Cardiac Output | Preload and Afterload EXPLAINED!

Summary

In this video, Dr. Mike Tadarovich explains the concept of cardiac output, which is the volume of blood ejected from the heart per minute. He breaks down the factors that contribute to cardiac output, including stroke volume (the amount of blood ejected per contraction) and heart rate (beats per minute). Dr. Tadarovich highlights how venous return, heart muscle contraction (affected by calcium), and afterload (resistance in arteries) all influence stroke volume. He also covers how the autonomic nervous system affects heart rate. This informative video clarifies the key components of cardiac output and their physiological interactions.

Takeaways

• 😀 Cardiac output (CO) is the volume of blood pumped by the heart per minute and is a key concept in cardiology.
• 😀 Cardiac output is determined by two factors: Stroke Volume (SV) and Heart Rate (HR).
• 😀 Stroke volume (SV) is the amount of blood ejected from the heart with each contraction, typically around 70 milliliters per beat.
• 😀 Heart rate (HR) is the number of times the heart beats per minute, which is around 70 beats per minute on average.
• 😀 The average cardiac output is 4.9 liters per minute (calculated as 70 ml stroke volume × 70 beats per minute).
• 😀 Stroke volume can be influenced by venous return, heart contraction, and afterload (the resistance blood faces when leaving the heart).
• 😀 Venous return is the amount of blood returning to the heart and can be increased by physical activity, which enhances blood flow back to the heart.
• 😀 Preload refers to the stretch of the heart's walls as it fills with blood, and this stretch directly influences stroke volume via the Frank-Starling mechanism.
• 😀 Ionotropic agents affect heart contraction: positive agents increase calcium availability, improving contraction, while negative agents decrease calcium, reducing contraction strength.
• 😀 Afterload is the resistance blood faces as it exits the heart. Increased afterload (e.g., due to narrow arteries) reduces stroke volume and cardiac output.
• 😀 Heart rate is controlled by the autonomic nervous system, with the sympathetic nervous system increasing heart rate and the parasympathetic nervous system decreasing it.

Q & A

What is cardiac output, and why is it important?

Cardiac output is the volume of blood ejected from the heart per minute. It's important because it directly influences blood pressure, which is a major factor in cardiovascular health and can be a critical contributor to life-threatening conditions.

What factors contribute to cardiac output?

Cardiac output is determined by two main factors: stroke volume (the volume of blood ejected per heart contraction) and heart rate (the number of heartbeats per minute). The equation is: Cardiac Output = Stroke Volume × Heart Rate.

What is stroke volume, and how is it calculated?

Stroke volume is the amount of blood ejected from the heart per contraction. It can be calculated by subtracting the end-systolic volume (the amount of blood remaining after contraction) from the end-diastolic volume (the amount of blood in the heart at the end of relaxation). For example, 130 ml (EDV) - 60 ml (ESV) = 70 ml (stroke volume).

What is the average stroke volume in a healthy person?

On average, the stroke volume is around 70 milliliters, but this can vary depending on the individual.

How does heart rate affect cardiac output?

Heart rate is the number of heartbeats per minute. A higher heart rate increases cardiac output, and a lower heart rate decreases cardiac output. The average heart rate is typically around 70 beats per minute.

How do stroke volume and heart rate work together to determine cardiac output?

Cardiac output is the product of stroke volume and heart rate. For example, with a stroke volume of 70 ml and a heart rate of 70 beats per minute, cardiac output would be 4.9 liters per minute (70 × 70 = 4.9L).

What factors can affect stroke volume?

Three primary factors affect stroke volume: venous return (the amount of blood returning to the heart), the contraction of the heart (which relies on calcium availability), and afterload (the resistance the blood faces as it leaves the heart).

What is venous return, and how does it influence stroke volume?

Venous return refers to the amount of blood returning to the heart via the veins. An increase in venous return, such as during exercise, increases stroke volume by filling the heart more completely, which improves cardiac output.

What is preload, and how does it relate to venous return?

Preload is the stretch of the heart muscle fibers due to the volume of blood filling the heart during diastole (relaxation). It is directly related to venous return, as more blood returning to the heart leads to a greater preload, which in turn increases stroke volume.

What role does afterload play in stroke volume?

Afterload is the resistance the blood encounters as it is ejected from the heart. If afterload is increased (e.g., due to narrowed arteries), stroke volume decreases, because the heart has to work harder to eject the blood. Conversely, reducing afterload (e.g., through vasodilation) can increase stroke volume.