1. what does the skeletal system protect?
2. in what ways do the skeletal system work with the muscular system?
3. how are muscles attached to bones?
4. what is a compact bone?
5. what is a spongy bone?
6. what is bone marrow and where is it found?

the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.