L47: Regulation of Urine and the Creation & Role of the Medullary Osmotic Gradient

What kind of epithelium does the PCT have?

Cuboidal leaky microvilli

What kind of epithelium does the DCT have?

Cuboidal tight

What kind of epithelium does the nephron loop for the thin descending limb have?

Squamous tight

What kind of epithelium does the nephron loop for the thick ascending limb have?

Cuboidal tight

What kind of epithelium does the collecting duct have?

Cuboidal tight

Only the … is squamous; it is the one part of the nephron that does not transport ions

thin limb

What is the percentage of filtrate that the Loop of Henle is responsible for reabsorption?

25%

What is the amount of water that the Loop of Henle is responsible for reabsorption?

45L per day

What is the percentage of filtrate that the DCT and collecting reabsorbs?

5-14%

What is the amount of water that the DCT and collecting reabsorbs?

9L per day

Is the water (and solute) absorption in the Loop of Henle obligatory or facultative?

Obligatory

Is the water (and solute) absorption in the DCT obligatory or facultative?

Facultative

Where does each Loop of Henle descend into?

Hyperosmotic renal medulla

What kind of nephron loop extends further into the medulla

Juxtamedullary nephron

In the medulla there is an osmolarity gradient: … in the medulla = … osmolarity

deeper, higher

Tubular fluid flows from the PCT to the … limb and on into the … limb

thin descending, thick ascending

Counter-current: exchange occurs between fluids (in the limbs of the loop) moving in … directions

opposite

Multiplication: exchange … as movement of fluid continues

increases

The 2 limbs have different … for water and solute movement, which enhances overall reabsorption

permeabilities

The blood vessels of the … also contribute to counter-current multiplication

vasa recta

What phenomenon does the arrangement of the Loop of Henle create?

Counter-current multiplication

The thin descending limb is … to water, but … to solutes

permeable, impermeable

What water channel does thin limb cells express on their membrane?

Aquaporin-1

Because the osmolarity of the ISF … as the limb goes deeper, the direction of tubular flow … water reabsorption

increases, enhances

The thick ascending limb can … out of the tubular fluid but is … to water

pump ions, impermeable

The thick limb can pump sodium and chloride out of the tubular lumen and into ISF due to expression of what transporter on the apical surface?

NKCC transporter

What does NKCC stand for?

1 Na⁺ : 1 K⁺ : 2 Cl^-

Does osmolarity of tubular fluid increase or decrease in the thin descending limb?

Increase

Does osmolarity of tubular fluid increase or decrease in the thick ascending limb?

Decrease

What is the osmolarity gradient in the deep medulla partly maintained by?

Urea permeability

Where are urea transporters (and thus permeability) only present in?

Papillary duct

Where does the papillary duct run through?

Renal papilla

The distal convoluted tubule and collecting duct are sites for … water and solute reabsorption

facultative

Reabsorption will only occur in the DCT and collecting duct if stimulated by … signals

hormonal

The epithelia in the DCT and collecting duct have little to no … transport and are … to water ‘at rest’

paracellular, impermeable

Without hormone signalling: water would be … in the lumen of the DCT and CD even while passing through the high osmolarity in the deep medulla

retained

Which hormones bind to DCT and CD cells allowing them to promote the addition of aquaporins to their apical membranes?

ADH/vasopressin

Where is the anti-diurectic hormone (also called vasopressin or AVP) is secreted from?

Posterior pituitary

AVPR activation signals for insertion of what?

Aquaporin-2

What creates transmembrane permeability to water, allowing for osmotic reabsorption and concentration of urine?

Aquaporins

What are aquaporins?

Water channels that allow water to rapidly diffuse according to the local osmotic gradient

What are the body’s key effector for homeostatic regulation of body fluid osmolarity?

ADH levels

ADH neurons in the hypothalamus are intrinsically …., so when plasma osmolarity increases, they produce higher frequencies of APs

osmosensitive

What kind of feedback loop is the ADH regulation of plasma osmolarity?

Direct

What endocrine disorder does the loss of either ADH secretion or AVPR2 lead to?

Diabetes insipidus (DI)

Inherited diabetes insipidus can be caused by mutations in which genes?

AVP gene or AVPR2 gene

Physical damage to what can prevent ADH secretion?

Hypothalamus or infundibulum of the pituitary

… regulates … reabsorption and urine volume primarily through …

RAAS, ion, aldosterone

Reductions in blood volume trigger the RAAS, resulting in … secretion

aldosterone

What does aldosterone stimulate?

Sodium/potassium ion ATPase pump

Aldosterone stimulate sodium/potassium ATPase pump, enhancing potassium ion … and sodium ion …

excretion, retention

Which side of the membrane in the DCT does aldosterone stimulate sodium/potassium ion ATPase?

Basolateral

In the DCT: aldosterone stimulates the sodium/potassium ion ATPase in the basolateral membrane, amplifying the gradient for sodium ion reabsorption at the … membrane

apical

In the CD (principle cells): aldosterone causes addition of … channels (sodium leak channels) to the apical surface, enhancing sodium ion reabsorption

ENaC

In response to a loss of volume, what does aldosterone (and ADH) both contribute to?

Homeostasis of fluid

What is a diuretic?

Any drug that causes diuresis (increased production of urine)

Most diuretics are … for specific renal transporter proteins, and they are often prescribed to manage hypertension or oedema

inhibitors