Heart Function: Dual Circulation & Blood Pressure Regulation

The heart is a dual pump serving two serial circulations. Its coordinated action maintains perfusion pressure and matches output to metabolic demand.

1. Pulmonary Circulation (Right Heart)

• Path: Systemic veins → RA → RV → Pulmonary Artery → Lungs (gas exchange) → Pulmonary Veins → LA.
• Pressure: Low (mean PAP ~15 mmHg). RV is thin-walled, volume pump.
• Function: Oxygenate blood; remove CO₂. High compliance accommodates entire cardiac output at low pressure.

2. Systemic Circulation (Left Heart)

• Path: LA → LV → Aorta → Arteries → Arterioles → Capillaries (exchange) → Venules → Veins → RA.
• Pressure: High (MAP ~90 mmHg). LV is thick-walled, pressure pump.
• Function: Deliver O₂/nutrients; remove waste. Autoregulation matches flow to tissue needs.

3. Blood Pressure Regulation: The Heart's Role

BP = Cardiac Output (CO) × Systemic Vascular Resistance (SVR). The heart directly controls CO:

• CO = Heart Rate (HR) × Stroke Volume (SV)
• HR: SA node automaticity modulated by autonomic tone (sympathetic ↑, parasympathetic ↓).
• SV: Determined by Preload (venous return), Afterload (aortic pressure), Contractility (inotropy).

Baroreflex Arc: Carotid sinus/aortic arch baroreceptors → NTS → vagal/sympathetic output → adjust HR, contractility, vascular tone → stabilize BP.

4. Integrated Response to Demand

Condition	HR	Contractility	Venous Return	CO	SVR
Exercise	↑↑	↑↑	↑ (muscle pump)	↑↑↑ (4-6x)	↓ (vasodilation in muscle)
Hemorrhage	↑	↑	↓↓	↓/↔	↑↑ (compensatory)
Heart Failure	↑ (compensatory)	↓	↑ (fluid retention)	↓	↑ (neurohormonal)

5. Clinical Correlates

• Heart Failure: Inability to meet metabolic demand at normal filling pressures. HFrEF (EF ≤40%) vs. HFpEF (EF ≥50%).
• Shock: Inadequate tissue perfusion. Cardiogenic (pump failure), distributive (vasodilation), hypovolemic, obstructive.
• Hypertension: Chronic ↑ afterload → LVH → diastolic dysfunction → HFpEF.
• Arrhythmias: Disrupt coordinated filling/ejection → ↓ CO.

The heart is not just a pump—it's a dynamic pressure/flow regulator exquisitely tuned by neural, hormonal, and local metabolic signals to sustain life across wildly varying demands.