What is the large bowel?
The large bowel is the final section of the digestive system. It is a tube about 1.5m long.
It starts on the lower right side, where it connects to the small intestine. From there, it travels upward, across and then down the left side of the abdomen before ending at the rectum – just like a frame around your digestive organs. Hanging off the lower part on the right side is the appendix.
The large bowel has several parts:
- Caecum – a pouch where the small intestine empties. Hanging from it is the appendix.
- Colon, divided on ascending colon (going up the right side), transverse colon (stretching across the upper belly), descending colon (going down the left side) and the sigmoid colon (S-shaped section leading to the rectum).
- Rectum – stores stool.
What is it made of?
As the rest of the gastrointestinal system, the large bowel has four main layers.
Mucosa – innermost layer
- Lines the inside and produces mucus.
- Nerve endings into its folds, or crypts, act like ‘taste buds‘ sensing what’s inside and helping the bowel respond.
- It absorbs water and salts, helping turn leftover digested material into solid stool.
- It separates bacteria from the gut wall.
- Its immune cells protect against threats and it provides nutrition for helpful bacteria to flourish.
If this layer gets damaged, bacteria and other harmful agents could pass into the rest of the body through the blood and lymphatic vessels.
Submucosa
- It contains blood and lymphatic vessels to transport nutrients and immune cells.
- It contains nerves that control gland secretions and adjust water and salt absorption.
- It contains many nociceptors, sensory nerves that detect tissue damage and signal pain. They are part of the gut’s sensory system and trigger protective reflexes when damage is detected.
Muscularis
- Made up of two layers of muscle (longitudinal and circular).
The muscles contract in waves (peristalsis) to push the waste content down the rectum. - The circular layer is a full ring that pushes and mixed the bowel contents.
- The longitudinal muscle in the large intestine isn’t continuous all the way around. It forms three bands, called taeniae coli, which help move the stool and shape the colon.
- The myenteric plexus (a complex network that is part of the bowel’s nervous system) sits between the muscle layers of the large intestine. It controls bowel movement (peristalsis) and also responds to the autonomic nervous system, which can speed up or slow down gut activity. Its many sensory nerves monitor what’s happening in the large bowel and adjust movements accordingly.
Serosa – outermost layer
The serosa envelops the intestine and helps support its structure.
What does the large intestine do?
It absorbs water and takes up salts like sodium and potassium.
Sodium is actively absorbed and potassium can either be taken in or released depending on how much is already in the gut. This movement of salts creates a pull that draws water in, helping the body stay hydrated.
Chloride and bicarbonate are also exchanged to keep the balance of salts and keep everything working properly.
It also hosts trillions of bacteria that protect the gut and make vitamins, like vitamin K and B vitamins. These vitamins are absorbed into the body, which is especially helpful when dietary intake is low.
Monitors gut contents through sensory nerves, which help trigger reflexes to protect the lining and control movement.
The large intestine’s main job is to absorb water and salts and turn leftover food into stool.
How does the body know it’s loo time?
Your body has a clever way of knowing when it’s time to head to the bathroom. As stool fills the rectum, stretch receptors in the wall sense the pressure and send signals to your brain – basically shouting, ‘Hey, we need to go!’
At the same time, the myenteric plexus coordinate the large bowel muscle contractions to push stool toward the rectum.
Sphincters, circular muscles at the end of the gut, control release.
The internal sphincter works automatically. In contrast, the external sphincter is under voluntary control, keeps the anal canal closed by maintaining resting pressure, and is responsible for letting you choose when to go (and this includes farts)!
Together, reflexes and conscious control keep bowel movements smooth and coordinated.
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