Overview
This lecture explains cardiac output, how it is calculated, and the factors that affect it, including preload, contractility, and afterload.
Definition and Formula of Cardiac Output
- Cardiac output (CO) is the amount of blood pumped by the left ventricle in one minute.
- CO measures blood sent to body tissues per minute, providing them with oxygen and nutrients.
- The formula: Cardiac Output = Heart Rate (HR) × Stroke Volume (SV).
- Heart Rate is the number of heartbeats per minute.
- Stroke Volume is the amount of blood ejected from the left ventricle with each contraction.
Factors Influencing Cardiac Output
- Anything that alters heart rate or stroke volume will change cardiac output.
- Increased heart rate (e.g., during exercise) raises cardiac output; decreased heart rate (e.g., at rest) lowers it.
- Stroke volume is calculated as SV = End Diastolic Volume (EDV) − End Systolic Volume (ESV).
- EDV is the blood in the left ventricle at the end of diastole (filling); ESV is the blood left in the ventricle after systole (contraction).
Preload, Contractility, and Afterload
- Preload is determined by venous return; higher preload increases EDV, raising stroke volume and cardiac output.
- Contractility is the force of heart contraction; increased contractility lowers ESV, raising stroke volume and cardiac output.
- Afterload is the back pressure the heart must overcome to eject blood; higher afterload (as in hypertension) increases ESV, lowering stroke volume and cardiac output.
Effects of Preload, Contractility, and Afterload on Cardiac Output
- Increased preload (more blood returning to heart) → increased EDV → increased SV → increased CO.
- Decreased preload → decreased EDV → decreased SV → decreased CO.
- Increased contractility (stronger heart squeeze) → decreased ESV → increased SV → increased CO.
- Decreased contractility → increased ESV → decreased SV → decreased CO.
- Increased afterload (e.g., high blood pressure) → increased ESV → decreased SV → decreased CO.
- Decreased afterload (normal/lowered blood pressure) → decreased ESV → increased SV → increased CO.
Key Terms & Definitions
- Cardiac Output (CO) — Blood volume pumped by left ventricle per minute.
- Heart Rate (HR) — Beats per minute.
- Stroke Volume (SV) — Blood pumped per beat by left ventricle.
- End Diastolic Volume (EDV) — Blood volume in left ventricle at end of diastole.
- End Systolic Volume (ESV) — Blood volume in left ventricle at end of systole.
- Preload — The degree of stretch/volume from venous return before contraction.
- Contractility — Strength of heart muscle contraction.
- Afterload — Resistance/back pressure the heart must overcome to eject blood.
Action Items / Next Steps
- Review formulas for cardiac output and stroke volume.
- Prepare to discuss how hypertension affects afterload and cardiac output in upcoming blood pressure exercises.