The easiest example: the uterus shrinks to its normal size after a while, but how does the body get rid of the extra tissue? How does it get metabolized? Through the kidneys, liver, guts…?
Any extra tissue that the woman gains through pregnancy will be fat and skin. Excluding the mammary glands, which will grow and start producing milk as part of a process which continues long after pregnancy and is outside the scope of your question.
The fat will be gotten rid of the same way it always is. Some of the water in it will be processed through your kidneys and you will urinate it out, or become sweat. Some it will be exhaled as you breathe.
Skin will shrink, although often not to the size or shape it was before. This is really the only organ that should be affected by the pregnancy (again, excluding mammary glands). Other organs may grow more fat on them as well. If other organs are affected in some way, that’s a medical problem which needs to be addressed.
Much of the extra size is caused by blood and fluid in the tissue. A pregnant person’s total blood volume increases by 45% on average. After pregnancy excess blood volume is excreted via the kidneys, thus decreasing the tissue’s apparent size
Not to make light of your question but that’s like asking what happens to the extra tissue after you un-flex your bicep. The baby, amniotic fluid, and the placenta are the *extra* tissue. The uterus itself is basically muscle, and specialized cells that collectively help the uterus do what it does and these cells either slough off monthly or are part of the placenta/after birth. All *extra* cells come out of the vagina ether way.
Extra tissue, do you mean lochia? It’s the discharge that happens after giving birth. Other than that, the uterus just shrinks back to its normal size like a balloon that had all the air let out