L18 - Small intestine physiology Flashcards
What are the main components of the SMALL intestines?
- Duodenum - chyme enters from stomach via pyloric sphincter
- Jejunum - where most of the digestion occurs
- Ileum
What are the main components of the LARGE intestines?
- Caecum
- Colon - predominant
- Rectum
General structure of the small intestine
- Mucosa
- epithelium
- villi + microvilli
- intestinal glands (secretions)
- vascularised systems - Submucosa
- submucosal plexus (hormonal control)
- Brunner’s glands (secretes alkaline mucus - neutralising)
- glands (hormone secretions) - Muscularis
- circular muscles (segmentation)
- longitudinal muscles (peristalsis)
- myenteric plexus (neuronal control) - Serosa + Mesentery
What is the structure and function of the villi and microvilli in the small intestine?
- To create greater surface area - increases absorptive capacity ~600-fold!
In a villus:
- lined with epithelial cells - main functioning cells
- microvilli on epithelial lining - increases surface area further
- capillaries - blood supply
- arterioles connecting to capillaries (connects to artery in the end)
- nerve fibres
- venule (connected to vein)
- lacteal (connected to lymph duct) - transport of nutrients
What are the intestinal juices made of?
- Water
- Mucus
- Digestive enzymes
- 2-3L liquid (pH 7.6) produced per day
- Secretion regulated by reflex stimulated by presence of chyme
- CCK + secretin also stimulates juice secretion
What are the mechanical movements in digestion in the small intestine?
- Segmentation
- facilitates mixing of food
- done by circular muscles in muscularis layer - Peristalsis
- facilitates propulsion of food along the GI tract
- done by longitudinal muscles in muscularis layer
Chemical digestion in the GI tract
- Starts by salivary amylase in the mouth
- Pepsin breaks down proteins in the stomach
- In small intestine, digestive process is completed by combines actions of:
- pancreatic juice
- bile
- intestinal juice
Digestion of carbohydrates
- Starch –> disaccharides —- by pancreatic amylase
- Disaccharides (maltose, sucrose) –> monosaccharides (glucose, fructose) — by glycosidase
- Monosaccharides absorbed
Digestion of proteins
Polypeptides (proteins partially broken down by pepsin in stomach) arrive small intestine
- catabolised by pancreatic trypsin + chymotrypsin
- Peptides broken further down by peptidase (from glandular epithelium) —- into amino acids
- Amino acids absorbed
Digestion of fats
- Jejunum - where most fat gets absorbed
- Bile salts from bile - emulsifies large fat globules into smaller globules
- Emulsification = breakdown of triglycerides by increasing surface area for water soluble pancreatic lipases to act upon and produce monoglycerides + free fatty acids
Absorption of monosaccharides
Glucose + galactose:
- absorbed into epithelial cell by co-transport with Na+ via Na-glucose transporters
- leave epithelial cell - enter blood by facilitated diffusion via glucose transported (GLUTs)
Fructose:
- Absorbed (without co-transport) into and leave epithelial cells by facilitated diffusion via GLUTs
Absorption of amino acids + dipeptides
Dipeptides:
- Uptake into epithelial cell - coupled to H+ influx
- broken down into amino acids in epithelial cell
Amino acids:
- Uptake into epithelial cells by active transport coupled to Na+ ions (as co-transport)
- Leave epithelial cell and into blood by facilitated diffusion
Absorption of fats
- Monosaccharides + free fatty acids associate with bile salts ==> micelles/emulsion
- Passive diffusion into epithelial cells
- Bile salt portion of the micelles remain in lumen until terminal ileum - then recycled by enterohepatic circulation
- Resynthesis of triglycerides inside epithelial cell (by smooth ER)
- Triglycerides accumulate in vesicles of smooth ER of epithelial cell to form chylomicrons (transport) — leave the cell by fusing with plasma membrane
- Chylomicrons leave intestine in lymph ducts to be distributed around body - too large to enter bloodstream — eventually delivered into venous circulation
The lymphatic system
Absorbs:
- nutrients (primarily fats)
- electrolytes
- fluid
- protein from interstitial spaces
- chylomicrons (triglycerides)
Function:
- removal of haemorrhaged red blood cells from tissue + invading bacteria
- Lymphatic flow - bypasses liver metabolism
- high capacity system - transports 100g triglycerides per day
- lymph flow - 100-200 mL/h
Absorption of vitamins
- Fat soluble vitamins (A, D, E, K) => absorbed in micelles
- Water soluble vitamins (B, C) => absorbed by diffusion
- Vitamin B12 => requires presence of intrinsic factor (produced by parietal cells in stomach)
