why does the blood returning to the heart have very little oxygen in it
the oxygen in the blood has been used by cells in the body.
The blood returning to the heart through the veins has very little oxygen in it because oxygen is delivered to the body's tissues and organs by the arteries, not the veins.
To understand why, let's start with the basics of blood circulation. Oxygen-rich blood flows out of the heart through the arteries, which then branch into smaller blood vessels called arterioles. As the arterioles reach the tissues and organs throughout the body, oxygen diffuses from the blood into the surrounding cells, providing them with the oxygen they need for their metabolic processes.
After releasing oxygen to the tissues, the blood becomes oxygen-depleted and laden with carbon dioxide and waste products. This deoxygenated blood is collected by tiny blood vessels called venules, which merge to form larger veins. Veins carry the deoxygenated blood back to the heart, where it is then pumped to the lungs to pick up fresh oxygen and get rid of carbon dioxide.
So, in summary, the blood returning to the heart through the veins has very little oxygen because the oxygen has already been delivered to the body's tissues through the arteries, and the veins are responsible for carrying the deoxygenated blood back to the heart for reoxygenation in the lungs.