Highly simplified, consider this....
When you breathe in, an oxygen molecule jumps into a red blood cell and gets transported around your body. Then when it finds its target cell, the oxygen molecule jumps off.... and a carbon-dioxide molecule jumps on. And then the red blood cell transports the C02 molecule back to the lungs where it is exhaled....
....red blood cells are like a taxi. Every cell in your body needs oxygen to stay alive, let alone grow and become bigger and stronger (in the case of skeletal muscle cells). And vice versa for C02, which is toxic and poisonous. The red blood cells take oxygen from the outside of your body, into the working cells.... and vice versa for C02.
When you smoke, you inhale carbon monoxide.... and it attaches to the red blood cell. This means oxygen can't get to the working cells, and carbon dioxide can't leave the body.
When you smoke, you're starving your body cells of the oxygen they need to live.
Not to mention what you're doing to your lungs.... coating the inside of the alveoli, so that gas exchange can't occur. Every cigarette you smoke, you're reducing the surface area your lungs has to breathe.