Topic 60 - Digestion in the colon, feces formation, defaecatio

Question 1

Words to include in carbohydrate digestion in the colon

Answer 1

• Volatile fatty acids
• Microbial digestion
  
  • Cellulose
  • Retention time
  • Fermentation product
• Osmotic pressure
• Buffer system
  
  • Bicarbonate buffer
  • Phosphate buffer

Question 2

Words to include in protein digestion in the colon

Answer 2

• Bacteria
• Urea
• Blood
• Nitrogen-metabolism
• Pseudo-caecotropia (rabbits)
  
  • Cecum

Question 3

Words to include in motions of the colon

Answer 3

• Retrograde flow (ru)
  
  • Retention
• Peristalsis
• Antiperistalsis
• Microbial synthesis
• Aboral-mass peristalsis (Car)
• Slow waves
• ANS
• Pacemaker areas
• Retrograde motions
• Endocrine effects
  
  • Gastrin
  • CCK
  • Secretin

Question 4

Words to include in feces formation

Answer 4

• Lieberkühn glands
  
  • Mucin
  • HCO₃^-
    
    • ​Buffer
    • Volatile fatty acids
• Stercobilin
• Protein
  
  • Detached epithelium
• Rectum
  
  • Storage
  • Evacuation
    
    • Stretch receptors
    • Afferent nerve fibers
    • S1-S4 segments of the spinal cord (feces evacuation center)
    • Feces evacuation reflex
      
      • Efferent stimulus (via pelvic nerve)
      • Somatic pudendal nerve
      • Aboral direction
  • Defecation
    
    • Hypothalamic control
    • Cortical control
  • Internal sphincter
  • External sphincter
    
    • Motor fibers from the brain cortex

Question 5

Topics to include in the essay

Answer 5

1. Role of colon
2. Carbohydrate digestion
   
   • Herbivore
   • Carnivore
   • Requirements of microbial digestion in large intestines
   • Buffer system
3. Protein digestion
4. Motions of the colon
5. Feces formation
   
   • In colon
   • In rectum
   • Defecation

Question 6

What is the role of the colon?

Answer 6

1. Microbial digestion
   
   • Significant in herbivores
2. Water and electrolyte reabsorption

Question 7

Carbohydrate digestion

In herbivores and carnivores

Answer 7

• Herbivore:
  
  • In the horse, about 75% of the energy is provided by volatile fatty acids
• Carnivores:
  
  • Microbial digestion of cellulose is important because: some carbohydrates are not absorbed in the small intestines
  • Carbohydrates not absorbed represents osmotic pressure in the colon, and would cause water loss if not digested down to volatile fatty acids

Question 8

Carbohydrate digestion

Requirements of microbial digestion in large intestines

Answer 8

• Neutralization of acidic products
• Ensuring retention time
• Dilution of fermentation products
• Continuous absorption of acidic end-products

Question 9

Carbohydrate digestion

Buffer system

Answer 9

• Buffer systems responsible for the proper pH of the large intestines:
  
  • Bicarbonate buffer
  • Phosphate buffer
• Eq, sus
  
  • High HCO₃^- content from pancreas/intestinal juice
• Ca
  
  • Phosphate buffer mainly
  • HCO₃^- concentration is low
• Ru
  
  • Phosphate buffer originates from saliva

Question 10

Protein digestion

Answer 10

• Common features with ruminant forestomach
• Bacteria in colon receive urea from blood to balance nitrogen-metabolism
• Rabbit: pseudo-caecotropia
  
  • Microbial digestion occurs in cecum
  • In order to not loose microbial vitamins and proteins, rabbit digest their food two times
  • They do not eat feces, they eat half digested food

Question 11

Motions of the colon

Answer 11

• Intestinal content moves slower in colon than in the previous section of the intestines
• Ru:
  
  • Retrograde flow (ileum – colon – caecum – colon)
  • This is to slow down the flow of the intestinal content (retention)
• Car:
  
  • Small microbial synthesis
  • Fast movement caused by an aboral-mass peristalsis
    
    • Causes evacuation of the majority of the colic content
• The motions of the colon:
  
  • The slow waves of the colon arise from the circular muscle layer and extend from there to the longitudinal fibers
  • The stimuli from the pacemaker areas (initial and terminal part of colon) spread both directions, which makes the retrograde motions and mixing of the intestinal content possible (antiperistalsis)
  • Peristalsis + antiperistalsis = mixing
• Endocrine effects:
  
  • Gastrin
  • CCK (facilitate)
  • Secretin (inhibit)

Question 12

Feces formation

Feces formation in colon

Answer 12

• Lieberkühn glands in the colon produces:
  
  • Mucin
    
    • For mechanical protection and feces formation
  • HCO₃^-
    
    • For buffering of volatile fatty acids
• Color: stercobilin
• Feces contain organic indigestible substances only to a small degree
• Proteins in feces originate from the detached epithelium of large intestines and from microbial origin

Question 13

Feces formation

Interspecies amount

1. Highest
2. Lowest

Answer 13

1. Highest: Cattle
2. Lowest: Carnivores

Question 14

Feces formation

Rectum

Answer 14

• Role: storage and evacuation of feces
• Most domestic mammals are unable to voluntarily regulate defecation
  
  • E.g. in the dog a conditioned response can be developed
• Internal sphincter
• External sphincter
  
  • Innervation: motor fibers from the brain cortex

Question 15

Feces formation

Defecation

Answer 15

• Empty rectum
  
  • A feces evacuation can be used if 15-20 mmHg pressure is exerted on the wall of the rectum
  • Spontaneous evacuation of feces is prevented by the closing of the inner and the outer sphincter
• Feces gets into the rectum
  
  • During the feces evacuation reflex, from the stretch receptors that are sensitive to the stretching of the wall of rectum
  • Afferent nerve fibers reach the S1-S4 segments of the spinal cord (feces evacuation center)
• Feces evacuation reflex
  
  • From the medullary center concerned with feces evacuation, the efferent stimulus (via the pelvic nerve) increases the contraction of the rectum and peristalsis, and relaxes the tone of the sphincter
  • Stimulus of the somatic n. pudenda causes the relaxation of the external sphincter
  • The consequence of this is the shortening of the rectum, and the processing of its content in the aboral direction