Blood vessels deliver oxygen and nutrients to every tissue in your body and carry waste products back out. They also regulate body temperature, control blood pressure, fight infections, and protect the brain from harmful substances. Laid end to end, an average adult’s blood vessels would stretch somewhere between 5,500 and 12,000 miles, with capillaries making up the vast majority of that length.
Three Types, Three Jobs
Your vascular system has three main vessel types, each built for a specific role. Arteries carry oxygen-rich blood away from the heart. The largest, the aorta, launches blood at roughly 30 centimeters per second. Arteries branch into smaller and smaller vessels until they become capillaries, where the real exchange happens. Capillaries are so thin-walled that oxygen, nutrients, carbon dioxide, and waste products pass directly through them into surrounding tissue and back again. From there, blood collects into venules and then veins, which return oxygen-depleted blood to the heart and lungs.
The speed difference across this network is dramatic. Blood races through the aorta at about 30 cm per second but slows to just 1 millimeter per second in capillaries. That slowdown is intentional. The slower the flow, the more time blood has to swap oxygen for carbon dioxide and deliver nutrients to cells that need them.
How Oxygen and Carbon Dioxide Are Exchanged
Gas exchange depends on diffusion, a passive process that requires no energy from your body. In the lungs, capillaries press up against tiny air sacs called alveoli. The barrier between inhaled air and your blood is about one micron thick, roughly one-thousandth of a millimeter. Oxygen slips through this ultra-thin wall into the blood, while carbon dioxide moves the opposite direction and gets exhaled.
The same principle works in reverse throughout the rest of the body. As blood reaches tissues that are actively burning fuel, oxygen diffuses out of capillaries into cells, and carbon dioxide diffuses in. Along with CO2, capillaries pick up other metabolic waste: lactate, urea, ammonia, and adenosine. These waste products travel through the venous system to be filtered by the liver and kidneys or exhaled through the lungs. All of these byproducts become toxic at high concentrations, so continuous removal is essential.
Controlling Blood Pressure
Your blood pressure isn’t just a product of how hard your heart pumps. It depends equally on how wide or narrow your blood vessels are. The formula is straightforward: blood pressure equals the heart’s output multiplied by the resistance your vessels create. Small arteries called arterioles are the main players here. When they tighten, resistance rises and blood pressure goes up. When they relax, resistance drops and pressure falls.
The inner lining of your blood vessels, called the endothelium, plays an active role in this process. Endothelial cells produce a signaling molecule, nitric oxide, that relaxes the smooth muscle in vessel walls and widens the opening. This keeps blood flowing smoothly and prevents pressure from climbing too high. Nitric oxide also discourages blood platelets from clumping together and sticking to vessel walls, which helps prevent the buildup of plaque that leads to atherosclerosis.
Regulating Body Temperature
Blood vessels act as your body’s radiator system. When you exercise or sit in a hot environment, vessels near the skin surface widen, a process called vasodilation. This pulls warm blood from your core toward the skin, where heat can radiate away. During whole-body heat stress, a dedicated nerve signaling system is responsible for 80% to 90% of this skin-level vessel widening.
In cold conditions, the opposite happens. Vessels near the skin constrict, keeping warm blood deeper in the body and reducing heat loss through the surface. This is why your fingers and toes go pale and cold first: your body is deliberately restricting blood flow to extremities to protect your core temperature.
Protecting the Brain
Not all blood vessels are built the same. In the brain, endothelial cells are packed so tightly together that almost nothing slips between them without help. This arrangement forms the blood-brain barrier, a selective filter that lets essential molecules through while blocking toxins, pathogens, and many drugs. The cell membranes in this barrier are lipid-based, meaning they naturally repel water-soluble molecules. Only substances the barrier actively transports, or molecules small and fat-soluble enough to pass through the membrane, can reach brain tissue.
This is why brain infections are relatively rare compared to infections elsewhere in the body, and why treating brain diseases with medication is often so challenging. Many drugs simply can’t cross the barrier.
Supporting the Immune System
When you get an infection or injury, blood vessels do more than just deliver supplies. They actively help white blood cells get where they need to go. White blood cells circulating in the bloodstream exit by squeezing between individual endothelial cells in the vessel wall, a process called extravasation. The endothelium doesn’t just passively allow this. It forms a ring of structural protein around the migrating cell, sealing the vessel wall like an elastic strap so blood doesn’t leak out.
After the white blood cell passes through, platelets gather at the exit point and release a signaling molecule that triggers the surrounding endothelial cells to extend small protrusions, closing the gap behind the departing cell. This coordination ensures the immune system can deploy its defenses to damaged tissue without compromising the integrity of the vessel itself.
The Endothelium as an Active Organ
Your body contains roughly 100 trillion endothelial cells lining the interior of every blood vessel. Far from being a passive wrapper, this lining functions as one of the body’s largest active organs. Endothelial cells produce nitric oxide to regulate vessel width, prevent clot formation, and stop white blood cells from sticking where they shouldn’t. They also block early steps in plaque formation by keeping platelets from exposing the deeper muscle layer of the vessel wall to growth-promoting signals.
When the endothelium is healthy, vessels stay flexible, blood flows freely, and clots form only when they’re needed. When it’s damaged by smoking, high blood sugar, chronic inflammation, or high blood pressure, the protective effects of nitric oxide decline. Platelets become stickier, vessel walls stiffen, and the conditions that lead to heart attack and stroke begin to develop. In this sense, the health of your blood vessels isn’t just about plumbing. It’s an active, living system that constantly adapts to keep everything else running.