The blood transports oxygen from the lungs to the cells&carbon dioxide from the cells to the lungs.
The heart is a muscular double pump. One side supplies the lungs; the other side supplies the body's organs.
Arteries carry blood away from the heart. They have very thick walls made from muscle tissue.
Veins carry blood towards the heart. Veins have valves inside them that allow the blood to flow in one direction only. Without valves gravity would pull the blood down to your feet.
Capillaries connect arteries and veins. The blood vessels carry blood deep into organs and tissues. The walls of a capillary are made of just a single layer of cells. This allows oxygen and carbon dioxide to pass through easily for respiration.
The heart, blood vessels&blood are part of the circulatory system.
The heart has valves that let the blood flow in one direction. A wall in the middle of the heart separates the left&right sides because they pump separately. The left side has thicker muscular walls to pump the blood all around the body.
The left side of the heart pumps blood from the lungs to the body.
The right side of the heart pumps blood from the body to the lungs.
From the heart the blood goes to the lungs to collect oxygen. Then it goes back to the heart to be pumped around the body to collect glucose from the gut. Blood cells deliver oxygen and glucose to the cells. The blood collects carbon dioxide and water from the cells, then through the heart to the lungs.