The small intestine is about 7.0 m long and 2.5 cm to 3 cm in diameter.
In contrast, the large intestine is about 1.6 m long and about 6 cm in diameter.
Why the difference?
There are two main reasons:
The small intestine is where most digestion occurs.
It takes from 6 h to 8 h for food to move through the small intestine.
This gives the food plenty of time to break down and be absorbed.
The length of the small intestine also maximizes the area of the intestinal mucosa (about ##30color(white)(l) “m”^2##) through which the nutrients are absorbed into the blood and lymph systems.
The primary job of the large intestine is to absorb any remaining nutrients and to push undigested food and waste through to be expelled.
The large intestine absorbs about 1.5 L of water from the waste each day, changing it from a liquid mush to a firmer stool, which is easier to pass.
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