Blood is not only in veins. It flows continuously through a closed loop that includes arteries, capillaries, veins, and the heart itself. At any given moment, about 70% of your total blood volume sits in the veins, which is likely why people associate blood so strongly with them. But the remaining 30% is actively moving through arteries, capillaries, heart chambers, and organs like the spleen and liver.
Where Blood Actually Goes
Your circulatory system has three main types of blood vessels, and blood passes through all of them in a continuous circuit. Arteries carry oxygen-rich blood away from the heart to your organs and tissues. Capillaries, the smallest vessels, connect arteries to veins and serve as exchange points where oxygen and nutrients pass into tissues while carbon dioxide and waste move back into the blood. Veins then carry the oxygen-depleted blood back to the heart, which pumps it to the lungs to pick up fresh oxygen before the cycle starts again.
The heart itself holds blood at all times. Its four chambers fill and empty in a rhythmic cycle, expanding to receive blood during relaxation and contracting to push it out. So even the heart is a temporary container for blood at every stage of the heartbeat.
Why Veins Hold Most of the Blood
Veins contain roughly 70% of your total blood volume at any given moment. Arteries hold about 10%, and capillaries just 5%. The rest is distributed between the heart and the lungs. Veins dominate because they’re far more flexible and stretchy than arteries. They act as a reservoir, expanding to accommodate large volumes of blood at low pressure while slowly returning it to the heart.
For an average adult weighing 150 to 180 pounds, total blood volume is approximately 1.2 to 1.5 gallons. That means your veins are holding close to a gallon of blood right now, while your arteries contain only a few cups.
Organs That Store Blood Too
Blood also pools in certain organs that act as reservoirs. The spleen is the most notable example. Its inner structure contains contractile tissue that can squeeze stored blood back into circulation when your body needs it, such as during heavy exercise or after significant blood loss. The spleen also stockpiles immune cells and platelets that can be released during infection or injury. The liver similarly holds a substantial reserve of blood that can be mobilized on demand.
What Happens in the Capillaries
Capillaries are where blood does its most important work, even though they hold the smallest share of it. Their walls are so thin that gases like oxygen and carbon dioxide pass directly through them. Nutrients like glucose and amino acids cross through specialized channels in the capillary walls, while larger molecules slip through tiny gaps between cells or get transported in small packages called vesicles.
Fluid also moves in and out of capillaries through pressure differences. Blood pressure inside the capillary pushes fluid out into surrounding tissue, delivering water and dissolved substances to cells. On the other end of the capillary bed, lower pressure allows fluid to flow back in. This constant exchange is why capillaries matter so much despite their tiny size. Only about 5% of your blood is in capillaries at any moment, but that small fraction is doing the bulk of the delivery and cleanup work.
Why Veins Look Blue but Blood Is Always Red
One reason people wonder whether blood behaves differently in veins is the color. Veins visible through your skin appear blue or greenish, which has fueled a persistent myth that blood in veins is blue and only turns red when exposed to air. That’s not true. Blood is always red. It’s a brighter red in arteries (where it’s oxygen-rich) and a darker, more maroon red in veins (where oxygen has been used up), but it’s never blue.
The blue appearance comes from how light interacts with your skin. Collagen fibers in the upper layer of skin scatter shorter wavelengths of light (blue) more than longer wavelengths (red), a process called Rayleigh scattering. It’s the same physics that makes the sky look blue. The skin filters out the red light reflected from superficial veins, leaving mostly blue wavelengths for your eyes to detect. Deeper arteries don’t show through the skin at all because they sit farther beneath the surface and are surrounded by thicker tissue.
When Blood Leaves the Vessels
Under normal conditions, blood stays inside the closed loop of your circulatory system. When it escapes, something has gone wrong. A bruise forms when small blood vessels break under the skin and blood leaks into surrounding tissue. Internal bleeding involves larger volumes escaping into body cavities. Even during medical procedures, blood can leak from vessels. During chemotherapy infusions, for example, drugs sometimes escape from the vein into nearby tissue, a complication called extravasation that affects up to 6 out of every 100 patients receiving these treatments.
The body treats escaped blood as a problem to solve. Clotting factors seal damaged vessels, immune cells clean up leaked blood in tissues, and the leaked blood is gradually reabsorbed. The color changes of a bruise, from red to purple to green to yellow, reflect this cleanup process over days to weeks.