Arteries, veins, and capillaries are the three types of blood vessels in your body, and they differ in wall thickness, size, direction of blood flow, and function. Arteries carry blood away from the heart under high pressure, veins return it under low pressure, and capillaries are the microscopic bridges between the two where oxygen and nutrients actually reach your tissues.
Wall Structure and Thickness
All three vessel types share the same basic blueprint of three layers: an inner lining (intima), a muscular middle layer (media), and a tough outer shell (adventitia). But the proportions of those layers vary dramatically based on what each vessel needs to do.
Arteries have the thickest muscular middle layer of any vessel type. This is especially pronounced in medium-sized arteries, where dense bands of smooth muscle wrap around the vessel in spirals, allowing the artery to expand and recoil with every heartbeat. Large arteries like the aorta also have a thick inner lining packed with elastic fibers that act like a rubber band, absorbing the surge of pressure each time the heart contracts. The outer layer of arteries is comparatively thin, often less than half the total wall width.
Veins flip that ratio. Their muscular middle layer is much thinner than in arteries of the same size, because veins operate under far less pressure. Instead, the outer layer is the thickest part of a vein’s wall, packed with collagen and elastic fibers that anchor the vessel to surrounding tissue. Large veins lack a clear boundary between their inner and middle layers entirely.
Capillaries are a different structure altogether. Their walls are just one cell thick, with no muscular or outer layer at all. A capillary is only about 5 micrometers wide, so narrow that red blood cells have to pass through in single file. That extreme thinness is the whole point: it allows oxygen, carbon dioxide, nutrients, and waste products to pass directly through the capillary wall.
How Blood Pressure and Flow Speed Differ
Blood pressure is highest in the arteries closest to the heart and drops steadily as it moves through the system. In the aorta, blood travels at roughly 30 centimeters per second. By the time it reaches the capillaries, flow slows to about 1 millimeter per second. That dramatic slowdown gives capillaries time to exchange gases and nutrients with surrounding cells. Once blood enters the veins, it speeds up again as vessels merge and funnel blood back toward the heart, though never reaching arterial speeds.
Because veins operate at low pressure, they need a mechanism to prevent blood from pooling or flowing backward, especially in the legs where gravity works against the return trip. Veins solve this with one-way valves, small flap-like structures (usually two leaflets) that open to let blood pass toward the heart and snap shut if it tries to fall back. Arteries don’t have valves because the heart’s pumping pressure keeps blood moving forward on its own. Capillaries don’t have them either.
Blood Volume Distribution
Your venous system holds the vast majority of your blood at any given moment, roughly three-quarters of your total circulating volume. The arterial system contains only about 10 to 15 percent. This makes veins the body’s primary blood reservoir. When you need to redirect blood quickly, such as during exercise or after blood loss, veins can constrict to push more volume back to the heart and into active circulation.
What Each Vessel Carries
The common shorthand is that arteries carry oxygen-rich blood and veins carry oxygen-poor blood. That’s true for most of the body, but there are two important exceptions. The pulmonary arteries carry deoxygenated blood from the heart to the lungs to pick up oxygen, and the pulmonary veins return oxygenated blood from the lungs to the heart. During fetal development, the umbilical vein carries oxygenated blood from the placenta to the fetus (with an oxygen saturation of 70 to 80 percent), while the two umbilical arteries carry deoxygenated blood back to the placenta.
The more accurate distinction is structural and directional: arteries carry blood away from the heart, veins carry blood toward it. The oxygen content depends on where in the circuit they sit.
How Capillaries Regulate Blood Flow
Despite being the smallest vessels, capillaries aren’t passive tubes. At the junction where small arteries feed into capillary beds, ring-like structures called precapillary sphincters act as gatekeepers. These sphincters are wrapped in contractile cells that can widen or narrow the opening, controlling how much blood enters a given capillary bed.
When cells in a particular area are active and need more oxygen, they release chemical signals that dilate local sphincters, flooding that tissue with blood. When demand drops, the sphincters tighten. Research on brain capillaries found that these sphincters can change diameter by more than 30 percent during stimulation, generating the largest shifts in blood flow resistance of any vessel segment in the brain. This mechanism protects delicate downstream tissue from pressure spikes while ensuring active areas get the supply they need.
A Quick Way to Check Capillary Health
You can get a rough sense of your capillary function with a simple test: press firmly on a fingernail until the skin beneath turns white, then release. The pink color should return within 2 seconds. In children, a refill time of 3 seconds or more is considered abnormal and can signal dehydration or poor circulation. If the test is done on the foot or chest rather than the finger, normal refill can take up to 4 seconds. Newborns in the first week of life tend to be slower, with normal times ranging up to 5 to 7 seconds.
What Goes Wrong in Each Vessel Type
The diseases that affect arteries and veins look quite different, largely because of their structural differences.
Arterial disease is driven primarily by atherosclerosis, where fatty deposits narrow and stiffen artery walls. Because arteries supply oxygen to tissues, blockages lead to oxygen starvation. In the legs, this shows up as thin, shiny skin, hair loss, thickened nails, and cold feet with weak or absent pulses. Arterial ulcers tend to form on the toes or forefoot (the points farthest from the heart), with well-defined edges and a pale or dead-looking wound bed that produces little fluid. Pain typically gets worse when the legs are elevated, since gravity is no longer helping push blood to the feet.
Venous disease stems from valve failure, often in the leg veins. When valves stop closing properly, blood pools and pressure builds in the lower legs. This affects up to a third of adults. The hallmarks are swelling, dark brownish-purple skin discoloration, and varicose veins. Venous ulcers form on the inner lower leg (the “gaiter” region above the ankle), tend to be shallow with irregular borders, and produce a large amount of fluid. Unlike arterial problems, venous pain improves with leg elevation because it helps blood drain back toward the heart.
Capillary problems are less visible but play a role in conditions like diabetes, where damage to capillary walls impairs nutrient exchange and wound healing, and in inflammatory conditions where capillaries become excessively leaky, causing tissue swelling.
Key Differences at a Glance
- Wall thickness: Arteries have thick, muscular walls. Veins have thinner walls with a thick outer layer. Capillaries are one cell thick.
- Diameter: Arteries range from about 1 centimeter (aorta) down to fractions of a millimeter. Capillaries are roughly 5 micrometers, smaller than a single hair.
- Blood pressure: Highest in arteries, lowest in veins, with capillaries in between.
- Flow speed: Fastest in arteries (30 cm/s in the aorta), slowest in capillaries (1 mm/s).
- Valves: Present in veins, absent in arteries and capillaries.
- Blood volume: Veins hold about 75% of circulating blood; arteries hold 10 to 15%.
- Direction: Arteries carry blood away from the heart, veins carry it back, capillaries connect the two.
- Primary function: Arteries distribute blood under pressure. Capillaries exchange oxygen, nutrients, and waste. Veins return blood and serve as a volume reservoir.