Renal System 2

Question 1

How much filtrate produces per day

Answer 1

180L

Question 2

What happens with the filtrate

Answer 2

• Most of the substances in the filtrate get quickly returned to the blood = peritubular capillaries of cortex, branch from efferent arteriole and adjacent to renal tubules

Question 3

Solute reabsorption process

Answer 3

1. solute in filtrate within proximal convoluted tubule
2. through apical membrane of epthelial cell
3. through basolateral membrane of epthelial cell
4. through basement membrane
5. out of proximal convoluted tubule
6. through peritubular space between PCT and capillary
7. Into peritubular space
8. Through capillary endothelial cells
9. Into peritubular capillary blood vessel

Question 4

How do substances cross epithelial cells of the tubule (2)

Answer 4

1. transcellular: through tubule cells- solute enters apical membrane of tubule cells, diffustion through cytosils of tubule cells
2. para cellular = between 2 tubule cells - movement through leaky tight junctions in PCT - H2O, Ca, Mg, K and Na

Question 5

How do substances cross epithelial cells of the tubule in transcellular transport

Answer 5

Active - passive

With channel proteins - without channel proteins

Question 6

Passive tubular reabsorption

Answer 6

• W/o expending metabolic energy
• with CH e.g. H2O from renal tubular fluid
• Substances pass through the plasme membrane until some sort of equilibrium is achieved

Question 7

Water reabsorption

Answer 7

• Always passive
• Diffuses to regions of greater osmolarity - higher solute concentration
• Active transport of solutes to peritubular fluid and plasma act to increase osmolarity, allowing water to follow

Question 8

Handling of urea by PCT

5

Answer 8

1. freely filtered at glomerulus = ends up in the filtrate
2. active reabsorption of solutes increases peritubulsr fluid and plasma osmolarity
3. water is reabsorbed by osmosis, following solutes
4. water reabsorption creates urea concentration gradient
5. passive urea movement from tubule to peritubular capillaries competely dependent upon water moevement

Question 9

How much urea is filters by glomerulus and reabsorbed in PCT

Answer 9

50%

Question 10

Active solute tubular reabsorption

Answer 10

• with expending metabolic energy
• uses protein pumps/ transporter in membrane
• Simple active transport, Na/K exchange pump, cotransporter

Question 11

Primary active transport = simple active transport

Answer 11

• occurs against a solute concentration gradient
• may occur at either a basolateral membrane or apical membrane
• energy from ATP used directly to transport a substance from low to high concentration

Question 12

Sodium/Potassium exchange pump

Answer 12

• an energy consuming ion pump in the basolateral membrane produces the gradient that facilitates Na+ entry across the luminal section of the cell, K+ movements are in the opposite direction

Question 13

Co transport = secondaty active transport

Answer 13

• transporter proteins move two molecule at the same time: one against a gradient and the other with its gradient
• energy released when ions transported simultaneously from high to low concentrations

Question 14

Glucose reabsorption

Answer 14

• Glucose is freely filtered at glomerulus and 100% reabsorbed at the proximsl tubule
• presence of glucose in urine is abnormal

Question 15

Sodium and water reabsorption in the PCT

Answer 15

• water follows salt

LOOK AT DIAGRAM

Question 16

Tubular transport maximum

Answer 16

• reabsorbed only in limited quantity over time
  -As solute concentration increases, more pumps are in use
  -There’s only a limited number of channels/transport proteins available in each membrane
  -To increase surface are for these, PCT epithelial cells have microvilli
  -[Tm] maximum rate at which a solute can be transported (reabsorbed) all channels occupied
  -This means all carrier proteins are all occupied and the excess solute cannot be transported across (saturation point reached)
  if more solute present than Tm – excretion of these solutes in urine
  -Most obvious in solutes that are 100% reabsorbed and should not be in the urine e.g. glucose

Question 17

Renal threshold

Answer 17

• The plasma concentration at which transport maximum is reached = active transport tubular carriers become saturated

Question 18

Renal handling of glucose

Answer 18

• reabsorption of glucose is proportional to plasma concentration, until saturation
• at normal filtered load (125mg/min), all glucose should be absorbed
• at renal threshold, Tm will be reached and glucose will apprear in urine

Question 19

Diabetes mellitus

Answer 19

-Diabetes mellitus: means sweet urine
-Normally, blood glucose levels are tightly controlled by insulin, a hormone produced by the pancreas
-Insulin lowers the blood glucose level
-Absence or insufficient production of insulin causes hyperglycaemia
•Type 1 diabetes mellitus: insulin is missing (loss of the insulin-producing beta cells)
•Type 2 diabetes mellitus: insulin resistance (cells don’t respond to insulin present)
-Glucose exceeds renal threshold/transport maximum, appearing in urine on urinalysis = glycosuria is abnormal
-Excessive urination (polyuria) an excessive drinking (polydipsia)

Question 20

Tubular secretion

Answer 20

• Transport of molecules from the plasma of peritubular capillaries to the lumen of renal tubules (filtrate)
• Membranes for solutes to cross are the same as for reabsorption
• Transport mechanisms same, but movement in reverse
• Secretion of ions (K+, H+), waste products and some drugs

Question 21

Excretion

Answer 21

-Metabolic process produce waste – in form of nitrogen usually
-Most nitrogenous waste from breakdown of amino acids = deamination results in the production of ammonia
-Three general ways that vertebrates get rid of nitrogenous waste
Ammonia (highly toxic), Urea, Uric acid (non-toxic)
-Mode of Nitrogen excretion depends on water availability in environment
-Elimination of a solute and water from the body in the form of urine
-Excretion rate: the rate at which a solute is excreted in urine
-A means of clinically assessing renal function
-Excretion rate depends on:
1.Filtered load
2.Secretion
3.Reabsorption rate
E = (F+S) – R

Question 22

Renal process

Answer 22

DIAGRAM

Question 23

Process of Loop of Henle

Answer 23

• Establish medullary osmotic gradient in order to create concentrated urine
• Main function of juxtamedullary nephrons
• Adjacent to vasa recta
• Outer medulla (near cortex) has the lowest osmolarity (300mOsm)
• Inner medulla has highest osmolarity (1200-1400mOsm)
• Gradient in the medulla necessary for water reabsorption
• Mechanism: counter current multiplier

Question 24

Counter current multiplier

Answer 24

• Counter-current: fluid flow in descending and ascending limbs move in opposite directions
• Creates osmotic gradient between inner and outer medulla as solutes actively pumped whilst water unable to follow
• Properties of different portions of the loop of Henle are critical to create counter current multiplier

Question 25

Counter current multipler established medullary osmotic gradient

Answer 25

• The more NaCl the ascending limb actively transports out into interstitial fluid, the more water diffuses out of the descending limb
• The more water that diffuse out of the descending limb, the saltier the filtrate becomes
• Ascending limb then uses salty filtrate to further raise osmolarity of medullary interstitial fluid
• Now the ascending limb starts with 400mOsm instead of 300mOsm

Question 26

Medullary osmotic gradient est

Answer 26

• Constant difference of 200mOsm always exists between two limbs of nephron loop and between ascending limb and interstitial fluid
• Osmolarity in descending LOH increases up to 1400mOsm – difference “multiplied”
• Osmolarity in ascending LOF always lower than descending LOH
• Osmolarity decreases in ascending LOH until hypo-osmotic (less than 300mOsm)
• Fluid entering distal convoluted tubule

Question 27

Counter current exchanger

Answer 27

• Preserves osmotic gradient
• Vasa recta reabsorbs water and solutes without undoing osmotic gradient (highly permeable)
• Flow in blood in vasa recta is also counter current (hairpin turn)
• Preservation of medullary gradient by:
  1. Preventing rapid removal of salt from interstitial space
  2. Removing reabsorbed water

Question 28

Water reabsorption in DCT and collecting ducts

Answer 28

• Fluid in distal convoluted tubule and collecting duct is hypo-osmotic
• tendency for water to move from lumen to interstitial fluid along an osmotic gradient
• Reabsorption depends on:
  1. Osmotic gradient established by counter current multiplier
  2. Water permeability of epithelium

Question 29

Urea recycling and the medullary osmotic gradient

Answer 29

-Urea helps form medullary gradient
1.Urea enters filtrate in ascending thin limb of nephron loop by facilitated diffusion
2.Cortical collecting duct reabsorbs water, leaving urea behind
3.In deep medullary region, now highly concentrated urea leaves collecting duct and enters interstitial fluid of medulla
Urea then moves back into ascending thin limb
Contributed to high osmolarity in medulla
-Water reabsorption increases urea concentration in tubular fluid
-Recirculation of urea makes it possible to achieve high concentrations of urea in urine
excrete necessary amounts of urea in a small volume of water