State Starling’s law of the heart. Explain its basis and significance.

Statement

Basis

• Length–tension relationship — stretch brings sarcomeres toward the optimal length of 2.2 µm, giving maximum actin–myosin overlap
• Stretch also increases the sensitivity of troponin C to Ca2+
• Cardiac muscle normally operates below its optimal length, so stretch improves performance — unlike skeletal muscle

The Starling Curve

↑ Venous return → ↑ End-diastolic volume (preload) → ↑ Stretch of cardiac fibres → ↑ Force of contraction → ↑ Stroke volume → ↑ Cardiac output

• Ascending limb — output rises with filling
• Plateau — optimal
• Descending limb — overstretch beyond 2.2 µm → overlap falls → output falls

Significance

• Beat-to-beat matching of cardiac output to venous return
• Equalises the output of the two ventricles — the most important function. Any mismatch would flood one circulation
• Adapts to posture and to sudden changes in blood volume
• Compensates for increased afterload
• Acts intrinsically — needs no nerve supply, so it operates in the denervated and the transplanted heart

Laplace Relationship

• A dilated ventricle must generate greater wall tension to produce the same pressure → higher oxygen demand
• This is why dilatation is disadvantageous and hypertrophy is initially compensatory
• It also explains why the descending limb of the Starling curve is reached in failure

Applied Aspects

• Heart failure — the ventricle operates on the descending limb; further dilatation reduces output → a vicious cycle
• Rationale of diuretics in heart failure — reduce preload and bring the ventricle back onto a more favourable part of the curve
• Exercise — increased venous return by the muscle and respiratory pumps raises stroke volume through this mechanism
• Cardiac tamponade and constrictive pericarditis — filling is restricted, so the law cannot operate