S4) Basic Renal Processes Flashcards
Which ion is transport together with Na+?
Cl-
How much sodium is reabsorbed in the kidney?
Up to ~99% Na+ ions, Cl- ions and H2O are reabsorbed in kidney
Sodium reabsorption varies with the ingestion of sodium.
Why?
The kidney needs to match excretion of sodium to ingestion to remain sodium balance
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
Sodium input only occurs through ingestion of food.
Identify 3 means of sodium output
- Sweat
- Faeces
- Urine
What is the main site of absorption in the nephron?
Proximal convoluted tubule – absorbs Na+ (67%) and H2O (65%)
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%)
What stimulates proximal tubule reabsorption?
RAAS
Which cells are the target for aldosterone?
Principal cells of distal convoluted tubule and collecting duct
What happens when renal artery blood pressure increases?
In proximal tubule:
- Number of Na-H antiporter reduces
- Activity of Na-K ATPase reduces
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
What is the significance of the pressure diuresis and natriuresis that follow increased BP?
- ECF volume decreases
- Initial BP rise diminishes
Distinguish between paracellular and transcellular reabsorption
- Paracellular reabsorption: lumen → tight junction → capillary
- Transcellular reabsorption: lumen → apical membrane → basal membrane → capillary
Distinguish between paracellular and transcellular secretion
- Paracellular secretion: capillary → tight junction → lumen
- Transcellular secretion: capillary → basal membrane → apical membrane → lumen
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
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
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
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)
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
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)
Describe the movement of chloride ions in S1 of the PCT
- 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?
Using aquaporins
How is sodium reabsorbed in the apical and basolateral membranes of S2-S3 in the PCT respectively?
- 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
