic and pulmonic). Marks onset of diastole. |
Loudest at base. Duration 0.12 seconds at approximately 50 Hz. Physiologic splitting normal on inspiration. |
| S3 |
Early diastole; 0.1-0.2 seconds after S2 |
Rapid ventricular filling causing ventricular wall vibration. |
Normal in young animals. Pathologic in older animals - indicates volume overload or ventricular dysfunction (gallop rhythm). |
| S4 |
Late diastole; just before S1 |
Atrial contraction against a stiff or hypertrophied ventricle. |
Always pathologic. Indicates diastolic dysfunction or decreased ventricular compliance. Low-pitched; best heard with bell. |
Section 1: Cardiac Cycle and Heart Sounds
The cardiac cycle represents the sequence of electrical and mechanical events from one heartbeat to the next. Understanding this cycle is essential for interpreting ECGs, heart sounds, and hemodynamic parameters.
Phases of the Cardiac Cycle
The cardiac cycle consists of two main periods: systole (ventricular contraction and ejection) and diastole (ventricular relaxation and filling). These are further divided into distinct phases.