Here's why:

* Arterial blood carries oxygenated blood from the lungs to the rest of the body. It has a bright red color due to the presence of oxygen bound to hemoglobin in red blood cells.

* Venous blood carries deoxygenated blood back to the lungs from the body. It has a darker red color because it has less oxygen bound to hemoglobin.

In summary:

* Arterial blood = high oxygen content

* Venous blood = low oxygen content