Structure and function of the renal tubule Flashcards
How does reabsorption and secretion occur in the renal tubule
reabsorption- movement of nutrients from tubular lumen to peritubular plasma
Secretion- movement of nutrients from peritubular plasma to tubular lumen
Describe active and passive transfer
active transfer (primary):
-moving molecules against concentration gradient
-operates against electrochemical gradient
-requires ATP
Active transfer (secondary):
Movement of one substance down its concentration gradient generates energy
Give the definitions of symports and antiports
Symport = two molecules in
same direction
e.g. Na+-glucose
Antiport = two (or more)
molecules in opposite
directions
e.g. Na+-H+
Describe transcellular transport in the tubule
Combination of active & passive
mechanisms
transcellular transport across both
luminal & basolateral membranes of
epithelial cells (either direction)
What are different techniques to investigate tubular function
- Clearance studies
- Micropuncture & Isolated Perfused Tubule
- Electrophysiological Analysis
* Potential measurement
* Patch clamping
What are the 4 different stages of micro puncture
1) puncture
2) inject viscous oil
3) inject fluid for study
4) sample and analyse
Describe how electrophysiology is carried out (electrical potential)
- Combine with
microperfusion to alter
potential difference (PD) - Measure whether ion
moving with or against
electrochemical gradient
Describe how electrophysiology is carried out (patch clamping)
- Current flow through individual
ion channel measured - Measure electrical resistance
» Across patch of cell membrane
» Changes when channels
open/close - Types of channels & response to
drugs & hormones
Explain the loops of henle in a kangaroo rat
- Barely drinks!
- Most concentrated urine of
any mammal - Recovers almost all dietary
water because it has
remarkably long loops of
Henle
Give a summary of the proximal convoluted tubule
High capacity for reabsorption
* Epithelial cell characteristics:
» highly metabolic, numerous
mitochondria for active
transport
» extensive brush border on
luminal side large surface
area for rapid exchange
What is the major site of reabsoprtion and what is fanconi’s syndrome
THE MAJOR SITE OF
REABSORPTION
65-70% of filtered load reabsorbed
in proximal convoluted tubule
* Fanconi’s syndrome:
» all PCT reabsorptive
mechanisms defective
Give an overview of the loop of henle
three distinct segments:
1) thin descending- thin epithelial cells, no brush border
2) thin ascending - few mitochondria and low metabolic activity
3) thick ascending:
-thick epithelial cells
-extensive lateral intercellular folding
-few microvilli
-many mitochondria
-high metabolic activity
-impermeable to water
What are functions of Loop of henle (LOH)
- Critical role in
concentrating/diluting urine - Adjusts rate of water
secretion/absorption
Actively
reabsorbs
Na
Site of action of powerful
“loop” diuretcs
e.g. Furosemide
~20% filtered Na+ excreted
explain the medullary osmotic gradient
1) LOH creates an osmolality gradient in medullary interstitium
2) collecting duct transverse medulla: urine concentrated as water moves out via osmosis
Explain counter current flow and vasa recta (VA)
1) VA freely permeable to solutes and H20
2) VA acts as a counter current exchange system
3) as the blood descends into medulla, H20 diffuses out and salts diffuse in
4) reverse occurs as it ascends
What does it mean when the VR has a low blood flow
-minimises solute loss from interstitium and maintains medullary interstitial gradient
Explain the early distal convoluted tubule
*Location of macula densa
(juxtaglomerular apparatus)
Provides feedback control
of glomerular filtration
rate & tubular fluid flow
in the same nephron
*Relative straight (i.e. not the
most convoluted bit of
convoluted tubule)
What are functions of late DCT
- Solute reabsorption continues, w/out H2O
reabsorption - Further dilution of tubular fluid
- High Na+,K+-ATPase activity in basolateral
membrane - Very low H2O permeability
- Anti-diuretic hormone (ADH) can exert actions
- Role to play in acid-base balance via secretion of NH3
Discuss the Collecting tubule
Connects end of DCT to collecting duct – mainly in outer cortex
* Relatively straight in shape
* Overlap in functional characteristics with both late DCT and
collecting duct
Explain the collecting ducts
- Collecting ducts formed by joining of
collecting tubules - Cuboidal-to-columnar epithelia, very few
mitochondria - 2 types of cells:
- Intercalated cells
- Involved in acidification of urine and
acid-base balance - Principal cells
- Role to play in Na balance & ECF
volume regulation* Final site for processing urine - Made very permeable to H2O by ADH
- Also permeable to urea
What is ADH secretion triggered by
ADH secretion is triggered by changes in plasma osmolality
Explain how ADH effects permeability in the kidneys
1) Dehydration or low blood pressure triggers ADH release from the posterior pituitary.
ADH travels to the kidneys via the bloodstream.
ADH acts on the collecting ducts, making them more permeable to water by inserting aquaporins.
Water is reabsorbed from the urine back into the blood.
Result: Blood volume increases, urine becomes concentrated, and the body conserves water.
Explain urea in the collecting duct
- Urea also
contributes to
medullary
interstitial gradient - Urea levels
monitored using
BUN (blood urea
nitrogen) test
What are 4 major factors contributing to build up of solute concentration in renal medulla
FOUR MAJOR FACTORS CONTRIBUTING
TO BUILD UP OF SOLUTE CONCENTRATION IN RENAL MEDULLA:
1) Active transport of Na+ and co-transport of K+ & Cl- out of
thick ascending limb into medullary interstitium
2) Active transport of ions from collecting ducts into medullary interstitium.
3) Facilitated diffusion of large amounts of urea from collecting ducts into medullary interstitium
4) Very little diffusion of water from ascending limbs of tubules
into medullary interstitium
