7 - Reabsorption along the Nephron Flashcards

1
Q

Fill in the following table to show where the majority of sodium and water is reabsorbed in the nephron

A
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2
Q

How do we change plasma volume? (ECF)

A
  • Can’t just add or take water as this would alter osmolarity
  • Secrete or excrete Na ions and water will follow. Keeps osmolarity the same and changes the volume and therefore blood pressure
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3
Q

Which ion is absorbed with Na?

A

Cl- = up to 99% of Na, Cl and Water reabsorbed

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4
Q

What happens if the amount of Na+ ions in ECF changed due to diet changes?

A

⇒ Amount of water in the ECF would change

⇒ ECV would change

⇒ BP would change

Need to keep in sodium balance so vary the amount of sodium excreted

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5
Q

How is a decrease in the ECF volume detected and corrected?

A
  • Low b.p so baroreceptors in atria detect and lower sympathetic activty
  • Send signals via vagus nerve to brainstem
  • ADH secretion
  • Water uptake
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6
Q

How is an increase in the ECF volume detected?

A
  • B.P rises
  • High pressure baroreceptors in the carotid sinus and aortic arch detect
  • Vagus and glossopharyngeal nerves
  • Increases sympathetic activity, stops ADH secretion and more water excreted
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7
Q

What is natriuresis?

A

Excretion of sodium into the urine

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8
Q

How does secretion and absorption happen in the nephron?

A

- Paracellular secretion: capillary → tight junction → lumen

- Transcellular secretion: capillary → basal membrane → apical membrane → lumen

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9
Q

Label the parts of the collecting duct and explain what/where the aquaporins are.

A
  • Aquaporins allow water to move passively in and out of cell
  • Make an area permeable to water
  • None in DCT and ascending limb so cannot absorb water here
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10
Q

Briefly distinguish between Cl- and Na+ reabsorption.

A
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11
Q

What substances that are reabsorbed in the PCT and how is this is achieved?

A

I. 65% H2O

II. 100% glucose and AA

III. 67% Na+

- Osmotic gradient established by solute absorption (osmolarity in interstitial spaces increases)

- Hydrostatic force in interstitium increases

- Oncotic force in peritubular capillary increases (loss of 20% glomerular filtrate leaving cells & proteins in blood)

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12
Q

What is the histology of the PCT and what are the different sections of the PCT?

A

- Simple cuboidal containing lots of mitochondria for Na transport

  • S1-S3
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13
Q

Identify the 4 different transporters which facilitate sodium reabsorption in the apical membrane in S1 of the PCT.

A

Concentration gradient set up by Na/K ATPase on basolateral membrane

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14
Q

Identify the 2 different transporters which facilitate sodium reabsorption in the basolateral membrane in S1 of the PCT

A
  • Cl and Urea concentration increase in S1 to maintain osmolarity. Allows passive reabsorption in S2-S3
  • Water follows Na and absorbed through aquaporins back into capillary
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15
Q

If the kidneys retain 100% of the glucose that they filter then why do diabetics have glucose in their urine?

A

Above renal thresold and reached transport maximum so it is excreted

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16
Q

Where and how is bicarbonate reabsorbed, and what drug can interrupt this process?

A

Mostly in S1 PCT

17
Q

How is sodium, chloride and water reabsorbed in the apical and basolateral membranes of S2-S3 in the PCT respectively?

A
  • Chloride reabsorbed para and transcellularly
  • Aquaporins with ~4mOsmol gradient favours water uptake from lumen
18
Q

How is sodium and chloride absorbed in the late late PCT?

A

No energy for active absorption

19
Q

Describe, briefly, how the concentration of the following molecules change to establish an iso-osmotic solution in the PCT:

  • Cl-
  • Pi
  • Glucose, AA, lactate
  • HCO3-
A
20
Q

Where does amino acid uptake occur in the nephron?

A
  • 100% in PCT
  • Sodium-dependent amino acid transports on luminal (set up by Na/K pump)
  • Passive transporters on basolateral
21
Q

What effect does an increased renal artery BP have on reabsorption?

A
  • All to allow the ECF volume to decrease to decrease B.P
  • Less Na reabsorbed due to glomerular tubular balance and changing expression (downregulation) of transporters (NHX due to less AngII)
  • Less H2O as less ADH
22
Q

In general, what occurs in the loop of Henle and what is the histology?

A

- Simple squamous in descending to allow water resorption (few mitochodria)

- Simple cuboidal in ascending for active transport at top of the limb

- Descending limb: reabsorbs H2O but not NaCl

- Ascending limb: reabsorbs NaCl but not H2O

23
Q

Describe the reabsorption that occurs in the descending limbs of the Loop of Henle?

A
  • Increase in [Na+] in the papilla allows paracellular reuptake of H2O from descending limb.

- Na+ and Cl- are concentrated in the lumen of the descending limb ready for passive transport in the thin ascending limb

24
Q

What is the difference between the thin and thick ascending limb?

A

Thin: passive absorption of Na

Thick: active absorption of Na so presence of transporters again

25
Q

What are the active transport processes that occur in the thick ascending limb?

A
  • Active transport from lumen to cells via NKCC2 transporter (also in MD)
  • Na+ move into interstitium via 3Na-2K-ATPase
  • K+ secreted via ROMK back into lumen (maintain activity of NKCC2 transporter)
  • Cl- move into interstitium
  • Ca and Mg paracellularly transport
26
Q

How do loop diuretics work?

A
  • Causes biggest diuresis e.g furosemide

- Block NKCC2 in thick ascending limb and macula densa

  • Less Na reabsorbed so less water (NKCC2 loop)
  • Constriction of AA doesn’t work so increased GFR so more urine (NKCC2 blockage in MD so no ATP release)

(risk of hypokalaemia)

27
Q

What state is the filtrate in when leaving the ascending limb?

A

Hypo-osmotic to plasma, been through diluting segment of ascending limb

28
Q

What happens in the DCT?

A
  • Hypo osmotic fluid (from LoH) enters
  • Active transport of Na+ (5-8%) by NCCT in DCT1 and ENaC/NCCT in DCT2
  • Low H2O permeability, depending on ADH
29
Q

What medications can affect sodium transporters in the DCT?

A

Thiazides: NCCT

Amiloride: ENaC

30
Q

Apart from Na, K and Cl, what other important ion is reabsorbed in the DCT?

A

- Cytosolic Ca2+ is bound by calbindin, shuttling it to the basolateral membrane

  • It is transported out by NCX
  • Tightly regulated by hormones e.g. PTH and 1,25- dihydroxy vitamin D
31
Q

How can the collecting duct be divided into segments?

A

- Cortical CD: containing principal and intercalated cells

- Medullary CD

32
Q

How does reabsorption in the principal cells of the collecting duct occur?

A

- Reabsorption of Na+ via ENaC on apical membrane

- Driving force: 3Na-2K-ATPase in basolateral membrane

  • Lumen(–) charge due to singular transport of Na+ drives paracellular Cl- uptake

- Variable water uptake depending on ADH

33
Q

What are the two types of intercalating cells?

A

- Acid-secreting Type A intercalating cells

- Bicarbonate-secreting Type B intercalating cells

Helps with acid base balance

34
Q

What is the histology of the collecting duct?

A
  • Simble cuboidal
  • No brushborder and has wide lumen unlike PCT
  • Minor calyx has transitional epithelia
35
Q

What structure is formed from the merging of the collecting ducts, and acts as the gateway to the cavity known as the minor calyx?

A

Collecting ducts merge and terminate to form papillary ducts of Bellini, which open sieve like at the area cribosa