Explain the Frank Starling mechanism of the heart and its clinical relevance in heart failure. (Exaplain meachanism in easy language ) b) Describe the determinants of cardiac output and their regulation. [
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Frank-Starling curve stroke volume versus end-diastolic volume diagram
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A. Frank-Starling mechanism of the heart
Simple meaning
The more blood that enters the heart, the more strongly the heart pumps out, within normal limits.
Think of the heart muscle like a rubber band:
- If it is filled a little more, its muscle fibres stretch a little more.
- A slightly stretched fibre contracts more strongly.
- Therefore, the heart ejects more blood with the next beat.
This is the Frank-Starling law:
Stroke volume is directly related to end-diastolic volume.
- End-diastolic volume (EDV): amount of blood inside a ventricle after it has filled, just before contraction.
- Stroke volume (SV): amount of blood pumped out by one ventricle in one beat.
- Greater venous return -> greater EDV, also called preload -> greater SV.
Mechanism step-by-step
- More blood returns to the heart through veins.
- The ventricles fill more during diastole.
- Ventricular muscle fibres, called sarcomeres, are stretched.
- This stretch improves interaction between actin and myosin contractile proteins and increases the muscle's sensitivity to calcium.
- The next ventricular contraction is stronger.
- More blood is ejected, so stroke volume and cardiac output rise.
[
\text{Venous return} \uparrow \rightarrow \text{Preload/EDV} \uparrow \rightarrow \text{Stretch} \uparrow \rightarrow \text{Force of contraction} \uparrow \rightarrow \text{Stroke volume} \uparrow
]
The mechanism works automatically inside the heart. It does not need nerve stimulation to start.
Why it is useful normally
It helps the heart pump out nearly the same volume that it receives.
For example, during exercise:
- More blood returns from muscles to the heart.
- Ventricles fill more.
- The heart pumps more blood per beat.
It also maintains equality of right- and left-sided cardiac output. If the right ventricle temporarily sends more blood to the lungs, the left ventricle soon receives more blood and increases its own output. This prevents blood from accumulating in either circulation. The relation between filling volume and stroke volume is described in Costanzo Physiology.
Limit of the mechanism
The rule applies only up to a limit.
At very high ventricular filling volumes, further stretching no longer gives a useful increase in force. The curve flattens. Excess stretch may instead raise filling pressure and cause congestion.
Clinical relevance in heart failure
1. Early compensation
In systolic heart failure, the ventricle contracts weakly. Less blood is ejected, so more blood remains inside the ventricle after systole.
This causes:
- Increased ventricular volume
- Increased preload
- More fibre stretch
- Temporary increase in stroke volume through the Frank-Starling mechanism
Thus, initially, the body partly maintains cardiac output. This is called compensated heart failure.
[
\text{Weak contraction} \rightarrow \text{more residual blood} \rightarrow \text{preload} \uparrow \rightarrow \text{temporary SV improvement}
]
2. Why the same mechanism becomes harmful later
With worsening heart failure:
- Contractility falls further.
- The Frank-Starling curve shifts downward.
- The ventricle is already on the flat part of the curve.
- Further increase in preload produces little or no increase in stroke volume.
However, the higher filling pressure is transmitted backward:
- Left heart failure -> raised left atrial and pulmonary venous pressure -> pulmonary congestion and pulmonary edema
- Right heart failure -> raised systemic venous pressure -> pedal edema, ascites, hepatomegaly
So, in advanced heart failure:
[
\text{Preload} \uparrow\uparrow \not\Rightarrow \text{Cardiac output} \uparrow
]
Instead:
[
\text{Preload} \uparrow\uparrow \Rightarrow \text{congestion and edema}
]
Textbook descriptions similarly note that a failing congested heart may operate on the flat part of the relation, where extra filling does not generate enough extra force and promotes pulmonary or peripheral edema. Goodman and Gilman's explanation is consistent with this principle.
3. Treatment implication
Many heart-failure treatments reduce excessive preload:
- Diuretics remove sodium and water.
- Venodilators, such as nitrates in selected settings, reduce venous return.
- RAAS-blocking drugs reduce salt and water retention over time.
These treatments move the patient away from excessive filling pressures, relieving breathlessness and edema. The goal is not to remove all preload, because the heart still needs adequate filling to pump effectively.
B. Determinants of cardiac output and their regulation
Definition
Cardiac output (CO) is the amount of blood pumped by each ventricle per minute.
[
\boxed{\text{CO} = \text{Heart rate (HR)} \times \text{Stroke volume (SV)}}
]
Example:
[
70\ \text{beats/min} \times 70\ \text{mL/beat} = 4900\ \text{mL/min}
]
So normal resting cardiac output is approximately 5 L/min in an average adult.
Main determinants
Cardiac output depends on:
- Heart rate
- Stroke volume
Stroke volume is determined mainly by:
- Preload
- Contractility
- Afterload
[
\boxed{\text{CO} = \text{HR} \times \text{SV}}
]
[
\boxed{\text{SV is influenced by preload, contractility, and afterload}}
]
| Determinant | Simple meaning | Effect when it increases |
|---|---|---|
| Heart rate | Beats per minute | CO rises initially |
| Preload | Filling/stretch of ventricle before contraction | SV rises, within limits |
| Contractility | Strength of contraction independent of filling | SV rises |
| Afterload | Resistance or pressure the ventricle pumps against | SV falls |
1. Heart rate
Definition
Number of heartbeats per minute.
Effect on cardiac output
- Moderate rise in HR -> CO rises.
- Very high HR -> diastole becomes too short.
- Less time for ventricular filling -> preload and SV fall.
- Therefore, extremely fast tachycardia can decrease cardiac output.
Regulation
Sympathetic stimulation
- Increases HR.
- Noradrenaline acts mainly on cardiac beta-1 receptors.
- Occurs during exercise, stress, fear, hemorrhage, and low blood pressure.
Parasympathetic stimulation
- Vagus nerve decreases HR.
- Acetylcholine acts mainly on M2 receptors at the sinoatrial node.
- Important at rest and during sleep.
Other factors
- Fever, thyroxine, anemia, and exercise increase HR.
- Hypothermia and some drugs, such as beta-blockers, decrease HR.
2. Preload
Definition
Preload is the degree of ventricular filling and myocardial fibre stretch just before systole.
Clinically, it is related to:
- Ventricular end-diastolic volume
- Ventricular end-diastolic pressure
- Venous return
Effect on cardiac output
- Increased preload -> increased stretch -> increased SV by Frank-Starling mechanism.
- Decreased preload -> reduced filling -> reduced SV and CO.
Factors increasing preload
- Increased venous return
- Increased blood volume
- Venoconstriction due to sympathetic stimulation
- Skeletal muscle pump during exercise
- Lying down
- Slow heart rate, because filling time is increased
- Fluid infusion
Factors decreasing preload
- Hemorrhage
- Dehydration
- Diuretics
- Venodilation
- Standing suddenly, with pooling of blood in the legs
- Positive-pressure ventilation
Regulation of preload
Preload is mainly regulated by:
- Venous tone
- Total blood volume
- Renal sodium and water balance
- Skeletal muscle pump
- Respiratory pump
- Posture
The sympathetic nervous system raises venous tone. The kidneys regulate blood volume through the renin-angiotensin-aldosterone system, ADH, and natriuretic peptides.
3. Contractility
Definition
Contractility is the inherent strength of myocardial contraction at a given preload and afterload.
It is also called the inotropic state of the heart.
Effect on cardiac output
- Increased contractility -> more complete ejection -> decreased end-systolic volume -> increased SV and CO.
- Reduced contractility -> less ejection -> increased residual ventricular volume -> reduced SV and CO.
Factors increasing contractility: positive inotropes
- Sympathetic stimulation
- Circulating adrenaline and noradrenaline
- Exercise
- Increased intracellular calcium
- Drugs such as digoxin and, in selected acute settings, dobutamine
Factors decreasing contractility: negative inotropes
- Myocardial ischemia or infarction
- Heart failure
- Hypoxia
- Acidosis
- Severe electrolyte abnormalities
- Some drugs, such as beta-blockers and certain calcium-channel blockers
Regulation
The main regulator is sympathetic stimulation:
[
\text{Sympathetic activity} \uparrow \rightarrow \text{Ca}^{2+} \text{ availability} \uparrow \rightarrow \text{contractility} \uparrow
]
This increases stroke volume and cardiac output.
4. Afterload
Definition
Afterload is the force or pressure the ventricle must overcome to eject blood.
For the left ventricle, afterload mainly depends on:
- Aortic pressure
- Systemic vascular resistance
- Aortic valve stenosis
- Ventricular wall stress
Effect on cardiac output
- Increased afterload -> heart has to pump against greater resistance -> less blood is ejected -> SV and CO decrease.
- Reduced afterload -> easier ejection -> SV and CO increase.
Factors increasing afterload
- Hypertension
- Vasoconstriction
- Aortic stenosis
- Increased systemic vascular resistance
Factors decreasing afterload
- Arteriolar vasodilation
- Reduced systemic vascular resistance
- Drugs such as ACE inhibitors, ARBs, ARNI therapy, and other vasodilators in appropriate patients
Regulation
Afterload is regulated mainly by arteriolar tone, which is influenced by:
- Sympathetic nervous system
- Angiotensin II
- Local tissue metabolites
- Nitric oxide
- Blood pressure control reflexes
The three major influences on stroke volume are preload, afterload, and contractility, as summarized in Harrison's cardiovascular physiology overview.
Integrated regulation during exercise
During exercise:
- Sympathetic activity increases.
- HR increases.
- Contractility increases.
- Skeletal muscles push more venous blood back to the heart.
- Preload and SV increase.
- Vasodilation in active muscles reduces resistance in their blood vessels.
Therefore, cardiac output rises markedly to meet increased oxygen demand.
One-line exam summary
[
\boxed{\text{Cardiac output} = \text{Heart rate} \times \text{Stroke volume}}
]
[
\boxed{\text{Stroke volume depends on preload, contractility, and afterload}}
]
- Preload increases SV through the Frank-Starling mechanism.
- Contractility increases SV.
- Afterload decreases SV.
- Heart rate increases CO only until it becomes so fast that ventricular filling is reduced.
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