Regulation of Body Fluid Flashcards
Why is important to regulate extracellular fluid volume?
ECFV determines the plasma volume. Plasma volume determines the circulatory filling pressure and therefore the cardiac output.
What determines the ECFV?
ECFV is determined by the total body Na content.
- ECFV is equal to amount of ECF Na divided by plasma Na concentration.
- So at constant plasma Na concentration, ECFV is proportional to total body Na.
T or F: Plasma Na concentration reflects the ECFV
FALSE
- Plasma Na concentration does not reflect the ECFV, because plasma Na is always kept constant by ADH-mediated regulation of water reabsorption in the kidney.
-*ECFV is INDEPENDENT of plasma Na
When is plasma Na concentrations altered
Only when gain or loss of Na exceeds the thirst mechanism and kidney’s ability to correct it.
What is a good indicator of changes in Na balance?
a change in BW over a short period
- patients with renal failure and under dialysis are required to monitor their BW on a daily basis.
- can use the values of BW changes to calculate how much dialysis might be required.
What are a few pathophysiologic conditions that can cause Na-imbalance?
- Diarrhea
- Excessive sweating
- Diuretics
signs and changes in ECFV deficit (hypovolemia)?
Causes: diarrhea/vomiting, severe burn or excessive sweating.
- associated with hypotension (both systolic and diastolic), especially orthostatic hypotension.
- The pulse is increased and low body temperature.
Signs and changes in ECFV expansion (Hypervolemia)?
Causes: chronic renal failure, heart failure due to excessive salt and water retention
- associatd with edema , but this requires an increase of ECFV by 2-3 L
- More severe increase in ECFV is likely to result in pulmonary edema.
- change in heart sounds (S3 gallop)
- distention of large veins
Fact:
The body rapidly responds to changes in plasma osmolarity via water excretion or retention
Thirst and ADH or AVP mechanisms respond quickly
- 1-2 hours
How does the speed of renal water excretory system compare to that of the renal sodium excretory system?
Unlike the renal water excretory response, Renal Na excretory response is relatively slow and takes several days to fix total body Na content.
High salt intake leads to what?
Increased ECFV => inc plasma volume => Increased BP
Low salt intake leads to what?
Decreased ECFV => dec Plasma volume => Increased BP
Types of ECFV receptors
- stretch receptors
a. Neural Stretch receptor
b. Atrial stretch receptor - Baroreceptors
How does the neural stretch receptors function in regards to body fluid balance?
Respond to mechanical stretch due to venous distention.
- Stimulated nerves sends signal to pituitary gland to suppress ADH secretion into plasma
- Reduced plasma ADH suppresses water reabsorption in the CD and Na reabsorption in the thick ascending limb of LOH
- result is increased excretion of Na and water in the urine.
How does the atrial stretch receptors function in regards to body fluid balance?
Respond to distention.
- sends signal via parasympathetic fibers to a variety of centers to suppress ADH secretion
- sends signals via synthetic firing to kidney and cardiovascular centers.
- Distention of atria also secrete atrial natriuretic peoptide (ANP) - stimulates renal Na excretion.
How do baroreceptors function in regards to body fluid balance?
They are located in arteries and respond to increase in arterial blood pressure.
- send signals to pituitary gland and control ADH secretion and regulate Na excretion.
Factors that regulate Sodium excretion in the kidney?
- Changes in GFR
- proportional to Na secretion - Aldosterone
- prevents Na secretion - ANP
- promotes Na secretion - renin-angiotensin system
- prevents Na Secretion - Others: Sympathetic NS and prostaglandins
What is pressure natriuresis?
a feedback regulatory mechanism for an increase in arterial pressure
- changes in BP leads to changes in Na excretion
T or F: Large changes in BP are required to alter sodium excretion
FALSE
- Small change in pressure at physiologic condition results in dramatic change in Na excretion
- due to steep renal function curve
T or F: Aldosterone response is slow
True
- The effect of aldosterone is slow as it involves changes in gene expression and not likely to play role in rapid changes in Na excretion (e.g. Bolus IV of isotonic saline or Hemorrhage)
ANP:
- Source?
- Target?
- Actions on that target?
- cardiac atria - when plasma Na high, or atrial distension
- NPR1 and 2 in both the DT AND the renal arterioles
- Renal arterioles: (Dilates aff. and constricts eff arterioles => inc GFR and Na secretion)
- DT: Inhibits reabsorption (Na-Cl cotransporter in DCT and ENAC in CD via phosphorylation)
Effects of ANP
- Inhibits renin secretion (macula densa)
- Inhibits aldosterone secretion form adrenal cortex
- It increases GFR and inhibits Na reabsorption and therefore, increases urinary Na excretion.
MOA of RAAs (specifically on nephron)
Ang II increases Na reabsorption by two mechanisms.
- Activates Na-H Exchanger in the PT to increase Na reabsorption.
- stimulates Aldosterone secretion from the adrenal cortex => inc Na reabsorption via Na channels in the DT.
- Net result is expansion of ECFV.
Sympathetic effect on renal Na reabsorption (relatively non-physiologic conditions)
- Decreases GFR (may be affected by autoreg)
2. inc proximal Na-reabsorption
Effect of the following on renal Na-rabsorption?
- prostaglandins
- bradykinin
- dopamine
- ouabain-like factors
diuresis and natriuresis
