Why does blood have to go first the lungs before entering heart?

This statement is not accurate. Blood does not have to go to the lungs before entering the heart. The heart is, in fact, the first organ that blood flows into after leaving the lungs.

Here's a corrected explanation of the circulatory system:

1. Pulmonary Circulation:

- Deoxygenated blood from the body flows into the right atrium of the heart.

- From the right atrium, it moves to the right ventricle.

- The right ventricle pumps the deoxygenated blood into the pulmonary arteries, which carry it to the lungs.

- In the lungs, the blood releases carbon dioxide and picks up oxygen, becoming oxygenated.

2. Systemic Circulation:

- Oxygenated blood from the lungs returns to the heart via the pulmonary veins, which empty into the left atrium.

- From the left atrium, the blood flows into the left ventricle.

- The left ventricle pumps the oxygenated blood out into the body through the aorta, which is the main artery.

- The aorta branches into smaller arteries that deliver oxygenated blood to various tissues and organs.

So, the heart receives deoxygenated blood from the body through the vena cava, pumps it to the lungs for oxygenation, and then pumps the oxygenated blood to the rest of the body. The lungs are part of the pulmonary circulation, which is a separate circuit from the systemic circulation that supplies oxygen to the body's tissues.