Structure & Function of the Renal Tubule

Question 1

What is the ultrafiltrate

Answer 1

• The ultrafiltrate is the fluid that forms in the bowman’s space and proceeds into tubule of the nephron
• The glomerular filtrate is almost identical to plasma in composition except it contains no cells and very little protein
• Yet plasma is very different in composition to urine so the GF undergoes various modifications

Question 2

What is the blood supply to the nephron?

Answer 2

The blood supply to the nephron includes the afferent arteriole, glomerulus and efferent arteriole, but also tubular capillaries that allow constant exchange of solutes etc between the lumen of the tubule and plasma

Question 3

What are the membranes of the tubule epithelial cells?

Answer 3

The epithelial cells have 2 distinct membranes - the luminal membrane on the lumen side and the basolateral membrane on the external side

Question 4

What is adjacent to the tubules?

Answer 4

Peritubular capillaries

Question 5

Where do molecules move in reabsorption and secretion?

Answer 5

• Reabsorption is movement from the tubular lumen into the peritubular capillary lumen
• Secretion is movement from peritubular capillary lumen into the tubular lumen

Question 6

Describe active transport, passive transfer and cotransport

Answer 6

• Active transfer/primary active transport -
• Moving molecule/ ion against the concentration gradient
• Operates against the electrochemical gradient
• Requires energy- driven by ATP
• Passive transfer or flux -
• Passive movement down the concentration gradient
• Removal of one component makes other components at a higher concentration
• Cotransport / secondary active transport-
• Movement of one substance down its concentration gradient generates energy which allows transport of another substance against its concentration gradient
• Requires a carrier protein
• 2 types - symport and antiport

Question 7

What are symporters and antiporters?

Answer 7

• Symport - two molecules in the same direction e.g. Na+ and glucose
• Antiport - two molecules in opposite directions e.g. Na+ and H+

Question 8

Describe transcellular transport in the tubule

Answer 8

• Combines active and passive mechanisms
• Na+ in GF moves passively into the peritubular capillaries - this can cotransport glucose with it
• The glucose is then in a high [ ] so then travels via a GLUT transporter back into the blood supply
• The sodium gradient between the epithelial cells of the tubule and the capillaries is created by the Na+/K+ ATPase - 3Na+ ions enter capillary from epithelial cells and 2K+ ions enter epithelial cells from capillaries
• As the Na+ has left the epithelial cells there is a gradient between the tubule lumen and epithelial cells so Na+ ions enter the epithelial cells and transports amino acids into the epithelial cells

Question 9

What are the techniques used to investigate tubular function?

Answer 9

1. Clearance studies - applied to patients (observational)
2. Micropuncture and isolated perfused tubule
3. Electrophysiological analysis - potential measurement, patch clamping

2&3 are applied to lab animals (mechanistic)

Question 10

Describe micropuncture, electrophysiology and patch clamping in more detail

Answer 10

• Micropuncture -
• Use a pipette to make a puncture in a tubule
• Inject viscous oil into the tubule
• Then inject a fluid that is the subject of the study
• Then remove the sample and analyse to see how the composition is changed after it has moved through the tubule
• Electrophysiology -
• Measure the electrochemical gradient on either side but putting electrodes on the outside and inside of the membrane
• This is combined with microperfusion of substances to see how they alter the gradient and therefore if they are being transported actively or not
• Patch clamping -
• Use a pipette to suction a small section of the membrane and a solution is added into the pipette
• Current flow through an individual ion channel is measured
• You measure electrical resistance across a patch of cell membrane
• This can be used to look at the effect of drugs / hormones on ion channels

Question 11

Describe the 2 types of nephron

Answer 11

• Juxta medullary nephrons - have a long loop of Henle - 15% of nephrons in the kidney - most of the capillaries are around the loop of Henle (vasa recta)
• Cortical nephrons - have a short loop of Henle - 85% of nephrons in the kidney - most peritubular capillaries are around the tubules

Question 12

Describe how PCT cells are specialised

Answer 12

Epithelial cells are highly metabolic (have numerous mitochondria for active transport) they also have an extensive brush border on the luminal side (large surface area for rapid exchange)

Question 13

Describe reabsorption in PCT

Answer 13

• The Na+/K+ ATPase pumps 3 Na+ into the blood and 2 K+ into the epithelial cells creating a gradient that allows Na+ to be reabsorbed
• 100% of glucose and amino acids are reabsorbed via transportation
• The removal of these substances from the tubule means the water potential is higher so it creates a water potential gradient that water then goes down so that it is reabsorbed into the capillaries
• There is also reabsorption of Cl- cotransported with Na+ ions
• Proteins are taken up by pinocytosis - proteins are engulfed into a vesicle which then passes from the tubule to the epithelial cells- this vesicle then binds to lysosomes which degrade them into amino acids and sugars
• 65-70% of filtered load is reabsorbed in the proximal convoluted tubule

Question 14

Describe the structure of the loop of Henle

Answer 14

3 distinct segments:
- Thin descending
- Thin ascending
- Thick ascending

• Both then thin segments have thin epithelial cells, no brush border, few mitochondria and low metabolic activity
• The thick segment has thick epithelial cells, extensive lateral intercellular folding, few microvilli, many mitochondria and high metabolic activity

Question 15

Functions of the loop of Henle?

Answer 15

• Has a critical role in concentrating/diluting urine
• It adjusts the rate of water secretion/ absorption as a byproduct of this
• The descending arm is very permeable to water
• The ascending limb is virtually impermeable to water so there is the active reabsorption of Na+ ions

Question 16

Describe how an osmotic gradient is created by the loop of Henle

Answer 16

• Na+ ions move from the tubular lumen into the epithelial cells via as symporter that also transports Cl- and K+ ions across
• Na+ moves across because of a Na+ gradient set up by a Na+/K+ ATPase between the epithelial cells and capillaries
• There are also uniporters that remove K+ and Cl- ions from the epithelial cells and transport them into the capillaries
• By taking up Na+ and Cl- ions the loop of Henle creates as osmotic gradient in the medulla
• The collecting duct transverses the medulla so urine becomes concentrated as water moves out by osmosis using this gradient created

Question 17

Describe countercurrent multiplication by the loop of Henle

Answer 17

• A countercurrent mechanism is one in which a solution is passed through an environment with a different osmotic gradient so composition can be altered
• Multiplication refers to the fact that once established this system will sustain itself
• As fluid moves through the loop of Henle , NaCl is reabsorbed in the ascending limb - in the thin arm this is by passive flux and in the thick arm it is active transport
• This means there is more NaCl in the area surrounding the nephron with the environment becoming higher in [NaCl] as you go down the descending arm (osmolality increases from 300 to 1200)
• Hence because of this H2O is reabsorbed in the descending arm and moves out of the tubule
• This then has the effect of concentrating the fluid in the tubule as less water is present
• As this more concentrated fluid passes up the ascending limb the osmolality decreases again - has an osmolality which is less than that of plasma by the time it leaves the loop of Henle
• This then facilitates the reabsorption of water in the collecting duct

Question 18

Describe the role of the vasa recta in the loop of Henle

Answer 18

• The movement of blood is the vasa recta is in the opposite direction the movement of fluid through the nephron
• The vasa recta is freely permeable to solutes and water and acts as a countercurrent exchange system
• As blood descends into the medulla water diffuses out into the tubule and salts diffuse into the blood
• As blood ascends the medulla water diffuses into the blood and salts diffuse out into the tubule
• Blood flow in vasa recta is very flow (5% of renal blood flow) - minimises solute loss from interstitium and maintains medullary interstitial gradient - alteration of blood supply to kidney changes the gradients

Question 19

Describe the roles of the first and second parts of the DCT

Answer 19

• 1st part of DCT -
• First part of the distal convoluted tubule is where we have the macula densa (juxtaglomerular apparatus)
• Provides feedback control of glomerular filtration rate and tubular fluid flow in the same nephron
• Relatively straight
• 2nd part of DCT -
• Solute reabsorption continues without water reabsorption
• Involved in further dilution of tubular fluid by removing solutes
• There is high Na+/K+ ATPase activity in the basolateral membrane
• Very low water permeability unless under the action of antidiuretic hormones
• Has a role in acid base balance via secretion of ammonia

Question 20

Describe the role of the collecting tubule

Answer 20

• Connects the end of the distal convoluted tubule to the collecting duct - mainly in outer cortex
• Relatively straight in shape
• Overlap in functional characteristics with both late DCT and collecting duct

Question 21

Describe the role of the collecting duct (structure and function)

Answer 21

• Formed by the joining of collecting tubules
• Cuboidal to columnar epithelia with very few mitochondria
• 2 types of cells- intercalated cells (involved in acid base balance and acidification of urine) and principal cells (role to play in Na balance and ECF volume regulation)
• Acts as final site of urine processing
• Made very permeable to water by ADH and also permeable to urea which maintains the counter current mechanism

Question 22

Describe the role of ADH in urine processing

Answer 22

• ADH is secreted by the brain (posterior pituitary)
• It is secreted in response to change in plasma osmolality
• It is secreted into the bloodstream and enters the peritubular capillaries where it is then transported into the collecting duct epithelial cells
• ADH binds to a V2 receptor on the principal cell of the collecting duct
• This stimulates G alpha s which stimulates adenylate cyclase which causes more conversion of ATP to cAMP
• The cAMP then activates protein kinase A
• This then has 2 effects- increases the transcription of aquaporin 2 and then causes that aquaporin 2 to move to the basolateral membrane which can then facilitate water uptake from the collecting duct
• This increases the osmolality so the urine is concentrated again
• It also allows urea to be taken up which contributes to the medullary interstitial gradient
• Urea levels can be monitored using BUN (blood urea nitrogen) test
• Water deprivation -
• In the collecting duct Na+/Cl-/ urea leave the collecting duct to be reabsorbed
• ADH is released and aquaporins are inserted to allow H2O to be reabsorbed
• Hence there is a small urine volume which is a high concentration
• Water excess -
• In the collecting duct Na+/Cl-/ leave the collecting duct to be reabsorbed
• ADH is not released so aquaporins are not inserted
• Hence there is a large volume of urine which is at a low concentration

Question 23

State the 4 major factors that contribute to build up of solute concentration in renal medulla

Answer 23

1. Active transport of Na+ and cotransport of K+ and 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 24

Describe some common kidney conditions

Answer 24

• Polycystic kidney disease - Polycystic kidney disease is a genetic disorder characterised by growth of numerous cysts
• 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
• This is a distinct condition to nephrotic syndrome
• Diseases of the tubule
• Obstruction (reducing glomerular filtration)
• Impairment of transport functions (reducing water & solute reabsorption) eg. Fanconi’s syndrome
• Hypertension
• 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 nephrogenic diabetes insipidus*
- Due to reduction of AQP2 expression

• Diabetes insipidus (DI) is a condition characterised 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.