large intestinal transit

Definition

Target type: biologicalprocess

Migration of ingested material along the length of the large intestine. [GOC:sl, PMID:28157109]

Large intestinal transit, also known as colonic transit, is the movement of food residue, known as chyme, through the large intestine. This complex process involves a combination of muscular contractions, water absorption, and microbial activity.

**1. Arrival of Chyme:**
* Chyme, a semi-liquid mixture of partially digested food, arrives from the small intestine into the cecum, the first part of the large intestine.
* The cecum serves as a holding area for the chyme, allowing time for bacterial fermentation and further digestion.

**2. Colonic Motility:**
* The large intestine exhibits a variety of muscular contractions that propel the chyme along its length.
* **Segmental contractions (haustral contractions):** Localized contractions of circular muscle fibers create pouches called haustra, which mix the chyme and increase surface area for water absorption.
* **Peristalsis:** Coordinated contractions of longitudinal and circular muscle layers create waves that propel the chyme forward.
* **Mass movements:** Powerful, infrequent contractions that move large quantities of chyme over significant distances. These occur primarily after meals and facilitate the emptying of the colon.

**3. Water Absorption:**
* As the chyme travels through the large intestine, water is reabsorbed, resulting in the formation of solid feces.
* This water absorption is critical for maintaining fluid balance in the body.

**4. Microbial Activity:**
* The large intestine harbors a vast and diverse community of bacteria, collectively known as the gut microbiota.
* These bacteria play a crucial role in the digestion of indigestible carbohydrates, such as fiber, and the production of short-chain fatty acids (SCFAs), which have various beneficial effects on the host.
* Microbial fermentation also contributes to the production of gas, which is expelled through flatulence.

**5. Formation and Elimination of Feces:**
* As the chyme progresses through the large intestine, it becomes progressively more solid, forming feces.
* Feces consist of undigested food residue, bacteria, water, and other waste products.
* The rectum, the final segment of the large intestine, stores feces until it is eliminated through defecation.

**Factors Influencing Colonic Transit:**
* **Diet:** High-fiber diets tend to increase transit time, while low-fiber diets decrease it.
* **Hydration:** Adequate hydration is crucial for proper colonic transit.
* **Medications:** Certain medications, such as laxatives and antibiotics, can affect colonic motility.
* **Stress:** Stress can alter gut motility and affect transit time.
* **Physical activity:** Exercise can promote colonic motility.

**Disorders of Colonic Transit:**
* **Constipation:** Slowed colonic transit, leading to infrequent bowel movements and difficulty passing stool.
* **Diarrhea:** Accelerated colonic transit, resulting in frequent, loose stools.
* **Irritable bowel syndrome (IBS):** A chronic disorder characterized by abdominal pain, bloating, and altered bowel habits.
* **Inflammatory bowel disease (IBD):** A group of inflammatory conditions that affect the digestive tract, often impacting colonic transit.'
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