Digestion in the colon, feces formation, defaecation

Question 1

Carbohydrate digestion

Answer 1

• The microbial digestion happens in the colon in the simple stomach herbivores.
• In the horse about 75% of the energy is provided by volatile fatty acids.
• Microbial digestion of cellulose is important because:
• Some carbohydrates are not absorbed in the small intestines
• Carbohydrates not absorbed represents osmotic pressure in colon, and would cause water loss if not digested down to volatile fatty acids.
• They also neutralize acetic products, ensure long retention time, and dilute fermentation products.

Question 2

Buffer system:

Answer 2

• Eq + Su: High HCO3- content from pancreas/intestinal juice
• Ca: Phosphate buffer mainly. HCO3- concentration is low.
• Ru: Phosphate buffer originates from saliva.

Question 3

Protein digestion

Answer 3

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

Question 4

Motions of the colon

Answer 4

• The slow waves of the colon arise from the circular muscle layer and extend from there to the longitudinal fibers.
• The influence of the transmitters in the autonomic nervous system is more significant when considering frequency and the period of time.
• 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
• The most important endocrine effects: Gastrin, CCK (facilitate), secretin (inhibit)
• Dog and cat: aboral mass peristalsis

Question 5

Feces formation

Answer 5

• Lieberkühn glands in the colon produces mucin for mechanical protection as well as feces formation, and HCO3- for buffering of volatile fatty acids.
• Composition, dry mass and pH of feces vary greatly among the domestic species.
• Its color is based on stercobilin
• Feces contain organic indigestible substances only to a small degree. These substances can get into feces when food contains a lot of them, or if the amount of food exceeds digestion/absorption ability of the GI-tract.
• Proteins in feces originate from detached epithelium of large intestines, as well as from microbial origin.
• Amount of feces per day:
• Horse: 15-20 kg/day
• Cattle: 20-30 kg/day
• Pig: 1-2 kg/day
• Carnivores: 0,3-0,5 kg/day

Question 6

Rectum

Answer 6

• Main activity of the rectum is the regulation of the storage and evacuation of the feces.
• Most domestic mammals are unable to voluntarily regulate defecation, but e.g. in the dog a conditioned response can be developed.
• Defecation is under hypothalamic, as well as cortical control.
• Internal sphincter: circular smooth muscle
• External sphincter: striated muscle, innervated directly by motor fibers from the brain cortex.

Question 7

Defecation

Answer 7

1. Empty rectum
   • A feces evacuation can be used if 15-20 mmHg pressure is exerted on the wall of the rectum, and therefore feces are pushed from the previous intestinal section, to the rectum.
   • Spontaneous evacuation of feces is prevented by the closing of the inner and the outer sphincter.
2. 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).
3. 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.
   • Simultaneously with this the stimulus of the somatic pudendal nerve causes the relaxation of the external sphincter.
   • During the feces evacuation reflex caused by the dilation and stretching of the wall of rectum, longitudinal muscles also contract. The consequence of this is the shortening of the rectum, and the processing of its content in the aboral direction.