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HUBS192 2024 > Lecture 27 > Flashcards

Lecture 27 Flashcards

Absorption in GIT

1
Q

What is absorption?

A

Movement of solutes and water from the GIT lumen across the epithelium, into the interstitial fluid and into the blood or lymph

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2
Q

How much absorption occurs in the mouth, oesophagus and stomach?

A

Very minimal

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3
Q

What is the main site of absorption?

A

The small intestine

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4
Q

What does the small intestine absorb?

A

About 90% of water and sodium, and all nutrients

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5
Q

How much absorption, and of what, happens in the large intestine?

A

9% of water and sodium

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6
Q

How does absorption occur in the small intestine?

A

Bulk absorption in leaky epithelium

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7
Q

How does absorption occur in the large intestine?

A

Regulated / fine tuning absorption in tight epithelium

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8
Q

What are the factors that affect absorption? (5)

A

Reduction in nutrient size, motility, transport across epithelium. surface area available and removal from interstitial fluid

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9
Q

Why must food and chyme travel at the correct rate through the GIT?

A

To allow mechanical and chemical digestion and absorption

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10
Q

What does the rate of food/chyme travel through the GIT depend on?

A

Storage in stomach and SI and, peristalsis in the stomach, SI and LI

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11
Q

What is segmentation?

A

Motility breaking down food to affect the exposure of nutrients to absorptive surfaces

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12
Q

What are the two pathways of transport across GIT epithelium absorption can take?

A

Paracellular or transcellular

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13
Q

What is the transcellular pathway?

A

Across the cell membrane and through the cytoplasm

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14
Q

What is the paracellular pathway?

A

Movement through the tight junctions between cells

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15
Q

What does movement across GIT epithelium require?

A

A driving force

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16
Q

What are the three different driving forces?

A

ATP, electrochemical gradients, osmotic gradients

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17
Q

Is the movement of nutrients via the paracellular pathway passive or active?

A

Passive

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18
Q

How selective is the paracellular pathway?

A

Dependent on type of tight junction - tight / leaky - and size of particle

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19
Q

How do lipid-soluble particles diffuse through the transcellular pathway?

A

Able to diffuse directly through the membrane if there is a gradient

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20
Q

How do water-soluble particles diffuse through the transcellular pathway?

A

They require a channel or transporter membrane protein with a driving force

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21
Q

How does surface area affect absorption?

A

The rate of particle absorption is proportional to the surface area

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22
Q

How do anatomical adaption maximise the surface area of the small intestines?

A

Long length, circular folds, villi and microvilli

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23
Q

How is absorption maximised within a fixed surface area?

A

Through reducing nutrients to their smallest unit via chemical digestion and through using specific transport proteins to absorb particles

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24
Q

Why do particles need to be removed from the interstitial fluid after absorption?

A

To prevent build up

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25
Q

How are particles removed from interstitial fluid after absorption?

A

Through having high blood supply to the intestines and blood vessels & lacteals close to the basolateral side of epithelial cells

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26
Q

What absorption mechanism do carbohydrates use in the SI?

A

Passive and active

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27
Q

What is the driving force allowing carbohydrates to absorb passively?

A

A glucose concentration gradient, allowing them to diffuse down through the leaky tight junctions

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28
Q

How do carbohydrates absorb actively across the apical membrane? (transcellular)

A

Using secondary active transport via the sodium-glucose cotransporter

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29
Q

How do carbohydrates absorb actively across the basolateral membrane? (transcellular)

A

Using facilitated diffusion via the glucose carrier

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30
Q

What is the driving force allowing carbohydrates to travel actively through the transcellular pathway?

A

The sodium gradient generated by Na+ / K+ ATPase

31
Q

What absorption mechanism do amino acids from proteins use in the SI?

A

Passively and actively

32
Q

What is the driving force allowing amino acids to absorb passively?

A

An amino acid concentration gradient, allowing them to diffuse down through the leaky tight junctions

33
Q

How do amino acids absorb actively across the apical membrane? (transcellular)

A

Using secondary active transport via the sodium-amino cotransporter

34
Q

How do amino acids absorb actively across the basolateral membrane? (transcellular)

A

Using facilitated diffused via the amino acid carrier

35
Q

What is the driving force allowing amino acids to travel actively through the transcellular pathway?

A

The sodium gradient generated by Na+ / K+ ATPase

36
Q

What absorption mechanism do di- and tri- peptides from proteins use in the SI?

A

Only active transport via the transcellular pathway

37
Q

What is the driving force allowing di- and tri- peptides to travel actively through the transcellular pathway?

A

The negative membrane potential generated by Na+ / K+ ATPase, generates an electrical gradient for H+

38
Q

How do di- and tri- peptides absorb actively across the apical membrane? (transcellular)

A

Using secondary active transport via the H+ - peptide cotransporter

39
Q

What happens to di- and tri- peptides within the cytoplasm of the cell?

A

Peptidase digests them into amino acids

40
Q

How do di- and tri- peptides absorb actively across the basolateral membrane? (transcellular)

A

Using facilitated diffused via the amino acid carrier

41
Q

What happens to free fatty acids and monoglycerides in the lumen and apical membrane during lipid absorption?

A

Free fatty acids and monoglycerides move by simple diffusion from micelles into the cell

42
Q

Are micelles absorbed during lipid absorption?

A

No

43
Q

Where in the SI are bile salts absorbed in the digestive process?

A

Absorbed in the ileum.

44
Q

What happens to free fatty acids and monoglycerides intracellularly during lipid absorption?

A

They are transported to the endoplasmic reticulum and resynthesized into triglycerides, which are then packaged into chylomicrons

45
Q

What happens to triglycerides at the basolateral membrane during lipid absorption?

A

Triglycerides (TAGs) are exocytosed in chylomicrons and enter the lymph via lacteals of the villi

46
Q

How is water lost during the day?

A

Via sweat, urine, faeces, respiration and secretion into the GIT

47
Q

What forms of secretion are secreted into the GIT?

A

Salivary, gastric, pancreatic, biliary and small intestinal secretion

48
Q

How much water in secreted into the GIT per day?

A

~8L

49
Q

How much water arrives in the SI per day?

A

~9-10L

50
Q

How is water absorbed?

A

Passively via osmosis

51
Q

What is the driving force of water absorption?

A

Osmotic gradient generated by the absorption of nutrients and ions

52
Q

How is water absorbed in the SI?

A

Mostly via paracellular pathway through leaky tight junctions and some via transcellular pathways

53
Q

How is water absorbed in the LI?

A

All via the transcellular pathways because of the tight tight junctions in the paracellular pathway

54
Q

How is water absorbed via the transcellular pathways in the small and large intestines?

A

Through aquaporins

55
Q

What are the mechanisms of sodium absorption in the SI?

A

Passive and active

56
Q

What is the driving force of passive sodium absorption in the SI?

A

Sodium ions diffuse down their concentration gradient across leaky tight junctions

57
Q

What is the driving force for active sodium absorption in the SI?

A

Sodium gradient created by Na+ / K+ ATPase

58
Q

How is sodium transported actively across the apical membrane of the SI?

A

Secondary active transport using the sodium-glucose/amino acids cotransporter

59
Q

How is sodium transported actively across the basolateral membrane of the SI?

A

Pumped out by the Na+ / K+ ATPase

60
Q

What are the mechanisms of sodium absorption in the LI?

A

Only via Active transport

61
Q

What is the driving force for active sodium absorption in the LI?

A

Sodium gradient created by Na+ / K+ ATPase

62
Q

How is sodium transported actively across the apical membrane of the LI?

A

Via sodium channels, down its concentration gradient

63
Q

How is sodium transported actively across the basolateral membrane of the LI?

A

Pumped out by Na+ / K+ ATPase

64
Q

What are the fat soluble vitamins?

A

A, D, E, K

65
Q

How are fat-soluble vitamins absorbed?

A

Passively via the transcellular pathway

66
Q

What is the driving force for fat-soluble vitamin absorption?

A

A chemical gradient

67
Q

How do fat-soluble vitamins cross the apical membrane?

A

Dissolved in micelles with fat: via diffusion through the cell membrane

68
Q

How do fat-soluble vitamins cross the basolateral membrane?

A

Packages into chylomicrons with fat

69
Q

What are the water-soluble vitamins?

A

C and most of B

70
Q

What is the driving force for water-soluble vitamin absorption?

A

Sodium gradient created by Na+ / K+ ATPase

71
Q

How do water-soluble vitamins cross the apical membrane?

A

Secondary active transport via the sodium cotransporter

72
Q

How do water-soluble vitamins cross the basolateral membrane?

A

Via a transporter

73
Q

How does vitamin B12 absorb in the ileum?

A

Through binding to intrinsic factor which is then endocytosed and released from the cell to travel in the blood bound to transcobalamin

74
Q

What happens to the nutrients we do not absorb?

A

Removed from body via the defecation reflex

HUBS192 2024 (38 decks)

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