Digestion; The small intestines

Question 1

Small intestine

Answer 1

The small intestine is a 6.5 meter long structure that acts as a key site for the digestion and absorption.
* 90% of nutrient absorption takes place in the small intestine
The small intestine has three regions:
1. Duodenum
2. Jejunum
3. Ileum (3.5m)

Question 2

Small intestine:
Duodenum

Answer 2

1. Duodenum (30cm) – Most digestion occurs here, aided by the influx of pancreatic juices and bile. Some vitamins and minerals are absorbed too

Question 3

Small intestine:
Jejunum

Answer 3

2. Jejunum (2.5m) – Most absorption occurs here, i.e. sugar, fatty acids, amino acids, Vitamins and minerals

Question 4

Small intestine:
Ileum

Answer 4

3. Ileum (3.5m) – Vitamin B12 is absorbed

Question 5

Small Intestine: Villi and microvilli

Answer 5

To aid absorption, the small intestine creates a vast surface area, the size of a tennis court.
* The large surface is created mostly by the folding of the small intestine, forming finger-like projections known as ‘villi’ that contain blood and lymphatic capillaries. Villi appear throughout the entire small intestine
* To further increase the surface area, the cell membranes of small intestinal cells fold to create microscopic projections called ‘microvilli’. This is also known as the brush border
* Nutrients contact the microvilli and are absorbed into the blood capillaries, with fatty acids entering lymph

Question 6

Small intestine:
Brush border enzymes

Answer 6

‘Brush border enzymes’ are attached to the small intestinal lining and are imperative for absorption.
* The brush border enzymes break double-chained carbohydrates and proteins (disaccharides and dipeptides), allowing them to be absorbed
Brush border enzymes:
Maltase, sucrase and lactase = breakdown sugars into glucose, fructose, galactose, etc
Dipeptidase = Break down proteins into (single) amino acids

Di = two, peptide = routine, -ase = enzyme

Question 7

Small intestine:
Intestinal absorption

Answer 7

Carbohydrates, proteins and fats are only absorbed once they have been digested into their simplest forms. This emphasises the importance of optimal function in areas such as the mouth, stomach, pancreas and gallbladder especially.

• E.g. proteins need to be chemically digested in the stomach and then in the small intestine into amino acids
• Carbohydrates (Single sugars) and amino acids enter blood capillaries of the small intestine to be sent to the liver
• Fatty acids including fat-soluble vitamins (A, D, E, K) are digested in the small intestine and enter lymphatic capillaries

Question 8

Digestion summary

Answer 8

Ingestion – Chemical and mechanical digestion – Absorption – Elimination

Propulsion =
* Swallowing (Oropharynx)
* Peristalsis (oesophagus, stomach, small intestine, large intestine)
Chemical digestion =
* Enzymes (amaylase, pepsinogen, lipase - break down food)
* Digestive juice
Mechanical digestion =
* Chewing (mouth)
* Churning (stomach)
* Segmentation (small intestine)
Absorption =
* Nutrients & water to blood vessels and lymph vessels (small intestine)
* Water to blood vessels (large intestine)