The heart.
Nothing, only the heart was left in the body.
Ancient egyptians believed it was to be weighed in the afterlife.
The organ that is not removed from the body is the heart so that they can weigh it to see if you go to the underworld or not.
Ancient egyptians left the heart in the chest cavity furing mummification since they believed that the heart was what could tell the gods is you where a good person or not in life.
Heart
Oh honey, the Egyptians didn't remove the heart during mummification because they believed it was the center of a person's being, not just a blood-pumping machine. They wanted to keep that bad boy intact for the afterlife journey. Plus, who wants to deal with a squishy heart when you're trying to preserve a body for eternity? Not the Egyptians, that's for sure.
During mummification in ancient Egypt, the brain, lungs, liver, stomach, and intestines were typically removed from the body. The only organs left in the body were the heart and kidneys, which were believed to be crucial for the afterlife. The removed organs were preserved separately in canopic jars.
It wasn't. Mummification was used by anyone who could afford it and for animals.
The liver is the only organ that crosses into all four quadrants of the abdomen: right upper quadrant, left upper quadrant, right lower quadrant, and left lower quadrant.
Mummification was the process of preserving the "Khet" (physical body) for the afterlife. It consisted of removing vairous internal organs and then dehydrating while embalming the body, before wrapping it in linen and placing it in the burial tomb. During Old Kingdom Egypt, afterlife was a royal privilege only, in the New Kingdom Egypt the afterlife to those who could afford it. The preperations for mummification were designed to guarantee a resting place for their spirit for all time and a place for their mortuary cult to be maintained by the relatives they left behind.
The priests performed mummification. No-one else did mummification.
I will assume you mean the organ that makes the left lung slightly smaller than the right and causes the apparent dent in the medial side. This organ is the heart. The reason the heart only causes the space in the left lung is that the left ventricle is far larger than the right and protrudes into the space where the lung would otherwise grow. This is the reason that people mistakenly think the heart is in the left of the chest. In reality it sits directly behind the sternum (very center of the thorax) and only the left ventricle sticks over to the left.