Structure and Function of the Renal Tubule

Question 1

What is the glomerular filtrate?

Answer 1

GF = same composition as plasma
Except no cells, v. little protein
BUT composition of urine ≠plasma
WHY & HOW?
Clue: 
GF formed at 120ml/min
Urine flow ~1ml/min

Question 2

Where do reabsorption and secretion happen?

Answer 2

Reabsorption and secretion
Reabsorption  = 	tubular lumen 
					→ peritubular plasma
Secretion  	=  	peritubular plasma 
					→ tubular lumen

Question 3

What are the types of active and passive transfer?

Answer 3

Active Transfer/Primary Active Transport:
Moving molecule/ion against conc gradient (low→high)
Operates against an electrochemical gradient
Requires energy - driven by ATP
Passive Transfer (or flux):
Passive movement down concentration gradient (requires suitable route)
Removal of one component ⇒ concentrates other components
Co-transport/Secondary Active Transport:
Movement of one substance down its concentration gradient
⇒ generates energy ⇒ which allows transport of another substance
against its concentration gradient
Requires carrier protein
2 types: symport and anti-port

Question 4

What is the difference between symports and antiports?

Answer 4

Symport = transport in same direction e.g. Na+-glucose
Antiport = transport in opposite directions e.g Na+-H+

Question 5

How does transport happen in the tubule?

Answer 5

Combination of active & passive mechanisms ⇒ transcellular transport across both luminal & basolateral membranes of epithelial cells (either direction)
This can happen through a symporter which allows glucose to be brought into the cell
Then the glucose through a uniporter into the peritubular capillary
In renal glycosuria, the symporter SGLT2 is missing, so they are unable to take glucose back out of the glomerular filtrate so glucose is found in the urine
ATP is used to expel the sodium from the epithelial cells also causing potassium to enter the epithelial cell
SGLT2 inhibitors are used to treat diabetes so more glucose gets excreted

Question 6

What techniques are used to investigate tubular functions?

Answer 6

Clearance studies 
Micropuncture & Isolated Perfused Tubule
Electrophysiological Analysis
Potential measurement
Patch clamping
1 = applied to man (observational)
2 & 3 = applied to lab animals (mechanistic)

Question 7

What are the different types of electrophysiology?

Answer 7

Electrophysiology- electrical potential
Combine with microperfusion to alter potential difference (PD)
Measure whether ion moving with or against electrochemical gradient
Actively transported?
Electrophysiology- 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

Question 8

What are the two types of nephron?

Answer 8

Juxta-medullary nephron (15%)
Cortical nephron (85%)
The cortical nephron has a short loop of Henle whereas the other has a long loop of Henle that descend deep into the medullary tissue
The second difference is the location of the peritubular capillaries
In the cortical nephrons these capillaries run around the distal and proximal convoluted tubules in the case of the juxta-medullary nephron there is little blood supply around the tubules instead it is around the loop of Henle and we have this structure called the vasa recta
The role of the vasa recta and juxta-medullary nephrons is to facilitate the uptake of water (e.g. if you are dehydrated)

Question 9

What are the PCT’s epithelial cell characteristics?

Answer 9

Proximal convoluted tube
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
Directly adjacent to Bowman’s capsule

Question 10

What are the functions of PCT?

Answer 10

Major site of reabsorption
~65-70% of filtered load reabsorbed here
Fanconi’s syndrome:
all PCT reabsorptive mechanisms defective

Question 11

What are the different segments of the Loops of Henle?

Answer 11

Three functionally distinct segments :
Thin descending + thin ascending:
Thin epithelial cells (squamous), 
No brush border, 
Few mitochondria
Low metabolic activity
Thin descending arm role is the uptake of water 
Thick ascending:
Thick epithelial cells, 
Extensive lateral intercellular folding, 
Few microvilli,
Many mitochondria,
High metabolic activity
Function is for ions to be reabsorbed from the tubular fluid and for example hydrogen ions to pass into the tubular fluid

Question 12

What are the functions of the LOH?

Answer 12

Critical role in concentrating/diluting urine
Adjusts the rate of water secretion/absorption
IN order to achieve this:
Only the descending arm is very permeable to water
Within the thick ascending arm we get this active reabsorption of Na
These sodium transporters are the site of action of some powerful and commonly used drugs known as diuretics

Question 13

How does transport occur across the thick ascending loop of Henle?

Answer 13

Here its symporter also allows the cotransport of chloride and potassium ions
The sodium concentration is generated by sodium pumps that move into the peritubular capillary with potassium being moved into the epithelial cell at the same time
So the excess of potassium and chloride pass through uniporters from the epithelial cell to the peritubular capillary

Question 14

How is the medullary osmotic gradient created?

Answer 14

1. LOH creates an osmolality gradient in the medullary interstitium
   
   2. Collecting duct traverses medulla; urine concentrated as water moves out by osmosis
      This means that as the fluid passes down it will always be surrounded by interstitial fluid that has a higher osmolality.
      This means that water will be moved out by osmosis.

Question 15

How does counter-current multiplication by LOH happen?

Answer 15

Essentially you get some kind of feedback
The salt being pulled out in the ascending arm affects the gradient as well as the water coming out in the descending arm
Starting with the glomerular filtrate entering the loop of Henle it will go in with an osmolality of around 300mM/Kg
On the ascending arm we get sodium and chloride ions passing through the walls for reabsorption and this creates the osmolality within the interstitium
As the fluid rises up, because its concentration of salt has decreased there’s less that can be transferred across to the interstitium
At the same time, because of this descending increase in osmomolarity caused by the sodium ions, water is taken from the descending arm
By the time it gets into the outer medulla, where the surrounding osmomolarity is around 600 that will cause water to be drawn out by osmosis
This mechanism would happen until there is an equilibrium but as the fluid descends it goes into ever higher osmomolarity in the surrounding tissue such that we end up concentrating the ultrafiltrate as we descend
The fluid leaving has a lower volume but also a lower osmomolarity than started

Question 16

What happens in the vasa recta?

Answer 16

VR freely permeable to solutes & H20
VR acts as a counter-current exchange system
As blood descends into the medulla H2O diffuses out and salts diffuse in
The reverse occurs as it ascends
Blood flow in VR is low ~5% of renal blood flow » minimizes solute loss from interstitium & maintains a medullary interstitial gradient
Alteration of blood flow in VR can change the gradient

Question 17

What does the early distal convoluted tubule do?

Answer 17

Location of macula densa (juxtaglomerular apparatus)
Provides feedback control for blood pressure and of glomerular filtration rate & tubular fluid flow in the same nephron
Relative straight (i.e. not the most convoluted bit of convoluted tubule)

Question 18

What are the functions of the late DCT?

Answer 18

Solute reabsorption continues, w/out H2O reabsorption
High Na+, K+-ATPase activity in the basolateral membrane
Very low H2O permeability and involved in further dilution of tubular fluid
Anti-diuretic hormone (ADH) can exert actions
Role to play in acid-base balance particularly via the secretion of NH3

Question 19

What does the collecting tubule do?

Answer 19

Connects end of DCT to collecting duct – mainly in the outer cortex
Relatively straight in shape
Overlap in functional characteristics with both late DCT and collecting duct

Question 20

What is the collecting duct formed of?

Answer 20

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
These contribute to the counter-current mechanism:
Final site for processing urine
Made very permeable to H2O by ADH
Also permeable to urea

Question 21

What does ADH do in the collecting duct?

Answer 21

ADH is secreted within the posterior pituitary and induced by changes in osmolality of the plasma, due to a lack of water in the extracellular fluid
It makes its way via the peritubular capillaries to act on epithelial cells surrounding the collecting duct and it will trigger water to be released causing urine to be more concentrated
It does this by binding to vasopressin receptor 2 triggering a G protein-coupled signalling cascade
It triggers the synthesis of AQP (aquaporin 2) as well as causing it to be transported to the plasma membrane where it will act as a pore for water
ADH also triggers epithelial cells to be permeable to urea

Question 22

How does urea affect the collecting duct?

Answer 22

Urea also contributes to medullary interstitial gradient

Urea levels monitored using the BUN (blood urea nitrogen) test

Question 23

How can ADH change with different levels of water intake?

Answer 23

When we are water-deprived we need the water to be reabsorbed
In a situation of water excess, there will be no action of ADH
Some urea will pass but most of it will go

Question 24

What are the major factors contributing to a buildup of solute concentration in the renal medulla?

Answer 24

Active transport of Na+ and co-transport of K+ & Cl- out of a thick ascending limb into the medullary interstitium
Active transport of ions from collecting ducts into the medullary interstitium
Facilitated diffusion of large amounts of urea from collecting ducts into the medullary interstitium
Very little diffusion of water from ascending limbs of tubules into the medullary interstitium

Question 25

What is PKD?

Answer 25

Polycystic kidney disease (PKD)
Polycystic kidney disease is a genetic disorder characterised by the growth of numerous cysts
The autosomal dominant mode of inheritance

Question 26

What is glomerulonephritis?

Answer 26

Inflammation of glomeruli in some or all of million nephrons in kidney
Can be primary or secondary to systemic disease like diabetes mellitus
This is a distinct condition to nephrotic syndrome

Question 27

What are the diseases of the tubule?

Answer 27

Obstruction (reducing glomerular filtration)

Impairment of transport functions (reducing water and solute reabsorption) e.g. Fanconi’s syndrome

Question 28

What other acquired kidney diseases are there?

Answer 28

Hypertensions:
Kidneys regulate ECF volume and hence influence blood pressure
Compensatory mechanisms in response to high BP can lead to chronic kidney damage
Congestive cardiac failure:
Fall in cardiac output ⇒ renal hypoperfusion ⇒ registered as hypovolaemia, compensation results in pulmonary oedema
Diabetic nephropathy:
As a consequence of diabetes (poor glucose control and hypertension), filtering system of kidneys gets destroyed over time
Lithium treatment results in acquired nephrotic diabetes insipidus:
Due to reduction of AQP2 expression
Diabetes insipidus (DI)is a condition characterized by excessive thirst and excretion of large amounts of severely diluted urine, with reduction of fluid intake having no effect on the concentration of the urine. There are different types of DI, each with a different set of causes.
The most common type in humans is the neurological form, called Central DI (CDI), which involves a deficiency of arginine vasopressin (AVP), also known as antidiuretic hormone (ADH).
The second common type of DI is nephrogenic diabetes insipidus (NDI), which is due to kidney or nephron dysfunction caused by an insensitivity of the kidneys or nephrons to ADH.