Epithelia

Question 1

Are the endothelial surfaces in the lungs considered to be epithelium? Why or why not?

Answer 1

No. The main task of the lungs is to transport O2 into and CO2 out of the blood. There are no membrane transporters (active or otherwise) for O2 and CO2; they are lipid soluble molecules that diffuse freely through all membranes. Thus, the endothelial cells in the lung that separate blood from air in the alveoli of the lungs do not possess special transporters like those in epithelia to transport O2 or CO2.

Question 2

The ____ surface faces the ‘special’ fluid and usually contains the special transporters that endow the epithelium with its specialized transport properties. Synonyms include (2).

The ____ surface is exposed to the _____ and usually has generic transport properties like the plasma membranes of non-epithelial cells. Synonyms include (2).

Answer 2

apical (e.g., food in the gut, urine in the kidney, saliva in the parotid duct); mucosal and lumenal

basolateral; interstitial fluid; serosal and peritubular

Question 3

To get across an epithelium, a substance must follow one (or both) of two possible routes:

Answer 3

1) cross two membranes by entering the epithelial cell on one side and leaving on the other
2) cross no membranes at all by passing in between cells through the pericellular shunt pathway.

Question 4

Where is the Na/K pump always located?

Answer 4

Basolateral membrane

Question 5

Transepithelial Potential Difference (TransPD) =

Answer 5

Vm (Basolateral) – Vm (Apical)

PD = Vm(Bl) - Vm(Ap) —> “Blap”

*by convention, the Basolateral fluid is defined as “0”

Question 6

How are Na and Cl absorbed into the blood?

Answer 6

Both diffuse passively across the apical membrane. Sodium is then actively pumped out across the basolateral membrane, while Cl- follows passively across.

Question 7

The apical solution is ____ with respect to the basolateral solution.

Answer 7

negative (This makes perfect sense, because the movement of sodium out of this solution creates the electrical gradient that draws the chloride along with it.)

Question 8

What are examples of leaky epithelia in the body (3)? Tight epithelia (2)?

Answer 8

Leaky: GI tract, proximal tubule of kidney, gallbladder
Tight: Sweat glands, distal tubule of kidney

Question 9

What drives secretion in epithelial cells?

Answer 9

Specialized cells concentrate Cl- by transporting it across the basolateral membrane along with Na+/K+ (this is driven by the leakage of Na across BL). Cl- leaks downhill (across the apical membrane), drawing Na+ along with it thanks to the electrical negativity created by the leakage.

Question 10

What four substances are NEVER pumped?

Answer 10

1. H2O
2. Urea
3. O2
4. CO2

Question 11

What two illnesses take advantage of (or mutate) the apical Cl- channels responsible for secretion?

Answer 11

1. Vibrio cholerae

2. Cystic fibrosis

Question 12

Describe the mode of action of V. cholerae.

Answer 12

Cholera toxin acts by locking open this channel, causing a massive efflux of fluid from the cell, leading to profound diarrhea and dehydration.

Question 13

How does cystic fibrosis cause illness?

Answer 13

While there are probably multiple types of Cl- channels (different gene products) involved in fluid secretion by epithelia, one of the most important is the ‘Cystic Fibrosis Transmembrane Conductance Regulator.’ (CFTCR, or more commonly CFTR).

Cystic fibrosis is a genetic disease; it is this Cl- channel that is mutated, which reduces the ability of epithelia to secrete ‘serous’ (watery) fluid, leading to thickened mucous secretions, infections, and other life shortening complications.

Question 14

Of the 15mol of waste product produced by our body each day, 14.5mol are ____ which is ____, and ingenious solution.

Answer 14

CO2, volatile

Question 15

When kidney function is lost, death from __ can follow.

Answer 15

uremia; literally means “urine in the blood.”

Question 16

How does the kidney get rid of waste? What is the driving principle?

Answer 16

Kidney knows what it likes, keeps that stuff.

Rather than trying selectively to pump waste products out of the plasma, it forms an ultrafiltrate of plasma in the glomerulus, which contains water, salts, sugars, amino acids, and all other beneficial compounds, as well as the non-volatile metabolic waste products. Then, as this plasma ultrafiltrate passes along the renal tubules, the epithelial cells lining the tubules reabsorb (pump back into the blood) the things that it wants to keep (glucose, salts, bicarbonate, etc), allowing the wastes to pass on. (ATP intensive)

Question 17

The kidney expels 500 mmols of waste per day, the GI tract a paltry 30 mmols. What comprises the majority of the waste expelled by the GI?

Answer 17

mostly breakdown products of red blood cells (delivered from the liver to the GI lumen), and are highly toxic if not promptly eliminated.

Question 18

Getting rid of extra water is relatively easy (haha). How does it happen?

Answer 18

Solutes in the lumen of the kidney tubule are reabsorbed as usual, but the epithelium is made water-impermeable, so water cannot follow the solutes osmotically. Thus, the extra water stays in the lumen, and passes into the urine.

(The epithelium is made impermeable to water when the hypothalamus, sensing extra water on board as a drop in plasma osmolarity, stops secreting anti-diuretic hormone (vasopressin), which causes certain kidney epithelial cells to remove water channels (aquaporins) from their apical
membranes, thereby reducing water reabsorption.)

Question 19

Retaining water is quite a bit more complex, but what is the gist?

Answer 19

Most salt and water are removed from the tubule at separate locations, and the salt is kept around to provide a special, hyperosmotic interstitium to draw a little extra water from the lumen of the collecting duct.