What is the effect of increased calcium in the sarcoplasm on stroke volume?

• A. Increased calcium enhances contractility, leading to increased stroke volume.
• B. Increased calcium reduces contractility, decreasing stroke volume.
• C. Calcium in the sarcoplasm has no effect on stroke volume.
• D. Increased calcium increases afterload.

Answer: (A)

The main idea is that cytosolic (sarcoplasmic) calcium directly strengthens cardiac contraction, which increases the amount of blood ejected with each beat. When calcium rises in the sarcoplasm, it binds to troponin C, moving the tropomyosin blockade away from actin’s binding sites. This allows more cross-bridge formation between actin and myosin, producing a stronger systolic squeeze (positive inotropy). With a stronger contraction, the ventricle ejects more blood, so stroke volume goes up, assuming preload and afterload stay the same.

Calcium does not mainly change afterload, and it does affect contractility, so the other ideas don’t fit the mechanism as well.