Regulation of Na, osmolarity and ECFV Flashcards
Unregulated fluid loss occurs due to
- sweat
- stool
- respiratory
(insensible)
Regulated fluid loss occurs due to
Renal
- urine can vary from 50-1200 mOsm
- measured as specific gravity (1.002-1.050)
- obligate urine volume to eliminate waste products
Fluid intake
Thirst
- stimulus generates desire for and allow the intake of water
Electrolyte loss
Occurs with accompanying water loss
- unregulated: sweat, stool
- regulated: renal
Electrolyte intake
Cravings or hunger
- depends on availability
Why is the electrolyte balance across cell membranes essential?
- partitioning water between intra and extracellular fluid spaces
- keeping cells from shrinking/swelling
- allows for electrical charge related actions
In a person after drinking 1 liter of water:
Little change in - plasma mOsm - urine electrolyte conc. Big change in - urine mOsm - urine production *primarily regulated by ADH in the distal segments of the nephron*
What happens when there is excess water in the body?
Body fluid osmolarity is reduced, kidney excretes urine with a low osmolarity
What happens when there is a deficit of water in the body?
Extracellular fluid osmolarity is high, kidney excretes urine with a high concentration
Kidney can excrete a large volume of dilute urine or a small volume of concentrated urine without major changes in ______
Rates of excretion of solutes (Na and K)
____ of sodium and water excretion is regulated reabsorption
10%
- one can be regulated independently of the other
Formation of dilute urine
Decrease reabsorption of water
How do you decrease reabsorption of water?
Decrease ADH release, which decreases water permeability of the distal tubule, cortical and medullary duct
In the proximal tubule, water and solutes are reabsorbed ______
At the same pace (iso-osmotic)
Water reabsorption in the thin descending loop
Passive due to interstitial concentration gradient
- unregulated
- could get renal medullary washout due to overconsumption of water
When the renal medulla is no longer hypertonic, the kidney can no longer ______
Concentrate urine
Remaining urine after the thick ascending loop becomes more ____ without _____
Dilute; any increase in volume
The ability to form concentrated urine is dependent on what 2 things?
- ADH: production in the CNS and appropriate tubular response
- renal medullary hypertonicity
Continuous reabsorption of electrolytes adds to _____
Increasing renal medullary hypertonicity
What are the 2 basic requirements for forming concentrated urine?
- high level of ADH
- hihg osmolarity of renal medullary interstitial fluid
How does increased reabsorption of water occur?
Increase ADH release
- increases water permeability of distal tubule, cortical and medullary collecting ducts
- increases urea permeability of medullary collecting duct, which increases medullary tonicity
Countercurrent multiplier mechanism
Process by which renal medullary interstitial fluid becomes hyperosmotic
- depends on arrangement of loops of Henle and vasa recta
Factors contributing to renal medullary hypertonicity
- active transport of solutes from thick ascending limb into medullary interstitium without reabsorption of water
- active transport of Na from medullary collecting ducts into interstitium (increased with aldosterone)
- facilitated diffusion of urea from medullary collecting ducts into interstitium (ADH)
- diffusion of small amounts of water into medullary interstitium, which is rapidly removed
- counter-current multiplier mechanism
Loop of Henle is only able to establish a ______ gradient
200 mOsm, before back diffusion of electrolytes results in equilibration
Why are juxtamedullary nephrons most susceptible to NSAID toxicity?
O2 tension decreases the deeper you go into the renal medulla, but the tubular epithelial cells are the most metabolically active as we progress to dehydration
What is a major reason for high medullary osmolarity?
Active transport of sodium and co transport of potassium, chloride, etc from thick ascending loop into the interstitum
Parallel current flow transfer is dependent on
- diffusion constant
- time/flow ratio
- -> less transfer than counter current flow
- -> once equilibrium is achieved there is no further exchange
Counter current flow is dependent on
- diffusion constant (same)
- time/flow ratio
- -> overall greater transfer than parallel flow (utilizes whole contact area)
- -> never reaches equilibrium, will always have a gradient directing flow
What helps preserve a high medullary interstitial fluid osmolarity?
The fact that the large amounts of water reabsorbed from the cortical collecting tubule is being reabsorbed into the cortex, instead of the medulla
What causes urea to diffuse out of the tubule into the renal interstital fluid?
High concentration of urea in the tubular fluid of the inner medullary collecting duct
- diffusion facilitated by urea transporters that are activated by ADH
Rate of urea excretion is determined by what 3 things?
- concentration of urea in the plasma
- glomerular filtration rate
- renal tubular urea reabsorption
Where does secretion of urea occur?
Thin loop of Henle, from the medullary interstitium
- also facilitated by urea transporter
What are 2 features of medullary blood flow that prevent the increase in solute concentration from being absorbed and washed away?
- renal medullary blood flow is only <5% of total renal flow
- vasa recta serves as countercurrent exchangers, minimizing washout of solutes from medullary interstitium
Due to hypertonicity of renal medulla, the concentration gradient favors _____
Electrolyte movement into the capillary as blood flows out
The faster the blood flows, the less time there is to equilibrate which leads to _____
Renal medullary washout
Does the vasa recta create the medullary hyperosmolarity?
No, but it prevents it from being dissipated
There is an obligatory ____ required to eliminate metabolic waste products
Water load
What is the obligatory urine volume dependent on?
- solute load that needs to be eliminated
- max concentrating ability of the kidney
_____ the amount of solute and you have to _____ the amount of volume needed to excrete the solute
Increase; increase
What controls ADH secretion?
- Na
- osmoreceptor cells stimulate secretion of ADH in response to increases in osmolarity
- projections from baroreceptors in aortic and carotid sinuses in response to decreased blood volume/pressure
- neurons that project to thirst centers stimulate drinking
Alpha-2-agonist
Common class of sedative, causes increased urination due to blocking of ADH
Osmolarity has a ______ impact on ADH secretion and thirst stimulation than pressure/volume
Greater
- 2-3% increase mOsm = 10-12% decrease ECF in effecting increase ADH
Isotonic volume depletion has ______ than hypotonic volume depletion
Less impact
Blood loss results in loss of fluid and Na ____
Equally
- no change in osmolarity
How to correct dehydration in a horse undergoing an endurance race?
- sweat is isotonic, so effect of the osmole receptor neurons is not triggered
- give oral electrolytes to increase osmolarity of the ECF and stimulate the thirst response
Why does water follow Na when aldosterone stimulates Na reabsorption?
Reabsorption of Na increases plasma Na concentration
Blocking the ADH system can result in large changes in _______
Osmolarity, as Na intake changes
Impairments of urine concentrating ability can cause large changes in _____
Osmolarity
Loss of solute gradient in renal medulla, causes failure of the counter-current exchange mechanism is the cause of what?
Renal medullary washout, or tubular failure to extract electrolytes
Diabetes insipidus
Lack of production or response to, ADH
- patient will take in large amounts of water and salt in an effort to maintain blood volume
- will lead to renal medullary washout, impairing ability to conserve water
ECF volume regulation: local renal mechanisms
- changes in GFR
- changes in tubular reabsorption
- changes in tubular secretion
- changes in hormones (local)
ECF volume regulation: systemic mechanisms
- changes in hormones (systemic)
- changes in sympathetic activity
- changes in blood pressure
- changes in blood composition
ECF volume is mainly determined by _____
Intake and output of water and electrolytes (esp. Na)
____ of the water we take in is excreted as urine
30-50%
If total GFR were to drop by 50%, the remaining nephrons will
Have to double their GFR to compensate
Angiotensin summary
- stimulates aldosterone
- constricts efferent arterioles
- directly increases sodium reabsorption
- increases systemic bp (vasoconstrictor)
Angiotensin helps regulate what 3 things by regulating sodium retention?
- blood pressure
- blood volume
- extracellular fluid volume
Aldosterone summary
- stimulate Na reabsorption
- H2O tends to follow
- complete lack of results in relying only on pressure natriuresis, requires close management
ADH summary
- promotes water retention
- excess ADH does not result in great increases in blood volume
- complete lack of results in relying only on pressure diuresis, requires close management
ANP summary
- excretion of more water and sodium
- increased levels directly inhibits reabsorption of sodium and water by the renal tubules, especially in the collecting ducts
- also inhibits renin secretion and angiotensin formation, reducing renal tubular reabsorption
Sympathetic input plays a role in ____
Moderate to severe dehydration or loss of blood volume due to hemorrhage
- pressure receptors in large blood vessels activate sympathetic nervous system when blood volume decreases
Pressure natriuresis and diuresis are important regulators of
Over consumption and have an impact over a very wide range of intakes and will be effective even without autoregulation
The balance between ECF and blood volume is controlled by what?
Starlings forces
Causes of edema formation
- increased hydrostatic pressure
- decreased colloidal pressure
- increased capillary permeability due to inflammation/prostaglandins or endotoxemia
- lymphatic drainage due to trauma
Glomerulonephritis is a common cause of __________
Hypoalbuminemia
