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small bowel Flashcards

1
Q

Q. What is the function of the small bowel?

A

To absorb nutrients, salt & water

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2
Q

what is the duodenum

A

The duodenum is the first segment of the small intestine. It’s largely responsible for the continuous breaking-down process

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3
Q

whta is the jejunum

A

The middle part of the small intestine. It is between the duodenum (first part of the small intestine) and the ileum (last part of the small intestine). The jejunum helps to further digest food coming from the stomach.

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4
Q

what is the ileum

A

The ileum is the final section of the small intestine. The function of the ileum is mainly to absorb vitamin B12, bile salts, and any products of digestion that were not absorbed by the jejunum

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5
Q

what is the transition between them

A

no sudden transition between them

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6
Q

how do the histological organistaion differ

A

All have same basic histological organisation

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7
Q

what is the mesentry

A

The mesentery attaches your intestines to the wall of your abdomen. This keeps your intestines in place, preventing it from collapsing down into your pelvic area.

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8
Q

functions

A

Suspends small & large bowel from posterior abdominal wall
anchoring them in place
whilst still allowing some movement

Provides a conduitfor blood vessels, nerves & lymphatic vessels.

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9
Q

where can you find villi

A

only occur in the small intestine

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10
Q

what is the blood and lymph supply like

A

Have a rich blood supply & lymph drainage for absorption of digested nutrients

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11
Q

what is the nerve innervation like

A

Have good innervation from the submucosal plexus.

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12
Q

describe the structure of the villi

A

Have simple epithelium
1 cell thick
dominated by enterocytes (columnar absorptive cells)
motile

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13
Q

what are enterocytes

A

(columnar absorptive cells)

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14
Q

what are villi lined with

A

simple columnar epithelium consisting of:
primarily enterocytes (absorptive cells)
scattered goblet cells
enteroendocrine cells

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15
Q

what is in the crypt epithelium

A

Crypts of Lieberkühn - epithelium includes:
Paneth cells
Stem cells

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16
Q

what is the most abundant cell in the small bowe,

A

enterocystes

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17
Q

describe their structure

A

Tall columnar cells with microvilli & a basal nucleus.

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18
Q

function

A

Specialised for absorption & transport of substances.

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19
Q

lifespan

A

Short lifespan of 1-6 days.

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20
Q

how is the surface area increased

A

Folds, villi & microvilli ^ surface area

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21
Q

what does the microvilli make up

A

brush border

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22
Q

what are the microvilli made up of

A

glycocalyx

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23
Q

what is a glycocalyx

A

rich carbohydrate layer on apical membrane

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24
Q

function

A

serves as protection from digestional lumen
yet allows for absorption.
regulates rate of absorption from intestinal lumen

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25
what is the unstirred layer
the glycocalyx traps a layer of water & mucous known as “unstirred layer”
26
what do mucous cells contain apical
Mucous containing granules accumulate at apical end
27
what is mucous
→ large glycoprotein that facilitates passage of material through bowel.  
28
how does the compisition of goblet cell change through the legnth of the bowel
^ abundance of goblet cells along entire length of bowel
29
what are enterendocrine cells | what is their structure
Columnar epithelial cells
30
where are they commonly found
most often found in lower part of crypts.
31
function
Hormone secreting | e.g. to influence gut motility (see Regulation of function lecture)  
32
where are paneth cells found
Found only in the bases of crypts
33
what do they contain
Contain large, acidophilic granules
34
what are the contents of the granules
antibacterial enzyme lysozyme (protects stem cells) | Glycoproteins & zinc (essential trace metal for a no. of enzymes)
35
what is another function
Also engulf some bacteria & protozoa May have a role in regulating intestinal flora
36
what are stem cells
Undifferentiated cells which remain capable of cell division to replace cells which die
37
why does the GI tract have stem cells
Epithelial stem cells are essential in the GI tract to continually replenish the surface epithelium Differentiate into various cell types (pluripotent)
38
describe their life cycle
Migrate up to tip of villus, replacing older cells that die by apoptosis -> digested and reabsorbed
39
what is the relative life cycle of enterocytes and goblet cells of the small bowel
short life span (about 36 hrs)
40
what is their relative lifespan in comparison to other epithelial cells
Rapid turnover contrasts with lifespan of weeks/months for other epithelial cell types (e.g. lung, blood vessels)
41
why are the lifespans so different
Enterocytes are first line of defense against GI pathogens & may be directly affected by toxic substances in diet. Effects of agents which interfere with cell function, metabolic rate etc will be diminished. Any lesions will be short-lived. If escalator-like transit of enterocytes is interrupted through impaired production of new cells (e.g. radiation) severe intestinal dysfunction will occur
42
how can you distniguish between the duodenum and the jeunum and iluem
Distinguished by presence of Brunner’s glands
43
how does the duodenum secreate alkaline fluid
Submucosal coiled tubular mucous glands secreting alkaline fluid
44
why the alkaline secreation
Neutralizes acidic chyme from stomach, protecting proximal small bowel Help optimise pH for action of pancreatic digestive enzymes.
45
Functions of small intestine motility
To mix ingested food with digestive secretions & enzymes To facilitate contact between contents of intestine & the intestinal mucosa To propel intestinal contents along alimentary tract
46
what are the 3 phases of motility
Segmentation (mixing) 2. Peristalsis (propelling) 3. Migrating Motor Complex
47
what is the segmentation
Mixes contents of lumen Occurs by stationary contraction of circular muscles at intervals. More frequent contractions in duodenum cf. ileum allow pancreatic enzymes & bile to mix with chyme Although chyme moves in both directions, net effect is movement → colon
48
what is peristalisis
. Peristalsis (propelling) Involves sequential contraction of adjacent rings of smooth muscle Propels chyme towards colon Most waves of peristalsis only travel about 10cm Segmentation & peristalsis result in chyme being segmented, mixed & propelled → colon
49
3. Migrating Motor Complex
Cycles of smooth muscle contractions sweeping through gut Begin in stomach → small intestine → colon → next wave starts in duodenum Prevents migration of colonic bacteria into ileum
50
describe the digestion in the small bowel
Digestion in small bowel occurs in an alkaline environment
51
how do digestive enzyme enter the duodenum and bile enter
Pancreatic digestive enzymes & bile enter duodenum from MPD & CBD
52
why doesn't the duodenal epithelial produce any of its oen digestive enzymes
Duodenal epithelium also produces its own digestive enzymes
53
where does digestion occur in the dueodnum
Digestion occurs in lumen & in contact with the membrane
54
what are the 2 carbihydrate strcutures
``` Simple carbohydrates (monosaccharides - glucose & fructose disaccharides - sucrose & maltose) ``` Complex carbohydrates (starch, cellulose, pectins → sugars bonded together to form a chain)
55
what is the first step in digetsion of carbs | and where does most of it take place
Digestion begins in mouth by salivary -amylase (destroyed in stomach (acid pH) Most of digestion of carbohydrates occurs in small intestine
56
when is Pancreatic a-amylase released
Secreted into duodenum in response to a meal
57
what is the optimum envoroment
Needs Cl- for optimum activity & neutral/slightly alkaline pH
58
where does pancreatic a-amylase act
Acts mainly in lumen (some also adsorbs to brush border) | Digestion of amylase products & simple carbohydrates occurs at the brush border
59
how is glucose and galactose absorbed
2o | active transport
60
what carrier protien
= SGLT-1 on apical membrane
61
how is fructose absorbed
by facilitated diffusion.
62
how
= GLUT-5 on apical membrane
63
what is the function of glut 2
facilitates exit at basolateral membrane
64
where are protiens first digested
Protein digestion begins in lumen of stomach by pepsin | pepsin then inactivated in alkaline duodenum
65
what then occurs in the small bowel
5x pancreatic proteases secreted as precursors → lumen of small bowel (e.g. trypsinogen)
66
what activates trypsin
enterokinase
67
where is enterokinase
an enzyme located on duodenal brush border
68
what does trypsin do
activates other proteases | hydrolyse proteins → single amino acids (AA) & oligopeptides (AA)n
69
what is the junction between the duodenam and jejunum
duodenojejunal flexure
70
what is the outer part of the small bowel called
serosa
71
what os under that
longitudinal and then circular muscle layer
72
what is cu=ircular muscle iimportant for
motility
73
what is brunners gland
submucosal coiled tubular mucous glands secreting alkaline fluid
74
where do they open
in the cypt
75
what is the difference between the jejunum and ileum
jejunum is wider and thicker walled | redder
76
where is the cypt
base of the villi
77
why is the jejunum more thicker
more plicae circulares
78
where do peptidases act
Action of luminal, brush-border & cytosolic peptidases
79
what do enterocytes absorb
Enterocytes directly absorb some of small (AA)n via action of H+/oligopeptide cotransporter PepT1
80
are lipids soluble in water
Lipids are poorly soluble in water | more complicated to digest.
81
how are they digested
Secretion of bile salts & pancreatic lipases Emulsification (↑s surface area for digestion) Enzymatic hydrolysis of ester linkages (Colipase complexes with lipase – prevents bile salts displacing lipase from fat droplet) Solubilisation of lipolytic products in bile salt micelles
82
how are lipids absorbed
Unlike AAs & simple sugars -> lipids transformed as absorbed via enterocytes
83
describe their absoroption
Fatty acids (FAs) & monoglycerides (MG) leave micelles and enter enterocytes FAs & MG resynthesized into tri-glycerides (TGS) by 2x pathways: ``` Monoglyceride acylation (major) Phosphatidic acid pathway (minor) ``` Chylomicrons - lipoprotein particles synthesised as an emulsion (80-90% TGs, 8-9% phospholipids, 2% cholesterol, 2% protein, trace carbohydrate) in Golgi apparatus Chylomicrons secreted across basement membrane by exocytosis Chylomicrons enter a lacteal (lymph capillary) → lymph transports them away from bowel
84
what is the Ileocaecal valve
Ileum is separated from the colon by the ileocaecal valve
85
function
Relaxation & contraction controls passage of material into colon Also prevents back flow of bacteria into ileum

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