7 - Reabsorption along the Nephron Flashcards
Fill in the following table to show where the majority of sodium and water is reabsorbed in the nephron
How do we change plasma volume? (ECF)
- 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
Which ion is absorbed with Na?
Cl- = up to 99% of Na, Cl and Water reabsorbed
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
Need to keep in sodium balance so vary the amount of sodium excreted
How is a decrease in the ECF volume detected and corrected?
- Low b.p so baroreceptors in atria detect and lower sympathetic activty
- Send signals via vagus nerve to brainstem
- ADH secretion
- Water uptake
How is an increase in the ECF volume detected?
- 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
What is natriuresis?
Excretion of sodium into the urine
How does secretion and absorption happen in the nephron?
- Paracellular secretion: capillary → tight junction → lumen
- Transcellular secretion: capillary → basal membrane → apical membrane → lumen
Label the parts of the collecting duct and explain what/where the aquaporins are.
- 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
Briefly distinguish between Cl- and Na+ reabsorption.
What substances that are reabsorbed in the PCT and how is this is achieved?
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)
What is the histology of the PCT and what are the different sections of the PCT?
- Simple cuboidal containing lots of mitochondria for Na transport
- S1-S3
Identify the 4 different transporters which facilitate sodium reabsorption in the apical membrane in S1 of the PCT.
Concentration gradient set up by Na/K ATPase on basolateral membrane
Identify the 2 different transporters which facilitate sodium reabsorption in the basolateral membrane in S1 of the PCT
- 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
If the kidneys retain 100% of the glucose that they filter then why do diabetics have glucose in their urine?
Above renal thresold and reached transport maximum so it is excreted
Where and how is bicarbonate reabsorbed, and what drug can interrupt this process?
Mostly in S1 PCT
How is sodium, chloride and water reabsorbed in the apical and basolateral membranes of S2-S3 in the PCT respectively?
- Chloride reabsorbed para and transcellularly
- Aquaporins with ~4mOsmol gradient favours water uptake from lumen
How is sodium and chloride absorbed in the late late PCT?
No energy for active absorption
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-
Where does amino acid uptake occur in the nephron?
- 100% in PCT
- Sodium-dependent amino acid transports on luminal (set up by Na/K pump)
- Passive transporters on basolateral
What effect does an increased renal artery BP have on reabsorption?
- 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
In general, what occurs in the loop of Henle and what is the histology?
- 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
Describe the reabsorption that occurs in the descending limbs of the Loop of Henle?
- 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
What is the difference between the thin and thick ascending limb?
Thin: passive absorption of Na
Thick: active absorption of Na so presence of transporters again
What are the active transport processes that occur in the thick ascending limb?
- 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
How do loop diuretics work?
- 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)
What state is the filtrate in when leaving the ascending limb?
Hypo-osmotic to plasma, been through diluting segment of ascending limb
What happens in the DCT?
- 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
What medications can affect sodium transporters in the DCT?
Thiazides: NCCT
Amiloride: ENaC
Apart from Na, K and Cl, what other important ion is reabsorbed in 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 CD: containing principal and intercalated cells
- Medullary CD
How does reabsorption in the principal cells of the collecting duct occur?
- 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
What are the two types of intercalating cells?
- Acid-secreting Type A intercalating cells
- Bicarbonate-secreting Type B intercalating cells
Helps with acid base balance
What is the histology of the collecting duct?
- Simble cuboidal
- No brushborder and has wide lumen unlike PCT
- Minor calyx has transitional epithelia
What structure is formed from the merging of the collecting ducts, and acts as the gateway to the cavity known as the minor calyx?
Collecting ducts merge and terminate to form papillary ducts of Bellini, which open sieve like at the area cribosa
