Lecture 27

Question 1

What is absorption?

Answer 1

Absorption is 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

Question 2

What are the sites of absorption?

Answer 2

} Mouth, esophagus, stomach
} Minimal absorption
} Lipid soluble substances
} Small intestine
} Main site of absorption
} 90% of water and sodium
} All nutrients
} Large intestine
} 9% on water and sodium

Question 3

What factors affect absorption?

Answer 3

1. Motility
2. Surface area available for absorption
3. Transport across epithelium
   • Reduction in size (chemical digestion)
   • Membrane transporters
4. Removal from interstitial fluid

Question 4

How does Motility affect absorption>

Answer 4

Correct rate of propulsion to allow for digestion and absorption.
Storage
EExposure Ofproducts of digestion to surface of absorption
Segmentation
Peristalsis

Question 5

What is the affect of available surface area on absorption?

Answer 5

rate of absorption is proportional to surface area.

Anatomical adaptations maximize surface area.

Question 6

How can molecules be transported cross epithelium?

Answer 6

Two pathways:
Paracellular - between gaps in cells
Cellular - Across the cell membranes, through the cytoplasm

Question 7

Features of paracellular pathway?

Answer 7

Only barrier is tight junctions binding cells together .
Relatively non-selective.
Passive.

Question 8

Features of the cellular pathway?

Answer 8

Solutes must cross two cell membranes

Cell membranes are lipid bilayers so solutes must be lipid soluble.

Question 9

How can absorption be maximised?

Answer 9

Reduce nutrients into smallest possible units.

Specific transport proteins that absorb what is required and allow active transport

Question 10

How much water is delivered to the small intestine each day?

Answer 10

9-10 L

Question 11

Mechanism of water absorption?

Answer 11

} Osmosis
} Passive movement of water from lumen into
blood
} Osmotic gradient set up by absorption of
salts and nutrients

Question 12

How is sodium absorbed?

Answer 12

Passive movement via paracellular pathway of active transport via the cells.

Question 13

Movement of sodium into cells?

Answer 13

Entering - sodium moves down the concentration gradient from area of high to low concentration which requires transport proteins.
Exiting - it goes from areas of low to high concentration which uses active transport.

Question 14

How are carbohydrates absorbed>

Answer 14

• Active transport
• Cotransport with sodium
• Monosaccharides
• Across cell membranes

Question 15

How are amino acids absorbed?

Answer 15

• Passive or active absorption.
  Passive - Diffuse down concentration gradient via paracellular pathway.
  Active - cellular pathway

Question 16

Absorption of products of fat digestion?

Answer 16

products are Lipid soluble. 
} Can diffuse across the cell membrane
} Do not need a transporter
} Delivered to brush border by micelles
} Whole micelle is not absorbed

Question 17

what happens to the products of fat digestion inside the cell?

Answer 17

} Resynthesized into triglycerides
} Packaged into chylomicrons
} Chylomicrons exit cell by exocytosis
} Chylomicrons enter the lacteals

Question 18

How are bile salts absorbed?

Answer 18

Occurs after fat digestion is complete.
In the Ileum - Active absorption.
In the Colon ( absorbs majority of bile salts) - Passive boption

Question 19

How are fat soluble vitamins absorbed?

Answer 19

} Vitamins A, D, E, K

} Absorbed with fats

Question 20

How are water soluble vitamins absorbed?

Answer 20

Sodium dependent absorptions

- Vitamin C

Question 21

How is Vitamin B12 absorbed?

Answer 21

} Absorbed in the ileum
} Binds to intrinsic factor
} Produced in stomach
} Specific transporter for Intrinsic factor
vitamin B12 in the ileum
} Pernicious anemia

Question 22

What happens to the stuff we don’;t absorb?

Answer 22

Elimination:
} Expulsion of residues of digestion
} Faeces formed in large intestine
} Transferred to rectum via peristaltic waves
} Mass movements
} Elimination from body by defecation reflex