Physiology and Anatomy of the Large intestines (The Colon)

After the ILEO-CECAL SPHINCTER materials move to the CECUM where the contents remain fluid. The proximal colon mucosal cells absorb Na+, Cl- and water.

The longitudinal muscle in the colon is gathered into 3 muscular bands called TENIAE COLI which contracts to form HAUSTRA:

The taeniae coli (also teniae coli or tenia coli) are three separate longitudinal ribbons (taeniae meaning ribbon in latin) of smooth muscle on the outside of the ascending, transverse, descending and sigmoid colons. They are visible and can be seen just below the serosa or fibrosa. There are three teniae coli: mesocolic, free and omental taeniae coli. The teniae coli contract lengthwise to produce the haustra, the bulges in the colon. (wikipedia).

Colonic Secretion and Motility

Na+ is absorbed by active transport and water absorption follows.

• Antidiuretic Hormone (ADH or Vasopressin) inhibit Na+ absorption
• Glucocorticoids (eg Cortisol) and Mineralcorticoids (Aldosterone) increase Na+ absorption

GOBLET CELLS - 

• Secrete mucous into the lumen
• Mucous acts as a lubricant for fecal matter and protects the mucosal cells
• Mucous acts as a protective barrier between the bacteria and the mucosal cells

BICARBONATE

• Also secreted in exhange for Cl-
• Provides an alkaline environment to neutralize acids produced by bacteria

Motility of Food:

1. Haustral contractions - Oscillating contractions of circular muscles at 2-6 contractions a minute regulated by the basic electrical rhythm of the smooth muscle
2. Peristalsis
3. Mass-action peristalsis like contactions of large sections of colon muscles move material from section to section
4. Defection is promoted by the neural reflex GASTROCOLIC reflex which responds to GASTRIN and the enteric nervous system promotes distal colon and rectal contraction

Materials in colon are progressively dehydrated in 12-72 hours. Typically 1L of materials enter colon daily and 150-200 g are defecated. 

Contents of stool:

• 75% water
• 25% solids
• Sloughed GI cells
• Digestive juice
• Fiber
• Unabsorbed fat
• Bile
• Bacteria (30% of solid weight)

Colonic Bacteria

Mouth with anaerobic bacteria
Stomach with few bacteria due to pH

• Acid resistant bacteria = lactobacilli and streptococci

Ileum and Colon:

• Gram- positive and negative, 1000s of species 
• Weight of bacteria up to 5lbs
• Mostly anaerobic (outweigh aerobic by 10 fold)
• Bacteroides
• Lactobacilli
• Clostridia
• Bifidbacteria
• Methanogens
• Eubacteria
• Streptococci

Bacteria obtain nutrition from undigested food:

• Starch that has survived pancreatic amylase is used
• Mucins from mucous can be used
• Digestive enzymes can be used
• Others: sugar alcohols (sorbitol, xylitlol), disacchraides (lactose), fibers

Bacterial products:

• B and K vitamins are produced and may be absorbed
• Carbohydrate fermentation by anaerobic bacteria produces lactic acid and 3 short-chain fatty acids (acetic acid, butyric acid, proprionic acid)

Benefits of Short-Chain Fatty Acids:

• Serve to acidify the lumen:
• Free bile acids become less soluble in acidic environments and making bacterial 7 alpha dehydroxylase (like pH ~6.0-6.5) less effective in converting bile to bile acids
• Calcium at lower pH is released from fiber and binds to bile acids thuse preventing conversion to secondary bile acids
• Lactobacilli and  bifidbacteria growth promoted, whereas pH sensitive bacterial growth inhibited
• Mucin production enhanced in acidic environment allowing better barrier protection
• Improved absorption of minerals released during fermentation at low pH
• Serve as signaling molecules
• Exhibit trophic effects
• Improve colonic and splanchnic blood flow
• Increase water and sodium absorption
• Provide energy and serve as substrate for use within cells
• Butyric acid is a major energy source for colonic clls
• Proprionic acid and acetic acid - absorbed and transported to liver
• May inhibit tumors
• Stimulate the immune system

Benefits of Colonic Bacteria

• Enhance host's immune system
• Displace, exclude, or antagonize pathogenic bacteria from colonizing
• Scavenge, sequester, transform, and/or promote excretion of harmful/carcinogenic substances
• Enhance fecal bulk and dilute fecal contents

Disadvantages of Colonic Bacteria

• Production of gas including:
• Methane (CH4)
• Hydrogen (H2)
• Hydrogen sulfide (H2S)
• Carbon dioxide (CO2)
• Products of lactose (as in deficiency of host lactase) fermentation
• Hydrogen gas (which can be detected by breath analysis)
• Products of Amino Acid degredation
• Branched chain amino acids broken into branched chain fatty acids isobutyric acid and isovaleric acid
• Denanimation (removal of the amino group) of aromatic amino acids leads to phenolic compounds
• Histidine bacterial decarboxylation leads to histamine production
• Ammonia is produced from bacterial deanimation of amino acids and bacterial action on urea by bacterial urease. Urea is secreted from blood into GIT.
• 25% of body's urea may be handled in this fashion
• The ammonia can be absorbed by the colon and recirculated to the liver and resued as amino acid
• Failure of liver as in cirrhosis can lead to hepatic encaphlopathy. 

Intestinal Conditions and Probiotics

Imbalances in the number and composition of gut microbiota linked to pathological conditions:

• Inflammatory bowel diseases (Crohn's or UC)
• Colon cancer
• Rheumatoid arthritis
• Diabetes

Probiotics - live organisms that heal by promoting beneficial bacterial colonization of colon

• Lactic acid bacteria (lactobacillus and bifidobacteria)
• 100 million live active bacteria per gram
• Yogurt, buttermilk, kefir, miso, tempeh, soy

Prebiotics - substances that are not digested by host enzymes but act as aids to beneficial bacteria.