Cardiovascular System Physiology - Cardiac Output (stroke volume, heart rate, preload and afterload)

Summary

This video explains cardiac output, defined as the amount of blood ejected by the heart per minute. Cardiac output is determined by heart rate and stroke volume, with normal output ranging between 4-6 liters per minute. Four main factors influence cardiac output: heart rate, preload, afterload, and contractility. Heart rate measures beats per minute, while stroke volume indicates blood pumped per beat. Preload involves blood entering the ventricles, afterload concerns resistance during blood ejection, and contractility reflects myocardium strength. Each factor can be affected by various physiological and chemical influences, including sympathetic stimulation, hypertension, and specific medications.

Takeaways

• 💓 Cardiac output is the amount of blood ejected by the heart in one minute.
• 🧠 Cardiac output is calculated by multiplying heart rate and stroke volume.
• ❤️ Heart rate refers to the number of beats per minute, while stroke volume is the amount of blood pumped by each ventricle per beat.
• 🩸 A typical cardiac output is around 5 liters per minute (4-6 liters is normal).
• 📊 The four main determinants of cardiac output are heart rate, preload, afterload, and contractility.
• ⏳ Chronotropic factors affect heart rate: positive factors like adrenaline increase it, while negative factors like acetylcholine decrease it.
• 🏃 Preload refers to the amount of blood entering the ventricles during diastole, influenced by venous return, blood volume, and atrial contraction.
• 💥 Afterload is the resistance the ventricles must overcome to eject blood, influenced by factors like hypertension and vessel constriction.
• 💪 Contractility is the strength of the heart's contraction, with positive inotropic factors (e.g., noradrenaline) increasing it and negative ones (e.g., beta blockers) decreasing it.
• 🩺 The four determinants work together to regulate the efficiency and output of the heart during circulation.

Q & A

What is cardiac output?

Cardiac output is the amount of blood ejected by the heart in one minute.

How is cardiac output calculated?

Cardiac output is calculated by multiplying stroke volume (amount of blood pumped by each ventricle per beat) by heart rate (number of beats per minute).

What are normal values for stroke volume and heart rate?

A normal stroke volume is about 70 mL per beat, and a normal heart rate is about 75 beats per minute.

What is the normal range of cardiac output in adults?

The normal cardiac output for adults is about 4 to 6 liters per minute.

What are the four determinants of cardiac output?

The four determinants of cardiac output are heart rate, preload, afterload, and contractility.

What are chronotropic factors, and how do they influence heart rate?

Chronotropic factors are elements that influence heart rate. Positive chronotropic factors, such as sympathetic stimulation, increase heart rate, while negative chronotropic factors, like parasympathetic activity, decrease heart rate.

What is preload and how does it affect stroke volume?

Preload is the amount of blood entering the ventricles during diastole (also known as end-diastolic volume). It affects stroke volume because a higher preload means more blood is available to be pumped out, thus increasing stroke volume.

What is afterload, and how does it influence cardiac output?

Afterload is the resistance the ventricles must overcome to circulate blood during systole. Increased afterload, due to factors like hypertension or atherosclerosis, increases resistance and can decrease cardiac output.

What is contractility and how is it regulated?

Contractility refers to the strength of the heart's contraction for a given preload. It is regulated by inotropic factors: positive inotropic agents like sympathetic stimulation increase contractility, while negative inotropic agents like parasympathetic stimulation reduce it.

How do drugs affect cardiac output determinants?

Certain drugs can affect cardiac output by influencing heart rate or contractility. For example, atropine can increase heart rate, while beta-blockers and calcium channel blockers can reduce contractility.