How your digestive system works – Emma Bryce

Across the whole planet, humans eat on average between
one and 2.7 kilograms of food a day. That’s over 365 kilograms
a year per person, and more than 28,800 kilograms
over the course of a lifetime. And every last scrap makes its way
through the digestive system. Comprised of ten organs
covering nine meters, and containing
over 20 specialized cell types, this is one of the most diverse
and complicated systems in the human body. Its parts continuously work in unison
to fulfill a singular task: transforming the raw materials
of your food into the nutrients and energy
that keep you alive. Spanning the entire length of your torso, the digestive system
has four main components. First, there’s the gastrointestinal tract, a twisting channel
that transports your food and has an internal surface area
of between 30 and 40 square meters, enough to cover half a badminton court. Second, there’s the pancreas, gallbladder, and liver, a trio of organs that break down food
using an array of special juices. Third, the body’s enzymes, hormones, nerves, and blood all work together to break down food, modulate the digestive process, and deliver its final products. Finally, there’s the mesentery, a large stretch of tissue that supports and positions all your digestive organs
in the abdomen, enabling them to do their jobs. The digestive process begins
before food even hits your tongue. Anticipating a tasty morsel, glands in your mouth start
to pump out saliva. We produce about 1.5 liters
of this liquid each day. Once inside your mouth, chewing combines with the sloshing saliva to turn food into a moist lump
called the bolus. Enzymes present in the saliva
break down any starch. Then, your food finds itself at the rim of a 25-centimeter-long tube
called the esophagus, down which it must plunge
to reach the stomach. Nerves in the surrounding
esophageal tissue sense the bolus’s presence
and trigger peristalsis, a series of defined muscular contractions. That propels the food into the stomach, where it’s left at the mercy
of the muscular stomach walls, which bound the bolus,
breaking it into chunks. Hormones, secreted by cells in the lining,
trigger the release of acids and enzyme-rich juices
from the stomach wall that start to dissolve the food
and break down its proteins. These hormones also alert the pancreas, liver, and gallbladder to produce digestive juices and transfer bile, a yellowish-green
liquid that digests fat, in preparation for the next stage. After three hours inside the stomach, the once shapely bolus is now
a frothy liquid called chyme, and it’s ready to move into
the small intestine. The liver sends bile
to the gallbladder, which secretes it into the first portion of
the small intestine called the duodenum. Here, it dissolves the fats
floating in the slurry of chyme so they can be easily digested
by the pancreatic and intestinal juices that have leached onto the scene. These enzyme-rich juices break the fat
molecules down into fatty acids and glycerol for easier
absorption into the body. The enzymes also carry out
the final deconstruction of proteins into amino acids and carbohydrates into glucose. This happens in the
small intestine’s lower regions, the jejunum and ileum, which are coated in millions
of tiny projections called villi. These create a huge surface area
to maximize molecule absorption and transference into the blood stream. The blood takes them on the final
leg of their journey to feed the body’s organs and tissues. But it’s not over quite yet. Leftover fiber, water, and dead cells
sloughed off during digestion make it into the large intestine,
also known as the colon. The body drains out most of the remaining
fluid through the intestinal wall. What’s left is a soft mass called stool. The colon squeezes this byproduct
into a pouch called the rectum, where nerves sense it expanding and tell the body when
it’s time to expel the waste. The byproducts of digestion
exit through the anus and the food’s long journey, typically lasting between 30 and 40 hours, is finally complete.