A dog experiment shows that acutely raising aortic pressure by clamping the aorta transiently reduces stroke volume, but over several minutes stroke volume returns toward baseline despite the elevated afterload. This delayed recovery is best explained by:

• A Baroreceptor-mediated reduction in systemic vascular resistance alone
• B The Bowditch staircase phenomenon driven by increased heart rate
• C Increased venous return secondary to skeletal muscle pumping
• D The Anrep effect, an increase in ventricular contractility independent of fibre length ✓

Explanation

The Anrep effect is homeometric autoregulation: a sustained rise in afterload produces a gradual increase in contractility over minutes, restoring stroke volume without any change in end-diastolic fibre length. It is thought to involve stretch-induced release of angiotensin II and other local mediators. The Bowditch effect requires a rate change, which does not occur here, and baroreflex changes alone cannot restore stroke volume against a fixed higher load.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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