Question

Which organ pumps the blood throughout the body?

Answer 1

Hint: Multicellular organisms need an organ system to conduct the body function as their bodies are complex in structure as compared to unicellular organisms. The blood is pumped to the body as it carries and transports various nutrients and oxygen which are needed by the tissues to work. Blood supplies the oxygenated blood and nutrients to the different parts of the body and also helps in the removal of waste ($CO_2$) generated in them during metabolic reaction.

Complete answer:
Multicellular organisms like human beings need a pumping organ to push blood around the body, and a network of tubes to reach all the tissues and body cells. Our heart is the pumping organ which pumps blood throughout the body. It is a muscular organ whose size is equal to the size of our first. Heart is partitioned into 4 chambers to prevent mixing of oxygenated and deoxygenated bloods. The upper chambers of the heart are small in size and have thin muscular walls, they are known as auricles of atria. The two lower chambers are large and thick walled, they are known as ventricles. The deoxygenated blood from the whole body enters the right atrium and then into the right ventricle. Right ventricle pumps the blood into the pulmonary artery which carries this deoxygenated blood to lungs making them oxygenated blood. Through pulmonary veins the blood from the lungs reach the left atrium of the heart and then into the left ventricle. Left ventricle pumps the blood into the artery through which the oxygenated blood is supplied to the whole body.

Note:
When the body size of the animal is large the diffusion pressure alone cannot take care of oxygen delivery to all parts of the body. Instead, respiratory pigments come into action and take up oxygen from the air in the lungs and carry it to the tissue. In human beings, respiratory pigment is haemoglobin which has a very high affinity for oxygen.