Cardiac Output Overview

Overview

This lecture explains cardiac output, how it is calculated, and the physiological factors that influence it, including contractility, preload, and afterload.

Cardiac Output and Related Concepts

• Cardiac output is the amount of blood pumped by each ventricle in one minute.
• Cardiac output = stroke volume × heart rate.
• Increasing stroke volume or heart rate increases cardiac output.
• During exercise, heart rate rises to boost cardiac output.

Components of Ventricular Volumes

• The ventricle holds about 100ml of blood at the end of filling (end-diastolic volume, EDV).
• Only about 60ml is ejected per beat (stroke volume).
• Ejection fraction = (stroke volume ÷ EDV) × 100%, typically 60%.
• The remaining 40ml after contraction is the end-systolic volume (ESV).

Factors Affecting Stroke Volume

• Stroke volume = EDV – ESV.
• Stroke volume is determined by contractility, preload, and afterload.

Contractility

• Contractility is the forcefulness of the heart’s contraction.
• Stronger contractions eject more blood from the ventricle.

Preload

• Preload refers to the stretch of cardiac muscle cells at the end of ventricular filling.
• Preload is estimated by the end-diastolic volume (EDV).
• Greater preload (more ventricular stretch) leads to stronger contractions (Frank-Starling mechanism).

Afterload

• Afterload is the resistance the ventricle must overcome to eject blood.
• High vascular pressure (e.g., hypertension) increases afterload and reduces blood ejection.
• Valve damage, such as stenosis, also raises afterload and decreases stroke volume.

Key Terms & Definitions

• Cardiac output — blood volume pumped by each ventricle per minute.
• Stroke volume — blood pumped out of a ventricle in a single heartbeat.
• Heart rate — number of heartbeats per minute.
• End-diastolic volume (EDV) — blood in the ventricle at the end of filling (~100ml).
• End-systolic volume (ESV) — blood left in the ventricle after contraction (~40ml).
• Ejection fraction — percentage of EDV ejected per beat (usually 60%).
• Contractility — strength of the heart’s contraction.
• Preload — degree of stretch in ventricular muscle at the end of filling.
• Afterload — resistance the ventricle must overcome to eject blood.

Action Items / Next Steps

• Review the Frank-Starling mechanism and its impact on cardiac function.
• Prepare for next class by reading about heart valve disorders and their effect on cardiac output.