In a physiologist’s isolated rat heart perfused with an oxygenated solution, if the volume entering the heart through the atria is decreased below normal, what happens to the force of contraction and cardiac output?

• A. Increased
• B. The force of contraction and cardiac output decreased
• C. No change
• D. The force of contraction and cardiac output increased

Answer: (B)

Preload (venous return) sets how stretched the cardiac muscle fibers are at the start of contraction. When atrial filling is reduced, end-diastolic volume falls, so the sarcomeres are shorter. According to the Frank–Starling relationship, the heart generates a stronger contraction when fibers are more stretched; with less stretch, the contraction is weaker. In an isolated heart, afterload is relatively fixed by the perfusion setup, so a weaker contraction leads to a smaller stroke volume and thus lower cardiac output. So both the force of contraction and the cardiac output decrease with reduced atrial filling.