Intestinal fluid and electrolyte balance

Question 1

How much water do we typically ingest each day?

Answer 1

Approx 2L

Question 2

How much fluid is generally released into the GI tract from pancreatic secretions and various other secretions?

Answer 2

Approx 7L

Question 3

How much fluid is approx absorbed in the small intestine?

Answer 3

Approx 7L

Question 4

How much fluid is approx presented to the large intestine and then how much fluid is actually left to be excreted in the faeces?

Answer 4

Only about 2L of water enters the large intestine and of this about 1.9L are absorbed so only about 100mL is actually lost in the faeces.

Question 5

What are some definitions of diarrhoea?

Answer 5

Diarrhoea is really a symptom. It can be defined as an increase in the number of bowel movements (more than 3 a day), an decrease in stool consistency (becomes looser/more fluid), or an increase in the volume of the stool movement.

Question 6

Which is more voluminous, diarrhoea originating in the small or large intestine?

Answer 6

Small intestine

Question 7

What is osmotic diarrhoea?

Answer 7

Osmotic diarrhoea results from a malabsoprtion syndrome. This means as there is not many solutes/nurtrients being absorbed from the ingested food it creates a disruption in the osmotic balance which mean water is retained in the chyme and even more water moves out to balance out the osmolarity.

Question 8

What are some common causes of osmotic diarrhoea?

Answer 8

Lactose intolerance - adults lacking the lactase enzyme are unable to break down and absorb the sugar found in milk (lactose) and so it creates an osmotic imbalance which retains water.

Coeliac disease - an autoimmune disease/ reaction to gluten which results in the villi in the small intestine being destroyed. With no villi there is a lack of nutrient absorption which once again creates an osmotic imbalance so water is retained.

Question 9

What are some common causes of secretory diarrhoea?

Answer 9

Secretory diarrhoea is generally caused by bacteria/micro-organisms such as E.coli or cholera toxin.

The enterotoxins produced by the bacterial micro-organisms raise the intracellular concentrations of CAMP or Ca2+ which leads to excessive stimulation of the CFTR channel in the apical membrane. This increases the Cl- pumped out into the lumen and therefore creates a charge gradient which draws out Na- and water then follows as a result of an osmotic gradient.

Question 10

What is secretory diarrhoea?

Answer 10

Secretory diarrhoea generally results from an increase in the secretion of electrolytes/ions into the lumen of the GI tract which creates an osmotic gradient for the secretion of water.

Question 11

How is the small intestine adapted for increased SA for absorption and secretion?

Answer 11

Plicae circularis (big folds of the mucosa and submucosa)
Villi
Microvilli (brush border on top of the coloumnar absorptive cells of the villi)
Crypts of lieberkuhn

Question 12

How is the large intestine adapted for increased SA for absorption and secretion?

Answer 12

Semilunar folds
Microvilli (but not villi - small intestine is the only region where there is villi)
Crypts of lieberkuhn

Question 13

What are the functional differences between the small and large intestine (in terms of what are their secretions and absorptions?

Answer 13

Small intestine

• the main site of dietary nutrient absorption
• absorbs net amounts of water, and Na, Cl, K
• secretes HCO3

Large intestine

• absorbs net amounts of water, Na, Cl
• secretes K and HCO3

(remember this is NET amounts as the small and large intestine but secrete water and electrolytes aswell but there is MORE absorption so overall NET absorption)

Question 14

What are the two methods by which water and solutes can move into and out of the cell?

Answer 14

Transcellular transport - the solutes/water must actively pass across the cell membrane which is a high resistance pathway and often requires the use of ATP

Paracellular transport - the solutes/water passively pass into the cell via the tight junctions between cells. Because it is a low resistance pathway it dose not require ATP and it is often favoured where possible.

Question 15

How is water absorbed?

Answer 15

Water can be absorbed purely through osmosis or by being coupled to the transport of a solute eg sodium.

It can occur via both paracellular or transcellular transport although paracellular is the preffered method as it is the low resistance pathway.

Question 16

Where does the majority of water absorption occur?

Answer 16

In the jejunum of the small intestine.

Question 17

What is solvent drag?

Answer 17

When water moves into/out of a cell it can often carry with it solutes that have been dissolved in the water. This can account for a lot of the Na and urea absorption in the small intestine.

Question 18

What are some of the differences between lactose intolerance and crohns disease?

Answer 18

Like lactose intolerance, cramping and persistent diarrhea usually accompany Crohn’s disease. However, a person with Crohn’s may also find blood or mucus in the stool. Other symptoms of Crohn’s that aren’t typically found with lactose intolerance are a loss of appetite, unintentional weight loss, fever, fatigue, and anemia.

Crohn’s disease may go into remission for weeks or months at a time with few or no symptoms. A person with lactose intolerance will experience symptoms every time they consume dairy products.

Question 19

How is dietary sodium taken up into the cell?

Answer 19

On the basolateral membrane (closest to the blood stream/extracellular fluid) there is a Na/K/ATPase pump which is costantly working to pump 3Na out of the cell and 2K in.

This maintains a low concentration of Na inside the cell which means there is always a Na gradient for dietary sodium to move into the cell.

The way in much sodium moves into the cell depends on where in the GI tract the absorption is occuring.

In the jejunum/ a little bit of ileum it is via Na/glucose and Na/amino acid cotransporters. (secondary active transporters)

In the duodenum and a little bit of the rest of the small intestine and the proximal colon it is via the Na/H exchanger or the parallel Na/H and Cl/HCO3 exchangers. In the case of the latter the Na and Cl form NaCl inside the cell and the H and HCO3 form H20 and CO2 in the lumen which is then excreted.

In the distal colon it is absorbed by epithelial sodium channels.

Question 20

How does the absorption of Cl- occur?

Answer 20

The absorption of Cl- is largely dependent on the movement of Na as the Cl will just move in response to charge gradients created by the Na in order to balance out the charge that the positive Na ions disrupt.

It can move passively via the paracellular pathway or directly across the cell surface.

It can also be absorbed via the Cl/HCO3 exchanger or the parallel Na/H, Cl/HCO3 exchangers.

Question 21

How is Cl secreted?

Answer 21

Cl is secreted through the CFTR channel which is in the apical membrane and is activated/stimulated by raised intracellular concentrations of CAMP and Ca2+.

As Cl is secreted into the lumen it creates an osmotic imbalance which results in Na+ and water moving passively via the paracellular pathway.

Question 22

How is K+ absorbed and secreted?

Answer 22

K+ is absorbed from the lumen from food either passively through paracellular transport or via the K/H exchanger.

K+ is moved into the cell from the extracellular fluid by the NKCC1 pump (Na/Cl/K) and also via the Na/K/ATPase pump.

It is then pumped back out/secreted into the extracellular fluid via the BK transporter (which just allows K to cross the membrane)

Question 23

How is the absorption and secretion in the GI tract controlled?

Answer 23

Enteric nervous system - via Ach
Endocrine system - via alsoterone
Paracrine system - via SHT

Question 24

Why does your heart rate increase when you have diarrhoea?

Answer 24

When you lose fluid in the diarrhoea, the fluid is coming from the extracellular fluid (your blood). As the blood volume decreases the heart must pump harder and faster in order to try and restore the blood pressue.

Question 25

What is oral hydration therapy?

Answer 25

People who have severe diarrhoea and are very dehydrated are often given an ORS which is a little sachet containing glucose and sodium and as the glucose and sodium is drawn back into the cells, it also draws water back in.

It also contains a lot of bicarbonates as when you are loosing a lot of fluid from the blood, your blood becomes acidotic.