Deoxygenated blood is blood that has delivered most of its oxygen to the body’s tissues and is traveling back to the heart and lungs to pick up a fresh supply. It makes up roughly half of the blood circulating through your body at any given moment. Despite what many people assume, it is not blue. Deoxygenated blood is dark red, while oxygen-rich blood is bright red.
How Blood Loses Its Oxygen
Every cell in your body needs oxygen to produce energy. When oxygen-rich blood reaches the tiny capillaries surrounding your tissues, oxygen naturally moves from the blood (where its concentration is higher) into the cells (where its concentration is lower). This passive movement is called diffusion, and it requires no energy. At the same time, carbon dioxide, a waste product of cell metabolism, moves in the opposite direction, from the tissues into the blood.
This swap is governed by pressure gradients. Gases always move from areas of higher pressure to areas of lower pressure, much like air rushing out of an inflated balloon. The oxygen pressure in blood arriving at your tissues is high, while the oxygen pressure inside working cells is low, so oxygen flows out of the blood. Carbon dioxide works the same way in reverse. By the time blood leaves a capillary bed, it has traded much of its oxygen for carbon dioxide, and it’s now considered deoxygenated.
What Makes It Look Different
The color change comes down to hemoglobin, the protein in red blood cells that carries oxygen. When hemoglobin binds oxygen, it shifts into a relaxed shape that reflects light as bright red. When it releases oxygen, it snaps into a tenser, more compact shape that absorbs more light and appears dark red. This is why blood drawn from a vein (which carries deoxygenated blood) looks noticeably darker than blood from an artery.
So why do veins look blue through your skin? That’s an optical trick. Skin scatters much of the red portion of light before it can reflect off the blood beneath. The remaining blue light bounces back to your eyes, making the veins appear bluish. The blood inside is still dark red, never blue.
How It Travels Through the Heart
Deoxygenated blood follows a specific route back to the lungs. Two large veins, the superior vena cava (draining the upper body) and the inferior vena cava (draining the lower body), funnel it into the heart’s right atrium. From there it passes into the right ventricle, which pumps it through the pulmonary valve and into the pulmonary artery. The pulmonary artery carries it to the lungs, where carbon dioxide is exhaled and fresh oxygen is absorbed. The now oxygen-rich blood returns to the heart’s left side and gets pumped out to the body again.
This means the right side of your heart handles only deoxygenated blood, while the left side handles only oxygenated blood. A muscular wall called the septum keeps the two sides separated so the streams never mix.
Pressure Differences From Oxygenated Blood
Deoxygenated blood travels under much lower pressure than oxygenated blood. Arterial blood is propelled by the powerful left ventricle at pressures typically ranging from 80 to 120 mmHg. Venous blood, by contrast, flows at around 5 to 10 mmHg. The right atrium of the heart, where deoxygenated blood collects, operates at roughly 5 mmHg. This low pressure is why veins have one-way valves inside them: without those valves, gravity would pull blood backward, especially in your legs.
Gas Levels in Venous Blood
Deoxygenated blood still contains oxygen. It hasn’t been completely depleted. In healthy adults, the oxygen pressure in venous blood typically ranges from 25 to 70 mmHg, compared to around 75 to 100 mmHg in arterial blood. Carbon dioxide pressure in venous blood runs between 35 and 59 mmHg, slightly higher than the 36 to 44 mmHg range in arterial blood. The pH of venous blood falls between 7.29 and 7.43, making it just slightly more acidic than arterial blood (7.35 to 7.45), because dissolved carbon dioxide forms a weak acid.
These differences are relatively small. Deoxygenated blood is not “empty” of oxygen or dangerously loaded with carbon dioxide. It simply has less of one and more of the other, enough of a difference that the body’s tissues can extract what they need and offload what they don’t.
Exceptions to the Usual Rules
Most people learn that arteries carry oxygenated blood and veins carry deoxygenated blood. That’s true for the vast majority of your circulatory system, but there are two notable exceptions.
The pulmonary arteries carry deoxygenated blood from the heart to the lungs, while the pulmonary veins carry freshly oxygenated blood back. This is the one place in your body where the roles are reversed. The same reversal happens during pregnancy: the umbilical arteries carry deoxygenated blood from the fetus to the placenta, and the umbilical vein carries oxygenated, nutrient-rich blood back to the fetus. In both cases, the naming follows the direction of flow relative to the heart (arteries flow away, veins flow toward), not the oxygen content.
When Oxygen Levels Drop Too Low
Your body monitors blood oxygen levels constantly. A healthy reading on a pulse oximeter falls between 95% and 100%. When saturation drops below 90%, the condition is called hypoxemia, and it means tissues aren’t getting enough oxygen to function properly. Early signs include shortness of breath, a rapid heartbeat, confusion, and a bluish tint to the lips or fingertips (called cyanosis).
Hypoxemia can result from lung disease, heart problems, severe anemia, or high altitude. It essentially means too much of the blood circulating through arteries resembles what should only be found in veins: dark, oxygen-poor, and loaded with carbon dioxide. A pulse oximeter, the small clip placed on your fingertip, works by shining light through your skin and measuring how much is absorbed by hemoglobin, distinguishing between its oxygenated and deoxygenated forms based on their different colors.