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PHYSIOLOGY III > LECTURE 3 (Small intestine) > Flashcards

LECTURE 3 (Small intestine) Flashcards

1
Q

Why is the stomach a poor absorptive area of the GI tract?

A
  • Lacks the villi type of absorptive membrane
  • Junctions between the epithelial cells are tight junctions

[however, highly lipid-soluble substances (alcohol, aspirin) can be absorbed in small quantities]

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2
Q

What increases the mucosal absorptive area?

A
  • Folds of Kerckring
  • Villi
  • Microvilli
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3
Q

What is absorbed by the small intestine?

A
  • All products of carbohydrate, protein and fat digestion
  • Ingested electrolytes
  • Vitamins
  • Water
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4
Q

Where does absorption usually occur and where does it occur very little?

A

Most absorption occurs in the DUODENUM and JEJENUM

Very little occurs in the ILEUM

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5
Q

What is Malabsorption caused by?

A

Damage to or reduction of the surface area of the small intestine

[one of the most common causes -> CELIAC DISEASE]

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6
Q

Describe absorption in the small intestine

A
  • Sodium is actively transported through the intestinal membrane
  • Water osmosis occurs because a large osmotic gradient has been created by the elevated concentration of ions in the PARACELLULAR SPACE
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7
Q

Aldosterone greatly enhances ___________ _____________

A

Sodium Absorption

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8
Q

In the upper part of the small intestine, chloride ion absorption is rapid and occurs mainly by ___________

A

Diffusion

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9
Q

The epithelial cells on the surfaces of the villi in the ileum and on all surfaces of the large intestine have what special capability?

A

Secrete bicarbonate ions in exchange for absorption of chloride ions

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10
Q

Describe the absorption of Bicarbonate ions in the Duodenum and Jejenum

A

1) When SODIUM IONS are absorbed, moderate amounts of H+ ions are secreted into the lumen of the gut in exchange for some of the SODIUM
2) The H+ ions combine with the BICARBONATE IONS to form CARBONIC ACID (H2CO3) which dissociates to form H2O and CO2
3) H2O remains part of the CHYME in the intestines but CO2 us readily absorbed into the blood and expired through the lungs

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11
Q

The bicarbonate ion is absorbed in an _____________ way

A

indirect

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12
Q

___________ ______ are actively absorbed into the blood, especially from he duodenum

A

Calcium ions

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13
Q

________ _____ are actively absorbed from the small intestine

A

Iron ions

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14
Q

______________, ____________ and ___________ can be actively absorbed through the intestinal mucosa

A

Potassium, Magnesium and Phosphate

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15
Q

Describe carbohydrate absorption in the small intestine

A

1) Dietary carbohydrates are presented to the small intestine in forms of DISACCHARIDES (maltose, sucrose and lactose)
2) DISACCHARIDASES located in the brush border of the small intestine reduce disaccharides/polysaccharides into MONOSACCHARIDE units (glucose, galactose, fructose)
3) Glucose and galactose are both absorbed by SECONDARY ACTIVE TRANSPORT using SYMPORT CARRIERS located at the LUMINAL MEMBRANE (e.g sodium-glucose co-transporter). Fructose enters cell by PASSIVE FACILITATED DIFFUSION via GLUT-5.
4) Glucose concentrated in the cell by these SYMPORTERS leaves the cell down its concentration gradient by FACILITATED DIFFUSION via GLUT-2
5) These monosaccharides enter the blood by simple diffusion

[Symporters do not use energy themselves but depend on Na+ conc established by sodium-potassium pump]

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16
Q

_________ is transported by almost exactly the same mechanism as glucose

A

Galactose

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17
Q

_________ is transported by facilitated diffusion all the way through the intestinal epithelium and is not coupled with sodium transport

A

Fructose

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18
Q

Describe protein absorption in the small intestine

A

1) Proteins are absorbed through the LUMINAL MEMBRANES of the INTESTINAL EPITHELIAL CELLS in the form of dipeptides, tripe-tides and a few free amino acids by SODIUM CO-TRANSPORT MECHANISM
2) A few amino acids no not require sodium co-transport mechanism but instead are transported by FACILITATED DIFFUSION

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19
Q

Describe fat absorption in the small intestine

A

1) Fats are digested to form MONOGLYCERIDES and FREE FATTY ACIDS -> both first become dissolves in the central lipid portions of BILE MICELLES
2) Bile micelles are carried to the surfaces of the microvilli of the intestinal cell brush border and penetrate into the recesses among the moving, agitating MICROVILLI
3) After entering the epithelial cell, FATTY ACIDS and MONOGLYCERIDES are taken up by the cell’s SMOOTH ENDOPLASMIC RETICULUM -> used to form new TRIGLYCERIDES that are released in the form of CHYLOMICRONS through the base of the epithelial cell, to flow upward through the THORACIC LYMPH DUCT and empty into the CIRCULATING BLOOD

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20
Q

What is the difference between water-soluble vitamins and fat-soluble vitamins?

A

Water-soluble vitamins = primarily absorbed passively with water

Fat-soluble vitamins = carried in the micelles and absorbed passively with the end products of fat digestion

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21
Q

What is the normal iron intake?

A

15 to 20 mg/day

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21
Q

What is unique about Vitamin B12?

A

It must be in combination with GASTRIC INTRINSIC FACTOR for absorption by RECEPTOR-MEDIATED ENDOCYTOSIS in the terminal ileum

22
Q

How does Iron exit the small intestine epithelial cell?

A

A membrane iron transporter known as FERROPORTIN

23
Q

Iron absorption is controlled by which hormone?

A

HEPCIDIN

Hepcidin is released from the liver when iron levels in the body become too high -> Binds with FERROPORTIN (iron transporter) and promotes endocytosis + degradation of lysosomes

24
Q

Iron that exits the small intestine epithelial cell is transported in the blood by a plasma protein carrier known as ____________

A

Transferrin

25
Q

What is Ferritin?

A

Iron not immediately needed is irreversibly stored within the small intestine epithelial cells in a granular form called FERRITIN

26
Q

How does Calcium enter the luminal membrane of the small intestine?

A

Down its electrochemical gradient through a specialised Ca2+ channel

27
Q

Which vitamin greatly enhances Ca2+ absorption?

A

Vitamin D

28
Q

Secretion of which hormone increases in response to a fall in Ca2+ concentration?

A

Parathyroid hormone

29
Q

Where does most of the absorption in the large intestine occur?

A

In the proximal one half of the colon called the ABSORBING COLON

30
Q

What does the Distal colon function as?

A

Faeces storage until faeces excretion and is called the STORAGE COLON

31
Q

What does active absorption of sodium and the electrical potential gradient created by sodium absorption cause?

A

Chloride absorption

32
Q

What is the importance of the absorption of sodium and chloride ions?

A
  • Creates an osmotic gradient across the large intestinal mucosa
  • Causes absorption of water
33
Q

What is the brown colour of faeces caused by?

A

Stercobilin and urobilin (derivatives of bilirubin)

34
Q

What is the odour of faeces caused by?

A

Products of bacterial action

35
Q

What is Segmentation?

A

The small intestine’s primary motility during digestion of a meal, which both mixes and slowly propels the chyme

36
Q

What are Segmentation contractions initiated by?

A

Basal electrical rhythm produced by small intestine’s pacemaker cells

[Small intestine’s BER brings circular smooth muscle layer to threshold -> segmentation contractions induced -> frequency of segmentation following frequency of BER]

37
Q

What is the circular smooth muscle’s degree of responsiveness/intensity of segmentation contractions influenced by?

A
  • Distention of intestine
  • Hormone gastrin
  • Extrinsic nerve activity
38
Q

What is the Gastroileal reflex?

A

Segmentation of the empty ileum brought about by gastrin secreted in response to presence of chyme in the stomach

[on the other hand, DUODENUM segments in response to local distension]

39
Q

Describe segmentation innervation

A

Parasympathetic stimulation ENHANCES SEGMENTATION whereas sympathetic stimulation DEPRESSES SEGMENTAL ACTIVITY

40
Q

What are the functions of segmentation?

A
  • Mixing the chyme with the digestive juices secreted into the small-intestine lumen
  • Exposing all chyme to the absorptive surfaces of the small-intestine mucosa
41
Q

What are intestinal segmentation contractions replaced with during fasting?

A

Migrating motility complex (MMC) or “intestinal housekeeper”

42
Q

Describe the Migrating Motility complex (MMC)

A

The MMX cycles through three phases every 1.5 hours as long as a person is fasting

PHASE I: A long period every 40-60 mins relatively quiet with very few contractions
PHASE 2: 20-30 min period with some peristaltic contractions
PHASE 3: Shortest phase where intense peristaltic contractions begin in upper stomach propagating through to end of small intestine -> “gurgling noises” & pushes remnants of last meal forwards

Regulated by hormone MOTILIN -> Motilin inhibited by feeding

43
Q

Do small intestine secretions contain digestive enzymes?

A

NO

Explanation: Small intestine does synthesise digestive enzymes that act intracellularly within the brush-border membrane of epithelial cells but are not directly secreted into lumen

44
Q

Which membrane-bound enzymes span the microvilli?

A
  • Enteropeptidase
    [activates pancreatic proteolytic enzyme trypsinogen]
  • Disaccharidases
  • Aminopeptidases
    [hydrolyse peptide into amino acids]

Explanation: Carbohydrates and proteins are digested via the microvilli but fat digestion is completed in the lumen via bile secretion and lipidases

45
Q

Most of the small intestine can be removed with no absorption interference apart from which region?

A

Terminal Ileum

Explanation: Specialised transport mechanisms for B12 and bile salts are only located in this region

46
Q

What are the Crypts of Lieberkuhn?

A

Shallow invaginations that secrete water and salt which along with the mucus secreted by the villus surface make up the SUCCUS ENTERICUS

47
Q

Describe how energy-dependent Na+ absorption drives passive H2O absorption

A

1) When electrochemical gradient favours movement of Na+ from lumen to blood, PASSIVE DIFFUSION occurs by PARACELLULAR TRANSPORT between intestinal epithelial cells through TIGHT JUNCTIONS (if not -> Secondary active transport)
2) Na+ actively transported out of cell by Na-K pump at BASOLATERAL MEMBRANE into interstitial fluid then DIFFUSES into capillaries
3) Cl- passively follows down electrical gradient created by Na+ absorption/secondary active transport if needed. H2O is absorbed by active carrier that pumps Na+ into lateral spaces -> results in concentrated area of HIGH OSMOTIC PRESSURE
4) Localised high osmotic pressure induces H2O to move from lumen through cell into lateral space -> Decrease OSMOTIC PRESSURE but increase HYDROSTATIC PRESSURE -> Flushes H2O into interior of villus where it is picked up by CAPILLARY NETWORK

48
Q

Describe Iron absorption

A

1) Only a portion of ingested iron is in a form that can be absorbed (either heme iron or ferrous iron) -> Iron is absorbed across LUMINAL MEMBRANE of small-intestine epithelial cells by different energy-dependent carriers for heme and Fe2+
2) Dietary iron that is absorbed into small-intestine and is immediately needed for RBC PRODUCTION is transferred into blood by FERROPORTIN -> in blood, iron is carried to bone marrow by TRANSFERRIN
3) Absorbed dietary iron not needed is stored in EPITHELIAL CELLS as FERRITIN -> cannot be transferred to blood -> released in faeces

49
Q

Describe calcium absorption

A

1) Calcium enters the LUMINAL MEMBRANE of small-intestine epithelial cells down its electrochemical gradient through specialised Ca2+ channel
2) Calcium is FERRIED within the cell by CALBINDIN
3) Exits the BASOLATERAL MEMBRANE by a primary active transport Ca2+ ATPase pump and a secondary active transport Na+-Ca2+ antiporter

50
Q

What does excessive loss of intestinal contents during diarrhoea cause?

A
  • Dehydration
  • Loss of nutrient material
  • Metabolic acidosis resulting from loss of HCO3-
51
Q

What are the causes of diarrhoea?

A
  • Excessive small-intestinal motility from irritation or emotional stress -> inadequate time for absorption of fluid
  • Excess osmotically active particles -> Cause H2O to enter and be retained in lumen -> increase fluidity of faeces
  • Toxins of the bacterium VIBRIO CHOLERA promote secretion of excessive amounts of fluid by small-intestine mucosa -> diarrhoea
52
Q
A

PHYSIOLOGY III (7 decks)

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