Urinary9 - Corticopapillary Gradient & Countercurrent Exchange

Question 1

4 features of urine osmolarity

Body Fluids
Normally Hydrated Person
Variable Excretion
Extreme Values

Answer 1

1.) Body Fluids - most body fluids are isotonic to cells osmolarity at 280-310 (300) mOsm/kg

2. ) Normally Hydrated Person
   - urine osmolarity is 500-700 mOsm/kg
   - it fluctuates depending on water intake
3. ) Variable Excretion - urine can be dilute or conc.
   - 1000 mOsm/kg can be excreted as 100 mOsm/kg in 10 litres (dilute) or as 1000 mOsm/kg in 1 litre of urine (concentrated)
   - urine concentration is inversely proportional to volume of urine produced
4. ) Extreme Values
   - minimum concentration is 50 mOsm/kg
   - maximum concentation is 1200 mOsm/kg

Question 2

4 features of generating a vertical concentration gradient (medullary countercurrent mechanism) in the kidney

Osmolality Gradient
3 Structures Involved

Answer 2

1. ) Osmolality Gradient - vertical osmotic gradient in the interstitial fluid of the medulla from the corticomedullary border to the papilla
   - isotonic (300 mOsm/kg) at the cortex
   - hyperosmotic (up to 1200 mOsm/kg) at the papilla
2. ) Juxtamedullary Nephron (JMN) - longer loop of Henle helps to create the vertical osmotic gradient
   - responsible for making concentrated urine
3. ) Vasa Recta - capillary network that supplies the JMN
   - it helps to maintain the generated osmotic gradient

4.) Collecting Ducts - use the gradient along with ADH to produce urine of varying concentration

Question 3

Process of generating the vertical concentration gradient

Initial Scene
Ascending Limb
Descending Limb
Fluid Movement
Recalibration
Final Osmotic Gradient

Answer 3

1. ) Initial Scene - osmolality of the descending limb, interstitium, ascending limb are isotonic (300 mOsm)
   - this is seen in transplant patients or patients on prolonged loop diuretics
2. ) Ascending Limb - active Na+ reabsoprtion only
   - establises a maximum 200 mOsm gradient at each horizontal level between the interstium and the nephron
3. ) Descending Limb - passive water reabsorption only
   - water enters interstitium to match the osmolarity of the interstitium on the same horitzontal level

4.) Fluid Movement - as fluids flows fowards, new isotonic fluid enters the descending limb whilst the old, now hyperosmotic fluid moves up the ascending limb

5. ) Recalibration - reabsorption of Na+ and water to reestablish the 200 mOsm gradient at each horizontal level
   - fluid flows fowards and the cycle (4 and 5) repeats
6. ) Final Osmotic Gradient - process continues until:
   - the intersitial fluid at the top of the descending limb (corticomedullary border) is isotonic (300 mOsm)
   - the fluid at the bottom of the loop of Henle (papilla) reaches maximum concentration (1200 mOsm)

Question 4

3 features of urea in the juxtamedullary nephron

Effective Osmole
Uptake
Recycling

Answer 4

1. ) Effective Osmole - exerts an osmotic force on membranes as it can’t freely pass them in the kidney
   - relies on urea transporters
   - aids water movement by concentrating interstitium
2. ) Urea Uptake - occurs in the PCT
   - basolateral Na pump generates Na conc gradient
   - urea enters the cell w/ Na+ via the Na-urea symporter
   - urea then enters ECF via facilitated diffusion
3. ) Urea Recycling - urea can leave the CD via aquaporin-1 and concentrate the interstitium
   - this aids water reabsorption from the descending limb
   - transported back into the ascending limb (‘recycled’)
   - dehydration = more ADH = more urea recycling

Question 5

6 features of the vasa recta

Location
Function
Flow Direction
Flow Speed
Descending Limb
Ascending Limb

Answer 5

1. ) Location - found only in juxtamedullary nephrons
   - branch from the EA which descends into the medulla

2.) Function - provides nutrients and oxygen to keep the medullary tissue alive

3. ) Flow Direction - opposite to tubular fluid flow
   - hair-pin configuration allows blood to equilibrate at each horizontal level, maintaining the conc gradient
4. ) Flow Speed - very low flow, only 5-10% of total RPF
   - slow flow gives time for the ions to equilibrate at each horizontal level, maintaining the conc gradient
   - comprimises need to deliver nutrients and need to maintain hypertonicity
5. ) Descending Limb - Na+, Cl-, and urea diffuse into the blood from the ascending loop of Henle
   - osmolarity of blood increases as it reaches the tip of the hairpin loop

6.) Ascending Limb - water moves into highly concentrated blood from the descending loop of Henle

Question 6

4 features of osmoreceptors and their effect on plasma osmolarity

Location
Structure
Primary Response
Secondary Response
Blood Volume vs Osmolarity

Answer 6

1. ) Location - OVLT of the hypothalamus
   - lies close to cells of the supraoptic nucleus which have input from baroreceptors to control BP

2.) Structure - fenestrated leaky endothelium exposed directly to systemic circulation on plasma side of BBB

3. ) Primary Response - detects small changes in osmolarity, altering ADH release from posterior pituitary
   - controls the concentration of urine
4. ) Secondary Response - triggers thirst sensation during severe dehydration (or increase in salt)
   - causes the brain to signal to you to drink more water
5. ) Blood Volume vs Osmolarity
   - during low BP, maintaining volume is more important
   - during high BP, osmolarity is more important

Question 7

3 features of diabetes insipidus

Definition
2 Types

Answer 7

1.) Definition - kidneys unable to retain water causing polyuria and polydipsia

2. ) Central Diabetes Insipidus - low plasma ADH levels
   - caused by damage to hypothalamus or pituitary gland
   - e.g. brain injury, tumour, sarcoidosis, TB, aneurysm, forms of encephalitis or meningitis
   - managed by ADH injections or ADH nasal sprays

3.) Nephrogenic Diabetes - acquired insensitivity of the kidney to ADH

Question 8

2 features of syndrome of inappropriate ADH secretion (SIADH)

What is it?
Defining Characteristic

Answer 8

1. ) Excessive release of ADH from the PP gland or another source
2. ) Dilutional Hyponatriema - plasma sodium levels are lowered but total body fluid is increased