❤️ Hemodynamics: The Physics of Blood Pressure
Blood pressure is not magic; it is pure physics. At its core, it is governed by a single, master equation:
Mean Arterial Pressure (MAP) = Cardiac Output (CO) × Total Peripheral Resistance (TPR)
1. Cardiac Output (CO)
This is the volume of blood the heart pumps per minute. It is the product of:
CO = Stroke Volume (SV) × Heart Rate (HR)
Factors affecting Stroke Volume (SV):
- Preload: Venous return / blood volume. (Frank-Starling Law: More stretch = stronger squeeze).
- Contractility: How hard the heart muscle squeezes (influenced by sympathetic tone, calcium, and thyroid hormone).
- Afterload: The resistance the heart must push against (high afterload lowers SV).
| Condition | Effect on SV | Mechanism |
|---|---|---|
| Hypervolemia (Fluid overload) | ⬆ Increases | High preload stretches the ventricle. |
| Hyperthyroidism | ⬆ Increases | High contractility (Beta-receptor upregulation). |
| Heart Failure (HFrEF) | ⬇ Decreases | Weak myocardium cannot squeeze effectively. |
| Aortic Stenosis | ⬇ Decreases | High afterload blocks ejection. |
2. Total Peripheral Resistance (TPR)
This is the resistance to flow offered by the arterioles. It is governed by Poiseuille's Law:
Resistance ∝ 1 / r4 (where r = radius of the arteriole)
Clinical Pearl: A tiny 20% decrease in arteriolar radius increases resistance by 144%. This is why vascular remodeling is so devastating.
3. The Clinical Prediction
| Presentation | Physiological Prediction | Likely Cause |
|---|---|---|
| High BP + High HR (Tachycardia) | High Cardiac Output | Hyperthyroidism, Anemia, Sympathetic overdrive. |
| High BP + Normal/Slow HR | High TPR | Essential HTN, Renal stenosis, Vascular remodeling. |
| Wide Pulse Pressure (e.g., 180/70) | Stiff Aorta (Loss of elastin) | Isolated Systolic HTN (Aging). |