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Term 2: Renal > Renal Physiology: Countercurrent and pH > Flashcards

Renal Physiology: Countercurrent and pH Flashcards

1
Q

What does osmolar clearance (Cosm) represent?

A

-how much urine we would be excreting if we added or subtracted enough water to make it isosmotic with blood plasma

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2
Q

What is free water clearance?

A
  • the difference between actual urine volume and iisosmolar volume
  • found by subtracting the osmolar clearance from the actual volume of urine
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3
Q

Where is water permeable in the nephron?

A
  • proximal tubule
  • Descending limb
  • Collecting duct if ADH present
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4
Q

Why is the distal convoluted tubule referred to as the diluting segment?

A
  • filtrate reaches its point of minimal osmolarity at the end of the distal tubule
  • this is due to active reabsorption of solutes in the thick ascending limb and the DCT
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5
Q

What contributes to the medullary interstitial hyperosmolartiy?

A
  • active solute reabsorption in the thick ascending limb
  • passive sodium reabsorption in the thin ascending limb
  • urea reabsorption from medullary collecting duct
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6
Q

What is the only limit to the effect of the countercurrent multiplier?

A

-lenght of the loop of henle in comparison to cortical thickness

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7
Q

How does AVP/ADH work?

A
  • increases water permeability in all segments past DCT by inserting aquaporins
  • stimulates solute reabsorption in the thick ascending limb and medullary collecting duct but upregulating Na/K/CL cotransporter and UT1
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8
Q

What is the main stimulator of AVP/ADH release?

A

-plasma hyperosmolarity

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9
Q

Why must there be a mechanism to preserve the existence of the hyperosmotic gradient in the medullary interstitium?

A

-without a mechanism to remove water from the interstitium, water reabsorption in the medullary collecting ducts would wash out the medullary osmolar gradient

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10
Q

What structure allows for a mechanism to maintain the osmolar gradient of the medullary interstitium?

A

-the Vasa Recta

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11
Q

WHy is the total solute content in the ascending vasa recta ultimately higher than the descending vasa recta?

A

-passive solute equilibrium is fast but not that fast

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12
Q

What happens if you infuse someone with a liter of IV saline solution of fluid that is isosmolar to plama?

A
  • Their ECF volume will also increase by 1 Liter

- neither the volume or composition of the ICF changes

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13
Q

What happens if you infuse someone with a liter of IV distilled water?

A
  • the plasma osmolarity will decrease slightly

- most of the water will enter into cells

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14
Q

What are the two categories of vascular volume receptors?

A

-low and high pressure receptors

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15
Q

Where are low-pressure receptors found?

A
  • places where hydrostatic pressure is comparatively low
  • cardiac atria
  • pulmonary artery
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16
Q

What are the two subcategories of atrial baroreceptors? What do they do?

A
  • Type A: Sense heart rate

- Type B:monitor atrial filling

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17
Q

What stimulates Type B atrial baroreceptors?

A

-volume overload in the atria

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18
Q

What inhibits Type B atrial baroreceptors?

A

-volume underload in the atria

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19
Q

Where are high-pressure baroreceptors found?

A
  • aortic arch
  • carotid sinus
  • renal afferent arteriole
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20
Q

What happens when a threat to volume homeostasis has been detected?

A
  • effector pathways are activated that will work to restore ECF volume to normal
  • most important: RAAAS
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21
Q

Where is renin produced?

A

-granular cells of the juxtaglomerular apparatus

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22
Q

What does renin do?

A

-catlyzes the conversion of angiotensinogen to angiotensin I

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23
Q

Where is angiotensin converting enzyme found?

A
  • surface of vascular endothelium

- particularly in the lungs

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24
Q

What are the effects of angiotensin II?

A
  • arteriolar vasoconstriction
  • stimulation of aldosterone release
  • increasing TG feedback sensitivity
  • stimulate Na-H countertransport
  • stimulates thirst centers
  • stimulates ADH release
25
Q

Where is aldosterone released?

A

adrenal cortex

26
Q

Where is ADH released?

A
  • hypothalamus

- posterior pituitary

27
Q

What does locally produced angiotensin II do in the kidneys?

A
  • preferential vasoconstriction of efferent arterioles, increasing GFR
  • decreases peritubular capillary hydrostatic pressure
  • this favors fluid and sodium reabsorption
28
Q

What factors stimulate renin release?

A
  • endothelin

- prostaglandins E2 and I2

29
Q

What factors inhibit renin release?

A
  • angiotensin II (feedback)
  • AVP
  • increased plasma [K]
  • NO
30
Q

What are three other efferent systems that control volume homeostasis besides RAAAS?

A
  • sympathetic nerve activity
  • Arginine Vasopressin (ADH)
  • ANP
31
Q

How does sympathetic nerve activity influence volume homeostasis?

A
  • stimulates Renin release
  • increases vascular resistance
  • stimulates proximal tubular reabsorption via stimulation of Na-H countertransport and the Na/K ATPase
32
Q

How does ADH influence volume homeostasis?

A
  • stimulates the Na/K/Cl cotransporter
  • increases sodium permeability of principal cells in cortical collecting tubules
  • increases water permeability of collecting ducts via aquaporins
33
Q

How does ANP influence volume homeostasis?

A

-promotes renal Na excretion by increasing GFR and inhibiting Na reabsorption in the inner medullary collecting duct

34
Q

What areas of the brain detect changes in water balance?

What doe they respond to?

A
  • organum vasculosum of the lamina terminalis (OVLT)
  • Subfornical organ (SFO)
  • Respond to changes in Posm
35
Q

What do the OVLT and SFO do when they detect an increased Posm?

A

-the fire on the anterior hypothalamus, stimulating ADH synthesis and release

36
Q

Given the choice, do the kidneys prefer to maintain normal volume or Posm?

A

-volume

37
Q

Why is the proximal tubule a high capacity, low gradient system for H+ secretion?

A

-it reabsorbs large quantities of HCO3 but is unable to generate a large pH difference between blood and lumen

38
Q

Why is HCO3 a good buffer when its pKa is 6.1?

A
  • there’s a bunch of it

- it is an open system, thus there can be a continuous addition and removal of CO2 from the ECF

39
Q

What are the main intracellular buffers?

A
  • hemoglobin

- Organic phosphates

40
Q

How does hemoglobins pKa change when its deoxygenated? What is the significance of this?

A

-its pKa goes from 6.7 to 7.9 when oxygen leaves

  • this is good, because it will account for the increase of CO2 in venous blood after dumping oxygen
  • the blood pH will only drop a little bit
41
Q

Why is the effect of compensatory respiratory acidosis limited?

A

-hypoxemia caused by hypoventilation probably stimulates an increase in respiration

42
Q

When is net secretion of HCO3 possible?

A
  • during circumstances of metabolic alkalemia
  • Acid-secreting alpha-intercalated cells are replaced by HCO3 secreting Beta-intercalated cells in the collecting tubules
43
Q

Why does ECF volume depletion effect efficient renal excretion of an alkali load?

A
  • volume depletion stimulates Na-H exchange
  • Also stimulates RAAAs axis, causing aldosterone induced enhancement of distal H+ secretion (and thus HCO3 reabsorption)
44
Q

How do we know that increasing pCO2 directly stimulates H+ secretion in the proximal tubule of the kidneys?

A

-it does this even in an absence of a change in [HCO3] or pH

45
Q

What is the calculation for net acid excretion in the kidneys?

A

Net acid excretion = (titratable acidity + Ammoniagenesis) - HCO3 reabsorption

46
Q

What are the three known transporters that perform H+ secretion into the tubular fluid?

A
  • Na-H countertransporter
  • H-ATPase
  • H/K-ATPase
47
Q

Why must the rate of H+ secretion and HCO3 basolateral transport be nearly identical?

A

-otherwise, whichever ion remains inside the tubular cell will inhibit further dissociation of CO2

48
Q

Where in the nephron does HCP3 reabsorption occur?

A
  • proximal Tubule (80%)
  • Thick ascending limb (10%)
  • Distal Convoluted tubule (6%)
  • Collecting duct (4%)
49
Q

What enzyme catalyzes the conversion of filtered HCO3 and secreted H+ into CO2 and H2O in the PROXIMAL TUBULE, for subsequent absorption into the luminal cell?

A

-Carbonic anhydrase IV

50
Q

Once CO2 is in the proximal tubule cell, what enzyme converts it back to HCO3 and H+?

A

-Carbonic anhydrase II

51
Q

How does the majority of proximal tubule H+ secretion occur?

A
  • Na/H countertransport

- there are a few H-ATPase pumps here too

52
Q

How is H-ATPase activity stimulated by aldosterone?

A

1) aldosterone-stimulated Na reabsorption in collecting ducts makes lumen voltage more negative, creating a gradient that favors H+ secretion
2) aldosterone stimulates the insertion of proton pumps into the luminal membrane of collecting ducts

53
Q

What is the most important of the titratable acids?

A

-HPO4

54
Q

In simple terms, how does formation of a titratable acid work?

A
  • We’re breaking a water molecule in the luminal cell in order to form HCO3 and H+. The H+ is excreted, and a brand new HCO3 is reabsorbed into the blood stream.
55
Q

How does ammoniagenesis secrete protons? What portion of the nephron does it occur in?

A
  • Glutamine is converted to alphaketoglutarate, releasing two ammonium ions
  • the ammonium ions dissociate to cross the apical membrane, then reform in the lumen
  • occurs in PCT
56
Q

Describe NH4 transport in the thin descending and ascending limbs.

A
  • Secretion of NH4 occurs in cortical portion of thin descending limb
  • reabsorption occurs in medullary portion of thin descending and ascending limb due to local concentration gradeints
57
Q

Describe NH4 transport in the Thick ascending limb.

A
  • apical membrane has low diffusive NH3 permeability
  • NH4 uptake occurs via Na/K/Cl cotransporter, substituting for K
  • also flows through K channels
58
Q

This is strange, you say! Why are we reabsorbing all this ammonium? How do we excreted it? List the three ways in which we ultimately remove NH4 from the blood stream.

A

1) Taken to the liver where it is converted to urea. NOTE: this process consumes HCO3, so it is actually acidifying.
2) Some NH4 diffuses laterally to late proximal tube and thin descending limb where it can be secreted. This is called ammonia recycling.
3) Some NH4 enters cells of collecting duct by nonionic diffusion, and then secreted into lumen of collecting duct

Term 2: Renal (3 decks)

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