Large intestine Flashcards
Describe the innervation of the large intestine
- Complex system, intramural ganglia
- Plexuses in submucosal layer (submucosal plexus) and in muscular layer (myenteric plexus)
- Connected to CNS by fine network of nerve fibres
- Belong to ANS, under sympathetic and parasympathetic control
- Responsible for muscular and secretory activity of gut
Describe the blood supply of the large intestine
- Mainly cranial and caudal mesenteric arteries
- Also abdominal aorta
- Internal pudendal artery supplies caudal part of rectum
Describe the venous drainage of the large intestine
- Veins parallel to artiers, most supply portal vein
- Veins of caudal rectum and anal region join caudal vena cava
What are the divisions of the large intestine?
- Caecum
- Colon
- Rectum
Describe the histological features of the large intestine
- Caecum, colon, rectum very similar
- Mucosa, submucosa, muscularis and serosa (same as SI)
- No villi, only crypts
- Crypts are tubular glands, deeper than SI
- Many goblet cells
- Presence of variable GALT within mucosa and submucosa
Describe the difference in goblet cell density through the large intestine
- Density increases from beginning to end
- Goblet cell density:
Rectum >colon>caecum
Describe the appearance and location caecum
- Blind tube that communicates with ileum and colon
- Left of abdomen in pigs
- Right of abdomen in carnivores, ruminants and horse
- In dog short and drawn into spiral
- In cat shorter and comma shaped
- Very large in horse
Describe the caecum of the horse
- Large capacity (30L, 1m long)
- Base (dorsally located)
- Body
- Blnd ending apex pointing cranioventrally (close to xiphoid cartilage, ventral abdominal floor)
- 4 taenia (medial, lateral, dorsal, ventral) over most of horse caecum
Describe the caecum of pigs
- Cylindrical blind sac laying on left of abdomen
- Apex caudoventrally
- 3 taenia (lateral, medial ventral)
Describe the caecum of ruminants
- Relatively small
- No taenia or haustra
What are taenia?
Visible bands of smooth muscle and elastiv fibres on external surface
What are haustra?
Sacculations between taenia
Describe the structure and appearance of the colon
- Divided into ascending, descending, transverse
- Short ascending, passes cranially on right
- Transverse right to left, cranial to root of mesentery
- Long descending, passes left of mesenteric root caudally
- Continues as rectum in pelvic cavity
- Form and topography complex in some animals
- Ascending can have many modifications
Describe the colon of the horse
- Ascending arranged in 2 U-shaped loops on top of each other
- Short transverse, long ascending
- Ascending referred to as large colon
Describe the large colon of the horse
- 4 limbs 3 flexures
- Is same as ascending colon
- right ventral colon -> sternal flexure -> left ventral colon -> pelvic flexure -> left dorsal colon -> diaphragmatic flexure -> right dorsal colon
- RVC and LVC: 4 taenia
- LDC: 1 taenia
- RDS: 3 taenia
What is the clinical importance of the the flexures in the large colon of the horse?
Sudden changes in diameter, likely sites of impaction causing colic
Describe the colon of the pig
- Big, cone shaped coiled ascending colon
- BAse attached to left abdominal roof, apex pointing ventrally
- 2 taeniae, 2 rows of haustra
- External loops have 2 taeniae, not present on internal loops
Describe the colon of ruminants
- Long ascending, 2 sigmoid flexures, double spiralled area
- Short transverse, straight descending
- Sigmoid flexure at end
- No taenia or haustra
Describe the structure and appearance of the rectum
- Desceding colon enters pelvis, becomes rectum
- Most dorsal pelvic viscera
- Joins anal canal (terminal part of alimentary tract) opens outside with anus
Describe the structure and appearance of the anus
- Mucosa of anal canal keratinised at ends, continuous with skin
- Circumanal galnd only present in canidae and humans
- Anal sacs present in carnivores
Describe the anal sacs of carnivores
- Paired diverticula
- Located within lamina propria/submucosa
- Contain apocrine glands and ducts opening into anal canal
- Prone to occlusion in dogs
Descrieb the large intestine of the dogs and cats
- Caecum between SI and LI
- A, T and D parts
- Caecum small
- Caecum no direct connection with ileum
- Caecum into spiral, held against ileum by fold of peritoneum
- Caecum to colon beyong ileocolic junction
- Guarded by ileocolic sphincter
- Colon suspended by mesocolon
- Transverse R to L of body
- Stomach cranially, SI and cranial mesenteric artery caudally
- Descending is lonest part, to pelvic cavity
- Then is rectum
- LI smooth, villi lacking, many scattered lymph nodules, noo mucosal folds
Describe the large intestine of the pig
- Coiled LI
- Spiral ascending colon
- Sacculated
- 2 taenia on colon, 3 on caecum
- Caudal limp of loop (incl, cecocolic junction) sisto left of mesenteric axis
- Ascending on left, transverse on right due to reversal of course of coil
- Caecum and colon considered together as combine in conical, ventrally tapering mass suspende from roof of abdomen
- Caecum has origin below left kidney, extends ventrally agains left flank
- Rounded blind apex
- Ascending colon arranges around mesentery in cone
- Outer part wide sacculatied continuing from caecum
- Pirals ventrally clockwise centripetally before reverse at apex to ascend in narrower, smoother and tighter centrifugal coils
Describe the large intestine of ruminants
- Colonn divided into ascending, transverse and descending
- Ascending wound in complex manner
- Narrows, turns ventrally, 2 centripetal turns, then 2 centrifugal turns
- Joins short transverse crossing midline in front of mesenteric artery, leads to descending
- No taeniae or haustra
- Smaller ruminants 3 or 4 turns in each direction, centrifugal turns pearl necklace appearance = segmentation of pellets
Describe the large intestine of horses
- Ascending divided into RVDC, LVC, LDC, RDC
- Sternal, then pelvic then diaphragmatic flexures
- RVC, caecum and LVC have 4 taenial bands
- LDC has 1
- RDC has 3
- Descending has 2
- Caecum on RHS
- Apex more ventral, curled like shell
- 4 taenia - medial, lateral, dorsal and ventral
- Short transverse and long descending
Describe water exchange in the GI tract
- Continuous water exchange (secretion and absorption ongoing)
- Essential for digestion and absorption
- Intracellular and extracellular water in body
- Most water absorbed from GIT is from intestinal secretions
- Most water secreted in proximal SI, absorbed in SI and LI
Define the muscularis layer of intestine
- Beneath submucosa, 2 layers smooth muscle, inner circular outer longitudinal
Define chyme
The mixture of feed and digestive secretions in the lumen of the digestive tract
Define peristalsis
Circular contractions travelling along the wall of a tube
Define anti-peristalsis
Contractions that propagate in the opposite direction to peristalsis
Define segmentation
- Causes content to move back and forth, while at same time is transported distally
- Most common form of motility in caecum and colon, lower frequency in intestines
Define mass movements
- At intervals of 3-5 minutes segmentation replaced by movements where larger sections of caecum contract simultaneously
- Represent form of peristalsis, distinguished from that in that contractions of individual muscule cells lasts longer
- Forces caecal content into colon
Explain why regions of the large intestine have haustra/sacculations
- In horses do not have continuous longitudinal muscle layer
- Instead have powerful muscle bands
- Causes series of sac like invaginations = haustra
- Contribute to mixing of colonic content
- Delays transport of chyme
What are the anatomical openings in the large intestine?
- Ileocaecal (between ileum and caecum)
- Ileocolic (between ileum and colon)
- Caecocolic between caecum and colon)
Describe digestion and absorption in hind-gut fermenters
- Adapted to high roughage diet
- Development of large intestine
- Major siteof microbial digestion and water absorption
- LI does not produce digestive enzymes
- Microbial fermentation of fibre, starcha and protein = VFAs and lactic acid
- Fermentation of fructans = fructose and lactic acid
- Absorption of VFAs accompanied by absorption of water and electrolytes
Why do hind gut fermenters require microbial digestion?
- LI does not have digestive enzymes
- Up to have fibre in grass cannot be digested in horse without microbial digestion
- Bacteria snthesise essential AAs
- Microbial digestion produces water soluble B and K vitamins
- Rabbits and rats coprophagic to increase intake of protein AAs and B vitamins
What factors are important in digestion in hind-gut fermenters?
- Type of bacteria present
- Correct pH 5-6
- Anaerobic environment
- Fluid movement
- Retention of food (up to 65 hours)
Where does most microbial digestion and absorption occur in the horse?
- Caecum
- Ventral colon
Where does most microbial digestion and absorption occur in the rabbit?
Caecum
What are the effects of infrequent feeds in horses?
- Decrease in pH and increased lactate
- Massive influx of water rather than continuous low level
- Large concentrate meal can cause 15% change in circulating fluid volumes
