Concentration and Dilution of Urine (B2: W5) Flashcards
What is the osmolarity throughout the whole length of the proximal tubule?
300 mOsm
What is the purpose of using energy to move salt back into the body in the proximal tubule?
Creating an isotonic urine
- Water follows salt in equal proportion
- Osmolarity is 300 the entire length
- Isotonic reabsorption
Describe the movement of salt in the ascending limb of the loop of Henle
Na/K/Cl transporter moves salt back into the body
- As you move salt, water is stuck
- Hypotonic urine comes out from ascending limb
- 150-250 mOsm
Why do we need a counter current multiplier system in the kidney?
To produce hypertonic urine!
What are the key components for creating hypertonic urine?
ADH and interstitial osmotic gradient
Describe the interstitial osmotic gradient that is present from the cortex to the papillary region of the kidney
- There is a gradient in the entire kidney
- In the cortical region, it is 300 mOsm
- As you go down, increases up to 1200
How is the interstital osmotic gradient generated by the counter current multiplier?
- Entire loop begins at same concentration (300)
- Pump in ascending limb moves NaCl out
- Water gets trapped
- Equilibration in the permeable descending limb
- Passive secretion
- Tubular flow shifts gradient, and cycle repeats
- Ultimately: an increase in osmolarity from 300-600
- Also, consider urea
Loop of Henle creates ionic osmotic gradient
What is the role of the vasa recta?
- Provide nutrients
- Remove excess salt and water from the medullary/papillary region
- Do so in a manner as not to wash out the interstitial gradient
- Does not create the interstitial gradient
How are the vasa recta able to remove excess byproducts?
Starling forces
- Oncotic pressure and hydrostatic pressure allow reabsorption to occur
- Flow out is twice the amount going in
- Take away salt and water
How much of the plasma flow exiting the glomerular capillaries goes to the tubule vs the peritubular capillaries?
20% to tubule
80% to peritubular capillaries
What portion of the renal blood flow goes to the renal cortex vs the medullary/papillary region?
90% to crotical region
10% to medullary region
Is the filtration coefficient (Kf) of glomerular capillaries greater than that of skeletal muscle capillaries?
Yes
How is urea reabsorbed?
Urea is reabsorbed in the inner medullary collecting duct during antidiuresis by facilitated diffusion
- UT1
- UT4
What activates the urea transporter?
ADH
- When saving water, urea goes back into body
- Urea is flow dependent
Describe the movement of urea throughout the nephron
- Freely filtered - 100% in the glomerulus
- Passively reabsorbed in the proximal tubule - 50%
- Permeability is low in the distal tubule - 110%
- Reabsorbed in the inner medullary collecting duct
- Some goes back to vasa recta
- Some goes to loop of Henle
- 40% excreted
From where is ADH released?
Posterior pituitary
- Paraventricular neurons
- Supraoptic neuron
Controlled by osmolarity
How does ADH create aquaporins?
- Binds to V2 receptors: coupled to Gs
- Adenylyl cyclase → cAMP → PKA → protein phosphorylation
- Phosphorylation of aquaporins present in vesicles
- Exocytosis - sit themselves in the luminal membrane
- Also enhances synthesis of aquaporins
How does water move in the case of diuresis (low ADH)?
- Hypotonic concentration in distal tubule
- Osmolarity decreases because water is not moving out (some salt does)
- Results in dilute, copious urine
- Urea concentration is low
- Urea clearance is high
- Overall osmolarity is low
- Flow is high
How does water move in the case of antidiuresis (high ADH)?
- Hypotonic solution enters from distal tubule
- Aquaporins allows water to leave through collecting duct
- Salt is removed
- Water goes toward particles
- Urea is stuck until inner medulla
- Urea particles go into vasa recta and into loop of Henle
- Results in very little urine
- Urea concentration is high
- Urea clearance is low
- Urine osmolarity is high
- Flow is low
Is the osmotic gradient in the interstitiam greater during water diuresis or antidiuresis?
During antidiruesis
- Urea contributes more to the gradient during antidiuresis than during water diuresis
What is the major non-electrolyte coming out of urine?
Urea
It is non-charged
What determines plasma osmolarity?
Plasma osmolarity is related to how much you are drinking
- If you drink a ton, your osmoalrity should not be in the normal range - should be lower
Why is urine yellow?
Bilirubin - breakdown of heme
