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HUBS192 2024 > Lecture 27: ABSORPTION > Flashcards

Lecture 27: ABSORPTION Flashcards

1
Q

What is absorbance?

A

The (net) passage of substances from the GI lumen across the lining of the intestine into the interstitial fluid and then into the blood or lymph

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2
Q

Where are the sites of absorption?

A

Mouth, esophagus, stomach, small intestine and large intestine

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3
Q

What absorption happens at the mouth, esophagus and stomach?

A

Minimal (lipid soluble substances only)

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4
Q

What is the main site of absorption?

A

Small intestine

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5
Q

What absorption happens at the large intestine?

A

9% of water and sodium

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6
Q

What factors affect absorption?

A

Motility, surface area available for absorption, transport across epithelium (reduction in size by chemical digestion and membrane transporters), removal from interstitial fluid

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7
Q

What is required to allow digestion and absorption?

A

The correct rate of propulsion

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8
Q

What motility affects absorption?

A

Peristalsis at an appropriate rate and segmentation to expose products of digestion to absorptive surfaces

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9
Q

What is the rate of absorption proportional to?

A

Surface area - the greater the surface area the faster the rate of absorption

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10
Q

What are the anatomical adaptations to maximise surface area for absorption?

A

Length of intestine (6m), circular folds, villi and microvilli

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11
Q

What is the problem with transporting molecules across the epithelium?

A

The lumen of the intestine is continuous with the outside world and the intestinal epithelium is a barrier

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12
Q

What is the paracellular pathway?

A

The ‘gap’ between cells

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13
Q

What do solutes do in the paracellular pathway?

A

Don’t cross the cell membrane, the only barrier is tight junctions

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14
Q

How selective is the paracellular pathway?

A

Relatively non-selective - if the solute is small enough it can get across

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15
Q

Is the paracellular pathway passive or active?

A

Passive - requires a gradient (high concentration in the lumen to low concentration in the interstitial)

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16
Q

What do solutes do in the cellular pathway?

A

Travel across two cell membranes and through the cytoplasm

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17
Q

What is required if the solute isn’t lipid soluble?

A

A transport protein as membranes are lipid bilayers

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18
Q

What are specific transport proteins required for?

A

In order to absorb what is required and allow active transport against a gradient

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19
Q

What is required to remove substances from the interstitial fluid?

A

A large blood flow to the intestine (25% of cardiac output) and the arrangement in the villi of blood vessels and lacteals prevents build up in the interstitial fluid

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20
Q

How much water do we drink?

A

About 1.5L per day which replaces the water lost in sweat, urine, faeces and as we breathe

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21
Q

What is the total amount of water delivered to the small intestine?

A

About 9-10 L per day (including secretion)

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22
Q

What happens if we don’t replace the losses and secreted water?

A

There is a major problem

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23
Q

What is osmosis?

A

Passive movement of water from lumen (high water concentration) into the blood (low water concentration)

24
Q

How is the osmotic gradient set up?

A

By absorption of salts and nutrients

25
Q

How may sodium be absorbed?

A

Passive movement via the paracellular pathway or active transport via the cells

26
Q

HOw is active transport of sodium described?

A

Transcellular

27
Q

What does active transport of sodium require?

A

Transporters to cross the cell membrane

28
Q

What are some mechanisms of sodium transport?

A

Na+ transport alone, Na+ transport coupled to monosaccharides, Na+ transport coupled to amino acids

29
Q

How does Na+ move into the cell?

A

Down its gradient via transport proteins (Na+ channel or Na+/H+ exchanger)

30
Q

What does sodium need to do to get out of the cell?

A

Go against the concentration gradient by active transport, using energy (ATP) via Na+/K+ - ATPase

31
Q

What happens in passive absorption of carbohydrates?

A

Monosaccharides diffuse down their concentration gradient via the paracellular pathway (minor role in absorption)

32
Q

What is involved in active absorption of carbohydrates?

A

Cotransport with Na+ via the cellular pathway

33
Q

What carbohydrates are absorbed?

A

Monosaccharides

34
Q

What is the carbohydrate transporter on the apical membrane?

A

Na+/glucose cotrasnporter (low concentration to high concentration)

35
Q

What is the carbohydrate transporter on the basolateral membrane?

A

Glucose carrier (high concentration to low concentration)

36
Q

What happens in passive absorption of amino acids?

A

Amino acids diffuse down their concentration gradient via the paracellular pathway

37
Q

What pathway is active absorption of carbohydrates via?

A

The cellular pathway

38
Q

How are di and tri-peptides absorbed?

A

Via H+ dependent cotransport

39
Q

How are amino acids absorbed?

A

Via Na+ dependent cotransporters

40
Q

What is the amino acid transporter on the apical membrane?

A

Na+ amino acid cotransporter

41
Q

What is the amino acids transporter on the basolateral membrane?

A

An amino acid carrier

42
Q

What are the products of fat digestion?

A

Lipid soluble so can diffuse across the cell membrane freely without a transporter

43
Q

How are products of fat digestion delivered to the brush border?

A

By micelles

44
Q

Is the whole micelle absorbed?

A

NO, it releases its contents and the bile salts are absorbed later

45
Q

What products of fat digestion move into the cell?

A

Fatty acids and monoglycerides

46
Q

What happens to the fatty acids and monoglycerides in the cell?

A

They re resynthesized into triglycerides, packaged into chylomicrons which then exit the cell by exocytosis and enter the lacteals

47
Q

Where do the bile salts which are absorbed come from?

A

Micelles

48
Q

When does bile salt absorption occur?

A

After fat absorption is complete

49
Q

How is bile salt absorption in the ileum?

A

An active transport process by apical Na+ dependent bile acid cotransporter

50
Q

How is bile salt absorption in the colon?

A

Passive

51
Q

How are fat soluble vitamins absorbed?

A

(A, D, E, K) are absorbed with fats

52
Q

What do water soluble vitamins rely on for absorption?

A

Na+ dependent absorption (Vitamin C)

53
Q

Where is vitamin B12 absorbed?

A

In the ileum

54
Q

What does vitamin B12 do?

A

It binds to intrinsic factor in the stomach and a specific transporter found in the ileum allows it to be asorbed

55
Q

What is vitamin B12 deficiency?

A

Perinicious anemia

56
Q

Do we absorb everything?

A

No

HUBS192 2024 (38 decks)

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