5- Small Intestine

Question 1

What are the 3 sections of the SI

Answer 1

Duodenum (0.25 m) - key roles in digestion and gut regulation
The jejunum (2.5 m) - key roles for absorption
The ileum (3.75 m) - key roles for absorption

Question 2

What are 3 features of the small intestine

Answer 2

The epithelial layer has finger-like projections called villi which enormously increase the internal surface area. These villi are motile and have a rich blood supply and lymph drainage
The mucosa is arranged in folds (like a concertina)
It has invaginations called crypts of Lieberkühn

Question 3

What is the lifespan of an enterocyte

Answer 3

1-6 days

Question 4

Describe the apical surface of an enterocyte

Answer 4

2000 microvilli on its apical surface ( ‘brush border’)
The apical membrane also has a rich network of carbohydrates called the glycocalyx. This helps to trap a layer of water, mucus and enzymes on the surface of the cell to protect it from the luminal contents (especially enzymes which may damage the cell/membrane), and to regulate digestion and absorption. This layer is referred to as the ‘unstirred layer’.

Question 5

What happens to goblet cell distribution down the si

Answer 5

the number of goblet cells increases along the entire length of the intestine.

Question 6

Name 4 enteroendocytes and what they secrete

Answer 6

G-cells secrete gastrin
I-cells secrete cholecystokinin
S-cells secrete secretin
D-cells secrete somatostatin

Question 7

What are paneth cells

Answer 7

These are immunological cells. They are located at the bottom of the crypts and contain a high number of acidophilic granules. Granules contain:

lysozyme: an antibacterial enzyme
glycoproteins: to protect local cells from enzymes
zinc: a common cofactor for lysozymes

Question 8

Why is the turnover of cells high

Answer 8

The effects of gut-borne toxins/drugs to eventually run their course - it is ‘easier’ to continually replace these cells than protect the huge numbers of cells at risk
Lesions will be short-lived and repaired quickly

Question 9

How do the duodenum, jejunum and ileum differ

Answer 9

Duodenum

Coiled mucous-secreting submucosal gland which secrete a bicarbonate-rich alkaline solution which open up into the base of the Crypts. Helps to neutralise acid chyme and protect the lining of the proximal small intestine and provide an optimum pH for enzymes.

Jejunum

Large submucosal folds called plicae circularis. These are similar to the folds in the rest of the small intestine, but they are considerably larger and closer together. They look much more like frills than folds.

Ileum

The ileum has up to 100 Peyer’s patches, which are aggregates of specialised lymphoid tissue that are key to the immune response to gut-borne pathogens. They are able to initiate leukocyte and immunoglobulin responses to pathogens. These patches contain M-cells (key for immunity, see later lesson) which do not have microvilli like the neighbouring enterocytes.

Question 10

How is intestinal motility achieved

Answer 10

segmentation (mixing) peristalsis (propulsion) and the migrating motor complex (periodic contractions remove food residue)