Absorptive mechanisms in GI tract & electrolytes and water absorption

Question 1

Digestibility or digestive efficiency

Answer 1

-the percentage of food that an animal absorbs, available for utilization
-generally calculated as the amount of nutrient consumed minus the amount of nutrient in the feces

Question 2

Feces

Answer 2

1. undigested food
2. endogenous nutrients
   >basal
   >diet induced

Question 3

Types of digestibility

Answer 3

-True
-Apparent
- Standardized
**used primarily in amino acid nutrition

Question 4

Apparent digestibility

Answer 4

-no correction made for endogenous losses (they are included in total losses)
-influenced by the amino acid or phosphorus content of the test diets (less influence from endogenous as the nutrient content of the diet increases

Question 5

True digestibility

Answer 5

-correction made for endogenous losses
-requires a separation of fecal (or ileal) losses into endogenous (basal and diet induced) from diet
Ex. phosphorus in the feces.. could be derived from diet (truly indigestible) or endogenous

Question 6

Standardized digestibility

Answer 6

-correction only for basal endogenous losses
Ex. feed a phosphorus free diet. Phosphorus in feces then assumed to be endogenous

Question 7

Metabolizability

Answer 7

-correction has been made for nutrient loss in the urine
-typically used with respect to energy (metabolizable energy)
**always used in poultry because can’t separate urine and feces

Question 8

Bioavailability

Answer 8

-the proportion of the nutrient reaching the systemic circulation in a form that can be incorporated into body protein or metabolized
-digestibility does not equal bioavailability
-difficult to measure

Question 9

Nutrient availability in pet foods often estimated using other animal models

Answer 9

-ex.study used to determine if heat processing affects nutrient availability in pet foods using roosters

Question 10

Digestion and absorption

Answer 10

-chemical reactions needed for breakdown often need fluid environment
>so control of amount of fluid in intestinal lumen is critical. Fluid movement in and out of intestinal epithelium is in response to osmotic gradients established by the transport of electrolytes and solutes
-Kidneys (not intestines) are the major determinant of whole body fluid and electrolyte absorption

Question 11

Factors affecting fluid and electrolyte absorption

Answer 11

-a reflection of small intestinal surface area and transit time
>folds in mucosa
>crypts and villi
>microvilli
-amplification of 600x

Question 12

Movement of particles across intestinal epithelium through cell membranes

Answer 12

-must be transported across two membranes
1.apical membrane in contact with the luminal contents
2.basolateral membrane in contact with extracellular fluids

Question 13

Cell membranes of GI tract

Answer 13

-composed of phospholipid bilayer
-lipid soluble hydrophobic compounds cross
-water and hydrophilic materials usually require a transporter
-unstirred water layer which adheres to the glycocalyx may impede passage of large lipophilic materials

Question 14

Different methods of transport

Answer 14

-diffusion (facilitated or carrier-mediated diffusion, non-ionic diffusion, osmosis)
-active transport
-solvent drag
-pinocytosis

Question 15

Different particles and diffusion

Answer 15

-charged water soluble particles do not cross the lipid bilayer
-small uncharged particles will cross membrane and move down the concentration gradient
-generally, uncharged particles with a MW below 100 can freely cross

Question 16

Carrier mediated diffusion

Answer 16

-utilization of carrier proteins
-can be regulated
-typically very selective
-no energy expanded
-ex. amino acids, glucose

Question 17

Non-ionic diffusion

Answer 17

-the transport of weak acids and bases (pH of luminal digesta and the pKa of acid or base)
-non-dissociated (no charge) state are both water and lipid soluble
-dissociated have a charge and cant cross the lipid bilayer
**important for the absorption of short chain fatty acids

Question 18

pKa of butyric acid

Answer 18

4.8

Question 19

Active transport

Answer 19

-typically move a substance against electrochemical gradient; requires energy
-most common in Na/K ATP pump
-required to keep extracellular Na concentration high; 3 Na out, 2K in
-maintains electrochemical potential difference (inside=negative relative to outside)

Question 20

Pinocytosis

Answer 20

-utilized for very large or highly charged particles
-eg. Immunoglobulins

Question 21

Solvent drag

Answer 21

-small solutes (electrolytes) can be pulled by the bulk flow of water

Question 22

Paracellular

Answer 22

-passage of particles through tight junctions
-important for passage of small ions (Na, Cl, K, Ca, PO4, Mg)

Question 23

Tight junctions

Answer 23

-area between cells where a tight seal is formed
-impermeable to bacteria, viruses, large molecules

Question 24

Transcellular

Answer 24

-movement of larger molecules across the enterocytes by a variety of transport mechanisms