What is absorbed by the following: - Descending LoH - Ascending LoH

- Descending thin limb of LoH: H2O (10-15%) - Ascending thin and thick limb of LoH: Na+ (25%)

S4) Basic Renal Processes Flashcards by shreya Arun

Which ion is transport together with Na⁺?

Cl^-

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How much sodium is reabsorbed in the kidney?

Up to ~99% Na⁺ ions, Cl^- ions and H₂O are reabsorbed in kidney

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Sodium reabsorption varies with the ingestion of sodium.

Why?

The kidney needs to match excretion of sodium to ingestion to remain sodium balance

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

⇒ Amount of water in the ECF would change

⇒ ECV would change

⇒ BP would change

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Sodium input only occurs through ingestion of food.

Identify 3 means of sodium output

Sweat
Faeces
Urine

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What is the main site of absorption in the nephron?

Proximal convoluted tubule – absorbs Na⁺(67%) and H₂O (65%)

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What is absorbed by the following:

Descending LoH
Ascending LoH

Descending thin limb of LoH: H₂O (10-15%)

- Ascending thin and thick limb of LoH: Na⁺ (25%)

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What stimulates proximal tubule reabsorption?

RAAS

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Which cells are the target for aldosterone?

Principal cells of distal convoluted tubule and collecting duct

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what are principal cells

They are cells responsible for Na reabsorption via the ENac channels

they also secrete K via romK channels

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What happens when renal artery blood pressure increases?

In proximal tubule:

Number of Na-H antiporter reduces
Activity of Na-K ATPase reduces

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

Pressure natriuresis:

I. Reduction in sodium reabsorption in PCT

II. Increased Na⁺ excretion

Pressure diuresis:

I. Reduction in water resorption in PCT

II. Increased H₂O excretion

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What is the significance of the pressure diuresis and natriuresis that follow increased BP?

ECF volume decreases
Initial BP rise diminishes

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Distinguish between paracellular and transcellular reabsorption

Paracellular reabsorption: lumen → tight junction → capillary
Transcellular reabsorption: lumen → apical membrane → basal membrane → capillary

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Distinguish between paracellular and transcellular secretion

Paracellular secretion: capillary → tight junction → lumen
Transcellular secretion: capillary → basal membrane → apical membrane → lumen

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What are aquaporin channels and what do they do?

- Aquaporins are channels which selectively conduct water molecules in and out of the cell

They prevent the passage of ions/other solutes hence solely increase permeability to H₂O

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Briefly distinguish between Cl^- and Na⁺reabsorption

Na⁺ reabsorption: active, transcellular process, driven by 3Na-2K-ATPase pumps
Cl^- reabsorption: active, transcellular and also passive paracellular process, coupled to 3Na-2K-ATPase pumps

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

PCT reabsorbs:

I. 65% H₂O

II. 100% glucose and AA

III. 67% Na⁺

Proximal tubule is highly permeable to H₂O and reabsorption is isosmotic with plasma

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Which 3 factors govern sodium uptake into renal capillaries in the PCT?

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

Na-H (antiporter)
Na-Glucose (symporter)
Na-AA (cotransporter)
Na-Pi

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

Basolateral 3Na-2K-ATPase
NaHCO₃^- cotransporter (acids and bases)

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Describe the movement of chloride ions in S1 of the PCT

Study These Flashcards

Urea and Cl^- move down S1 compensating for loss of Glucose
Increasing Cl^- concentration creates a conc. gradient for chloride reabsorption in S2-3

How is water transported in S1 of the PCT?

Study These Flashcards

Using aquaporins

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

Study These Flashcards

Basolateral: 3Na-2K-ATPase
Apical: Na-H exchange

How are chloride ions reabsorbed in S2-S3 of the PCT?

Paracellular and transcellular Cl^- transport

How is water reabsorbed in S2-S3 of the PCT?

- Aquaporins - ~4mOsmol gradient favours water uptake from lumen

Describe sodium and chloride uptake in the late PCT

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^-

Identify the sodium transporter in the Loop of Henle

NaKCC (symporter)

Distinguish between the general actions of the descending and ascending limbs of the Loop of Henle

- **Descending limb** reabsorbs H₂O but not NaCl - **Ascending limb** reabsorbs NaCl but not H₂O

Describe the reabsorption that occurs in the descending and ascending limbs of the Loop of Henle

- Increase in [Na⁺]_i allows paracellular reuptake of H₂O from descending limb - Na⁺ and Cl^- are concentrated in the lumen of the descending limb ready for active transport in the ascending limb

Describe the reabsorption that occurs in the thin ascending limb of the Loop of Henle

- H₂O reabsorption in descending limb creates a gradient for passive Na⁺ reabsorption in thin ascending limb - This occurs through the paracellular route

Describe the 4 active transport processes which occur in the thick ascending limb of the Loop of Henle

- Active transport from lumen to cells via NaKCC transporter - Na⁺ move into interstitium via 3Na-2K-ATPase - K⁺ diffuse via ROMK back into lumen (maintain activity of NKCC2 transporter) - Cl^- move into interstitium

What is unique about the thick ascending limb of the Loop of Henle?

- Region uses the most energy in the nephron - Sensitive to hypoxia

Describe the osmolarity of the tubule fluid leaving the Loop of Henle

Tubule fluid leaving loop is hypo-osmotic compared to plasma

Identify the 2 regions of the distal convoluted tubule

- DCT 1 (early) - DCT2 ( late)

Describe the general reabsorption that happens in the DCT as a whole

- Hypo osmotic fluid (from LoH) enters - Active transport of Na⁺ (5-8%) - Low H₂O permeability

Identify the sodium transporter in the early DT

NaCC (symporter)

Describe the reabsorption that occurs in DCT1

- NaCl enters apical membrane via electro-neutral NCC transporter - NaCl leaves via 3Na-2K-ATPase in basolateral membrane - More hypo-osmotic fluid leaves, so further dilution occurs

Identify the sodium transporter in the late DT & CD

ENaC (epithelial sodium channels)

Describe the reabsorption that occurs in DCT2

- NaCl enters apical membrane via NCC and ENaC - NaCl leaves 3Na-2K-ATPase in basolateral membrane - Electrochemical gradient from ENaC drives paracellular Cl^- reuptake

Describe the action of diuretics on membrane transporters in the DCT

- NCC transporter: sensitive to **Thiazide diuretics** - ENaC transporter: sensitive to **Amiloride diuretics**

Describe calcium reabsorption across the DCT

- 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*

How can the collecting duct be divided into segments?

- Cortical region (CCD) - Medullary region (MCD)

Identify 2 cell types found in the CCD

- Principal cells (70%) - Intercalated cells

Describe reabsorption in the principal cells of the collecting duct

- Reabsorption of Na⁺ via ENaC on apical membrane - Driving force: 3Na-2K-ATPase in basolateral membrane - Lumen(–) charge drives paracellular Cl^- uptake

Identify the 2 types of intercalating cells

- **Acid-secreting** Type A intercalating cells - **Bicarbonate-secreting** Type B intercalating cells

Describe the different membrane transporters found in Type A intercalating cells of the collecting duct

- Express H⁺-ATPase and H⁺/K⁺-ATPase on apical membrane - Express Cl⁻/HCO₃⁻ exchanger on basolateral membrane

glucose reabsorption in the nephron

* normal plasma glucose conc = 2.5-5.5mmol/L * resorption via secondary active transport driven by energy released from active transport of Na down its conc gradient

Tm glucose

* when there is a maximum amount of glucose that can be absorbed by the body * due to a limited number of glucose transport carriers * glycosuria can develop and glucose can collect in the urine * people with diabetes may want to wee more often due to increased levels of glucose In the urine so water move

summary on proximal convoluted tubule

* reabsorption is isosmotic * responsible for the bulk reabsorption of many soutes * provides energy for Na/K ATPase * metabolically active

reasons for damage to hypothalamus or pituitary Gland

* brain injury * tumour * sarcoidosis * aneurysm * meningitis

what is central diabetes insipidus

* impaired ADH secretion or synthesis * water is inadequately reabsorbed from the collecting ducts so a large quantity of urine is produced * managed by ADH injections

nephrogenic diabetes insipidus

* insensitivity of the kidney to ADH * water inadequately reabsorbed and a large amount of water is released in the urine * low salt and low protein diet reduces urine output

S4) Basic Renal Processes Flashcards

(54 cards)