Cardiac Output, Stroke volume, EDV, ESV, Ejection Fraction

Summary

Cardiac output, the volume of blood pumped per minute, is determined by stroke volume and heart rate. An increase in either raises cardiac output, as seen during exercise. The heart doesn't fully eject its blood; ejection fraction is typically 60%. Stroke volume is influenced by contractility, preload, and afterload. Preload, analogous to a balloon's stretch, affects the force of contraction, while afterload, including vascular pressure and valve damage, represents the resistance the ventricles must overcome to pump blood.

Takeaways

• 💓 Cardiac output is the volume of blood pumped per ventricle per minute, calculated by multiplying stroke volume and heart rate.
• 🏃‍♂️ An increase in either stroke volume or heart rate leads to an increase in cardiac output, which is essential during physical activities.
• 🚫 Ventricles do not eject all the blood during one heartbeat, with an average ejection fraction of 60%.
• 🔢 The end-diastolic volume (EDV) is the amount of blood in a ventricle at the end of its load, typically around 100ml.
• 🔄 The end-systolic volume (ESV) is the remaining blood in the ventricle after contraction, which is 40ml in the given example.
• ➖ Stroke volume is determined by subtracting ESV from EDV and is influenced by three main factors.
• 💪 Contractility is the force of the heart muscle contraction, with stronger contractions ejecting more blood.
• 🎈 Preload is associated with EDV and represents the stretch of cardiac myocytes at the end of ventricular filling, affecting the force of contraction.
• 🚫 Afterload is the resistance the ventricle faces to eject blood, including vascular pressure and valve conditions.
• 🌡 High vascular pressure, as in hypertension, increases afterload, making it harder for the heart to eject blood.
• 🛠️ Valve damage, such as stenosis, also increases afterload and can reduce the amount of blood ejected by the heart.

Q & A

What is cardiac output?

Cardiac output is the volume of blood pumped by each ventricle of the heart in one minute, calculated as the product of stroke volume and heart rate.

What is stroke volume and how is it related to cardiac output?

Stroke volume is the amount of blood pumped in one heartbeat. It directly affects cardiac output, as cardiac output is the product of stroke volume and heart rate.

How does heart rate influence cardiac output?

Heart rate, which is the number of beats in one minute, influences cardiac output. An increase in heart rate results in increased cardiac output, and a decrease would result in decreased output.

What happens to cardiac output during physical exercise?

During physical exercise, the heart beats faster to meet the body's increased demand for blood, thus increasing cardiac output.

Why don't ventricles eject all the blood they contain in one beat?

Ventricles do not eject all the blood due to the presence of an end-systolic volume (ESV), which is the remaining volume of blood in the ventricle after contraction.

What is the ejection fraction and how is it calculated?

The ejection fraction is the percentage of blood that is ejected from the ventricle during contraction. It is calculated as (end-diastolic volume - end-systolic volume) / end-diastolic volume.

What is the end-diastolic volume (EDV) and its significance?

End-diastolic volume (EDV) is the volume of blood in the ventricle at the end of its filling phase, before contraction. It is significant because it influences the preload and, through the Frank-Starling mechanism, the force of contraction.

What is preload in the context of cardiac function?

Preload refers to the degree of stretch of cardiac myocytes at the end of ventricular filling, often represented by the end-diastolic volume. It affects the force of the heart muscle contraction.

What is the Frank-Starling mechanism and how does it relate to preload?

The Frank-Starling mechanism states that the greater the stretch of the heart muscle (preload), the greater the force of contraction, allowing the heart to pump more blood with each beat.

What is afterload and how does it affect the heart's ability to eject blood?

Afterload is the resistance the ventricle must overcome to eject blood, including vascular pressure and any damage to the valves. Increased afterload, such as in hypertension or valve stenosis, makes it more difficult for the heart to eject blood.

How does hypertension affect cardiac output?

In hypertension, the higher vascular pressure increases afterload, making it more difficult for the heart to open the aortic and pulmonary valves, which can result in reduced cardiac output.