Small and Large Intestine – Anatomy, Function, Flora, Pictures

How a Healthy Bowel Works?

Terms explanation:

  • The GUT refers to the complete digestive tract from the mouth to the anus.
  • The BOWEL consists of small intestine (and this from duodenum, jejunum and ileum), and from large intestine (and this from cecum, appendix, colon, and rectum)
  • The STOMACH is an organ lying between esophagus (gullet) and duodenum; stomach also refers to the whole abdomen.

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Bowel anatomy (Picture 1). The stomach lies right below the bottom end of the breastbone. The small intestine (duodenum, jejunum and ileum) fills up the most of the abdominal cavity. The large intestine starts in the lower right abdomen with the appendix, runs up as the ascending colon toward the liver, then as the transverse colon below the stomach, then on the left side down as the descending colon, in the pelvis it widens into the sigmoid colon, and ends as the rectum and the anal opening (anus).

Anatomy of digestive system, small and large intestine (bowel)

Picture 1. The bowel consists of the small intestine (orange) and large intestine (green)
(source: Wikimedia )

Gut motility. Swallowed food excites the nerves in the esophagus wall and triggers rhythmic contractions of the smooth gut muscles that slowly push the food through the stomach, small and large intestine toward the anus; this process is called peristalsis. Food factors that stimulate peristalsis: food volume, temperature, fiber and liquid content, and low calorie value (4). The gut and the brain are connected with nerves (‘the brain- gut axis’), so the ingested food may influence your mood; on the other hand the pain, thoughts, and emotions, which act through the brain, may stimulate or depress your peristalsis (5). Defecation can be consciously controlled by the muscle, which encircles the anus (external anal sphincter).

The liver, gallbladder and pancreas. In the liver, the bile is produced, temporarily stored in the gallbladder, and then delivered into duodenum via cystic and common hepato-biliary duct. In the pancreas, digestive enzymes are produced and delivered into duodenum via pancreatic duct. The common hepato-biliary and the pancreatic duct join before they open into the duodenum (3). This opening, called papilla Vateri, is embraced by a smooth muscle – sphincter of Oddi.

Digestive enzymes. Proteins are broken down in the stomach by enzyme pepsin, and in the duodenum by the pancreatic enzymes trypsin and chymotrypsin, into amino acids. Carbohydrates are broken down by pancreatic enzyme amylase, and by enzymes from the small intestinal mucosa: sucrase break down sucrose (table sugar); lactase breaks down lactose (milk sugar) into glucose and galactose. Fats are broken down by pancreatic enzyme lipase into fatty acids and glycerol. Bile acids are needed to emulsify fats for digestion.

Nutrient absorption. Most nutrients, water and electrolytes (Na, K, Ca, Mg, Fe) are absorbed in the duodenum and jejunum, while vitamin B12 is absorbed mainly in the terminal ileum. Bile acids are absorbed through the entire small intestine, but mainly in the terminal ileum; after absorption they enter the liver and may be used again. Fats can only be absorbed in the presence of bile, while vitamins A, D, E and K may only be absorbed together with fats. Additional sodium and water are absorbed in the colon. From the intestinal hollow, nutrients enter intestinal mucosal cells and from there venous blood and lymph. From the intestinal mucosa many small finger-like projections (villi) bulge toward the intestinal hollow, and thus greatly increase absorptive surface.

Intestinal Flora (Normal Intestinal Bacteria)

Many bacteria (and some yeasts and protozoa) normally live in a healthy human gut; mostly in the colon, fewer in the small intestine, but none in the stomach. Newborn gets its first bacteria by swallowing them during parturition.

Normal (beneficial) intestinal bacteria can:

  • Prevent the growth of harmful bacteria;
  • Prevent intestinal inflammation;
  • Chemically change bile acids so they can be absorbed when they reach the distal small intestine;
  • Break down fibers, lactose and other carbohydrates into short chain fatty acids, which are absorbed in the colon, enhancing colonic water absorption;
  • Synthesize some vitamin K and vitamin B12.

The composition of the gut flora differs from person to person and depends on age, diet, environment and use of antibiotics (6). Bacteria in the first part of the small intestine (jejunum) are mostly aerobic lactobacilli and enterococci, while those in the distal small intestine (ileum) and colon are mostly anaerobes (Bacteroides, Bifidobacterium bifidum,E. coli, enterococci, clostridia, anaerobic lactobacilli) (6). In breast-fed infants, Bifidobacteria predominate. Some fungi, like Candida albicans, are also a part of normal intestinal flora.

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