💧Urology💧 - Regulation of Water & Acid-Base Balance Flashcards
What is osmolarity?
Osmolarity = Concentration x No. of dissociated particles
= Osm/L OR mOsm/L
Calculate the osmolarity for 100 mmol/L glucose and 100mmol/L NaCl
Osmolarity for glucose = 100 x 1 = 100 mOsm/L
Osmolarity for NaCl = 100 x 2 = 200 mOsm/L
What is osmotic pressure proportional to?
Osmotic pressure ∝ No. of solute particles
How is body fluid distributed?
What is transcellular fluid?
Transcellular fluid is a component of extracellular fluid that is found in specialized compartments, separated from other extracellular fluids by epithelial layers. These fluids are typically involved in lubrication, cushioning, or specific physiological functions.
What are the unregulated sources of water loss?
Sweat
Faeces
Vomit
Water evaporation from respiratory lining and skin
What are the regulated sources of water loss?
Renal regulation - urine production
Outline renal regulation of water
Note [Na+] refers to concentration of sodium, not amount
How does osmolarity change within the medulla?
The deeper into the medulla, the higher the osmolarity
What is required for the movement of water?
Since water is reabsorbed through the passive process of osmosis, it requires a gradient
What conditions are needed for water reabsorption to occur in the LOH and collecting duct?
The medullary interstitium needs to be hyperosmotic for water reabsorption to occur from the Loop of Henle and Collecting duct
Where is the majority of water reabsorbed?
67% water absorbed in the PCT
Where in the nephron is water reabsorbed?
PCT (majority)
Passively in the descending LOH
Variable amounts of water in the collecting duct
What is countercurrent multiplication?
Countercurrent multiplication is the process in the loop of Henle where active reabsorption of sodium and chloride in the thick ascending limb creates an osmotic gradient in the medullary interstitium. This gradient allows passive water reabsorption in the thin descending limb, concentrating the tubular fluid.
Outline the process of countercurrent multiplication in more detail
Initial Conditions (A):
The osmolarity of the fluid in both limbs and the interstitium starts at ~300 mOsm/L.
There is no gradient yet.
Active Salt Reabsorption (B):
The thick ascending limb actively transports NaCl into the interstitium.
The ascending limb is impermeable to water, so osmolarity in the ascending limb decreases (e.g., to 200 mOsm/L), while the interstitial fluid becomes more concentrated (e.g., to 400 mOsm/L).
Passive Water Reabsorption (C):
The thin descending limb is permeable to water but not to solutes.
Water moves out of the descending limb into the hyperosmotic interstitium, increasing the tubular osmolarity in the descending limb (e.g., to 400 mOsm/L).
Gradient Amplification (D to F):
With continuous flow and repeated active salt pumping and water movement, the osmolarity gradient is progressively amplified.
The medullary interstitium becomes increasingly concentrated (up to ~1200 mOsm/L at the tip of the loop), and tubular fluid osmolarity varies along the loop.
What is meant when urea is described as an “ineffective osmole”?
Urea can diffuse easily across most cell membranes with the help of urea transporters.
As a result, urea concentrations equilibrate between intracellular and extracellular compartments, minimizing any osmotic gradient it might create
Summarise urea recycling
Urea recycling involves urea reabsorption in the collecting duct via UT-A1 and UT-A3 transporters, enhanced by vasopressin. Recycled urea contributes to the medullary interstitium’s high osmolarity, supporting water reabsorption and urine concentration. This process minimizes water loss and is essential for maintaining the osmotic gradient for concentrated urine production.
What is the first step in urea recycling?
Thin Descending Limb: Urea enters the nephron from the medullary interstitium via UT-A2 transporters.
Urea travels through the nephron to the collecting duct, where it is reabsorbed into the medullary interstitium through UT-A1 (apical membrane) and UT-A3 (basolateral membrane) transporters.
Why is urea recycled into the medullary interstitium?
Recycled urea contributes to the high osmolarity of the medullary interstitium (600 mmol/L), which is essential for water reabsorption and urine concentration
What is the role of vasopressin in urea recycling?
Vasopressin (ADH) increases the expression of UT-A1 and UT-A3 in the collecting duct, enhancing urea reabsorption
Give a brief overall view of urea recycling (diagram)
What is ADH?
Protein hormone
Promotes water reabsorption from the collecting duct
What factors stimulate ADH production and release?
Increase plasma osmolarity
Hypovolemia - decreased blood pressure
Nausea
Angiotensin II
Nicotine
What factors inhibit ADH production and release?
Decreased plasma osmolarity
Hypervolemia - increased blood pressure
Ethanol
Atrial natriuretic peptide (ANP)
Outline the mechanism of action of ADH
Binds to V2 receptor
G-protein reaction cascade
Activates adenylate cyclase, converts ATP to cAMP
cAMP promotes activity of protein kinase A
Leads to insertion of AQP2 into apical membrane
Explain the concept of acid-base balance
Diet and metabolism contribute to the production of acids and bases.
Acid is balanced against base, with base excreted in faeces.
However, there is a net addition of metabolic acid of approximately 50–100 mEq/day, which must be neutralised
What is the role of carbonic anhydrase?
Aids in the production of bicarbonate
What is the Henderson-Hasselbalch equation?
What equation is used to calculate H+ concentration?
How are new bicarbonate ions produced?
In the PCT from glutamine
Ammoniogenesis
Glutamine produces 2 ammonium ions and a divalent ion - produces 2 bicarbonate ions
Ammonium ions must be excreted to allow net addition of HCO3-
Occurs via Na+/H+ antiporter and via diffusion in the form of NH3 gas
In the DCT - protons can be neutralised by another urinary buffer (e.g. phosphate)
