absorption Flashcards
absorption
products of digestion move across the intestinal
mucosa, and into the vascular system
what membranes do nutrients pass through
apical and basolateral
active transport consumes
ATP
in intestines, most important active membrane transport pathway is the
Na+/K+ ATPase pump
where does Na+/K+ ATPase pump sit
basolateral membrane
Na+/K+ ATPase pump how does it work
- Pumps Na out of cell across basolateral membrane
- Exchanges 3 Na+ for 2 K+ (uses 1 ATP)
- Keeps intracellular Na+ concentration low
- Creates Na+ electrochemical gradient
Na+/K+ ATPase pumps 3 Na+ out of cell in exchange for 2 K+ into cell, this keeps the inside of the cell
electrically negative compared to the extracellular fluid.
describe co-transport or symport (secondary active transport using potential energy of Na+ electrochemical gradient)
At the apical surface
- Binds 1 or 2 Na+ ions
- Binds another specific molecule, e.g., glucose
- Fills all binding sites –> Co-transporter protein reconfigures –> Binding sites and ligands all flip to the interior of the cell –> Na+ + co-transported molecule (glucose) released
intracellularly
(recall Na+-K+ ATPase is at basolateral membrane ie other side and pumps Na+ back out)
co-transport aka symport is able to transport glucose against its concentration gradient if
Na+ electrochemical gradient is high enough
what are other nutrients, other than glucose, that are absorbed by the Na+ co-transporter (secondary transport)
amino acids, bile acids and several vitamins
what is another form of secondary transport (other than symport)
- Involve proteins known as exchangers or antiports
- Exchange extracellular Na+ for intracellular molecule, such as H+
sodium/hydrogen exchanger
Na GOES IN (following is chemical gradient due to the Na+-K+ ATPase pump)
H goes OUT
- apical membrane at top
- recall Na+-K+ ATPase pumps Na out at basoletaral membrane
- secondary transport; antiport
- The protein has a binding site for sodium, and another binding site for hydrogen. When no ligands – the ions – are bound, the sodium site faces the lumen of the intestine, and the hydrogen site faces
into the cell. When both sites are occupied, the exchanger changes shape ; it flips and releases sodium into the cell, and hydrogen outside the cell. - it’s driven by the sodium electrochemical gradient across the
membrane
tertiary active transport
- Uses electrochemical gradients formed by secondary active transport mechanisms
- Cl- /HCO3 - exchanger
chloride-bicarb exchanger
This tertiary active transport mechanism is driven by the gradients that are set up by the sodium/hydrogen
exchanger, which is secondary active transport, as it is driven by the primary Na+/K+-
ATPase pump.
Passive transport through the intestinal epithelium either takes place
through ion
channels in the cell membranes, or directly through the tight junctions
transcellular absorption ie passive diffusion
passive transport through ion channels; ions move along their electrochemical gradient
how are ion channels regulated (passive transport)
opened or closed
transcellular absorption vs paracellular absroption
- transcellular: Passive
diffusion through the cells (transcellular absorption) takes place through ion channels (ions) - paracellular; passice diffusion AROUND the cells through tight junctions (water and small inorganic molecules)
paracellular absorption (ie passive transport through tight junctions) has movement in response to
osmotic pressure and electrochemical gradients
what are the products of membranous-phase digestion of carbs and how are they absorbed
glucose and galactose
absorbed by sodium co-transport (secondary active transport, symport) in apical membrane
- Close proximity to membranous phase digestive enzymes in glycocalyx
- Pass through basolateral membrane by facilitated diffusion (driven by concentration gradient)
how is fructose absorbed
Transported across both apical and basolateral surfaces by facilitated diffusion
how are free amino acids absorbed
using Na+ cotransporters (secondary active transport; symport)
3-5 diff types of cotransport proteins used (those for neutral, basic and acidic amino acids)
how are dipeptides and tripeptides absorbed
- H+ co-transporters; secondary active transport, symport
- Once inside the enterocyte, digested into free amino acids
how are lipids absorbed
- by carrier proteins and simple diffusion
- Fatty acids and monoglycerides carried in micelles to surface of enterocytes
- Highly lipid-soluble –> diffuse through enterocyte cell membrane
- Aided by fatty acid-binding proteins in glycocalyx
all componnets of micelle diffuse into enterocytes during absorption of lipids EXCEPT
bile acids
these are left behind and recycled
when are bile acids absorbed
aren’t absorbed until they reach the ileum, where they are reabsorbed by a sodium co-transporter (secondary transport, symport)
once in enterocytes absorbed lipids are transported to the _____ and packaged into _____
endoplasmic reticulum
chylomicrons
chylomicrons
- how absorbed lipids are packaged
- Chylomicrons are spherical structures, that have a lipid core containing triglycerides and cholesterol ester, and the surface contains phospholipids and cholesterol.
- arranged so that the hydrophobic ends are pointed into the core, and hydrophilic ends face the surface.
- water soluble.
after lipids are absorbed and packaged into chylomicrons, how do they leave the cells, and where do they go
it’s transported across the basolateral
membrane of enterocytes, into the lateral space. However, the chylomicrons are too
big to pass through the endothelium lining the capillaries. So instead, they pass into the more porous lymphatic vessels, the lacteals, where they can eventually drain into
the blood stream (why u see milky blood plasma after animal eaten a fatty meal)
3 mechanisms of sodium absorption
- Na+ co-transport proteins
- Na+/H+ exchanger, coupled with Cl-
/HCO3 - exchanger
–> “Coupled sodium chloride transport” - Simple diffusion through ion channels
digestion and absorption of lipids SUMMARY
(individual cards on all of this info just here to refer back to)
In the stomach the fats are warmed up, and broken down in size so they can pass into the intestine. In the small intestine bile acids and phospholipids act as detergents, finishing the process of emulsification. The lipids are then acted on by lipiddigesting pancreatic enzymes, leading to micelle formation. The micelles
diffuse through the unstirred water layer and glycocalyx, until they reach the apical surface of the enterocytes. Most of the components of the micelles, except for the bile acids, then enter the enterocyte. The fatty acids re-form triglycerides, and are packaged into chylomicrons, which exit the basolateral membrane and enter a lacteal, which transports the chylomicrons away from the intestine
3 mechanisms of chloride absorption
1) coupled sodium chloride absorption
2) paracellular Cl- absorption
3) Cl-/ HCO3- exchanger
coupled sodium chloride absorption
Hydrogen in the cell is formed by the carbonic anhydrase reaction, and is exchanged for sodium within the lumen across the apical membrane.
As hydrogen is exchanged for sodium, bicarb also builds up in the cell, which drives Cl-
/HCO3- exchangers on the apical membrane.
Paracellular Cl- absorption
- As Na+ pumped to lateral spaces creates electrical gradient
- Cl- ions pass through tight junctions
- Neutralise gradient
Cl- /HCO3- exchanger: what membranes and results in luminal pH increasing or decreasing
- both apical and basolateral membranes
- increased luminal pH
bicarbonate absorption
- HCO3 - ions absorbed in ileum and colon, however anions are not absorbed directly
- Carbonic anhydrase in enterocytes form H+ and
HCO3- ions - H+ exchanged for Na+ at luminal surface
- H+ ions neutralise the HCO3 - in the lumen
- HCO3 - ions generated in enterocytes plus
absorbed Na+ cross basolateral membrane
HCO3 - not directly absorbed across luminal surface
- New HCO3- is generated in the enterocytes and the
HCO3 - in the lumen is neutralised
HCO3- secreted into the intestinal lumen by several digestive glands and reacts with H+ and Cl- from stomach to form
H2O, CO2 and NaCl
Potassium Absorption
- Diet usually has sufficiently high levels of K+–> Concentration gradient
- K+ then absorbed by passive paracellular diffusion
how would impaired water absorption affect potassium (K+) absorption
With more water in the lumen, the potassium in the lumen is diluted, and the concentration gradient required for potassium absorption isn’t formed.
Water Absorption and Secretion
Passive absorption by osmosis, either paracellular or transcellular
Water movement occurs in whatever direction necessary to keep ingesta iso-osmotic
Water movement occurs in whatever direction necessary to keep ingesta ____
iso-osmotic
there is active secretion of H2O in ____ cells
crypt
how is the coupled sodium chloride transport mechanism in crypt cells different
Crypt cells have the coupled sodium chloride transport mechanisms on the basolateral membrane. This is opposite to what we see
in the mature cells on the tips of the villi, where we see it on the apical membrane. The effect of having the coupled sodium chloride transport mechanism on the basolateral membrane is to pump sodium and chloride into the enterocytes in the crypt.
Sodium is quickly pumped back out by the sodium potassium ATPase pump.
The chloride ions remain within the enterocyte, increasing the intracellular concentration. Chloride channels are stimulated to open by cAMP, and chloride ions flow down their concentration gradient, into the lumen. The chloride ions in the lumen create an electrical attraction for sodium ions, which move into the lumen from the lateral space, via the paracellular route. Water follows sodium and chloride
osmotically. So the crypt epithelium secretes sodium, chloride, and water.
crypt epithelium secretes
sodium, chloride, and water
Nutrient-rich blood leaves intestine and travels
via hepatic portal vein to liver before returning to the heart
blood pressure of nutrient rich blood leaving intestines and travelling to liver
The pressure is relatively low as blood has to pass through 2 capillary beds, in the gut wall and in the liver, before it returns to the heart
what are 2 ways to compensate for the low blood pressure to allow blood to flow from intestines to liver
- Sinusoids relatively large and offer little resistance to flow
- Hepatic vein goes directly to thoracic caudal vena cava, negative pressure created in thorax
helps ‘suck out’ blood from liver
liver disease affect on blood flow
increased resistance to flow in sinusoids
right sided heart failure affect on blood flow
decreased venous return
In the intestine, division and replication of enterocytes only takes place in the
crypts
maturing enterocytes move up the
crypts to the villi as they mature
how long do enterocytes last
4-7 days
length of villi depends on what 2 factors
how quickly the enterocyte cells are multiplying in the crypts, and how quickly the cells at the tips are being lost
how do hormones in GI tract effect villi length
GIT hormones stimulate increase in mitosis in crypts (where replication and division of enterocytes occur) (trophic effect)
* Results in longer villi, increasing digestive and absorptive capacity
for neonates, what is the primary source of antibodies
Antibodies secreted in colostrum are primary source of antibodies
what are 3 ways that ensure antibodies are not digested in neonates
- Acid secretion delayed for several days after birth
- Pancreatic secretion delayed
- Special population of enterocytes in jejunum and ileum
can neonates absorb intact proteins after birth
yes but only for a few days, then SI loses capacity, called gut closure
true or false: Animals that do not receive colostrum will likely die due to opportunistic
infections
true
animals that are closer to weaning: what happens to lactase activity, and maltase activity?
- Lactase activity decreases (may be non-existent in adult ie lactose intolerant)
- Maltase activity increases
- Reflects diet shift from lactose (milk sugar) to starch as main CHO source
diarrhea is a mismatch between
secretion and absorption of water
where does water in gut come from (3 sources)
- Ingested water
- Water secreted by glands of the GI system
- Water secreted or lost directly through the mucosal epithelium
Malabsorptive diarrhoea is when there’s:
usually due to what
decreased absorption
Usually due to a loss of GIT epithelium
Loss may be due to viral, bacterial or protozoal infection
* Results in shorter villi
* Cell loss > cell replacement in crypts
* Decreased absorptive capacity
diarrhea resulting from a problem w digestion is often due to issues with
exocrine pancreas
- EPI: exocrine pancreatic insufficiency
- Decreased digestion causes solutes to remain in the lumen
- Increases osmotic pressure
- Causes osmotic diarrhoea
secretory diarrhea, what and cause
Secretory diarrhoea = secretions overwhelm absorptive capabilities
Often due to toxins (enterotoxins) produced by some pathogenic bacteria such as E. coli
Cause increase cAMP –> opening of Cl- channels –> increase NaCl and H2O
secretion
A drug that blocks the activity of the Na+/K+-ATPase pump could be expected to have what effect on sodium-glucose co-transport?
Decreased sodium-glucose co-transport
Which of the following statements regarding the absorption of carbohydrates is CORRECT?
A.
Fructose is absorbed by sodium co-transport at the apical membrane
B.
Disaccharides are absorbed by sodium co-transport at the apical membrane
C.
Glucose passes through the basolateral membrane by facilitated diffusion
D.
Galactose is transported across both the apical and basolateral surfaces of the enterocyte by facilitated diffusion
C.
Glucose passes through the basolateral membrane by facilitated diffusion
Before entering the intestinal capillaries, all nutrients pass through the:
Lateral space
During sodium absorption by glucose co-transport: chloride…
Chloride is absorbed by the paracellular route
In some species, maternal antibodies do not cross the placenta, and the primary source of antibodies for the neonate are ingested in colostrum. Which of the following statements is INCORRECT?
A.
Acid and pancreatic secretions are delayed for several days after birth
B.
Neonates can absorb intact proteins for a few weeks after birth
C.
The small intestine rapidly loses the capacity to absorb intact proteins
D.
Animals that do not receive colostrum will likely die due to opportunistic infections
B. Neonates can absorb intact proteins for a few weeks after birth
