Section 6 Flashcards
What are the main components of the large intestine?
The large intestine comprises the cecum, appendix, and colon.
Describe the structure of the large intestine.
Unlike the coiled small intestine, the large intestine consists of three parts: the ascending colon, transverse colon, and descending colon.
The sigmoid colon forms from the end of the descending colon, which straightens out to become the rectum.
Define the cecum, appendix, and colon.
Cecum: A blind-ended pouch located below the junction of the small and large intestines.
Appendix: A small finger-like projection of lymphoid tissue extending from the bottom of the cecum.
Colon: The main portion of the large intestine.
What is the function of the large intestine?
The large intestine primarily processes indigestible food elements, unabsorbed biliary secretions, and some fluid. It also absorbs a small amount of salt and water, with the majority of chyme moving to the lower half of the colon for storage before elimination as feces.
What are haustral contractions?
Haustral contractions are slow, non-propulsive movements of the colon, akin to segmentation in the small intestine. They result from the rhythmic activity of colonic smooth muscle cells and are characterized by the formation and movement of haustra, or sacs, along the colon.
How are haustra formed in the colon?
The colon’s longitudinal smooth muscle layers organize into three bands called taenia coli. These bands are shorter than the underlying smooth muscle and mucosal layers, causing them to bunch up and form sacs known as haustra.
What controls haustral contractions?
Haustral contractions are controlled by locally mediated reflexes involving the intrinsic plexus. They occur at a much lower frequency compared to segmentation in the small intestine, with haustral contractions happening as few as 2 times per hour.
What are the key differences between motility in the small intestine and the colon?
Motility in the small intestine primarily involves weak peristaltic waves that move remnants of meals, debris, and bacteria forward. These waves start at the stomach and slowly progress down the intestine.
In contrast, the colon’s main motility is through haustral contractions, which are slow and non-propulsive. They are generated autonomously by colonic smooth muscle cells and occur at a much lower frequency compared to small intestine motility.
What are mass movements in the colon, and what are they driven by?
Mass movements are marked increases in haustral contractions that occur several times a day, often after meals. These contractions can propel feces 1/3 to 3/4 the length of the colon in a few seconds.
They are driven by the gastrocolic reflex, which is mediated by the release of gastrin from the stomach and extrinsic autonomic nerves.
Describe the defecation reflex.
- When fecal matter fills the rectum, stretch receptors send impulses to the spinal cord, which then sends a reflex signal via the parasympathetic system to the distal colon.
- This signal causes the internal anal sphincter (smooth muscle, involuntary) to relax and stimulates contractions in the sigmoidal colon and rectum.
- If the external anal sphincter (skeletal muscle, voluntary) is also relaxed, defecation occurs. If the timing isn’t appropriate, voluntary control of the external anal sphincter keeps it closed. During defecation, contraction of the abdominal muscles and forcible expiration against the closed glottis increase abdominal pressure to aid in expelling the feces.
What are the secretions of the large intestine?
The large intestine secretes an alkaline mucus containing sodium bicarbonate (NaHCO3) to lubricate and protect the colon by neutralizing acids produced from bacterial fermentation.
no digestion occurs in the large intestine so NO ENZYMES are secreted
How does the large intestine’s slow motility affect bacterial growth?
The slow motility of the large intestine favors bacterial growth. Unlike other digestive organs, the colon does not secrete antibacterial agents, allowing bacteria to thrive. More than 10,000 different species of bacteria comprise the microbiome in the colon.
What roles does the microbiome play in the gut?
The gut microbiome enhances intestinal immunity by competing with potentially pathogenic microbes for nutrients and space, promotes colonic motility, makes nutritional contributions through metabolism, and helps maintain the integrity of the colon mucosa.
How does the intestinal epithelium contribute to the support of the gut flora?
The intestinal epithelium and the intestinal mucosal barrier secreted by the epithelium co-develop in a way that not only provides a barrier to pathogenic organisms but also supports the gut flora.
What is absorbed in the large intestine, and how?
The large intestine primarily absorbs salt and water. Sodium is actively absorbed, while chloride and water follow passively.
