Test 2 (Mechanisms to Adjust Urine Concentration) Flashcards
Renal Control of Salt and Water Balance
Crucial for regulation of:
- Blood Pressure
- Extracellular Fluid Solute Concentration
- Concentrations of Na+, K+ in Body Fluids
Normal function of these mechanisms allows:
- Water retention doing Dehydration
- Excretion of Dilute Urine when well Hydrated
- Sodium EXCRETION when Blood Pressure rises
- Sodium RETENTION when Blood Pressure falls
Failure of Renal Control of Salt and Water Balance can cause:
- Edema
- Disorders of plasma K+ Concentration: HYPERkalemia, HYPOKalemia
- Undesirable changes in Blood Pressure
- Acid/Base disorders
- Neurological Problems: Shrinking or Swelling of the Brain
Sodium Reabsorption Mechanisms
1) Proximal Tubule (50-55%):
- Cotransport with Glucose, Amino ACIDS, Phosphate
- Countertransport with H+ (Na+/ H+ Exchange)
2) Thick Ascending Limb (30-45%):
- Na+, J+, 2Cl- Cotransport
3) Early Distal Convoluted Tubule (5-8%):
- Na+, Cl- Cotransport
4) Late Distal Convoluted Tubule, Collecting Duct (2-3%):
- Luminal Na+ Membrane Channels
Water and Chloride follow Sodium
1) WATER REABSORPTION:
- Always PASSIVE; can be Transcellular or Paracellular
- Follows Osmotic Gradients established by Reabsorption of Sodium, other solutes
2) CHLORIDE REABSORPTION:
- Always linked, either directly or indirectly, to Na+ Reabsorption (Cl- can balance the + Charges)
- Specific Mechanisms differ in different segments
Loop of Henle
1) DESCENDING LIMB:
- Freely PERMEABLE to WATER
- IMPERMEABLE to Na+, Cl-
2) ASCENDING LIMB:
- Always IMPERMEABLE to WATER
- THIN Segment: NaCl Reabsorption mechanism is Controversial
- THICK Segment: Active Na+, K+, 2Cl- Cotransport (Has K+ Leak Channels that allows K+ to follow its Concentration Gradient. This established the Positive Tubular Potential)
Late DCT and Collecting Duct
* THE MAJOR SITE OF PHYSIOLOGICAL CONTROL OF SALT AND WATER BALANCE!!!!!!!***
1) ALDOSTERONE: Stimulate Na+ Reabsorption, K+ Secretion, H+ Secretion in this Segment
2) ATRIAL NATRIURETIC PEPTIDE: Inhibits Na+ Reabsorption (Medullary Collecting Duct)
3) ANTIDIURETIC HORMONE aka ARGININE VASOPRESIN (AVP) Stimulates Water REABSORPTION
Cation Transport in Late Distal Tubule, Collecting Duct
- Not the Large TRANSEPITHELIAL POTENTIAL (50mV, Lumen NEGATIVE)
- This is part of the DRIVING FORCE of K+ and H+ Secretion by the PRINCIPAL CELLS!!!!!!!!
Aldosterone Mechanisms for Increasing Na+ Reabsorption in PRINCIPAL CELLS
1) Incorporation fo Na+ Channels in Luminal Membrane
2) Incorporation of Na+, K+, ATPase Ion pumps in Basolateral Membrane
Water Permeability of Collecting Duct is Physiologically Controlled
1) Well Hydrated Individuals:
- Collecting Duct is IMPERMEABLE TO WATER
- Water remains in Tubular Lumen; Dilute Urine is Excreted
2) DEHYDRATED INDIVIDUALS:
- Collectign Duct is HIGHTLY WATER-PERMEABLE
- Water is Reabsorbed; Low Volume of Concentrated Urine is Excreted
ADH (AVP) on Late Distal Tubule and Collecting Duct
- ADH (AVP) INCREASES H2O Permeability (Reabsorption) of Late Distal Tubule, Collecting Duct via V2 RECEPTORS and Insertion of Aquaporin Channels!!!!!!!!!!!
Solute Concentrations in Particular Interstitium
- INNER MEDULLARY INTERSTITIAL FLUID has a Very High Solute Concentration
Countercurrent Multiplies Mechanism
- Concentrates Solute in Medullary Interstitial
- High Solute Concentration enables kidneys to Excrete Highly Concentrated Urine, conserve Water during periods of Dehydration
This Mechanisms requires Integrated Function of 3 Components:
1) Descending, Ascending Limbs of Henle’s Loop
2) Vasa Recta Capillaries
3) Collecting Ducts
Components of Countercurrent Multiplier
1) Na Gradient that the Na, K, Clo Cotransporter can establish in TAL is 200 mOsm/ Kg
2) Interstitial becomes HYPEROSMOLAR and pulls WATER OUT of the Descending Limb
3) Urine in Descending Limb is CONCENTRATED
4) Process Repeats
5) Augmented by action of ADH in CD
Recycling Concentrated UREA in Inner Medulla
- AVP promotes UREA Reabsorption from Inner Medullary Collecting Duct
Role of Urea in the Countercurrent Mechanism
- In the presence of ADH, Water but nor UREA is Reabsorbed in the Cortical Collecting Tubule, resulting in an INCREASE in the Tubular Fluid UREA Concentration
- In the Presence of ADH, more Water but NOT UREA is Reabsorbed in the Medullary Collecting Tubule, further raising the Tubular Fluid Urea Concentration. The Inner Medullary Collecting Tubule is relatively PERMEABLE to UREA; as a result, Urea passively diffuses into the Interstitial, INCREASING the Interstitial Osmolarity
Vasa Rects
VASA RECTA maintain SOLUTE GRADIENT
- Water and NaCl are EXCHANGED between Descending and Ascending Limbs
- Solute gradient is maintained while SMALL amounts of NaCl and Water are RETURNED to SYSTEMIC CIRCULATION
**IF the Blood Flow INCREASES, there will be a Concentration Gradient that will make out more Solutes and abolish the Concentration Gradient and not allow us to Concentrate our Urine
Antidiuresis
- HIGH AVP
- AVP makes the Collecting Duct Epithelium HIGHLY WATER PERMEABLE.
- Water is Reabsorbed in this Segment as a LOW VOLUME, HIGHLY CONCENTRATED URINE is Excreted
Flows, Solute Concentrations in Antidiuresis
NOTE: The Vasa Recta remove more Water and Solute from the MEDULLA than they bring in
Diuresis
- LOW AVP
- Here, a HIGH Volume of DILUTE URINE is Excreted
- Collecting Duct Epithelium is IMPERMEABLE to WATER
- Note the LOWER SOLUTE Concentrations in the MEDULLARY INTERSTITIUM!!!!!!!!!!!!!!
Summary of NaCL and H2O Transport throughout the Nephron during an Antidiuresis and a Water Diuresis
- The tubular Fluid and Interstitial Concentrations are expressed in milliosmoles per Kilogram (mOsm/Kg)
- Note that the Composition and Volume of the Tubular Fluid are essentially the same at the end of the Loop of Henle as the Excretion of a Concentrated or Dilute Urine is determined primarily in the Collecting Tubules
Obligatory Urine Volume (WATER LOSS)
What is minimum Daily Required to Eliminate Wastes?
- Body generates 600 Milliosmol of Waste/Day
- Maximum Urine Concentration = 1200 mOsm/ L
(600 mOsm/day)/ (1200 mOsm/L) = 0.5 L/day
What if the Kidneys couldn’t concentrate Urine above Plasma Solute Concentration?
(600 mOsm/ Day)/ (300 mOsm/ L) = 2.0 L/ day
Osmolar Clearance
Osmolar Clearance (Cosm) = Uosm x V/ Posm
***When Kidneys EXCRETE Excess Solute = Cosm INCREASES!!!!!!!!!!
**When Solute is RETAINED = Cosm Falls!!!!!!!!!
Free Water Clearance (CH2O)
DEFINITION: EXCRETION of Water in Excess of amount needed to Excrete Ososmotic Urine
- Ex: Excretion fo Sulte Free Water by the Kidneys
CH2O = V- Cosm
1) If Uosm Posm CH2O is NEGATIVE!!!!!
- Pure Water is RETAINED
Fractional Excretion
DEFINITION: The fraction (percentage) of the filtered load of a Substance that is Excreted in Urine
Fex = (Amt Excreted)/ (Amt Filtered) ——–>
= (Ux x V)/ (Px x GFR)
- If CREATININE Clearance = GFR
Fex = (Ux x V)/ (Px x [(Ucr x V)/ Pcr]) ———–>
= (Ux x Pcr)/ (Px x Ucr)
Fraction Excretion Example
Ex: In a Spot Urine Sample, Fractional Excretion fo Na (FE Na) = (UNa x Pcr)/ (PNa x Ucr) x 100
***BELOW 1% = PRERENAL and AGN —> Na AVIDLY REABSORBED!!!!!!!!!!
***GREATRE Than 2% = ATN, RENAL!!!!!!!!!!
