Does blood have oxygen when it enters the heart?

Every cell, tissue, muscle, etc. needs oxygen to perform its function. The heart is a smooth muscle which needs oxygen and circulates oxygen throughout the body.

When we breath out, the lungs release carbon dioxide. When we breathe in, oxygen molecules collect on small sacs in the lungs called aveoli. The aveoli look like upside-down groups of grapes, at the farthest ends of the bronchioles. As blood passes the aveloi, oxygen attaches to hemoglobin on red blood cells. The arteries push the blood through the heart to the brain...and to the major abdominal organs...then to the legs and feet...

All along the way, red blood cells "drop off" nutrients and oxygen to tissues through special processes within cells.

Then, from your legs, abdomen (ribcage to privates), arms, and your brain, capillaries return carbon dioxide and cell waste up through your body using special valves in the blood vessels (the valves only open one way - back toward the heart). The blood passes through the heart, to the lungs where it drops off carbon dioxide which you breathe out. At the same time, the kidneys act to filter certain waste products carried by venous blood (venous is non-oxygenated--- arterial is oxygenated). Although a heartbeat happens in a split second, with every heart beat the blood makes this continuous route out through your body and back.

As such, the respiratory system and cardiac system work in conjunction with each other all the time. If one of these fails, death can result. If one system becomes diseased, both systems begin to work harder to do the same jobs.