BIOL 318: CH. 23 - URINARY SYSTEM: CONCENTRATION, EFFECTS OF HORMONES

What do comparative studies show about animals in drier climates?

Animals have longer and more loops of Henle.

How does the kangaroo rat manage water?

It doesn't drink water, gets water from food.

Why are comparative studies important?

They help understand adaptations to water availability, like the kangaroo rat's extreme case.

What is characteristic of a cortical nephron?

Short loops, no osmotic gradient, renal corpuscle near outer cortex.

What is characteristic of a juxtamedullary nephron?

Renal corpuscle near medulla, long loops of Henle into medulla, more concentrated filtrate, concentration gradient formed in medulla.

How does the kangaroo rat manage water loss?

Water scarce, produces hypertonic urine, doesn't drink water, 100% juxtamedullary nephrons, urine concentration up to 9000 mOsm/L.

How does the beaver manage water loss?

Water abundant, produces large volume of hypotonic urine, drinks to quench thirst, cannot concentrate urine more than ~500 mOsm/L, 100% cortical nephrons.

What happens in humans during dehydration?

Produces hypertonic urine, more concentrated than blood (300 mOsm/L), ADH and aldosterone involved, urine concentration may reach 1200 mOsm/L.

How does the permeability of the collecting duct affect urine concentration in dehydration?

Increased permeability allows water reabsorption, making urine more concentrated.

What is water diuresis?

Drinking large volumes of water leads to large volume of hypotonic urine.

What does an impermeable collecting duct do?

It rids excess water.

What happens to urine when it is more dilute than blood?

Urine concentration may be as low as 50 mOsm/L.

What is the role of ANP in urine concentration?

It helps regulate water balance and reduce urine concentration.

What is the primary function of the loops of Henle?

To generate an osmotic gradient to concentrate urine and conserve water.

How much of Na+, K+, Cl-, and water does the loop of Henle reabsorb?

About 25% of Na+, K+, Cl-, and 15% of the water in the glomerular filtrate.

What are the characteristics of the thick segments in the loops of Henle?

Simple cuboidal epithelium, active transport of salts, impermeable to water.

What happens in the thin segment of the loops of Henle?

Simple squamous epithelium, highly permeable to water, no active transport, active water diffusion.

What happens in the descending limb of the loop of Henle?

Impermeable to NaCl, highly permeable to water, water fuses out, tubular fluid concentration increases to 1,200 mOsm/L at lower end.

What happens in the ascending limb of the loop of Henle?

Actively pumps out NaCl, impermeable to water, tubular fluid concentration decreases to 100 mOsm/L at lower end.

What is the process of NaCl reabsorption in the loops of Henle?

NaCl is reabsorbed, and tubular fluid concentration changes from 300 to 1,200 to 100 mOsm/L.

What does an increasingly salty medulla create?

A high osmotic gradient, which increases the potential for water reabsorption in the collecting duct when needed.

Why is countercurrent flow of nephron loops not enough by itself?

The vasa recta is needed to deliver nutrients and O2 into the renal pyramids.

What is the difference between single effect and countercurrent multiplication?

Countercurrent multiplication involves fluid flowing in opposite directions in adjacent tubules of the nephron loop.

What does countercurrent refer to in countercurrent multiplication?

Fluid flowing in opposite directions in adjacent tubules of nephron loop.

What does multiplier mean in countercurrent multiplication?

Continually adding Na+ to extracellular fluid, multiplying medulla’s osmolarity.

What is the difference between countercurrent and countercurrent multiplication?

Countercurrent doesn’t contribute to osmolarity, has no active transport, and equal permeabilities.

How does countercurrent multiplication work in the nephron loop?

Na+ is pumped out of the ascending limb, creating an osmotic gradient that allows for water reabsorption in the descending limb.

How does the urea effect establish the final gradient in countercurrent multiplication?

Urea adds to the osmotic gradient, increasing it to 600 mOsm/L, which is 2x more concentrated than blood.

What role does urea play in the medulla osmotic gradient?

Urea is reabsorbed in the lower collecting duct, secretes in the nephron loop, and contributes to the osmotic gradient.

How does urea contribute to the renal medulla osmotic gradient?

Urea remains concentrated in the collecting duct, diffuses out, and adds to osmolarity, reaching 1,200 mOsm/L when ADH is high.

What is aldosterone's role in the kidney?

Secreted when Na+ is low or K+ is high, or BP is low. It stimulates Na+ reabsorption and K+ secretion in the tubules.

Where does aldosterone act in the nephron?

Thick ascending loop, distal convoluted tubule (DCT), collecting duct.

How does aldosterone affect blood volume and BP?

Increases Na+ reabsorption, water follows, increasing blood volume and BP.

What triggers ADH release in the body?

Arterial baroreceptors, hypothalamic osmoreceptors, and angiotensin II.

How does ADH affect water absorption in the collecting duct?

ADH increases aquaporins in collecting ducts, enhancing water reabsorption.

What happens in the short term with ADH release?

Aquaporins are inserted into the membrane of collecting duct cells.

What happens long-term with ADH release (over 24 hours)?

Aquaporins are continuously produced, increasing water reabsorption over time.

What hormone triggers aquaporin membrane insertion in collecting ducts?

ADH

What signaling pathway does ADH use for aquaporin insertion?

G protein to adenylyl cyclase to cAMP to protein kinase A

What happens to AQP2 after phosphorylation by protein kinase A?

vesicle fuses with apical membrane and is stabilized

What is the result of aquaporin insertion in the apical membrane?

increased water reabsorption and hypertonic urine (>300 mOsm/L)

Is the response to ADH fast or slow?

fast, doesn’t require gene expression

What organ secretes natriuretic peptides?

heart

What triggers the secretion of natriuretic peptides?

high blood pressure

How do natriuretic peptides affect afferent arterioles?

dilate them

What is the effect of afferent arteriole dilation on filtration?

increases filtration and fluid loss, lowering blood pressure

How do natriuretic peptides affect aldosterone secretion?

inhibit it

How do natriuretic peptides affect ADH secretion?

inhibit it

How do natriuretic peptides affect NaCl absorption in the collecting duct?

inhibit it

What causes insertion of AQP2 in apical membrane of collecting duct principal cells?

phosphorylation

What causes internalization of AQP2?

ubiquitination

What happens to AQP2 when phosphorylated at S261?

recycled

What happens to AQP2 targeted to multivesicular bodies?

either degraded in lysosomes or excreted in urine as exosomes

How much of human nephrons are juxtamedullary?

15%

What do juxtamedullary nephrons generate?

renal medulla concentration gradient

What mechanism is used by juxtamedullary nephrons to generate this gradient?

countercurrent multiplier

Where do both types of nephrons empty?

collecting ducts

What happens to filtrates from both nephron types?

joined and acted on together

Where do all collecting ducts descend?

into the medulla

What happens as filtrate travels down collecting ducts?

exposed to increasing osmolarity