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GI lectures > intestinal physiology 1: fluid & electrolytes and digestion in the intestines > Flashcards

intestinal physiology 1: fluid & electrolytes and digestion in the intestines Flashcards

1
Q

define absorption

A

the movement of nutrients, water and electrolytes
from the lumen of the small intestine into the cell,
then into the blood.

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2
Q

how do water and ions enter single-celled organisms

A

via diffusion

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3
Q

how do larger particles enter single-celled organisms

A

phagocytosis & endocytosis

digestion & absorption in lysozymes

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4
Q

how does shape affect exchange

A

larger surface area to volume ratio
for optimal digestion and absorption

multicellular organisms have a greater ratio than single-celled ones

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5
Q

terrestrial multicellular organisms

A

not living in an aqueous solution filled with nutrients

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6
Q

GI function

A

to take relatively large solids and digest them into smaller molecules that can be absorbed as nutrients, while still serving as a barrier to toxins, bacteria, parasites

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7
Q

what is the GI system

A

a hollow organ

a tube through the body

contains specialised organs for secretion of enzymes and biles

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8
Q

describe lumen of GI

A

it is ‘outside’ the bodys tissues

but its environment is tightly controlled by the body

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9
Q

what do epithelial cells do in GI

A

they line the entire the GI tract and serve as the primary barrier

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10
Q

how is structure of GI efficient

A

it maximises SA for secretion and absorption

contains folds, villi and crypts

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11
Q

what do epithelial cells do with water and electrolytes

A

they can secrete and aborsb them

the GI tract transports fluids and electrolytes

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12
Q

what is chyme

A

a thick semifluid mass of partially digested food and digestive secretions that is formed in the stomach and intestine during digestio

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13
Q

what is the most abundant substance in chyme

A

water

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14
Q

how much ingested and secreted water enters the small intestine every day

A

around 8000ml

In ingested food & drink, and through secretions (saliva, bile, pancreatic juices etc.)

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15
Q

how much ingested and secreted water is passed onto the large intestine

A

only around 1500 ml

since 80% of water
reabsorption occurs in the small intestine

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16
Q

what re absorbs the most amount of water

A

the jejunum

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17
Q

how much water is re absorbed in the stomach

A

small amounts

the stomach has a much
smaller surface area available for diffusion and lacks the solute-absorbing
mechanisms that create the osmotic-gradient necessary for absorbing water

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18
Q

how does water move

A

down an osmotic gradient

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19
Q

how do eletrolytes move

A

down electrochemical gradients

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20
Q

what supplies energy for water and electrolytes to move against concentration gradients

A

sodium gradients and proton gradients

generated by the sodium pump

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21
Q

function of small bowel/intestine

A

it is the main area for fluid absorption and secretion

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22
Q

what are the 3 segments of the small bowel

A
  1. duodenum
  2. jejunum
  3. ileum
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23
Q

in total how much of the fluid load is re absorbed

A

around 98%

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24
Q

how much of the fluid load is excreted in stool

A

only around 200ml (2%)

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25
Q

describe membrane of small intestine

A

The epithelial membranes of the small intestine are very permeable to water, and
net water diffusion occur across the epithelium whenever a water concentration difference is established by the active absorption of solutes

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26
Q

what is the most actively transported solute

A

Na+

because it constitutes the most abundant solute in chyme - it is actively transported from the lumen in the
cell membranes of the ileum & jejunum

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27
Q

describe luminal membrane transport

A

it is is variably coupled
- glucose, amino acids or other
substance

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28
Q

what is coupled transport

A

the simultaneous transport of two substances across a biological membrane

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29
Q

how does Na help with nutrient absorption

A

many nutrients are absorbed via Na+ cotransport systems

2 stage transcellular process

  1. membrane transport protein
  2. Na+K+ATPase transporter
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30
Q

describe contents of the colon

A

they are iso-osmotic

(concentration in lumen of colon = that of blood)

this means Na+ is actively pumped from the lumen and water follows

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31
Q

describe potassium re absorption in colon

A

In general K+ reabsorption is by passive diffusion

the net movement is determined by the potential difference between the lumen and intestinal capillaries.

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32
Q

what is hypokalaemia

A

loss of K+

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33
Q

what can cause severe hypokalaemia

A

diarrhoea

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34
Q

describe chloride re absorption in colon

A

Cl- is actively reabsorbed in exchange for bicarbonate

resulting in the intestinal contents becoming more alkaline

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35
Q

factors affecting absorption (4)

A
  1. number and structure of enterocytes
  2. blood and lymph flows
  3. nutrient intake
  4. GI motility
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36
Q

factors affecting secretion (3)

A
  1. irritants
  2. bile
  3. bacterial toxins
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37
Q

how do toxins cause dirrahoea

A
  1. cholera toxin released from bacteria in infected intestine
  2. binds to intestinal cells
  3. stimulates adenylate cyclase to produce CAMP
  4. dramatic efflux of ions and water
  5. watery diarrhoea
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38
Q

what happens if there is an imbalance between absorption and secretion

A

leads to disease

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39
Q

journey of digestion

A
  1. stomach
  2. small intestine
  3. blood
  4. liver
  5. large intestine
  6. leaves body
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40
Q

mechanical digestion

A

chewing & churning of food

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41
Q

chemical digestion

A

enzymes

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42
Q

what are carbohydrates broken down into

A

monosaccharides

43
Q

what are proteins broken down into

A

amino acids

44
Q

what are fats broken down into

A

fatty acids & glycerol

45
Q

what is small intestine the primary site for

A

digestion and absorption of food

46
Q

how does digestion occur in the GI lumen

A

by secreted enzymes

47
Q

how does digestion occur on the surface of enterocytes

A

by membrane-bound enzymes

48
Q

what are the methods that absorption occurs by

A
  1. simple diffusion
  2. facilitated diffusion
  3. active transport
  4. endocytosis
  5. paracellular transport
49
Q

what increases SA of small intestine

A

extensive folding
projection of villi covered with microvilli

50
Q

how are glucose and galactose absorbed

A

by enterocytes through an NA dependent secondary active transport process

51
Q

how is fructose absorbed

A

by facilitated transport

52
Q

what are the 2 functional states/periods the body undergoes in providing energy for cellular activities

A

absorptive & postabsorptive states

53
Q

what is absorptive state

A

During which ingested nutrients enter the blood from the GI tract

During this state, some of the ingested nutrients provide the energy
requirements of the body and the remainder is added to the body’s energy stores to be called upon during the next postabsorptive state

54
Q

what is post absorptive state

A

During which the GI tract is empty of nutrients and the body’s own stores must supply energy

55
Q

average daily intake of lipids

A

70 to 100 per day - most in the form of
triglycerides (glycerol with three fatty
acids attached)

56
Q

what are the 3 main important fatty acids we absorb

A
  1. Palmitic (most abundant),
  2. Stearic
  3. Oleic acid
57
Q

what is a triglyceride

A

a glycerol molecule with 3 fatty acids attached

58
Q

describe triglyceride digestion

A

occurs to a limited extent in the mouth & stomach but it predominantly occurs in the small intestine.

59
Q

major digestive enzyme for lipids

A

lipase - synthesised by pancreas

60
Q

what does lipase do

A

catalyses the splitting of bonds linking fatty acids to the 1st & 3rd carbon atoms of glycerol

producing two free fatty acids & a monoglyceride as products:
Triglyceride —> Monoglyceride + 2 Fatty acids , under the action of lipase

61
Q

what do lipids do in stomach

A

they aggregate into large lipid
droplets in the upper portion of the stomach

because

lipids in ingested food are insoluble in water

These lipid droplets are then converted into very small droplets (1mm in diameter) via the process of emulsification

62
Q

what does emulsification require

A

1 Mechanical disruption of the large droplets into smaller droplet - provided by the motility of the GI tract, in the lower portion of the stomach & in the small intestine, which grinds & mixes the luminal contents

  1. An emulsifying agent - provided by the phopho-lipids in food and by bile salts secreted in bile
63
Q

what are phospholipids

A

amphipathic molecules - (both hydrophilic & phobic)

(containing polar or ionised groups on one end of the molecule and non-polar
groups on the other)

2 non-polar fatty acid chains attached to glycerol with a charged phosphate group on one end

64
Q

where are bile salts formed

A

liver

65
Q

are bile salts amphipathic

A

yes

66
Q

what do the non-polar portion of the phospholipids and bile salts do

A

they associate with the non
polar interior of the lipid droplet - leaving the polar portions exposed at the water surface

  • here they repel other lipid droplets that are similarly coated with these emulsifying agents thereby preventing their reaggregation into larger fat droplets
67
Q

define absorption

A

the movement of nutrients, water and electrolytes
from the lumen of the small intestine into the cell,
then into the blood.

68
Q

how does bile aid lipid digestion

A

by emulsifying the fat goblets into smaller chunks, called micelles, which have a much larger surface area.

69
Q

how much of lipid digestion is started by lingual and gastric lipases

A

this only digests 10% of ingested lipids.

70
Q

what are 3 major enzymes involved in lipid digestion?

A
  • Pancreatic lipase
  • phospholipase A2
  • cholesterol ester hydrolase
71
Q

what do pancreatic lipase, phospholipase A2 and cholesterol ester hydrolase do

A

they hydrolyse the micelles, breaking them down into fatty acids, monoglycerides, cholesterol and lysolecithin.

72
Q

where are the products from digestion of lipids released

A

at the apical membrane and diffuse into the enterocyte.

73
Q

what happens to products of lipid digestion inside enterocyte cells

A
  • the products are re-esterified to form the original lipids, triglycerides, cholesterol and phospholipids.
  • the lipids are then packaged inside apoproteins to form a chylomicron.
  • the chylomicrons are too large to enter circulation, so they enter lymphatic system via lacteals.
74
Q

which carbohydrate products are absorbed by the small intestine

A

glucose, galactose and fructose

75
Q

where is starch digestion initiated

A

in the mouth, facilitated by salivary amylase

76
Q

where does the majority of carbohydrate digestion occur

A

small intestine

77
Q

what is the main enzyme for starch digestion

A

pancreatic amylase, which yields disaccharides from starch by digesting the alpha 1-4 glycosidic bonds.

78
Q

what happens to disaccharides produced from starch digestion

A

(maltose, maltotriose, and α-dextrins) are all converted to glucose by brush border enzymes.

79
Q

what happens to disaccharides occurring naturally in food

A

they do not require amylase to break them down.

Brush border enzymes (lactase, sucrase, trehalase) hydrolyse these compounds into molecules of glucose, galactose and fructose.

80
Q

how are glucose and galactose absorbed

A

across the apical membrane by secondary active transport (along with Na+) through the Sodium-Glucose cotransporter (SGLT1).

81
Q

how do glucose and galactose exit the cell

A

via GLUT2 receptors across the basolateral membrane into the blood.

82
Q

how does fructose enter the cell

A

by facilitated diffusion via GLUT5 and is transported into the blood via GLUT2 receptors.

83
Q

where does protein digestion begin

A

in the stomach with the action of pepsin, which breaks protein into amino acids and oligopeptides.

84
Q

where is protein digestion completed

A

in the small intestine with brush border and pancreatic enzymes.

they split the oligopeptides into amino acids, dipeptides and tripeptides.

85
Q

how are amino acids absorbed

A

via a Sodium cotransporter, in a similar mechanism to the monosaccharides.

86
Q

how are amino acids transported

A

rhey are transported across the basolateral membrane via facilitated diffusion

87
Q

how are Di and tripeptides absorbed

A

via separate H+ dependent cotransporters and once inside the cell are hydrolysed to amino acids.

88
Q

what are the fat soluble vitamins

A

A,D,E & K

89
Q

how are fat soluble vitamins absorbed

A

occurs in the ileum

they are incorporated into mixed micelles with other lipids and bile acids in the lumen of the small intestine and enter the enterocyte largely by diffusion.

Within the enterocytge, they are incorporated into chylomicrons and exported via exocytosis into lymph.

90
Q

how are water soluble vitamins absorbed

A

by diffusion or mediated transport in the JEJUNUM

but vitamin B12 is an exception

91
Q

what are the water soluble vitamins

A

vitamins B and C

92
Q

how is vitamin b12 absorbed

A

is a very large and charged vitamin.

To be absorbed B-12 must first bind to the protein intrinsic factor (secreted by
parietal cells of the stomach) - intrinsic factor with bound B-12 then binds to
specific sites on the epithelial cells in the LOWER PORTION OF THE ILEUM
where vitamin B-12 is absorbed via endocytosis.

93
Q

what is vitamin B12 needed for

A

erythrocyte formation

a deficient in B-12 can lead to pernicious anaemia and is usually caused due to a deficiency in intrinsic factor

94
Q

where does vitamin absorption occur

A

small intestine

95
Q

how are sodium ions absorbed (3)

A
  • Sodium-hydrogen antiporter on the luminal membrane
  • Epithelial sodium channels
  • Enhanced by absorption of short-chain fatty acids in the colon via specialised symporters
96
Q

how are chloride ions absorbed

A

the movement of sodium into the plasma produces an electrochemical gradient to allow absorption of chloride.

Chloride ions are exchanged for bicarbonate ions (causing net bicarbonate secretion).

97
Q

how are potassium ions absorbed

A

absorption of water along the length of the bowel concentrates potassium in the lumen.

This provides an electrochemical gradient for the movement of potassium into the plasma.

In the colon potassium may be absorbed or secreted depending on the remaining concentration in the lumen and the electrochemical gradient created by the active absorption of sodium.

Secretion usually occurs when the luminal concentration of potassium ions is below 25mM.

98
Q

how are short-chain fatty acids, crucial B vitamins (such as B6 and B12) and vitamin K produced

A

by the digestion of chyme by the commensal microbial flora of the colon

99
Q

impact of Aldosterone on absorption in the colon

A

increases the net absorption of water and electrolytes by stimulating the basolateral sodium-potassium ATP-ase. This increases the electrochemical gradient and driving force for sodium absorption. It also increases transcription of epithelial sodium channels.

100
Q

impact of Glucocorticoids and somatostatin on absorption in the colon

A

act to increase water and electrolyte absorption by increasing the action of the basolateral sodium-potassium ATP-ase.

101
Q

what innervates the intestines

A

enteric nervous system

102
Q

what does Parasympathetic innervation do to inestines

A

promotes net secretion from the intestines

103
Q

what does sympathetic innervation do to inestines

A

promotes net absorption from the intestines.

GI lectures (9 decks)

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