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The vital organs are the heart, lungs, liver, and kidney. And they are vital to the survival of all human beings. Here's how each of them develops in your growing baby.

Heart

The baby's heart begins to form around Week 4. The next week, the heart and circulatory system continue to develop and your baby will have a heartbeat. The blood starts pumping and the circulatory system is in action. This system is considered the first functioning organ system. By Week 9, the heart finishes dividing into four chambers and the valves start to form. In the second and third trimesters, the heart grows stronger and gets better at pumping blood....

The vital organs are the heart, lungs, liver, and kidney. And they are vital to the survival of all human beings. Here's how each of them develops in your growing baby.

Heart

The baby's heart begins to form around Week 4. The next week, the heart and circulatory system continue to develop and your baby will have a heartbeat. The blood starts pumping and the circulatory system is in action. This system is considered the first functioning organ system. By Week 9, the heart finishes dividing into four chambers and the valves start to form. In the second and third trimesters, the heart grows stronger and gets better at pumping blood.

Liver

The liver is responsible for cleaning the blood, storing nutrients, and providing needed chemicals to your body. Your baby's liver begins to work by the end of the first trimester. Around Week 22, the liver begins to metabolize bilirubin, a substance produced by red blood cells. (After your baby is born, the liver may have some trouble with this task, resulting in a buildup of bilirubin in the blood. This causes the skin to take on the yellowish hue of jaundice.) By Week 36, the liver has begun processing some waste products.

Lungs

The lungs begin to appear around Week 5 and throughout your pregnancy they will grow like branches on a tree. Instead of leaves or fruit, the branches end in tiny air sacs, called alveoli.

Around Week 13, your baby will start to inhale and exhale small amounts of amniotic fluid, which helps the lungs to develop and grow. (In the womb, your baby gets oxygen from the placenta.)

Around Week 23, your baby's lungs begin to produce surfactant, which allows the air sacs to inflate and keeps them from collapsing and sticking together when they deflate.

After about Week 24, there are enough alveoli for your baby to have a chance of breathing air (with medical help). But the lungs continue to mature up until birth.

Kidneys

The primary role of these bean-shaped organs is to keep the blood clean and chemically balanced. The kidneys remove wastes and extra water from the blood to form urine. In addition to removing waste, your kidneys also release important hormones. These hormones help regulate important functions, such as the production of red blood cells and the growth and maintenance of bones.

Kidneys begin to function around Week 11. By Week 19, your baby's kidneys are already producing urine, which is excreted into the amniotic fluid. By Week 36, the kidneys are fully formed.

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