N- GI tract Flashcards
How long is the GI tract, oesophagus to rectum
8m
Function of GI tract?
Breaks down ingested food into small molecules which can be taken into body tissue. This is achieved by:
• Digestion
- Chemical, secretion of enzymes
- Mechanical, motility (mixing and grinding)
• Absorption
- Movement of small molecules across the gut wall and into the circulation
Small intest. transit time?
3-6 hours
Large intest. transit time?
1-2 days
What is deglutination
swallowing. Process by which food passes from mouth to stomach.
What are the 3 phases of deglutination?
1) Oral phase- voluntary
2) Pharyngeal phase- involuntary
3) 3- Oesophageal phase
What happens in the Oral phase
a. Tongue moves upwards- compress bolus against hard palate. Respiration inhibited
b. Retraction of tongue forces bolus into pharynx
What happens in the 2- Pharyngeal phase
c. Involuntary movement pushes bolus from pharynx into oesophagus
d. Soft palate reflected backward closing nasal pharynx
What happens in the 3- Oesophageal phase
e. Upper oesophageal sphincter relaxes and bolus moves into oesophagus
f. Start primary peristaltic wave (Vagal)
g. Secondary peristaltic wave (Enteric)
h. Lower oesophageal sphincter relaxes
What is the Oesophagus- muscular tube
- 25cm in length
- 2cm in diameter
- Pharynx to stomach
- Fast transport (10s)
Lower oesophageal sphincter function?
prevents reflux of material back into oesophagus
Stomach- J shaped organ
what are the openings?
What are the 4 regions?
- 2 openings: Oesophagus and duodenum
* 4 regions: Fundus, Cardia, Body, Pylorus
Role of stomach?
store food and begin digestion (mechanical)
4 Stomach functions?
- Motility
- Gastric accommodation- temporary storage reservoir
- Trituration – Dissolve, mix and grind food particles
- Gastric emptying - Control delivery to small intestine - Digestion – initiate digestive process (proteins) via gastric juice
- Protection
- Foreign invasion (acid/proteases)
- Mechanical abrasion (mucus)
- Prevents autodigestion (mucus) - Absorption
- Alcohol and fat soluble drugs (diffusion)
What is present in the Gastric juice?
- Water and ions
- HCl
- Provides low pH (as low as 1 or 2!)
- Prevents bacterial growth
- Catalyses cleavage of pepsinogens to pepsin
• Pepsinogens
- proenzyme of pepsin
- Pepsin breaks down proteins into peptides
• Intrinsic factor
- Glycoprotein
- Binds to vitamin B12- allowing digestion in the ilium
• Mucus
- Protects gastric mucosa
• Gastrin
- From ‘G cells’
- Regulates acid secretion
Name the 3 gastric glands and what they release
- Cardiac glands- Mucus and HCl
- Oxyntic glands (acid secreting)- Mucus, HCl, Pepsinogen, Intrinsic factor
- Pyloric glands- Mucus, pepsinogen
Name some cells of the stomach and what they produce/release
Mucus neck cells- mucus
Parietal cells- HCl and intrinsic factor
Chief cell- pepsinogen
Endocrine cells- G cells: gastrin to stimulate acid secretion and D cells: somatostatin to inhibit acid secretion
Gastric secretory response to a meal:
3 phases…
- Cephalic phase 30%- sight, smell, thought of food triggers gastric secretion; instigated by vagal fibres
- Gastric Phase 60%– food entering stomach stretches it (the stretch!); triggers secretion via long (vagal) and short (myenteric) reflexes.
- Intestinal Phase 10%– intestinal gastrin release in response to distension of duodenum and/ or products of protein digestion
Small intestine, 3 regions:
- Duodenum
- Jejunum
- Ilium
Describe the SI
- Highly coiled
- 4-6 meters long
- No definable junction
- Highly folded- large SA
- Most absorption takes place
- Absorptive epithelium ( 3 orders/strata of folding increases the area available for absorption)
Large intestine, 5 regions:
- Cecum
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
Describe the large intestine
- 1.5m length
- No nutrients in chyme (all have been absorbed?)
- Removes water, salts, sugars and vitamins
- Tightly packed mucosa- goblet cells for mucus secretion
- Areas of lymphoid tissue- Peyers patches for local immune protection
- Teeming with bacteria
Control mechanisms of the GI tract…
- including long/ short reflexes,
- peristalsis and motility,
- secretion and absorption
There are three major control mechanisms: hormonal, paracrine, and neural.
- Autonomic nervous system
- Enteric nervous system
- Gut peptides
• Paracrine
• Hormonal
What is the ENS- Enteric nervous system?
Gut’s own NS.
2 nerve plexus in the gut wall.
- 100 million neurons! As many as in the spinal cord
- ‘senses luminal contents’ -controls muscle and glands
What is chyme?
Fluid that passes from the stomach to small intestine…
- Gastric juices + partly digested food
Long reflexes of the GI tract
- External stimuli (i.e. sight and smell of food)
- Involves CNS
- Alters activity of ENS
- Causes: changes in motility and secretion
Short reflexes of the GI tract
- Internal stimuli (i.e. molecules in lumen)
- ENS
- Local neural circuits
- Causes: changes in motility and secretion
what is Peristalsis and how is it controlled?
- Movement through the GI tract - peristalsis
* Mediated by neurones in myenteric plexus…
What happens in peristalsis?
Distention by bolus of food (chyme) - stimulates peristalsis
- Oral contraction
- Aboral relaxation
What is Hirschprungs disease
- Congenital disorder
- All or part of colon has no innervation
- 1:5000 children affected
- Surgical removal of the colon
What are Enteroendocrine cells
- Single cells scattered through the GI tract
- Link between luminal contents and capillaries
- Intestinal ‘taste’ cells
- Densely packed with vesicles
- Sense luminal contents and respond via release of peptide transmitters
Name some hormones secreted by Enteroendocrine cells in the mucosa
- Gastrin
- Secretin
- Cholecystokinin
- Glucagonlike
- Somatostatin-like
Functions of the buccal cavity- Mastication
- Teeth and tongue mechanically break down food and the addition of saliva results in a smooth soft bolus of food, lubricated and readily swallowed.
- Muscles of mastication are- masseter, temporalis and the pterygoids-innervated by V3 of trigeminal (CNV).
- Saliva contains alpha-amylase which begins breaking down starches in the bolus. Some people have more than other.
Stomach acts as:
3
- Reservoir- Store food
- Preparatory chamber – Breakdown injected materials
- Emptying regulator – Responds to feedback from duodenum. Controls rate of release of calorie, H+ and particles into the duodenum.
Stomach motility after a meal (4)
- Fasting state- quiescent
- Meal enters stomach
o LOS relaxes- relaxation of fundus and body to allow accommodation - Peristalsis begins
o Begins in the middle- push towards the pylorus
o Mixing occurs in antrum - Retropulsion due to Antral systole
o Peristaltic wave pushes contents back into body-
o Some chyme into duodenum
Intestinal- arrival of chyme into first part of small intestine causes slowing down as it passes onto the next phase (feedback mechanism)
Rate of Gastric emptying for liquids, solids
Liquids- empty faster than solids – 2-3mins
Solids – lag due to time to reduce particle size (1 mm diam)
What are Gastric/ Peptic Ulcers
Break in mucosal barrier exposing underlying tissue to corrosive action (acid, proteases)
Symptoms of stomach ulcers
Abdominal pain
Bloating
Nausea/vomiting
Bleeding- haemorrhage and anaemia
Endogenous Factors of stomach ulcers
> Anxiety Parasympathetic output (Ach, Gastrin, acid
>Stress Sympathetic output (Ad), HCO3/mucus
Exogenous Factors of stomach ulcers
> Diet
Alcohol - damages cells, stimulates parietal cell
Coffee – stimulates parietal cell
> Non-steroidal anti-inflammatory drugs (NSAIDs)
decreased prostaglandin production a and decreased inhibition of acid
Name the 3 accessory organs in the GI system
1- Liver: bile, exports to duodenum
2- Pancreas; makes pancreatic juices (digestive enzymes, bicarbonate ions)
3- Gallbladder; stores, concentrates and releases bile