systemic circulation
Think of systemic circulation as the heart’s main delivery route to the whole country of your body. Freshly oxygenated blood leaves the powerful left side of the heart, travels out through a vast road network of arteries, reaches every town and village (your tissues), drops off oxygen and nutrients, and then heads home through the veins to be sent off for refueling.
More precisely, the systemic circuit begins in the left ventricle, which pumps oxygen-rich blood into the aorta. From there blood flows through progressively smaller arteries and arterioles into capillary beds, where gas, nutrient and waste exchange happens. Deoxygenated blood is collected by venules and veins and returns to the right atrium through the superior and inferior vena cava. Because this loop must reach the entire body against substantial resistance, the left ventricle generates the high pressures we measure as blood pressure.
The systemic circuit is one of two circulations working in series with the pulmonary loop, so the same total volume of blood passes through each per minute. A useful caveat: although we speak of systemic blood as ‘oxygenated,’ it is never fully saturated everywhere, and certain organs (such as the gut and skin) receive variable flow depending on the body’s moment-to-moment needs.
When you sprint for a bus, the systemic circulation rapidly redirects blood toward your leg muscles while temporarily diverting flow away from your digestive tract.
The systemic loop is not a fixed pipe network—flow is constantly re-allocated by arteriolar tone.
Systemic and pulmonary circulations are arranged in series, not parallel: blood must pass through both on every full circuit, so over time their flows must match.