What happens to materials that are not absorbed during digestion, and how are they egested?
What happens to materials that are not absorbed during digestion, and how are they egested?
Answer
Materials that are not absorbed during digestion undergo a series of processes in the large intestine before being egested from the body. Here’s an overview of what happens to these materials and how they are eliminated:
1. Passage to the Large Intestine
After food is digested in the stomach and small intestine, any undigested materials, including dietary fiber and some nutrients, enter the large intestine (colon) as part of the chyme. This material consists primarily of water, electrolytes, undigested food, bacteria, and waste products.
2. Absorption in the Large Intestine
- Water Reabsorption: The primary function of the large intestine is to absorb water and electrolytes from the chyme. Approximately 90% of the water that enters the colon is reabsorbed, which helps to concentrate the waste material into feces .
- Fermentation by Gut Bacteria: The large intestine is home to a diverse microbiota that ferments some undigested carbohydrates (such as fiber) into short-chain fatty acids and gases. These fermentation products can be absorbed and used by the body for energy or other metabolic functions .
3. Formation of Feces
- Composition of Feces: Feces are composed of about 75% water and 25% solid material. The solid components include:
- Undigested food materials (primarily cellulose)
- Bacteria (both live and dead)
- Inorganic substances (such as calcium and iron phosphates)
- Lipids (including cholesterol)
- Cellular debris from the intestinal lining
- Bile pigments (which contribute to fecal color)
As water is absorbed and fermentation occurs, the remaining material is compacted into semi-solid feces.
4. Storage in the Rectum
- Storage Mechanism: Once formed, feces are stored in the rectum until elimination. The rectum has stretch receptors that signal when it is full, creating an urge to defecate. This process involves:
- Peristaltic contractions that move fecal matter toward the rectum.
- Relaxation of the internal anal sphincter (involuntary control) while maintaining contraction of the external anal sphincter (voluntary control) until an appropriate time for defecation.
5. Egestion Process
- Defecation Reflex: When ready to eliminate waste, a series of reflex actions occur:
- The rectal walls contract, pushing feces toward the anal canal.
- The internal anal sphincter relaxes involuntarily.
- If appropriate, voluntary relaxation of the external anal sphincter allows feces to exit through the anus.
This process is coordinated by both involuntary reflexes and conscious control, allowing individuals to choose when to defecate.
Sourav
