The Frank-Starling Law of the heart states that:

RML Lucknow - 2021

Applied Physiology

Easy

Appeared in: RML Lucknow - 2021

Explanation

The Frank-Starling law describes the heart's intrinsic ability to adapt its output (stroke volume) to its input (venous return).
It states that the stroke volume increases in response to a greater stretch on the heart muscle fibers before contraction.
This stretch is caused by an increased volume of blood in the ventricles at the end of diastole (end-diastolic volume or preload).
A greater stretch leads to a more forceful contraction, which ejects the increased volume of blood, thus matching output to input.

Option A: This statement describes the baroreceptor reflex, which is a neural response to changes in blood pressure, not the Frank-Starling law, which is an intrinsic property of the heart muscle.
Option C: In a healthy circulatory system, the right and left ventricles must pump equal volumes of blood over time to prevent fluid accumulation in either the pulmonary or systemic circuits.
Option D: This is the direct opposite of the Frank-Starling principle. The law fundamentally states that cardiac output is highly dependent on venous return, as venous return determines the end-diastolic volume and subsequent stretch.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Frank-Starling Law of the heart as background academic context rather than a clinical decision trigger.
Understanding the Frank-Starling law is crucial for nurses when administering IV fluids. Increasing a patient's blood volume (preload) can increase their stroke volume and cardiac output, which is beneficial in cases of hypovolemia.
The law also explains the pathophysiology of decompensated heart failure. When the ventricle is chronically over-stretched, it moves to the descending limb of the Starling curve, where further stretching leads to a weaker contraction and worsening heart failure.
What if? - If a patient is in cardiogenic shock with pulmonary edema, their heart is already over-filled and on the flat/descending part of the Starling curve. In this case, giving more IV fluids (increasing preload) would not improve cardiac output and would worsen the pulmonary congestion. Instead, treatments would focus on reducing preload (diuretics) and afterload, and increasing contractility (inotropes).

How to Approach the Question

Identify the key term in the question stem: 'Frank-Starling Law'.
This is a factual recall question that asks for the definition of a core physiological principle.
Recall that the law connects the stretching of the heart muscle to the force of its contraction.
Analyze the options to find the one that links a measure of stretch (end-diastolic volume) to a measure of output (stroke volume).
Option B directly states this relationship: increased end-diastolic volume (stretch) leads to increased stroke volume.
Eliminate the other options by identifying them as other concepts (baroreceptor reflex) or as physiologically incorrect statements.

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