Oxygen in the air is very important for human beings to survive. And it is the job of the blood to carry this oxygen to the body.
Blood is like a river that flows through the body, and it is spread throughout the body. However, not all blood is red. The red color is due to the red hemoglobin contained in red blood cells called red blood cells. Hemoglobin is made of iron and is red in color. This hemoglobin fulfills the blood's function of carrying oxygen throughout the body.
Simply put, blood is red because the hemoglobin in it is red.

Hemoglobin is made up of a protein called globin bound to a pigment called heme. Strictly speaking, it is this heme that is red. The source of this redness is thought to be the iron atoms in heme.