Digestion; The large intestine

Question 1

Large intestine

Answer 1

Any remaining materials enter the large intestine (pH 5–7) through the ileocecal valve, which permits a one-way flow of materials from small to large intestine.
* By the time materials enter the large intestine, most nutrients should have been digested and absorbed
* The large intestine is the location of the final stages of digestion and absorption. Digestion is assisted by the microflora. Water, as well as some minerals and vitamins are absorbed.
* Mucus is produced by goblet cells in the large intestine and lubricate the lining, but no digestive enzymes are released here (these finish in the small intestine)

Question 2

Large intestine:
Regions

Answer 2

Four regions:
1. Caecum; The appendix is attached, which is a twisted tube about 8 cm in length that is almost entirely formed of immune tissue in its wall (containing macrophages and lymphocytes). It also acts as a key reservoir for beneficial gut bacteria
2. Colon; ascending, transverse, descending regions
3. Rectum; pushes stool into anal canal
4. Anal canal; contains an involuntary internal anal sphincter and voluntary external anal sphincter

Question 3

Where are the nutrients absorbed?

Answer 3

Absorption takes place between the stomach and large intestine. By far the most absorption occurs in the small intestine (especially the jejunum)

Question 4

Gut associated lymphoid tissue (GALT)

Answer 4

70% of the immune system is based in the GIT as ‘Gut associated lymphoid tissue’ (GALT)
* GALT is found in the GIT wall (beneath the epithelium) and house the leukocytes – macrophages and lymphocytes. These are in close proximity to the trillions of microbes attaching to the intestinal epithelium (especially in the large intestine)
* These leucocytes learn to identify microbes through this close interaction with bacterial surface antigens (identity markers) located in their cell membranes
* GALT clusters in the tonsils, as well as in the walls of the oesophagus, stomach and intestines. The appendix serves as a key location for GALT
* Secretory IgA is secreted into the GIT mucosa, providing immune defense. IGA production is reduced during periods of stress
* Lipopolysaccharides (LPS) (a type of endotoxin), is released from some bacteria cell walls when they die. These toxins can produce certain symptoms and can damage the gut lining – leaky gut. The toxins enter the blood and can cause systemic inflammation = excessive immune reactions, allergies and autoimmune diseases
IgA = Immunogloblin A