Circulatory Pathways

Two circuits in series. Blood flows through two loops connected by the heart:

• The pulmonary circuit carries oxygen-poor blood from the right ventricle to the lungs, where it picks up oxygen and releases carbon dioxide, then returns oxygen-rich blood to the left atrium. This is the "refueling" loop.
• The systemic circuit carries oxygen-rich blood from the left ventricle to the entire body, delivering oxygen and nutrients and collecting wastes, then returns oxygen-poor blood to the right atrium.

Because the loops are in series, all blood passes through the lungs and then the body in turn. A common point of confusion: the pulmonary artery carries oxygen-poor blood (arteries are defined by flowing away from the heart, not by oxygen content), and the pulmonary veins carry oxygen-rich blood.

Tracing a drop of blood. Following one complete trip:

Right atrium → right ventricle → pulmonary artery → LUNGS → pulmonary veins
   → left atrium → left ventricle → aorta → BODY tissues → veins (vena cava)
   → back to the right atrium

Special circulations. A few pathways are worth noting:

• Coronary circulation: branches off the aorta to supply the heart muscle itself (blockage → heart attack, as covered earlier).
• Hepatic portal circulation: blood draining from the digestive organs doesn't go straight back to the heart. Instead it travels via the hepatic portal vein to the liver first, so the liver can process absorbed nutrients and detoxify substances before they enter the general circulation. This is why the liver is a "first stop" for absorbed food and many drugs.

Fetal circulation (brief). Before birth, the fetus doesn't breathe air, so its lungs are bypassed. Special shunts (like the ductus arteriosus and foramen ovale) route most blood around the lungs, and the placenta (not the fetal lungs) provides oxygen from the mother. At birth, these shunts normally close as the newborn begins breathing, switching to the adult pattern. (Failure to close causes certain congenital heart conditions.) This is developed further in the reproduction/development unit.

How it works

The full loop and detours:

Systemic body → right heart → pulmonary circuit (lungs, oxygenate) → left heart → systemic body
Detours: coronary (feeds heart), hepatic portal (gut blood → liver first)
Fetal: lungs bypassed via shunts; placenta oxygenates

Comparisons

Common confusions

• Pulmonary artery/vein oxygen. Pulmonary artery = oxygen-poor (away from heart to lungs); pulmonary veins = oxygen-rich (back to heart). Arteries/veins are defined by direction, not oxygen.
• Pulmonary vs systemic. Pulmonary = heart–lungs; systemic = heart–body.
• Hepatic portal detour. Gut blood goes to the liver before the heart.
• Fetal lungs are bypassed — the placenta, not the lungs, oxygenates before birth.

Memory aids

• "Pulmonary = to the Lungs (Puff); Systemic = to the System (body)."
• Blood path: "RA → RV → lungs → LA → LV → body."
• Hepatic portal = "gut checks in at the liver first."

Quick review

• Blood flows through two loops in series: the pulmonary circuit (right heart → lungs → left heart, picks up oxygen) and the systemic circuit (left heart → body → right heart, delivers oxygen).
• Trace: RA → RV → pulmonary artery → lungs → pulmonary veins → LA → LV → aorta → body → vena cava → RA.
• Special pathways: coronary (feeds the heart), hepatic portal (gut blood → liver first), and fetal shunts (bypass the lungs before birth).
• These pathways explain heart-failure congestion patterns and first-pass drug metabolism.