Why does an organ system need multiple cell and tissues to function?

Cells make up tissues, tissues make up organs, and organs make up organ systems. For instance, digestive system organs cooperate to process food. The survival of the organism depends on the integrated activity of all the organ systems, often coordinated by the endocrine and nervous systems.

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.