intestinal physiology 1: fluid & electrolytes and digestion in the intestines Flashcards
define absorption
the movement of nutrients, water and electrolytes
from the lumen of the small intestine into the cell,
then into the blood.
how do water and ions enter single-celled organisms
via diffusion
how do larger particles enter single-celled organisms
phagocytosis & endocytosis
digestion & absorption in lysozymes
how does shape affect exchange
larger surface area to volume ratio
for optimal digestion and absorption
multicellular organisms have a greater ratio than single-celled ones
terrestrial multicellular organisms
not living in an aqueous solution filled with nutrients
GI function
to take relatively large solids and digest them into smaller molecules that can be absorbed as nutrients, while still serving as a barrier to toxins, bacteria, parasites
what is the GI system
a hollow organ
a tube through the body
contains specialised organs for secretion of enzymes and biles
describe lumen of GI
it is ‘outside’ the bodys tissues
but its environment is tightly controlled by the body
what do epithelial cells do in GI
they line the entire the GI tract and serve as the primary barrier
how is structure of GI efficient
it maximises SA for secretion and absorption
contains folds, villi and crypts
what do epithelial cells do with water and electrolytes
they can secrete and aborsb them
the GI tract transports fluids and electrolytes
what is chyme
a thick semifluid mass of partially digested food and digestive secretions that is formed in the stomach and intestine during digestio
what is the most abundant substance in chyme
water
how much ingested and secreted water enters the small intestine every day
around 8000ml
In ingested food & drink, and through secretions (saliva, bile, pancreatic juices etc.)
how much ingested and secreted water is passed onto the large intestine
only around 1500 ml
since 80% of water
reabsorption occurs in the small intestine
what re absorbs the most amount of water
the jejunum
how much water is re absorbed in the stomach
small amounts
the stomach has a much
smaller surface area available for diffusion and lacks the solute-absorbing
mechanisms that create the osmotic-gradient necessary for absorbing water
how does water move
down an osmotic gradient
how do eletrolytes move
down electrochemical gradients
what supplies energy for water and electrolytes to move against concentration gradients
sodium gradients and proton gradients
generated by the sodium pump
function of small bowel/intestine
it is the main area for fluid absorption and secretion
what are the 3 segments of the small bowel
- duodenum
- jejunum
- ileum
in total how much of the fluid load is re absorbed
around 98%
how much of the fluid load is excreted in stool
only around 200ml (2%)
describe membrane of small intestine
The epithelial membranes of the small intestine are very permeable to water, and
net water diffusion occur across the epithelium whenever a water concentration difference is established by the active absorption of solutes
what is the most actively transported solute
Na+
because it constitutes the most abundant solute in chyme - it is actively transported from the lumen in the
cell membranes of the ileum & jejunum
describe luminal membrane transport
it is is variably coupled
- glucose, amino acids or other
substance
what is coupled transport
the simultaneous transport of two substances across a biological membrane
how does Na help with nutrient absorption
many nutrients are absorbed via Na+ cotransport systems
2 stage transcellular process
- membrane transport protein
- Na+K+ATPase transporter
describe contents of the colon
they are iso-osmotic
(concentration in lumen of colon = that of blood)
this means Na+ is actively pumped from the lumen and water follows
describe potassium re absorption in colon
In general K+ reabsorption is by passive diffusion
the net movement is determined by the potential difference between the lumen and intestinal capillaries.
what is hypokalaemia
loss of K+
what can cause severe hypokalaemia
diarrhoea
describe chloride re absorption in colon
Cl- is actively reabsorbed in exchange for bicarbonate
resulting in the intestinal contents becoming more alkaline
factors affecting absorption (4)
- number and structure of enterocytes
- blood and lymph flows
- nutrient intake
- GI motility
factors affecting secretion (3)
- irritants
- bile
- bacterial toxins
how do toxins cause dirrahoea
- cholera toxin released from bacteria in infected intestine
- binds to intestinal cells
- stimulates adenylate cyclase to produce CAMP
- dramatic efflux of ions and water
- watery diarrhoea
what happens if there is an imbalance between absorption and secretion
leads to disease
journey of digestion
- stomach
- small intestine
- blood
- liver
- large intestine
- leaves body
mechanical digestion
chewing & churning of food
chemical digestion
enzymes
what are carbohydrates broken down into
monosaccharides
what are proteins broken down into
amino acids
what are fats broken down into
fatty acids & glycerol
what is small intestine the primary site for
digestion and absorption of food
how does digestion occur in the GI lumen
by secreted enzymes
how does digestion occur on the surface of enterocytes
by membrane-bound enzymes
what are the methods that absorption occurs by
- simple diffusion
- facilitated diffusion
- active transport
- endocytosis
- paracellular transport
what increases SA of small intestine
extensive folding
projection of villi covered with microvilli
how are glucose and galactose absorbed
by enterocytes through an NA dependent secondary active transport process
how is fructose absorbed
by facilitated transport
what are the 2 functional states/periods the body undergoes in providing energy for cellular activities
absorptive & postabsorptive states
what is absorptive state
During which ingested nutrients enter the blood from the GI tract
During this state, some of the ingested nutrients provide the energy
requirements of the body and the remainder is added to the body’s energy stores to be called upon during the next postabsorptive state
what is post absorptive state
During which the GI tract is empty of nutrients and the body’s own stores must supply energy
average daily intake of lipids
70 to 100 per day - most in the form of
triglycerides (glycerol with three fatty
acids attached)
what are the 3 main important fatty acids we absorb
- Palmitic (most abundant),
- Stearic
- Oleic acid
what is a triglyceride
a glycerol molecule with 3 fatty acids attached
describe triglyceride digestion
occurs to a limited extent in the mouth & stomach but it predominantly occurs in the small intestine.
major digestive enzyme for lipids
lipase - synthesised by pancreas
what does lipase do
catalyses the splitting of bonds linking fatty acids to the 1st & 3rd carbon atoms of glycerol
producing two free fatty acids & a monoglyceride as products:
Triglyceride —> Monoglyceride + 2 Fatty acids , under the action of lipase
what do lipids do in stomach
they aggregate into large lipid
droplets in the upper portion of the stomach
because
lipids in ingested food are insoluble in water
These lipid droplets are then converted into very small droplets (1mm in diameter) via the process of emulsification
what does emulsification require
1 Mechanical disruption of the large droplets into smaller droplet - provided by the motility of the GI tract, in the lower portion of the stomach & in the small intestine, which grinds & mixes the luminal contents
- An emulsifying agent - provided by the phopho-lipids in food and by bile salts secreted in bile
what are phospholipids
amphipathic molecules - (both hydrophilic & phobic)
(containing polar or ionised groups on one end of the molecule and non-polar
groups on the other)
2 non-polar fatty acid chains attached to glycerol with a charged phosphate group on one end
where are bile salts formed
liver
are bile salts amphipathic
yes
what do the non-polar portion of the phospholipids and bile salts do
they associate with the non
polar interior of the lipid droplet - leaving the polar portions exposed at the water surface
- here they repel other lipid droplets that are similarly coated with these emulsifying agents thereby preventing their reaggregation into larger fat droplets
define absorption
the movement of nutrients, water and electrolytes
from the lumen of the small intestine into the cell,
then into the blood.
how does bile aid lipid digestion
by emulsifying the fat goblets into smaller chunks, called micelles, which have a much larger surface area.
how much of lipid digestion is started by lingual and gastric lipases
this only digests 10% of ingested lipids.
what are 3 major enzymes involved in lipid digestion?
- Pancreatic lipase
- phospholipase A2
- cholesterol ester hydrolase
what do pancreatic lipase, phospholipase A2 and cholesterol ester hydrolase do
they hydrolyse the micelles, breaking them down into fatty acids, monoglycerides, cholesterol and lysolecithin.
where are the products from digestion of lipids released
at the apical membrane and diffuse into the enterocyte.
what happens to products of lipid digestion inside enterocyte cells
- the products are re-esterified to form the original lipids, triglycerides, cholesterol and phospholipids.
- the lipids are then packaged inside apoproteins to form a chylomicron.
- the chylomicrons are too large to enter circulation, so they enter lymphatic system via lacteals.
which carbohydrate products are absorbed by the small intestine
glucose, galactose and fructose
where is starch digestion initiated
in the mouth, facilitated by salivary amylase
where does the majority of carbohydrate digestion occur
small intestine
what is the main enzyme for starch digestion
pancreatic amylase, which yields disaccharides from starch by digesting the alpha 1-4 glycosidic bonds.
what happens to disaccharides produced from starch digestion
(maltose, maltotriose, and α-dextrins) are all converted to glucose by brush border enzymes.
what happens to disaccharides occurring naturally in food
they do not require amylase to break them down.
Brush border enzymes (lactase, sucrase, trehalase) hydrolyse these compounds into molecules of glucose, galactose and fructose.
how are glucose and galactose absorbed
across the apical membrane by secondary active transport (along with Na+) through the Sodium-Glucose cotransporter (SGLT1).
how do glucose and galactose exit the cell
via GLUT2 receptors across the basolateral membrane into the blood.
how does fructose enter the cell
by facilitated diffusion via GLUT5 and is transported into the blood via GLUT2 receptors.
where does protein digestion begin
in the stomach with the action of pepsin, which breaks protein into amino acids and oligopeptides.
where is protein digestion completed
in the small intestine with brush border and pancreatic enzymes.
they split the oligopeptides into amino acids, dipeptides and tripeptides.
how are amino acids absorbed
via a Sodium cotransporter, in a similar mechanism to the monosaccharides.
how are amino acids transported
rhey are transported across the basolateral membrane via facilitated diffusion
how are Di and tripeptides absorbed
via separate H+ dependent cotransporters and once inside the cell are hydrolysed to amino acids.
what are the fat soluble vitamins
A,D,E & K
how are fat soluble vitamins absorbed
occurs in the ileum
they are incorporated into mixed micelles with other lipids and bile acids in the lumen of the small intestine and enter the enterocyte largely by diffusion.
Within the enterocytge, they are incorporated into chylomicrons and exported via exocytosis into lymph.
how are water soluble vitamins absorbed
by diffusion or mediated transport in the JEJUNUM
but vitamin B12 is an exception
what are the water soluble vitamins
vitamins B and C
how is vitamin b12 absorbed
is a very large and charged vitamin.
To be absorbed B-12 must first bind to the protein intrinsic factor (secreted by
parietal cells of the stomach) - intrinsic factor with bound B-12 then binds to
specific sites on the epithelial cells in the LOWER PORTION OF THE ILEUM
where vitamin B-12 is absorbed via endocytosis.
what is vitamin B12 needed for
erythrocyte formation
a deficient in B-12 can lead to pernicious anaemia and is usually caused due to a deficiency in intrinsic factor
where does vitamin absorption occur
small intestine
how are sodium ions absorbed (3)
- Sodium-hydrogen antiporter on the luminal membrane
- Epithelial sodium channels
- Enhanced by absorption of short-chain fatty acids in the colon via specialised symporters
how are chloride ions absorbed
the movement of sodium into the plasma produces an electrochemical gradient to allow absorption of chloride.
Chloride ions are exchanged for bicarbonate ions (causing net bicarbonate secretion).
how are potassium ions absorbed
absorption of water along the length of the bowel concentrates potassium in the lumen.
This provides an electrochemical gradient for the movement of potassium into the plasma.
In the colon potassium may be absorbed or secreted depending on the remaining concentration in the lumen and the electrochemical gradient created by the active absorption of sodium.
Secretion usually occurs when the luminal concentration of potassium ions is below 25mM.
how are short-chain fatty acids, crucial B vitamins (such as B6 and B12) and vitamin K produced
by the digestion of chyme by the commensal microbial flora of the colon
impact of Aldosterone on absorption in the colon
increases the net absorption of water and electrolytes by stimulating the basolateral sodium-potassium ATP-ase. This increases the electrochemical gradient and driving force for sodium absorption. It also increases transcription of epithelial sodium channels.
impact of Glucocorticoids and somatostatin on absorption in the colon
act to increase water and electrolyte absorption by increasing the action of the basolateral sodium-potassium ATP-ase.
what innervates the intestines
enteric nervous system
what does Parasympathetic innervation do to inestines
promotes net secretion from the intestines
what does sympathetic innervation do to inestines
promotes net absorption from the intestines.
