Cardiovascular System

Pump, Cycle, and Conduction

• Blood path: RA to RV to lungs to LA to LV to body; the left ventricle is thickest because it pumps against systemic resistance. S1 marks AV valve closure, S2 semilunar closure.
• Conduction: SA node (pacemaker) to AV node (0.1 s delay) to bundle of His to Purkinje fibers. On the ECG, P is atrial depolarization, QRS ventricular depolarization, T ventricular repolarization.

Core Equations

$\text{CO} = \text{HR} \times \text{SV} \qquad \text{BP} = \text{CO} \times \text{TPR}$

• Poiseuille: resistance scales as $R \propto \eta L / r^4$, so halving a vessel radius raises resistance 16-fold — arterioles are the resistance vessels controlling blood pressure.
• Frank-Starling: more preload stretches the ventricle and raises stroke volume; afterload is the pressure it must overcome; sympathetic tone raises contractility independently. Ejection fraction is SV/EDV.

Capillary Exchange

• Starling forces: $J_v \propto (P_c - P_i) - (\pi_c - \pi_i)$ Hydrostatic pressure filters fluid out at the arteriolar end; albumin's oncotic pull reabsorbs it at the venular end. Low albumin or blocked lymphatics produces edema.

Key Takeaways

• CO and TPR jointly set blood pressure; exercise raises both HR and SV.
• The $r^4$ dependence makes arteriolar tone the dominant BP lever.
• Preload up means SV up; afterload up means SV down.
• Starling force imbalance explains edema in liver, kidney, and lymphatic disease.