S8) The Digestive System Flashcards
Label the following components of the alimentary canal:
What are the four layers of the gut wall?
- Mucosa (innermost)
- Submucosa
- Muscularis externae
- Serosa (outermost)
Identify the 8 functions of the gut
- 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
What is digestion?
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
Identify 4 roles of saliva in the GI tract
- Starts digestion (amylase & lipase)
- Assists swallowing
- Protects mouth & teeth (high calcium)
- Bacteriostatic (IgA antibody)
Compare physical and chemical disruption in the mouth and stomach respectively
- 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
Describe the transport of food in the GI tract
- Mouth forms a bolus which enters the oesophagus
- Rapid peristaltic transport (even when upside down) moves bolus to stomach (8-9 seconds)
Describe the neural control of the oesophagus
- Upper end oesophagus – voluntary control (striated skeletal muscle)
- Lower end oesophagus – involuntary control (solely smooth muscle)
Identify and describe the different components of oesophageal mucosa
- 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
Identify and describe the structure of the layer of the oesophageal wall, apart from the mucosa
- Submucosa – connective tissue containing mucus-secreting glands
- Muscularis externa – smooth muscle layers which move food by peristalsis
- Adventitia – thin outermost layer of connective tissue
Identify the different smooth muscle layers in the muscularis externa of the oesophageal wall
- Inner layer is circular muscle
- Outer layer is longitudinal muscle
State 4 roles of the stomach
- Temporarily stores food
- Receptive relaxation to maintain pressure
- Proteolytic enzyme and acid secretion
- Produces hypertonic incompletely digested chyme
What are the three layers of smooth muscle in the stomach?
- Oblique
- Circular
- Longitudinal
How does the stomach protect its epithelium
The stomach protects its epithelium by secreting bicarbonate-rich mucus
What is rugae?
Rugae are folds of gastric mucosa, forming longitudinal ridges in the empty stomach
What is the gastric pit of the gastric gland ?
The gastric pit is the region of the gastric gland lined by mucus secreting cells, similar to goblet cells
What is the isthmus (of the gastric gland)?
The isthmus is the region in which stem cells divide to populate the gland by upward or downward migration
What do parietal cells do?
Parietal cells secrete H+ ions into the lumen and HCO3+ ions into nearby capillaries, which move it to the surface mucous cells
What do chief cells do?
Chief cells secrete pepsinogens which are converted into pepsins which partly hydrolyse proteins
What are enteroendrocrine cells?
Gastric enteroendrocrine cells (G cells) are cells found at the base of the gastric gland which secrete gastrin
What are surface mucous cells and what do they do?
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
Discuss the resistance/vulnerability of surface mucous cells
- Their secreted mucus is resistant to pepsin degradation
- They can be damaged by aspirin/alcohol
Describe the structure, location and function of the duodenum
- 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
Describe the dilution and neutralisation of chyme
- Water drawn in from ECF to dilute hypertonic chyme
- Pancreas & liver secrete alkali and bile to neutralise acidic chyme
Digestion is completed by enzymes from the pancreas and intestine.
What does this entail?
- Cleaves peptides to amino acids
- Cleaves polysaccharides to monosaccharides
- Breaks down and re-form lipids
- Breaks down nucleic acids
What are the 3 main requirements of absorption?
- Large amounts of energy (active process)
- High surface area (slow process)
- Good blood supply/drainage
Identify the components of the small intestine and their individual roles
- Duodenum absorbs iron
- Jejunum absorbs most sugars, amino acids and fatty acids
- Ileum absorbs Vitamin B12, bile acids and remaining nutrients
Plicae circulares are found in the jejunum. What are they?
Plicae circulares are circular folds of mucosa and submucosa that project into the gut lumen
What is a lacteal?
A lacteal is a dilated lymphatic capillary vessel
What are the components of the large intestine?
What are the roles of the bacteria of the large intestine?
- Synthesis of vitamins K, B12, thiamine and riboflavin
- Breakdown of bile acids
- Conversion of bilirubin to non-pigmented metabolites
Identify 2 consequences of disrupted fluid balance in the gut
- Life threatening dehydration (depletion of body fluids)
- Life threatening electrolyte imbalance
Describe the neural control of the gut
- Somatic:
I. Ingestion - mouth & upper oesophagus
II. Excretion - last spinchter of anus
- Autonomic:
I. Range of neurotransmitters
II. Post ganglionic neurones form plexuses
In the paracrine control of the gut, substances act locally.
Describe this
- Histamine controls production of acid in stomach
- Vasoactive substances affect blood flow in gut
Which 3 actions are controlled by endocrine hormones of the gut?
- Stomach acid secretion
- Alkali secretion from liver and pancreas
- Enzyme secretion
What does gastrin do?
Gastrin promotes production of HCl by stomach parietal cells
Which three structures produce gastrin?
- G cells of pyloric antrum of stomach
- Pancreas
- Duodenum
Where does CCK come from?
CCK is synthesised and secreted by enteroendocrine cells of the duodenum
Which 3 things does CCK do?
- Promotes release of digestive enzymes from the pancreas
- Promotes release of bile from gall bladder
- Suppresses hunger
Which 3 things does secretin do?
- Promotes HCO3 secretion from duct cells of pancreas
- Promotes bile production by the liver
- Inhibits acid secretion by parietal cells of stomach
Why don’t we inhale what we are trying to swallow?
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
What is bile?
Bile is a digestive juice that is secreted by the liver and stored in the gallbladder
Identify 3 roles of bile
- 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
