❤️ Cardiac Output: How Your Heart Keeps Up the Flow

Cardiac output (CO) is the volume of blood the heart pumps per minute. It is one of the most fundamental concepts in cardiovascular physiology and shows up on every major health science exam.

🩺 Formula:
Cardiac Output = Heart Rate × Stroke Volume

Understanding this concept helps you interpret vital signs, manage hypotension, and grasp how the body compensates during shock.

💡 Two Key Determinants

1️⃣ Heart Rate (HR) – beats per minute

• Controlled by the autonomic nervous system

  • 🧠 Sympathetic: increases HR

  • 🧘‍♂️ Parasympathetic: decreases HR

• Influenced by hormones like epinephrine

2️⃣ Stroke Volume (SV) – ml per beat

• The amount of blood ejected from the ventricle with each contraction

• Regulated by:

  • 🩸 Blood Volume (preload)

  • 🩻 Vascular Resistance (afterload)

🧪 MCAT/USMLE Integration Tip

Cardiac output is central to understanding:

• 🔁 Frank-Starling Mechanism

• 💉 Hemodynamic shock states

• 💊 Drug actions (beta-blockers, inotropes)

• 📊 Vital sign interpretation in clinical vignettes

🧠 Memory Aid

CO = HR × SV
"How Rapid × Squirt Volume" = Cardiac Output

This trick helps remember that increasing either heart rate or stroke volume will increase output (unless the other compensates).

📊 Comparison Table: HR and SV Factors

Factor	Influences	Effect on CO
Nerves	Heart rate	↑ with sympathetic, ↓ with parasympathetic
Hormones (Epinephrine)	Heart rate	↑ via beta-1 receptor stimulation
Blood Volume	Stroke volume	↑ preload = ↑ SV = ↑ CO
Vascular Resistance	Stroke volume	↑ afterload = ↓ SV = ↓ CO

🩺 Clinical Example

A patient with dehydration has ↓ blood volume → ↓ preload → ↓ SV → ↓ CO.
Compensatory: HR ↑ via sympathetic response to maintain CO.

This mechanism frequently appears in shock cases, volume depletion, and sepsis questions.

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