Systemic circulation

The systemic circulation carries blood from the left ventricle through body tissues and back to the right atrium. Its main arterial outflow is the aorta, and major venous return channels include the superior vena cava and inferior vena cava.^[ref] The separate pulmonary circulation then passes through the lungs before blood returns to the left heart.

The two circuits operate in series, while many systemic tissue beds receive arterial supply in parallel. In an original fixed-resistance model, beds sharing pressure difference $\Delta p$ haveFor a stipulated $\Delta p=90$ mmHg and two resistances 30 and $60\mathrm{mmHg}\min {\mathrm{L}}^{-1}$, flows are 3 and 1.5 litres per minute, totaling 4.5. The equivalent resistance is 20 in those resistance units. Adding flows is appropriate here; adding parallel resistances is not.

Real vascular resistance changes with vessel geometry and physiology. Some routes also contain successive exchange beds, as in a portal system. The parallel-bed calculation is therefore a deliberately reduced model, not a complete vessel map.

Conservation of mass constrains sustained output from the right and left heart. If their flows differ, a vascular compartment must gain or lose blood. For the pulmonary pool,A hypothetical imbalance of $0.2\mathrm{L}\mathrm{mi}{\mathrm{n}}^{-1}$ adds 0.2 litres in one minute if maintained. Long-term matching of flows is a consequence of finite storage, rather than a rule that both instantaneous waveforms must be identical.

For a whole-body oxygen balance, systemic cardiac output must be paired with compatible arterial and mixed venous oxygen content. A regional venous sample answers a regional question; it does not become whole-body return simply because it was taken from a large vein.