By Emily Pate, Updated Mar 24, 2022
A circulatory system is the body’s internal highway, consisting of organs that work together to transport blood, oxygen, and nutrients throughout every tissue. Humans possess two interconnected loops: the pulmonary circuit, carrying blood between the heart and lungs, and the systemic circuit, delivering blood to the rest of the body.
The heart is a muscular, hollow pump that propels blood through the entire body. It typically beats 60–100 times per minute, and over a 70‑year lifespan it will contract approximately 2.5 billion times. The organ comprises four chambers: two upper atria (left and right) and two lower ventricles. The heart’s rhythm adjusts automatically to meet the body’s demand for oxygen and nutrients.
Arteries carry oxygen‑rich blood away from the heart. They are the thickest vessels, with three layers—a protective outer layer, a muscular middle layer for contraction, and a smooth inner lining that facilitates flow. The aorta, the largest artery, branches into the coronary arteries and an extensive network of smaller vessels. The pulmonary artery, unique among arteries, transports deoxygenated blood to the lungs.
Veins return blood to the heart. Equipped with valves, they prevent backflow and help maintain forward momentum. The superior and inferior vena cavae are the major veins returning blood from the upper and lower body, respectively. Capillaries, the smallest vessels, connect arteries and veins, allowing exchange of gases, nutrients, and waste. When venous valves fail, blood can pool, leading to varicose veins.
Blood is the transport medium for oxygen, nutrients, hormones, and waste products. Oxygenated blood appears bright red, while deoxygenated blood has a bluish hue. It contains red blood cells packed with hemoglobin, white blood cells that defend against infection, and plasma—the clear liquid component that carries platelets for clotting. Roughly 55% of blood volume is plasma, and red blood cells have a lifespan of about 120 days, requiring continuous production in the bone marrow.
