test 10 part 2 Flashcards
(16 cards)
ECF volume determined by
Balance between intake and output of water and salt
Intake controlled by
Habit, not physiological
mechanisms
Kidneys must adapt based on
intake
- so ECF volume and osmolarity does not change
Sodium Intake and Excretion
- Must excrete the amount of sodium that is ingested
Excretion by kidneys determined by intake
When kidney function is compromised
Blood pressure changes
Circulating hormones change
Sympathetic nervous system activity changes
- Balance can be restored even if with major changes in renal function
Sodium Excretion controlled by
Altering GFR or tubular reabsorption rate
- Excretion = Glomerular Filtration – Tubular reabsorption
A 5% INCREASE IN GFR (189 L/DAY) WOULD CAUSE
- A 9 L/DAY INCREASE IN URINE VOLUME!!! (1.5 to 10.5 L/day)
• If no change in tubular reabsorption – There has to be a compensatory mechanism
• GFR 180 L/day
• Tubular Reabsorption 178.5 L/day
Buffering Mechanisms
- Limit a drastic change in sodium urinary excretion during disturbances altering GFR or tubular reabsorption
- do not eliminate the change
Do not, cannot provide 100% compensation!!!
Other feedback mechanisms are activated to then help bring intake rate and excretion rate back into balance
Affect sodium and water balance
Also affect extracellular fluid volume
Example of buffering mechanism
Renal vasodilation causing increased GFR
Glomerulotubular Balance: Increased reabsorption of filtered sodium
Macula Densa Feedback: Increased sodium chloride in distal tubule causes afferent arteriolar constriction, returning GFR to normal
Pressure Natriuresis and Diuresis: acute increase in pressure
Increase in blood pressure by 30-50 mmHg causes 2-3 fold increase in sodium output
Independent of changes in sympathetic tone or changes in hormone concentration
Pressure Natriuresis and Diuresis: chronic increase in pressure
Effect greatly enhanced
Suppressed renin release -> decreased formation of angiotensin II and
aldosterone
Inhibited renal tubular reabsorption of sodium
Can produce huge increase in sodium excretion
Renal-Body Fluid Feedback
Feedback mechanism involving: ECV; CBV; cardiac output; arterial pressure; and urine output
Controlled at same time but as separate parts
When sodium / fluid intake changes mechanism helps to reduce changes to CBV, ECV, blood pressure
Small change CBV -> Bigger change in CO
Small change CO -> Bigger change in BP
Small change BP -> Bigger change in urine output
Requires NORMAL renal function
Daily fluid intake changes
- we can increase our daily fluid intake per day causing little to no change to blood volume (NOT EXTRACELLULAR FLUID VOLUME)
- fluid intake to less than a liter per day = death (because you need to excrete a certain amount of water every day to get rid of wastes)
Additional nervous and hormonal mechanisms that will change sodium / water excretion to match intake without big changes in cardiac output or blood pressure
Low Fluid Intake / Blood Loss
Causes opposite sequence of events
Even a small decrease in BP causes a large decrease in urine output
Fluid balance maintained with minimal changes in blood pressure, blood volume, or ECF volume
Control of ECV and CBV
Usually controlled in parallel
Some circumstances cause accumulation of fluid in interstitial spaces
- pressure in and outside of the capillary, interstitial fluid, and plasma as well as capillary permeability and lymphatic obstructions
Distribution of ECF between Blood and Interstitial Spaces when blood volume increases
Small increases in blood volume
20-30% remains in blood and increases blood volume
Large increases in blood volume (30-50% above normal)
Almost all the fluid goes into interstitial space and little stays in blood
Once interstitial fluid pressure rises to a positive value, the tissue spaces become compliant and fluid pours in
Huge increase in volume with minimal increase in pressure
Act as an “overflow” reservoir for excess fluid to protect CV system (10 to 30 Liters)
