Oxygen-poor blood is found in the right side of the heart, the veins throughout your body, and the pulmonary arteries that connect the heart to the lungs. In a healthy person, this blood still carries about 60 to 80% of its oxygen capacity. It’s not empty of oxygen, just significantly lower than the near-100% saturation of freshly oxygenated blood leaving the lungs.
The Right Side of the Heart
The heart is split into left and right halves, and each half does a completely different job. The right side handles only oxygen-poor blood. After your body’s tissues have used up the oxygen they need, that darker, oxygen-depleted blood flows back toward the heart through two large veins: the superior vena cava (draining the head, arms, and upper body) and the inferior vena cava (draining the legs, abdomen, and lower body). Both empty into the right atrium, the heart’s upper-right chamber.
From the right atrium, blood is pumped down into the right ventricle, the lower-right chamber. The right ventricle then pushes it out through the pulmonary arteries toward the lungs. This is the only place in the body where arteries carry oxygen-poor blood. Everywhere else, arteries carry oxygen-rich blood. The naming follows the vessel type (artery means “away from the heart”), not the oxygen content.
Veins Across the Entire Body
Nearly every vein in your body carries oxygen-poor blood. Once arterial blood reaches the tiny capillaries embedded in your tissues, oxygen passes through the thin capillary walls and into surrounding cells. The blood left behind has given up a substantial portion of its oxygen and picked up carbon dioxide as a waste product. It then enters small veins called venules, which merge into progressively larger veins until everything funnels back to the vena cava and into the heart’s right atrium.
The oxygen level in venous blood typically ranges from 30 to 40 mmHg of partial pressure, compared to around 100 mmHg in arterial blood. That translates to a venous oxygen saturation of roughly 60 to 80%. The exact number depends on how hard a particular tissue is working. Muscles during exercise extract far more oxygen than resting skin, so the venous blood leaving active muscles is more depleted than blood draining from less metabolically active areas.
The Pulmonary Arteries
The pulmonary arteries are the final stop for oxygen-poor blood before it gets refreshed. These vessels carry blood from the right ventricle directly into the lungs, where it flows through a dense network of pulmonary capillaries surrounding the air sacs. In healthy individuals, pulmonary artery blood has an oxygen saturation of about 76%, even in people breathing pure oxygen. That’s because the pulmonary artery simply reflects the mixed venous blood from the entire body.
Once blood passes through the lung capillaries, oxygen diffuses across from inhaled air into the blood, and carbon dioxide moves in the opposite direction to be exhaled. The now oxygen-rich blood exits through the pulmonary veins (the only veins carrying oxygen-rich blood) and enters the left atrium of the heart, ready to be pumped out to the body again.
How Blood Loses Its Oxygen
Oxygen leaves the blood through a process called diffusion at the capillary level. Capillaries are so narrow that red blood cells pass through in single file, and their walls are only one cell thick. Oxygen naturally moves from areas of higher concentration (inside the blood) to lower concentration (in the surrounding tissue). The more capillaries that are open in a given area, and the higher the blood’s oxygen content, the more oxygen gets delivered per second.
At the molecular level, hemoglobin is the protein inside red blood cells responsible for carrying oxygen. Each hemoglobin molecule contains iron, which binds oxygen in the lungs and releases it in the tissues. When hemoglobin releases its oxygen, it shifts into a more rigid shape that makes it harder to pick up new oxygen molecules until it returns to the lungs. This structural change is also what makes oxygen-poor blood appear darker red.
Why Veins Look Blue (but the Blood Isn’t)
A common misconception is that oxygen-poor blood is blue. It’s not. All human blood is red, always. Oxygenated blood is bright red, and deoxygenated blood is a darker, more maroon red. The difference comes from hemoglobin’s iron-containing structure, which absorbs blue and green wavelengths of light and reflects red regardless of how much oxygen it carries.
Veins appear blue through the skin because of an optical illusion. Red light has a long wavelength and penetrates 5 to 10 millimeters deep into tissue, which is roughly how deep your veins sit. Your tissues absorb most of that red light before it can bounce back to your eyes. Blue light, with its shorter wavelength, can’t travel as far, so it reflects off the vein area and back to your eyes. The result: you see blue veins, even though the blood inside them is dark red.
One Exception: Fetal Circulation
Before birth, the system works differently. A fetus doesn’t breathe air, so the lungs aren’t involved in oxygenation. Instead, oxygen comes from the mother’s blood through the placenta. Oxygenated blood travels from the placenta to the fetus through the umbilical vein (with a saturation of about 70 to 80%), while the fetus sends its oxygen-poor blood back to the placenta through two umbilical arteries, with saturation dropping to around 40%. After birth, these vessels close off as the baby’s lungs take over the job of oxygenating blood, and circulation shifts to the adult pattern.