system
Given the following post, respond as if you're commenting on Reddit:
human
How exactly is unused breast milk absorbed back into the bloodstream?

I read somewhere that whatever milk is left in the breast after weaning gets absorbed back into the body. But what exactly is happening there? How is the body able to absorb nutrients without involving the digestive tract?

I hope these are not extraordinarily dumb questions. I only have middle school level background in biology (and most other things).
gpt
The cells in the breast that produce milk are going to either commit suicide (this is called apoptosis and is completely normal) or the connections between those cells and neighboring cells are removed and the cells die of starvation. Other cells come in and destroy the junk that’s left behind and will get rid of most of the harmful stuff left behind. The clean up cells and the remaining cells will most likely take up and use the nutrients left behind. The destroyed cells are replaced with fat cells. 

Breast milk contains protein, fat, and carbs. ~~All of these things can be pushed into the blood stream and easily degraded by the liver or just released in the urine.~~ 

Edit: a lot of the time, any milk left over after weaning will leak out of the breast. unless the ducts in the breast that carry milk to the nipple are blocked, there will be discharge. release of milk is caused by a few mechanisms and it takes a while (up to months) to destroy the majority of milk producing cells. 

Edit 2: those clean up cells will do all the work (macrophages), VERY little of those proteins, fats, or carbs will enter the blood.