Arteries carry blood away from your heart, and veins carry blood back to it. Together with capillaries, they form a closed loop that delivers oxygen and nutrients to every cell in your body and carries waste products away. In an average adult, the total length of all blood vessels is estimated between 5,500 and 12,000 miles.
Despite sharing the same basic blueprint, arteries and veins are built differently, operate under very different pressures, and face distinct health problems when things go wrong.
How Arteries and Veins Are Built
Both arteries and veins have three layers in their walls. The innermost layer is a smooth lining that lets blood flow with minimal friction. The middle layer is made of muscle and elastic tissue. The outermost layer anchors the vessel to surrounding tissue.
The key difference is thickness. Arteries have a much thicker middle layer because they need to handle the force of blood being pumped directly from the heart. Blood pressure inside arteries is high, typically peaking around 120 mmHg during a heartbeat. Veins, by contrast, operate in a low-pressure environment. Pressure in large named veins runs between 8 and 10 mmHg, and near the heart it drops to roughly 0 to 6 mmHg. Because of this lower pressure, vein walls are thinner and more flexible.
Different Types of Arteries
Not all arteries are identical. The largest ones, like the aorta (the main artery leaving the heart), are called elastic arteries. They contain a high proportion of stretchy elastic tissue in their middle layer, which allows them to expand when the heart pumps a burst of blood and then recoil between beats. This smooths out the pulsing rhythm of the heart into a steadier flow downstream.
Farther from the heart, arteries become muscular arteries. These have more muscle and less elastic tissue, which lets them contract or relax to direct blood toward specific organs based on demand. When you exercise, for example, muscular arteries feeding your legs dilate to deliver more oxygen to working muscles. These muscular arteries branch into smaller and smaller vessels called arterioles, which are the final gatekeepers before blood enters the capillary beds.
How Veins Push Blood Uphill
Getting blood back to the heart is a challenge, especially from the legs, where blood has to travel upward against gravity. Veins solve this problem with one-way valves. These small flaps open to let blood flow toward the heart and snap shut to prevent it from sliding backward. When a valve is fully open, its two flaps narrow the vein’s interior by about 35%, which speeds the blood into a jet that pushes it along. Behind each valve flap, a small swirling current prevents blood from stagnating in the pocket.
Veins also rely on surrounding skeletal muscles. Every time you walk, flex your calves, or shift your weight, the muscles around your veins compress them and squeeze blood upward through those one-way valves. This is why sitting or standing in one position for hours can cause your legs and feet to swell: the muscle pump isn’t active, and blood pools in the veins.
Where Arteries Meet Veins
Arteries and veins never connect directly. Between them sit capillaries, the smallest blood vessels in the body, with walls just one cell thick. The tiniest arteries (arterioles) feed into capillary beds, and the capillaries then drain into the tiniest veins (venules).
Capillaries are where the actual work of the circulatory system happens. Oxygen and carbon dioxide, which dissolve easily through the capillary wall, move by diffusion: oxygen passes from blood into tissue, and carbon dioxide moves in the opposite direction. Water-soluble nutrients pass through tiny channels between the cells of the capillary wall. Fluid is also filtered out of capillaries into surrounding tissue at the arterial end, and much of it is reabsorbed at the venous end, driven by differences in pressure inside and outside the vessel. This constant exchange is the entire reason the circulatory system exists.
The Pulmonary Exception
A common point of confusion: arteries don’t always carry oxygen-rich blood, and veins don’t always carry oxygen-poor blood. The pulmonary arteries are the exception. They carry oxygen-poor blood from the right side of the heart to the lungs, where it picks up fresh oxygen. Pulmonary veins then return that oxygenated blood to the left side of the heart, which pumps it out to the rest of the body through the aorta.
The defining feature of an artery is not the oxygen content of its blood. It’s the direction of flow. Arteries always carry blood away from the heart. Veins always carry blood toward it.
What Happens When They Stop Working Well
Because arteries and veins have different structures and pressures, they tend to develop different problems.
Arterial Problems
The most common issue is peripheral artery disease (PAD), where fatty deposits narrow the arteries supplying the legs. The hallmark symptom is leg pain or cramping during walking, especially in the calves, that goes away with rest. This happens because the narrowed artery can’t deliver enough oxygen-rich blood to meet the muscles’ demand during activity. Over time, reduced circulation can also cause coldness in the lower leg or foot, hair loss on the legs and feet, shiny or discolored skin, and slow-healing sores.
Venous Problems
Venous insufficiency occurs when the one-way valves in leg veins stop closing properly, allowing blood to flow backward and pool. Instead of the activity-related cramping seen with arterial disease, venous insufficiency produces a persistent aching or heaviness that worsens after prolonged standing or sitting. Visible signs include swelling in the ankles and lower legs, spider veins, and skin that gradually changes color or texture. In severe cases, ulcers can develop on the lower legs.
The pain patterns offer a practical way to tell these two conditions apart. Arterial pain shows up during movement and eases with rest. Venous pain lingers during inactivity and often improves with walking or elevating the legs, since both actions help push pooled blood back toward the heart.