S8) The Digestive System

Question 1

Label the following components of the alimentary canal:

Answer 1

Question 2

What are the four layers of the gut wall?

Answer 2

• Mucosa (innermost)
• Submucosa
• Muscularis externae
• Serosa (outermost)

Question 3

Identify the 8 functions of the gut

Answer 3

• Provides a port of entry for food
• Mechanically disrupts food
• Temporarily stores food
• Chemically digests food
• Kills pathogens in food
• Moves food along the tract
• Absorb nutrients from the resultant solution
• Eliminate residual waste material

Question 4

What is digestion?

Answer 4

Digestion is the conversion of what we eat by physical and chemical disruption into a solution that is relatively sterile, neutral in pH and isotonic; from which we can absorb our nutrients

Question 5

Identify 4 roles of saliva in the GI tract

Answer 5

• Starts digestion (amylase & lipase)
• Assists swallowing
• Protects mouth & teeth (high calcium)
• Bacteriostatic (IgA antibody)

Question 6

Compare physical and chemical disruption in the mouth and stomach respectively

Answer 6

- Begins in the mouth:

I. Physical - by teeth, tongue, muscles of mastication

II. Chemical - by salivary amylase and lipase

• Continues in the stomach:

I. Physical - by churning

II. Chemical - by HCl and enzymes

Question 7

Describe the transport of food in the GI tract

Answer 7

• Mouth forms a bolus which enters the oesophagus
• Rapid peristaltic transport (even when upside down) moves bolus to stomach (8-9 seconds)

Question 8

Describe the neural control of the oesophagus

Answer 8

• Upper end oesophagus – voluntary control (striated skeletal muscle)
• Lower end oesophagus – involuntary control (solely smooth muscle)

Question 9

Identify and describe the different components of oesophageal mucosa

Answer 9

- Epithelium – stratified squamous non-keratinized

- Lamina propria – loose connective tissue bearing blood & lymph vessels, smooth muscle and immune cells

- Muscularis mucosae – thin layer of smooth muscle cells

Question 10

Identify and describe the structure of the layer of the oesophageal wall, apart from the mucosa

Answer 10

- Submucosa – connective tissue containing mucus-secreting glands

- Muscularis externa – smooth muscle layers which move food by peristalsis

- Adventitia – thin outermost layer of connective tissue

Question 11

Identify the different smooth muscle layers in the muscularis externa of the oesophageal wall

Answer 11

• Inner layer is circular muscle
• Outer layer is longitudinal muscle

Question 12

State 4 roles of the stomach

Answer 12

• Temporarily stores food
• Receptive relaxation to maintain pressure
• Proteolytic enzyme and acid secretion
• Produces hypertonic incompletely digested chyme

Question 13

What are the three layers of smooth muscle in the stomach?

Answer 13

• Oblique
• Circular
• Longitudinal

Question 14

How does the stomach protect its epithelium

Answer 14

The stomach protects its epithelium by secreting bicarbonate-rich mucus

Question 15

What is rugae?

Answer 15

Rugae are folds of gastric mucosa, forming longitudinal ridges in the empty stomach

Question 16

What is the gastric pit of the gastric gland ?

Answer 16

The gastric pit is the region of the gastric gland lined by mucus secreting cells, similar to goblet cells

Question 17

What is the isthmus (of the gastric gland)?

Answer 17

The isthmus is the region in which stem cells divide to populate the gland by upward or downward migration

Question 18

What do parietal cells do?

Answer 18

Parietal cells secrete H⁺ ions into the lumen and HCO₃⁺ ions into nearby capillaries, which move it to the surface mucous cells

Question 19

What do chief cells do?

Answer 19

Chief cells secrete pepsinogens which are converted into pepsins which partly hydrolyse proteins

Question 20

What are enteroendrocrine cells?

Answer 20

Gastric enteroendrocrine cells (G cells) are cells found at the base of the gastric gland which secrete gastrin

Question 21

What are surface mucous cells and what do they do?

Answer 21

Surface mucous cells are cells which are abundant in gastric pits and release mucus in response to distension, stomach contents and acid secretion from gastric glands

Question 22

Discuss the resistance/vulnerability of surface mucous cells

Answer 22

• Their secreted mucus is resistant to pepsin degradation
• They can be damaged by aspirin/alcohol

Question 23

Describe the structure, location and function of the duodenum

Answer 23

• Structure: proximal portion of the small intestine
• Location: curves around the head of the pancreas
• Function: walls contain Brunner’s glands which secrete bicarbonate-rich mucus to neutralise acidic chyme

Question 24

Describe the dilution and neutralisation of chyme

Answer 24

• Water drawn in from ECF to dilute hypertonic chyme
• Pancreas & liver secrete alkali and bile to neutralise acidic chyme

Question 25

Digestion is completed by enzymes from the pancreas and intestine. What does this entail?

Answer 25

- Cleaves **peptides** to amino acids - Cleaves **polysaccharides** to monosaccharides - Breaks down and re-form **lipids** - Breaks down **nucleic acids**

Question 26

What are the 3 main requirements of absorption?

Answer 26

- Large amounts of energy (active process) - High surface area (slow process) - Good blood supply/drainage

Question 27

Identify the components of the small intestine and their individual roles

Answer 27

- **Duodenum** absorbs iron - **Jejunum** absorbs most sugars, amino acids and fatty acids - **Ileum** absorbs Vitamin B₁₂, bile acids and remaining nutrients

Question 28

Plicae circulares are found in the jejunum. What are they?

Answer 28

**Plicae circulares** are circular folds of mucosa and submucosa that project into the gut lumen

Question 29

What is a lacteal?

Answer 29

A **lacteal** is a dilated lymphatic capillary vessel

Question 30

What are the components of the large intestine?

Answer 30

Question 31

What are the roles of the bacteria of the large intestine?

Answer 31

- Synthesis of **vitamins** K, B₁₂, thiamine and riboflavin - Breakdown of **bile acids** - Conversion of **bilirubin** to non-pigmented metabolites

Question 32

Identify 2 consequences of disrupted fluid balance in the gut

Answer 32

- Life threatening **dehydration** (depletion of body fluids) - Life threatening **electrolyte imbalance**

Question 33

Describe the neural control of the gut

Answer 33

- **Somatic**: I. Ingestion - mouth & upper oesophagus II. Excretion - last spinchter of anus - **Autonomic**: I. Range of neurotransmitters II. Post ganglionic neurones form plexuses

Question 34

In the paracrine control of the gut, substances act locally. Describe this

Answer 34

- **Histamine** controls production of acid in stomach - **Vasoactive substances** affect blood flow in gut

Question 35

Which 3 actions are controlled by endocrine hormones of the gut?

Answer 35

- Stomach **acid** secretion - **Alkali** secretion from liver and pancreas - **Enzyme** secretion

Question 36

What does gastrin do?

Answer 36

**Gastrin** promotes production of HCl by stomach parietal cells

Question 37

Which three structures produce gastrin?

Answer 37

- G cells of pyloric antrum of stomach - Pancreas - Duodenum

Question 38

Where does CCK come from?

Answer 38

**CCK** is synthesised and secreted by enteroendocrine cells of the duodenum

Question 39

Which 3 things does CCK do?

Answer 39

- Promotes **release of digestive enzymes** from the pancreas - Promotes **release of bile** from gall bladder - Suppresses **hunger**

Question 40

Which 3 things does secretin do?

Answer 40

- Promotes **HCO₃ secretion** from duct cells of pancreas - Promotes **bile production** by the liver - Inhibits **acid secretion** by parietal cells of stomach

Question 41

Why don't we inhale what we are trying to swallow?

Answer 41

The **epiglottis** is a flap of elastic cartilage that projects behind the tongue and closes during swallowing, preventing aspiration forcing liquids/food down the oesophagus

Question 42

What is bile?

Answer 42

**Bile** is a digestive juice that is secreted by the liver and stored in the gallbladder

Question 43

Identify 3 roles of bile

Answer 43

- **Emulsify fat** to increase surface area for enzymes - Assists in **absorption of fat soluble** substances *e.g. Vitamin A D E K* - **Neutralise acidic pH** of the stomach contents as they enter the duodenum