9.1 - Renal regulation of water and acid-base balance Flashcards
What is osmosis?
Flow of water from area of low solute concentration to area of high solute concentration across a semi-permeable membrane
What is the driving force for osmosis?
Osmotic (AKA oncotic) pressure - depends on the number of solute particles (not the size)
What is the difference between osmolarity and osmolality?
- osmolarity - amount per L of solvent (calculated)
- osmolality - amount per kg of solvent (measured)
- in practice, they are very similar
How do you calculate osmolarity?
Osmolarity (Osm/L or mOsm/L) = concentration x no. of dissociated particles
Calculate the osmolarity for 100 mmol/L glucose and 100 mmol/L NaCl.
- osmolarity for glucose = 100 x 1 = 100 mOsm/L
- osmolarity for NaCl = 100 x 2 = 200 mOsm/L –> this is because NaCl’s dissociated particles is both Na+ and Cl-
What % of body weight is fluid?
60%
Describe the body’s fluid distribution in different compartments (in %).
- 2/3 intracellular fluid
- 1/3 extracellular fluid
- 1/4 intravascular (plasma in bloodstream)
- 3/4 extravascular:
- 95% interstitial fluid (surrounds and bathes cells)
- 5% transcellular fluid (including CSF, peritoneal fluid)
What are some ways of unregulated water loss? (4)
- sweat
- faeces
- vomit
- water evaporation from respiratory lining and skin
What is the regulated way of water loss?
Renal regulation - urine production (through positive and negative water balance)
Describe the steps for positive water balance.
- high water intake
- increases ECF volume –> lowers [Na+] –> lowers osmolarity
- kidney produces large volume hypo-osmotic urine to lose water
- osmolarity normalises
Describe the steps for negative water balance.
- low water intake
- lowers ECF volume –> increases [Na+] –> increases osmolarity
- kidney produces small volume hyper-osmotic urine (compared to plasma)
- osmolarity normalises
What happens in the PCT in terms of water (and sodium) reabsorption?
67% of water (and NaCl) is reabsorbed at PCT
What happens in the thin descending loop of Henle in terms of water/NaCl reabsorption?
- 15% water passively reabsorbed (permeable to water)
- NaCl is not reabsorbed (impermeable to NaCl)
What happens in the thin and thick ascending loop of Henle in terms of water/NaCl reabsorption?
- thin ascending - NaCl passively reabsorbed
- thick ascending - NaCl actively reabsorbed
- water cannot be reabsorbed (impermeable to water)
What happens at the DCT and collecting duct in terms of water reabsorption?
- variable amount of water reabsorbed depending on body’s needs
- action of ADH kicks in to modulate aquaporin channels to vary amount of water reabsorption
How and why does water reabsorption occur passively in the kidney?
- water is reabsorbed through the passive process of osmosis and requires a gradient
- this is done as the body does not want to spend too much energy absorbing water
- the medullary interstitium needs to be hyperosmotic for water reabsorption to occur from loop of Henle and collecting duct
What is the process of countercurrent multiplication?
- filtrate arrives at loop of Henle - isoosmotic with the plasma
- active salt reabsorption - thick ascending LoH
- passive water reabsorption - thin descending LoH
- process repeats again and again –> gradient down medulla
What is the detailed, step-by-step process of countercurrent multiplication?
- filtrate arrives at LoH at 300 mOsm/L = iso-osmotic with plasma
- active salt reabsorption in thick ascending LoH - salt into interstitium so osmolarity in tubular filtrate of ascending LoH decreases 300–>200 and medullary interstitium osmolarity rises 300–>400
- passive water reabsorption in descending LoH - since interstitium osmolarity is higher, water from descending loop moves into interstitium via osmosis to equilibrate the osmolarity = causes descending loop osmolarity to increase 300–>400
- repeat!! more filtrate arrives at descending loop (at 300 mOsm/L) and pushes rest of filtrate along loop, changing the osmolarities along the loop
- active salt reabsorption (thick ascending LoH) = increased osmolarity interstitium, decreased osmolarity ascending LoH
- passive water reabsorption (descending LoH) - water from descending loop into interstitium so descending osmolarity increases until it is equal to interstitium osmolarity
- gradient in medullary interstitium already developing from outer medulla to inner medulla
Why is the process called countercurrent multiplication?
- countercurrent since filtrate flows in opposite directions in ascending and descending loops
- multiplication since process repeats again and again to achieve a proper gradient down medulla
How does countercurrent multiplication help us?
Helps water passively reabsorb into body without spending a lot of energy
In collecting duct cells, what side does the basolateral cell membrane face?
The side with the blood capillaries
In collecting duct cells, what side does the apical cell membrane face?
The lumen of the collecting duct (the inside of the tube)
What is the vasa recta?
A series of blood capillaries that surround nephron mainly in medullary region
What happens to urea after being filtered through Bowman’s capsule (urea recycling mechanism)?
- travels through nephron and reaches collecting duct
- through UT-A1 and UT-A3 transporters, the urea is transported out into medullary interstitium (concentration of urea in interstitium can be has high as 600mmol/L)
- urea in interstitium can now either:
- go into vasa recta through UT-B1 transporter which surrounds nephron so urea circulates medullary region (instead of leaving region - needed here to maintain concentration gradient)
- go into descending LoH through UT-A2 transporter where it goes back through nephron and some exits collecting duct back into interstitium again (recycling)
What is the purpose of recycling urea?
- increases interstitium osmolarity:
- allows urine concentration to occur - as water moves from collecting duct into interstitium
- urea excretion requires less water - when filtrate reaches inner medullary collecting duct it equilibrates with urea in inner medullary interstitium (as high as 600mmol/L, so concentration of urea in collecting duct could also go that high) = this urea then requires less water to excrete
- both of these methods ultimately help us to conserve water in our body
What does vasopressin do to the urea recycling mechanism?
Vasopressin boosts UT-A1 and UT-A3 numbers - increases collecting duct permeability for urea to aid urea reabsorption and recycling
What is vasopressin?
Protein with length of 9 amino acids
What is the main function of vasopressin?
Promote water reabsorption from collecting duct
What are two other functions of vasopressin?
- helps in urea reabsorption (boost UT-A1 and UT-A3)
- helps in sodium reabsorption (increase Na+K+2Cl-, Na/Cl and Na+ transporters)
Where is vasopressin produced and stored?
Produced in hypothalamus by neurons in supraoptic and paraventricular nuclei, then stored in posterior pituitary
What factors stimulate ADH production and release? (5)
- increase in plasma osmolarity
- hypovolaemia
- reduced blood pressure
- angiotensin II
- nicotine
What factors inhibit ADH production and release? (5)
- reduction in plasma osmolarity
- hypervolaemia
- increased blood pressure
- ethanol
- atrial natriuretic peptide