What is Cardiac Output (CO)?

Definition:

• The amount of blood ejected by each ventricle per minute.
• Abbreviations: CO or Q.

Formula:

• Cardiac Output = Stroke Volume × Heart Rate
• Stroke Volume: Volume of blood pumped out of the ventricle with each contraction (beat).  Stroke Volume is also called Ejection Volume).
• Heart Rate: Number of contractions per minute.
• Normal Values for Healthy Adult:
  • Heart Rate at Rest: approximately 70 beats per minute
  • Stroke Volume at Rest: approximately 70mL per beat
  • Resulting Cardiac Output at Rest= HR x SV = 70 x 70 = 4900 mL per minute = 4.9L per minute

Key Terms:

1. Preload:

• The initial stretching of cardiac myocytes before contraction.
• Measured as venous return — the blood volume arriving in the right atrium via superior and inferior vena cava.

2. Afterload:

• The force or resistance the ventricle must overcome to eject blood into the aorta.
• Primarily determined by peripheral resistance in blood vessels.
• Increased afterload (e.g., constricted aorta) strains the heart, making it work harder.
• Decreased afterload (e.g., dilated arteries) eases the workload.

Pressure and Blood Flow:

• Blood flows from high pressure to low pressure.
• Pressure is highest in the left ventricle (~93 mm Hg) during systole.
• Pressure drops through the circulatory system:
  • Aorta: ~120 mm Hg systolic.
  • Capillaries: ~35 mm Hg.
  • Venous system: 18 mm Hg before returning to the right atrium (2 mm Hg).
• Blood flow is driven by this pressure gradient against gravity, with the right ventricle generating ~12 mm Hg for pulmonary circulation.

Influences on Cardiac Output:

1. Heart Rate:

• Controlled by the medulla oblongata via sympathetic and parasympathetic pathways.
• Sympathetic stimulation:
  • Releases epinephrine onto beta-1 adrenergic receptors on the SA node.
  • Speeds up depolarization, increasing heart rate and contraction force (fight or flight).
• Parasympathetic stimulation:
  • Via vagus nerve, releases acetylcholine onto SA node.
  • Opens potassium channels, hyperpolarizing heart cells, slowing heart rate (rest and digest).

2. Stroke Volume:

• Starling’s Law:
  • Increased venous return leads to greater end-diastolic volume (preload) and hence increased stroke volume.
• Contractility:
  • Sympathetic stimulation increases force of contraction, ejecting more blood.
• Afterload:
  • Increased resistance decreases stroke volume as the heart faces more opposition during ejection.

Venous Return & Blood Flow Against Gravity

Venous Return Support:

1. Respiratory Pump:
   • During inhalation, thoracic volume increases, decreasing pressure in veins (like the inferior vena cava), helping “suck” blood back to the heart.
2. Skeletal Muscle Pump:
   • Muscle contractions compress veins; valves prevent backflow, assisting blood return.
3. Valves:
   • Ensure unidirectional blood flow toward the heart, especially important in lower limbs.

Summary:

Cardiac output depends on heart rate and stroke volume, both influenced by neural, hormonal, and mechanical factors. Venous return, blood resistance, and external muscle activity all contribute to maintaining effective circulation. Proper regulation ensures tissues receive adequate oxygen and nutrients, especially during stress or activity.