Arteries are the blood vessels that carry blood away from the heart. Every artery in your body, from the massive aorta in your chest to the tiny arterioles feeding individual tissues, moves blood outward from the heart’s pumping chambers. Veins do the opposite, returning blood back to the heart.
The Two Main Arteries Leaving the Heart
Two large arteries exit the heart directly. The aorta leaves from the left side and carries oxygen-rich blood to the entire body. The pulmonary artery leaves from the right side and carries oxygen-poor blood to the lungs, where it picks up a fresh supply of oxygen before returning to the heart’s left side.
These two arteries mark the starting points of two separate loops. The aorta feeds the systemic circuit, which supplies every organ and tissue from your brain to your toes. The pulmonary artery feeds the pulmonary circuit, a much shorter loop that only travels to and from the lungs. One-way valves called semilunar valves sit at the base of each artery, preventing blood from flowing backward into the heart between beats. The aortic valve has three leaflets that open during each contraction and snap shut the moment pressure drops.
Why the Pulmonary Artery Is the Exception
Most people assume arteries always carry oxygen-rich blood, but that’s not quite right. What defines an artery is direction, not oxygen content. The pulmonary artery carries blood with an oxygen saturation of about 76%, far lower than the near-100% saturation in the aorta. This is blood that has already delivered its oxygen to the body’s tissues and needs to be refreshed in the lungs. So the pulmonary artery is a true artery (it carries blood away from the heart) that happens to carry deoxygenated blood.
The Aorta and Its Major Branches
The aorta is the largest artery in the body. It rises about 5 centimeters from the top of the heart (the ascending aorta), curves into an arch, then descends through the chest and abdomen before splitting into two branches that supply the legs. Along the way, it sends off branches to nearly every major organ system.
The first branches are the coronary arteries, which split off right at the aortic valve to feed the heart muscle itself. From the aortic arch, three arteries branch upward to supply the head, neck, and arms. As the aorta descends through the chest, it sends smaller branches to the lungs’ surrounding tissues, the esophagus, and the chest wall. In the abdomen, five major branches supply the digestive organs, kidneys, and liver. This branching pattern means the aorta doesn’t just deliver blood to one destination. It acts as a central highway with exits at every level of the body.
How Arteries Handle High Pressure
Blood leaves the heart under significant force. A healthy blood pressure reading of around 120/80 mmHg reflects the pressure inside your large arteries, first when the heart contracts (systolic) and then when it relaxes (diastolic). Veins, by comparison, operate under roughly 30 times less pressure. Arteries are built to handle this difference.
Arterial walls have three distinct layers. The innermost layer (the intima) is a smooth, single-cell-thick lining that directly contacts flowing blood. The middle layer (the media) is the thickest and contains smooth muscle cells and elastic fibers that allow the artery to expand and contract. The outer layer (the adventitia) is a tough coat of collagen fibers that anchors the artery in place and prevents it from overexpanding. This outer layer even contains its own tiny blood vessels and nerves that keep the arterial wall itself nourished and functional.
The elastic fibers in large arteries like the aorta serve a critical purpose. When the heart contracts, these fibers stretch to absorb the surge of blood. When the heart relaxes between beats, the fibers recoil like a rubber band, pushing blood forward. This mechanism, sometimes called the Windkessel effect, converts the heart’s pulsing output into a smoother, more constant flow by the time blood reaches your organs. Without it, blood would rush forward in spurts and stop between heartbeats.
Arterioles: Where Flow Gets Fine-Tuned
As arteries branch into progressively smaller vessels, they eventually become arterioles, the smallest arteries in the body. Despite their size, arterioles are responsible for about 80% of the total resistance to blood flow. They are the primary control point for how much blood reaches any given tissue at any given moment.
Arterioles adjust their diameter constantly. When a muscle is working hard and needs more oxygen, the local arterioles widen to increase flow. When an area needs less blood, they constrict. This happens through several mechanisms: the smooth muscle in their walls responds to changes in pressure, to chemical signals from nearby tissues, and to nerve signals from the sympathetic nervous system. This is also how your body maintains overall blood pressure. If arterioles throughout the body constrict slightly, blood pressure rises; if they relax, it falls.
What Happens When Arteries Narrow
Because arteries operate under high pressure and carry blood directly from the heart, they’re vulnerable to a specific kind of damage over time. Atherosclerosis is the gradual buildup of fatty deposits, calcium, and fibrous tissue inside arterial walls. It begins when the smooth inner lining of an artery becomes damaged or inflamed, often from high cholesterol, high blood pressure, smoking, or diabetes.
The process starts small. Cholesterol particles penetrate the inner wall and trigger an immune response. White blood cells arrive to clean up the cholesterol but become overwhelmed, forming a fatty streak beneath the artery’s surface. Over years, this streak grows into a larger plaque with a soft, fatty core covered by a fibrous cap. If that cap stays thick and stable, the plaque may simply narrow the artery and reduce blood flow, causing symptoms like chest pain during exertion or leg cramps when walking.
The real danger comes when a plaque becomes unstable. Plaques with large fatty cores, thin caps, and heavy inflammation are considered vulnerable. If the cap cracks or ruptures, the body treats it like a wound and forms a blood clot on the spot. That clot can partially or completely block the artery. In a coronary artery, this causes a heart attack. In an artery supplying the brain, it causes a stroke. This is why atherosclerosis in the arterial system is the leading driver of cardiovascular disease, even though the narrowing itself may develop silently over decades.