RENAL Structure and function of the renal tubule

Question 1

Renal tubule in relation to bowmans capsule and glomerulus

what do Bowman’s capsule and glomerulus do?
what does Bowman’s capsule and glomerulus do?

Answer 1

Bowman’s capsule and glomerulus filter large amounts of plasma.

Renal Tubule segments contain filtered fluid is converted to urine

Question 2

Glomerular Filtrate
composition vs plasma?
what is the GF formate rate?
urine flow rate?
when does urine formation begin? 

what does the fast filtration rate mean? (2)

Answer 2

• GF = same composition as plasma except
o No cells, v. little protein

• BUT composition of urine ≠plasma
o GF formed at 120ml/min
o Urine flow ~1ml/min

Urine formation begins when large amounts of fluid that is virtually free of protein is filtered from the glomerular capillaries into the Bowman’s capsule. In essence the GF is an ultrafiltrate of plasma.

This fast filtration rate coupled with so many nephrons means that can function with only 1 kidney and also reduced function in that kidney.

Question 3

Selective modification of filtrate as it passes through tubule

How?
Modification of GF done by?

Answer 3

The filtration process at the glomerulus is relatively non-selective, where modification occurs is along the tubule by the process of reabsorption and secretion of water and various solutes.

Modification done by tubular transport of solutes and water into and out of tubule.

Question 4

Reabsorption/Secretion
what is their job?

movement in relation to kidneys - name membranes

How do they move? (2)

Answer 4

When the direction of movement is from the tubular lumen into the peritubular capillary plasma it is called reabsorption

When the movement is in the opposite direction i.e. peritubular plasma into tubular lumen, it is called secretion

Clearing unwanted substances by excretion into urine & Returning wanted substances by reabsorption into blood

Hence to summarise – a substance can enter into tubule and be excreted into urine by glomerular filtration OR tubular secretion OR both

For a substance to be reabsorbed it must first cross the luminal membrane -> diffuse through the cytosol -> across the basolateral membrane and into the blood. Vice versa for secretion.

• 2 physiological processes involved in this: active and passive transfer.

Question 5

Active Transfer/Primary Active Transport
conc grad?
energy?

Answer 5

• Moving molecule/ion against conc gradient (low→high)
• Operates against electrochemical gradient
• Requires energy - driven by ATP

Question 6

Passive Transfer

conc grad?

Answer 6

• Passive movement down concentration gradient (requires suitable route)
• Active removal of one component -> concentrates other components

Question 7

Co-transport/Secondary Active Transport

how it works?
what is needed?
types?

Answer 7

• Movement of one substance down it’s concentration gradient -> generates energy -> Allows transport of another substance against its concentration gradient

• Requires carrier protein
• 2 types: symport and anti-port

Question 8

Passive transfer can be a consequence of active transport

How do ions and neutral substances move?

Does active transport have to take place across both membranes?

Answer 8

Suitable route i.e. lipid soluble substances move through lipid matrix

For ions and neutral substances move through water filled protein channels

Substance does not need to be actively transported across both luminal & basolateral membranes in order to be actively transported across the overall epithelium.

Question 9

Symport example

what moves?

Answer 9

Co-transport of Na and glucose i.e. because Na moves into cell down its concentration gradient i.e. high outside and low inside -> creates lots of energy.

This can pull other substances along with it = called cotransport.

One form of secondary active transport. For Na to pull another substance with it needs a coupling mechanism = carrier protein. E.g. with glucose.

Question 10

Counter-transport (antiport)

what moves? example

Answer 10

Counter-transport is when substance to be transported along with Na binds to carrier protein from inside of cell and comes out.

Na+ and H+

Question 11

Transport in Tubule
what mechansims and over what membranes?

How is NA electrochemical gradient established?
How does glucose move trancellularly?
How does glucose gain engery to move against gradient?
What other substances move with Na+? And which way?
Which co-transporter is used? where is it? what is the moevement?

genetic defect in transporter?
where else is the defetc seen? problem it leads to?

what other substances are co-transported with Na? (2)

Answer 11

Combination of active & passive mechanisms -> transcellular transport over luminal & basolateral membranes.

1. Combination of active & passive transport at different sides i.e. one side have active transport and on the other passive transport either by simple diffusion or facilitated diffusion.
2. High [Na] in tubule (140mEq/L) cf to low [Na] (12mEq/L) inside cell hence have movement of Na down its concentration gradient at luminal membrane also aided by greater intracellular negative potential (-70mv).
3. As Na diffuses down its electrochemical gradient, energy is released which drives another substance -> in this instance glucose uphill against its concentration gradient across the luminal membrane into the cells (Na-glucose symport via a specific carrier protein)
4. The energy generated from Na moving into the cell is ultimately generated by the primary active transport of Na moving out of the cell at the basolateral membrane i.e. the Na-K-ATPase keeps the cytoplasmic [Na] lower than tubular [Na] and maintains the electrochemical gradient for passive Na transport across luminal membrane.
5. Glucose just exits out at basolateral membrane by facilitated diffusion driven by the high [glucose] in the cell. Also known as SGLT2 (sodium-glucose cotransporter).

Genetic defect in this protein = familial renal glycosuria just like similar defect in intestinal protein SGLT1 -> glucose-galactose malabsorption

Other substances which are co-transported with Na are Cl- and aa (symport) and H+ (antiport).

Question 12

How do you treat diabetes (glucose in urine)?

Answer 12

SGLT2 inhibitors to treat diabetes - Dapagliflozin

Question 13

Techniques to investigate tubular function

3 technqiues
what is applied to human? what to animals?

Answer 13

1. Clearance studies - covered in later lectures
2. Micro puncture & Isolated Perfused Tubule
3. Electrophysiological Analysis
   a. Potential measurement
   b. Patch clamping
   1-> applied to man and 2 & 3 -> applied to lab animals

Question 14

Micro puncture
what do you do?
Techniques to investigate tubular function
what can be difficult?
only used ib?

Answer 14

• Direct sampling of tubular fluid in different parts of nephron
• Minute analysis of function, Difficult in inaccessible segments i.e. those deep in medulla, Combine with isolated tubule perfusion
• Only used in lab animals

(puncture -> inject viscous oil -> inject fluid for study -> sample and analyse)

Question 15

Micro puncture
In 1924 there were only theoretical mechanisms proposing that glomerular filtration, tubular reabsorption and tubular secretion occurred. How were they actually proven?

what proved gf?
what proved reabsorbtion?

Answer 15

Wearn came up with the idea of trying to puncture a glomerular capsule with a pipette and measure it’s composition.

Wearne & Richard then measured protein, glucose, Cl-, K+, urea and pH of blood, glomerular fluid and bladder urine. This proved the differences in composition (protein-free) between glomerular fluid and blood.

Also the absence of Na & glucose in urine cf to GF proved that reabsorption took place.

Question 16

Electric Potential
Techniques to investigate tubular function
what do you do?

Answer 16

• Combine with micro perfusion to alter PD

* Measure whether ions are moving with or against electrochemical gradient

Question 17

Patch Clamping
Techniques to investigate tubular function

what do you do? what do you measure?

Answer 17

• Current flow through individual ion channel measured
• Measure electrical resistance
o Across patch of cell membrane
o Changes when channels open/close

• Types of channels & response to drugs & hormones

Question 18

Nephron- Tubule Structure

from begining to end

Answer 18

Proximal convoluted tubule (PCT)
Thin Descending Limb, Loop of Henle
Thin Ascending Limb, LoH
Thick Ascending Limb, LoH
Distal convoluted tubule (DCT)
Collecting/Connecting tubule
Medullary Collecting duct

Question 19

Types of Nephron (2) what is major difference?
anatomical differences and where do they extend to?
% of each type in humans

How does the vascular system compare for the nephrons?

Answer 19

The major anatomic difference between the Cortical nephrons & Juxtamedullary
nephrons is the length of the loops of Henle.

Cortical nephrons have short-reach loops that just penetrate the boundary between the inner and outer zones of the medulla. These loops do not extend into the medulla.

Juxtamedullary nephrons have long-reach loops that penetrate deep into the medulla. These are better at concentrating urine
- In humans about 15 per cent of nephrons are juxtamedullary and 85 per cent are cortical.

Vascular system is also different.
o Cortical nephrons – entire tubular system is surrounded by and extensive network of capillaries
o Juxtamedullary nephrons – long efferent arterioles extend from glomeruli to outer medulla and divided into specialised capillaries (vasa recta) that extend downward into medulla and lie side by side with loops of Henle

Question 20

Proximal Convoluted Tubule

where is it?

What cellular characteristics allow high capacity for reabsoprtion? (2)

Functions of PCT?
what allows PCT to be major site of reabsorption? (2)

What is reabsorbed?
What happens to any loose mobile proteins?
Why is HCo3- reabsorbed as opposed to Cl-?
in what circumstances is Na+ passively reabsorbed? along what path?
Name a clincial condition related to the PCT

Answer 20

• Directly adjacent to Bowman’s Capsule
• High capacity for reabsorption due to special cellular characteristics:
  o Highly metabolic as a result of the numerous mitochondria for active transport
  o Extensive brush border on the luminal side which increases SA for rapid exchange

Functions:
Major site of reabsorption – 70% of filtered load reabsorbed here

Located in the luminal and basolateral membranes are enzymatic and protein carriers, primary and secondary active transport systems, which together with its permeability characteristics, make the proximal tubule the major site of reabsorption of the glomerular filtrate.

About 70 % of the filtrate including all essential nutrients are reabsorbed by the proximal tubule.
-> Glomerular filtrate is protein free, but some small proteins get through. These proteins are taken up by endocytosis → degraded by lysosomal enzymes -> amino acids and simple sugars -> reabsorbed into plasma

By the end of the early proximal tubule essentially all the glucose and amino acids and much of the HCO3- have been reabsorbed.

HCO3- is preferentially reabsorbed relative to Cl- -> concentration of Cl- rises in the tubular fluid. This establishes a Cl- concentration gradient from lumen to peritubular fluid and, as Cl- moves passively down its concentration gradient, the lumen acquires a positive electric charge relative to the peritubular fluid.

Na+, moves passively along the gradient with Cl-. This passive component of Na+ reabsorption occurs primarily along the paracellular path.

Clinical condition:
Fanconi’s Syndrome occurs when all the proximal tubule re-absorptive mechanisms are defective, so glucose, AA, Na, K etc all found in urine

Question 21

Loop of Henle
Structure
3 segments
types of cells at each segment and cellular structure?

how far do the segements travel?

Answer 21

LoH consists of 3 functionally distinct segments :

Thin Descending

Thin Ascending
thin epithelial cells, no brush border, few mitochondria & low metabolic activity

Thick Ascending
thick epithelial cells, extensive lateral intercellular folding, few microvilli, many mitochondria -> high metabolic activity

Thin descending segment can travel variable distance into medulla
Thin ascending segment can be quite short
Thick ascending extends back into cortex

Question 22

Loop of Henle
Functions
What do loop diuretics do? how?

Answer 22

Plays a critical role in concentrating and diluting urine by adjusting rate of water secretion and absorption.

• This depends on characteristics of the LOH which creates a zone within the medulla where the tissue fluid osmolality is high.
• Loop diuretics act here causing 20% of filtered Na to be excreted, by blocking Na-transport out of LoH (thick part)

Question 23

Medullary Osmotic Gradient

where do solutes accumulate? what happens to tubule fluid?

what 2 things happen?

Answer 23

Solutes accumulate in the renal medullary interstitium, maintained by a balanced inflow and outflow of solutes and water in the medulla.

‘’A high solute concentration (high osmotic pressure) is generated and maintained in the medullary interstitium and the tubule fluid becomes hypotonic.

1. LoH creates an osmolality gradient in medullary intersitium
2. Collecting Duct traverses medulla: urine concentrated as water moves out by osmosis

Question 24

Counter-current Multiplication by the loop of Henle

where does the fluid flow? locations?

osmotic conc in entering and leaving? How does it change through the journey?

permeability of ascending and descending limbs?

what happens at ascending limb? what effect does this have?

what happens at the descending limb? why?

what is the osmotic gradient between ascending and descending? How is this effect multiplied?

what does this create?

fluid that leaves LoH compared to plasma?

thin ascending limb vs thick?

Osmotic gradient from top to bottom? osmotic gradient between ascending and descending limbs?

Answer 24

LoH has 2 parallel limbs arranged so that tubular fluid flows into descending limb into medulla and out of medulla through the ascending limb i.e. Flow of fluid is in opposite directions = counter-current

The fluid that enters descending limb from proximal tubule has osmotic concentration approx. equal to that of plasma = 300mosm/kg.

The ascending limb is impermeable to water but reabsorbs solutes particularly NaCl.

Hence as tubular fluid travels up ascending limb it becomes more dilute – whilst the solute is accumulating in the interstitial fluid around the loop raising its osmolality.

On the other hand, the descending limb is freely permeable to water, thus the hyperosmotic ISF causes water to leave the descending limb.

This leads to osmotic gradient between ascending and descending limb of 200mOsm/kg. This effect is multiplied by the entry of new fluid into the descending limb which pushes fluid from around the loop to the ascending limb.

Thus, a continuous osmolality gradient is created the top of the loop (cortex) of about 300mOsm/kg to a peak of 1200mOsm/kg at the bottom of the loop (medulla).

Though not all of this is due to salt accumulation. Fluid that leaves the LoH is hypo-osmotic compared to plasma (~100mOsm/kg)

Thin ascending limb permeable to Na & Cl, but Thick ascending limb actively pumps Na & Cl out of tubular fluid.

Question 25

Thick Ascending Loop of Henle

Role? 
How does Na+ enter cell from lumen?
How does Na+ exit cell into blood?
what about entry and exit of K+ and Cl-?
what effects for loop diuretics have? examples of loop diuretics? what does this result in and why?

Answer 25

The thick ascending LoH reabsorbs approx. 25% of filtered Na and can compensate partially for any failure by PCT to reabsorb Na.

Na enters cell via Na: K: 2Cl cotransporter (or symporter), the driving force of which is large electrochemical difference favoring entry of Na into the cell.
• To exit, Na is transported actively via Na-K-ATPase, while K and Cl cross into the peritubular fluid passively.

The inhibition of the Na: K: 2Cl transporter by loop diuretics, results in inhibition of net NaCl reabsorption and increased excretion of these ions along with water. (Furosemide and bumetanide)

By disrupting the reabsorption of these ions, loop diuretics prevent the generation of the medullary osmotic gradient. Without such a concentrated medulla, water has less of an osmotic driving force to leave the collecting duct system, ultimately resulting in increased urine production.

Question 26

Counter-Current and Vasa Recta

What is the blood flow of VR compared to renal blood flow? WHY? What happens if this changes? effects of increased and decreased flow rate?

What maintains the osmotic gradient of the medullary?
Role of ther VR? permeability of VR? How does VR not disrupt the gradient?
What enters the VR and leaves the VR during descending limb? ascending limb?

what does VR remove and why? blood flow rate in VR? why?

what happens to Na+ in descending VR and effect of this on region?

what happens to Na+ in ascending VR?

what is the amount of solute in ascending VR a product of? (2)

effects of increased and decreased flow rate of VR?

Answer 26

Blood flow in VR is low ~5% of renal blood flow » minimizes solute loss from interstitium & maintains medullary interstitial gradient

Alteration of blood flow in VR can change gradient

The osmotic gradient of the medullary is maintained by the counter current and Vasa Recta

The vasa recta delivers O2 and nutrients to cells of the loop of Henle. The vasa recta, like other capillaries, is permeable to both H2O and salts and could disrupt the salt gradient established by the loop of Henle.

To avoid this, the vasa recta acts as a counter-current multiplier system as well. As the vasa recta descends into the renal medulla, water diffuses out into the surrounding fluids, and salts diffuse in.

When the vasa recta ascends, the reverse occurs. As a result, the concentration of salts in the vasa recta is always about the same, and the salt gradient established by the loop of Henle remains in place.

Water is removed by VR, so it doesn’t dilute the longitudinal osmotic gradient. The medullary blood flow in the VR is slow, which is sufficient to supply the metabolic needs of the tissue but minimise solute loss from the medullary interstitium.

Reabsorbed Na+ in descending VR is carried to inner medulla equilibrating with ISF -> ↑ regional osmolarity

Na+ in ascending VR returns to systemic circulation. The amount of solute in the ascending VR is a product of flow rate & concentration
- e.g. If blood flow in VR increases then solutes are washed out of the medulla and its interstitial osmolality is decreased. If blood flow is decreased, then the opposite happens.

Question 27

Distal Convoluted Tubule and Connecting Tubule
2 parts of DCT
1st part - what is it? function?
2nd part - function?

What is connecting tubule? mainly where? function?

Answer 27

DCT:
-> 1st part (macula densa) linked to juxtaglomerular complex
Provides feedback control of GFR & tubular fluid flow in the same nephron
->2nd part very convoluted

Connecting Tubule:
Connects end of DCT to collecting duct – mainly in outer cortex
Overlap in functional characteristics with 2nd part of DCT

Question 28

Distal Convoluted Tubule and Connecting Tubule
Functions - permeability and effect on tubular fluid?

what is reabsorbed? what isnt reabsorbed?
what activity takes place in basolateral membrane?
what happens to fluid?
acid-base balance?
what hormone can act here?

Answer 28

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

Question 29

Collecting Duct
how is it formed? type of cells?

” different types of cells? role?

Answer 29

Collecting ducts formed by joining of collecting tubules
o Cuboidal epithelia, very few mitochondria

2 types of cells:
o Intercalated cells
- Involved in acidification of urine and acid-base balance

o Principal cells
- Role to play in Na balance & ECF volume regulation

Question 30

ADH and the Collecting Duct

where is ADH made? stored? effect of ADH?

what variability regulates ADH secretion?
What senses osmalarity? where? Effect of high/low osmolarity?

other factors that affect ADH secretion?
Where does AVP bind? effect of this? what does it promote?

Other affect of vasopression binding to a receptor? what does it activate and where? effect of this and what happesn at thick ascending limb? effect of that?

Answer 30

Your body makes ADH in the hypothalamus and stores the hormone in your pituitary gland. ADH then concentrates the urine by triggering the kidney tubules to reabsorb water back into your bloodstream rather than excreting water into your urine.

The single most important effect of antidiuretic hormone is to conserve body water by reducing the loss of water in urine.

The most important variable regulating antidiuretic hormone secretion is plasma osmolarity, or the concentration of solutes in blood.

Osmolarity is sensed in the hypothalamus by neurons known as an osmoreceptors, and those neurons, in turn, simulate secretion from the neurons that produce antidiuretic hormone.

When plasma osmolarity increases above the threshold, osmoreceptors recognise this -> stimulate the neurons that secrete antidiuretic hormone. Secretion of ADH is mediated primarily by hypothalamic osmoreceptors, which monitor changes in plasma osmolality.

• ADH secretion can also be regulated by volume receptors and arterial baroreceptors.

At a cellular level, Binding of AVP to V2-receptors stimulates the synthesis of aquaporin-2 water channel proteins and promotes cAMP-dependent trafficking of aquaporin 2 water channels to the luminal membrane of principal cells allowing back diffusion of water down its concentration gradient.

Vasopressin via V2 receptors also activates urea transporters in the distal nephron to facilitate urea reabsorption and urea recycling, which allows maximisation of sodium reabsorption in the thick ascending limb, supporting the axial hyperosmotic gradient drawing water from the distal nephron.

Question 31

Function of the Collecting Duct
what is urea?
permeability to urea?
What happens to urea at collecting duct? effect of this?
what is BUN? what does increasing levels of urea in kidney indicate?

Answer 31

Urea is a waste product formed in the liver during metabolic breakdown of proteins. You would imagine that the majority of urea would be excreted from the body via urine, but it is not.

Urea filters freely through glomerulus and passes down the tubule. Unlike cortical collecting tubule, the medullary collecting duct is permeable to urea.

As water is reabsorbed from the CD the urea is concentrated so that it moves out of the CD and is absorbed into the surrounding capillaries and also into the interstitium of the medulla where it contributes to the osmotic gradient around the LoH.

Increasing levels of urea in kidney is a sign of pre-renal failure because reabsorption is enhanced. Monitored using blood urea nitrogen test (BUN).

Question 32

ADH and Diuresis/ Antidiuresis
osmolarity of fluid entering CD?
What is the effect of ADH on osmolarity? How does it act? where does it act? How is water reabsorbed in relation to the osmolarity gradient?
Affect of ADH on urea?
other soluties movement in CD? affect on osmolarity? what does this maintain and facilitate?
effect of absense of ADH? Volume of urine excreted? what continues to happen?
effect of water deprivation?
effect of water excess?

Answer 32

The tubular fluid which enters the CD system is always hypo-osmotic and its concentration or further dilution as it traverses the CD depends on the water permeability of the duct, which is determined by the action of ADH.

In the presence of ADH water permeability is increased. ADH acts by inducing synthesis and insertion of water channels (aquaporins) into the luminal membrane. Water is reabsorbed along the osmotic gradient and the urine osmolarity approaches that of the medullary ISF at the tips of the long loops of Henle.

Water reabsorption increases the CD urea concentration and ADH increases the duct permeability to urea and therefore its reabsorption is increased.

• Other solutes, particularly Na+ and Cl-, continue to be reabsorbed in the CD and this further solute reabsorption serves to maintain the medullary hyper-osmolarity and thus facilitates the reabsorption of water in the presence of ADH.
• The final urine volume may be as low as 0.5 to 1.0 % of the filtered load and its concentration as high as 1400 mOsm/L.

In the absence of ADH the CD becomes essentially impermeable to water and urea. Sodium reabsorption continues in the CD and the tubular fluid become progressively more dilute in its progress along the duct. The volume of urine excreted under these conditions is potentially very large.

Under conditions of water deprivation, ADH acting through its receptor opens water channels in the epithelium so that water is extracted osmotically by the osmotic gradient set up in the interstitium around the LoH.

However, under conditions of water excess, ADH secretion falls, the water channels close and this facultative reabsorption of water cannot occur -> get large volume of dilute urine resulting.

Question 33

Proportion of reabsroption in % along the kidney tract?

pct? LoH? CD?

Answer 33

About 65% of the glomerular filtrate is reabsorbed proximally and another 10 % in the loop of Henle. The 25 % remaining enters the CD.

Question 34

GFR means 36L of urine produced

effect of adh? if no adh what happens?

Answer 34

At the average normal GFR of 180 L/day this is 36 L/day. In the presence of ADH only 1 to 2 L is excreted. In the absence of ADH all of this volume may be excreted and may have an osmolarity as low as 50 mOsm/L.

Question 35

water diuresis

Answer 35

An individual excreting large volume of dilute urine is said to be in a state of water diuresis.

Question 36

Factors contributing to build up of solute concentration in renal medulla
(4 factors)

Answer 36

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

Question 37

Abnormalities of the Kidney

name 3 and their effects

Answer 37

• Polycystic Kidney Disease (PKD)
- Genetic disorder characterised by growth of numerous cysts in kidney

• Diseases of the glomerulus
-> Usually called glomerulonephritis (GN)
– inflammation of glomeruli of some or all of million nephrons in kidney
– Can be primary or secondary to systemic disease like diabetes mellitus
-> Inherited diseases of the glomerular
basement membrane

• Diseases of the tubules

• Obstruction (reducing glomerular filtration)
• Impairment of transport functions (reducing water & solute reabsorption) e.g. Fanconi’s syndrome.

Question 38

Acquired Kidney Diseases:

name 4 and effects

Answer 38

• Hypertension
  o 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
  o Fall in cardiac output  renal hypoperfusion  registered as hypovolaemia, compensation results in pulmonary oedema
• Diabetic nephropathy
  o As a consequence of diabetes, filtering system of kidneys gets destroyed over time
• Lithium treatment results in acquired nephrogenic diabetes insipidus
  o Due to reduction of AQP2 expression.