Lecture 2.2a - Control of cardiac output

Cardiac Output

• Cardiac output is the amount of blood pumped from the right or left ventricles per minute, calculated by multiplying heart rate and stroke volume.
• At rest, cardiac output is approximately 5L/minute, increasing to around 25L/minute during exercise.
• The cardiac output should meet the body's demand for oxygen and nutrients.

Heart Rate

• The average heart rate in a healthy adult is 60-80 beats per minute (bpm).
• The sinoatrial (SA) node sets the sinus rhythm, which can be influenced by various factors.
• Factors that increase heart rate (positive chronotropic): • Sympathetic nervous system (adrenaline/noradrenaline) signals • Hormones (T3, T4) increasing metabolism • Body temperature • Increased Ca2+ levels • Hypoxia and hypercapnia (low oxygen and high carbon dioxide)
• Tachycardia is a heart rate above 100bpm, while bradycardia is a heart rate below 60bpm.
• Factors that decrease heart rate (negative chronotropic): • Parasympathetic nervous system (acetylcholine) signals • Hyperkalaemia (increased K+ levels) • Decreased Ca2+ levels

Stroke Volume

• Stroke volume is the amount of blood pumped from the left or right ventricles per beat, calculated by subtracting end-systolic volume (ESV) from end-diastolic volume (EDV).
• The stroke volume is influenced by three factors: • Preload: • Amount of stretch of the ventricles at the end of diastole (EDV) • Factors that alter preload: - Venous return - Skeletal muscle pump - Respiratory pump - Venous tone - Gravity • Contractility: • The Frank-Starling Law, which states that the more the heart stretches, the more it contracts.