The Small Intestine Flashcards
What features of the small intestine increase its surface area?
Large circular folds
Villi
Microvilli
What is contained in the core of each villus?
Vessels
Lacteals (lymphatics)
Cells
What does the lamina propria of the small intestine contain?
Plasma cells Lymphocytes Macrophages Eosinophils Mast cells
What are the crypts of Leiberkuhn?
The spaces between the bases of the villi
Where are enterocytes formed?
At the bottom of the crypts of Leiberkuhn
What is present on the luminal side of the small intestine enterocytes?
A gelatinous layer called the glycocalyx
What are scattered between epithelial cells of the small intestine?
Mucin secreting goblet cells
Intraepithelial lymphocytes
Paneth cells
Which artery supplies most of the blood supply to the small intestine?
Superior mesenteric artery
Which two plexuses are the enteric nervous system’s neutrons split between?
Myenteric plexus
Submucosal plexus
When does the MMC occur?
During fasting
What are the three phases of the MMC?
Period of motor inactivity
Period of irregular contractile activity
Short burst of regular phasic contraction that propels secretion, residual food and desquamated cells towards the colon
How is starch absorbed in the small intestine?
- Starch broken up into oligo-, di- and monosaccharides
- Oligo- and disaccharides are further broken up on the microvilli to form monosaccharides
- The monosacchraides are then able to be transported across the enterocytes into the blood
How is protein absorbed in the small intestine?
- Protein is broken down into amino acids and peptides
- Enzymes are activated, allowing them to break down the peptides into oligopeptides
- Some di- and tripeptides are absorbed and the remainder are broken down to amino acids on the microvilli before being absorbed
What enzyme breaks down the majority of starch?
Pancreatic amylase
Which enzymes break down peptides to oligopeptides?
Trypsin
Chymotrypsin
Elastase
How is fat absorbed in the small intestine?
- Pancreatic lipase breaks down the triglycerides in fat to fatty acids and monoglycerides
- The products of fat digestion form mixed micelles along with bile acids and phospholipids
- The lipid contents of the micelles are absorbed into enterocytes and the bile salts remain in the lumen
- Inside the cells the monoglicerides and fatty acids are resynthesised to triglycerides which are incorporated into chylomicrons to be transported into the lymph
What does dietary fat consist of?
Mainly triglycerides
Cholesterol
Fat-soluble vitamins
What happens to the bile salts that remain in the lumen after lipid products are absorbed into enterocytes?
Bile salts pass through the small intestine to be absorbed in the terminal ileum.
They are then transported back to the liver
How can patients with pancreatic exocrine or bile salt insufficiency supplement their fat absorption?
With medium chain triglycerides that are transported via the portal vein
What happens when unabsorbed lipids reach the ileum?
They delay lipid emptying via peptide YY to allow more time for lipid absorption
What are the physical defences of the small intestine against pathogens?
Mucous layer
Continual shedding of surface epithelial cells
Physical movement of luminal contents
Colonisation resistance (the ability of indigenous microbiota to outcompete pathogens for survival
What are the innate chemical defences of the small intestine against pathogens?
Enzymes secreted by Paneth cells
Antimicrobial peptides
Trefoil peptides which stabilise mucous, promote cell migration to injured areas and promote repair
What are the innate immunochemical defences of the small intestine against pathogens?
IgA antibodies
B-cells
T-cells
Where are T cells present?
Gut associated lymphoid tissue
Lamina propria
Surface epithelium
What are the oral tolerances that protect against pathogens?
Maintenance of barrier function
Active inhibition via regulatory T cells
Dendritic cells
What are the common clinical features of small bowel disease?
Diarrhoea Steatorrhoea Abdominal pain and discomfort Weight loss Nutritional deficiencies
What is steatorrhoea?
Stools are pale, bulky, offensive and float, leaving a fatty film on the water and are difficult to flush away
What are the common nutritional deficiencies?
Iron
Vitamin B12
Folate
What can vitamin K deficiency cause?
Bruising
What can calcium deficiency cause?
Tetany (involuntary contraction of muscles)
What can vitamin D deficiency cause?
Osteomalacia
What can hypoproteinaemia cause?
Oedema
What can visible peristalsis and high pitched bowel sounds indicate?
Chronic subacute obstruction of the small intestine
What is the usual pathway for investigation of suspected small bowel syndrome?
- Coeliac serology
- Faecal elastase (followed by pancreatic investigations if positive)
- Small bowel imaging
- Gastroscopy and duodenal biopsies
- Video capsule endoscopy
- Breath tests for bacterial over growth
What are the blood tests that can be done for suspected small bowel syndrome?
FBC and film Inflammatory markers - ESR and CRP Serum calcium LFTs Immunological tests HLA testing
What may indicate the presence of osteomalacia?
Low serum calcium and raised alkaline phosphate
What serological test is helpful to diagnose Coeliac disease
Tissue transglutamse
What investigations can be done to assess small bowel anatomy?
MRI enteroclysis Small bowel barium follow-through Small bowel biopsy Ultrasound CT scan Video capsule enteroscopy
What does a small bowel barium follow-through detect?
Abnormalities such as diverticula, strictures, dilation and Crohn’s disease
When is an ultrasound useful in small bowel investigation?
As a preliminary investigation
Can show thickened small bowel or distended loops
What can a CT scan show in small bowel investigation?
Small bowel wall thickening
Diverticula
Extra-intestinal features such as abscess
What is video capsule enteroscopy used for in small bowel investigation?
To visualise the small bowel lumen and mucosa along the whole length
When is a hydrogen breath test used?
As a screening test to measure transit time and detect bacterial overgrowth
When are tests for pancreatic insufficiency used?
In the differential diagnoses for steatorrhoea
When are serum immunoglobulins measured?
To exclude immune deficiencies
When are gut peptides measured?
In high volume secretory diarrhoea
When are chromogranins A and B raised?
In endocrine tumours
When are tests for protein-losing enteropathy used?
If low serum albumin is a clinical feature
