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Alimentary System > Small Intestine Structure and Function > Flashcards

Small Intestine Structure and Function Flashcards

1
Q

how long is the duodenum?

A

25 cm long

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2
Q

What is the length of the small intestine?

A

About 6m

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3
Q

What does the small intestine consist of?

A

Duodenum

Jejunum

Ileum

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4
Q

What are some functions of the duodenum?

A

Gastric acid neutralisation

Digestion

Iron absorption

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5
Q

How long is the jejunum?

A

2.5m long (2/5 of total length)

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6
Q

What are some functions of the jejunum?

A

Nutrient absorption

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7
Q

What percentage of nutrient absorption occurs in the jejunum?

A

95%

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8
Q

How long is the ileum?

A

3.5m (3/5 total length)

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9
Q

What is the function of the ileum?

A

NaCl/H2O absorption

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10
Q

What is the absorptive surface of the small intestine enhanced by?

A

Folds, villi and microvilli

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11
Q

What are the circular folds of the small intestine called?

A

Plicae

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12
Q

What is the relative surface area increase of the small intestine due to plicae?

A

3

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13
Q

What is the relative surface area increase of the small intestine due to villi?

A

30

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14
Q

What is the relative surface area increase of the small intestine due to microvilli?

A

600

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15
Q

What is the classification of the epithelium in the small intestine?

A

Simple columnar

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16
Q

What does villus cells absorb?

A

NaCl

Monosaccharides

Amino acids

Peptides

Fats

Vitamins

Minerals

Water

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17
Q

What are the functions of crypt cells?

A

Secrete Cl and water

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18
Q

How much water does the small intestine secrete everyday?

A

1500ml

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19
Q

Where does the water that the small intestine secretes come from?

A

Epithelial cells lining the crypts of Lieberkuhn

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20
Q

How does the small intestine secrete water passively?

A

as a consequence of active secretion of chloride into intestinal lumen

21
Q

Why is H2O secretion important for normal digestive processes?

A

Maintains lumenal contents in liquid state

Promotes mixing of nutrients with digestive enzymes

Aids nutrient presentation to absorbing surface

Dilutes and washes away potentially injurious substances

22
Q

What happens to H2O secreted by crypts?

A

Normally H2O secreted by crypts reabsorbed by villi

23
Q

What are the 2 distinct kinds of movements that makes up intestinal motility?

A

segmentation
peristalsis

24
Q

What is segmentation?

A

occurs when the intestine divides into many sections due to simultaneous transient contractions, and when these contractions subside, contractions appear more or less at the sites in between the original contractions giving a checkered appearance.

Contraction ⇔ relaxation of short intestinal segments
Contraction (few seconds) moves chyme (up & down) into adjacent areas of relaxation
Relaxed areas then contract and push chyme back

25
Q

When is segmentation most common?

A

most common during a meal

26
Q

What is the function of segmentation?

A

Provides thorough mixing of contents with digestive enzymes

Brings chyme into contact with absorbing surface

27
Q

How long does each cycle of segmentation last?

A

few seconds

28
Q

Explain the mechanism of segmentation contractions?

A

Initiated by depolarisation generated by pacemaker cells in longitudinal muscle layer (cf gastric motility)

Intestinal basic electrical rhythm (BER) produces oscillations in membrane potential ⇒ threshold ⇒ action potential ⇒ contraction

Action potential frequency determines strength of contraction

Frequency of segmentation determined by BER

29
Q

What is BER?

A

The basal or basic electrical rhythm (BER) or electrical control activity (ECA) is the spontaneous depolarization and repolarization of pacemaker cells known as interstitial cells of Cajal (ICCs) in the smooth muscle of the stomach, small intestine, and large intestine.

30
Q

What determins the strength of contraction for segmentation?

A

Action potential frequency determines strength of contraction

31
Q

What is frequency of segmentation determined by?

A

Frequency of segmentation determined by BER

32
Q

How does BER change as chyme moves down the intestine to the rectum?

A

BER decreases as move down intestine → rectum

33
Q

What supplies the innervation for segmentation?

A

Parasympathetic NS (vagus) ⇒ ↑ contraction
Sympathetic NS ⇒ ↓ contraction
No effect of autonomic nervous system on BER

34
Q

Does the autonomic nervous system have an effect on BER?

A

no effect

35
Q

What effect does the parasympathetic nervous system have on segmentation?

A

↑ contraction

36
Q

What effect does the sympathetic nervous system have on segmentation?

A

↓ contraction

37
Q

What does segmentation cause?

A

Following absorption of nutrients: segmentation stops, and peristalsis starts

38
Q

What occurs following absorption of nutrients in terms of segmentation and peristalsis?

A

Migrating Motility Complex (MMC)

39
Q

What are the pattern of contractions that creates peristalsis known as?

A

Migrating Motility Complex (MMC)

40
Q

Explain the mechanism of peristalsis?

A

Pattern of peristaltic activity travelling down small intestine (starts in gastric antrum)

As one MMC ends (terminal ileum) another begins

Arrival of food in stomach ⇒ cessation of MMC and initiation of segmentation

41
Q

Where does peristalsis begin and end?

A

gastric antrum
terminal ileum

42
Q

How does the arrival of food into the stomach impact segmentation and peristalsis?

A

cessation of MMC and initiation of segmentation

43
Q

What is the function of peristalsis?

A

Move undigested material into large intestine

Limit bacterial colonisation of small intestine

44
Q

What is a hormone involved in the initiation of MMC?

A

Motilin (hormone) involved in initiation of MMC

45
Q

What happens when intestinal smooth muscle is distended such as due to chyme?

A

If intestinal smooth muscle is distended (eg by bolus of chyme):
Muscle on oral side of bolus contracts
Muscle on anal side of bolus relaxes
Bolus is moved into area of relaxation towards colon

Mediated by neurones in myenteric plexus

46
Q

What is the bolus moving towards the colon mediated by?

A

neurones in myenteric plexus

47
Q

Explain the mechanism of the gastroileal reflex?

A

Gastric emptying ⇒ ↑ segmentation activity in ileum

Opening of ileocaecal valve (sphincter)

Entry of chyme into large intestine

Distension of colon

Reflex contraction of ileocaecal sphincter (prevents backflux into small intestine)

48
Q

What reflex controls chyme entering the large intestine?

A

Gastroileal Reflex

Alimentary System (44 decks)

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