Review of Renal Physiology (pgs 1-17 in handout) Flashcards
In a 70kg person, what is the: TBW? ICF? ECF? Intravascular Fluid? Extravascular Fluid? Plasma?
TBW- 42L ICF-28L (40% of weight) ECF- 14L (20% of weight) IVF-4.9L (7% of weight) EVF-11L (75% of ECF) Plasma- 3L (25% of ECF)
How does body composition change as we age?
-as age increases total body fluid decreases
How does the osmolarity of the ECF compare to that of the ICF?
essentially the same (solute composition differs)
What do rapid gains and losses in body weight reflect?
a change in total body water
What percentage of CO perfuses the kidneys (RBF)?
20% (1-1.2L)
What percentage of renal blood flow (RBF) is renal plasma flow (RPF)?
55% (600-700ml)
What percentage of renal plasma flow (RPF) is GFR?
20% (125ml)
Approximately how many times per day is the ECF volume filtered through the kidney?
10x
Approximately what percent of the daily GFR (180L) is excreted in urine?
1-2L/day (0.5-1%)
Which solute in the ECF primarily determines ECF volume?
Na+ (more Na, more ECF volume)
When ECF volume expands how does the kidney compensate?
increases Na output and therefore water output leading to ECF volume contraction –> compensatory decrease in Na and water output
*This does not change GFR
Severe ECF volume contraction (dehydration) can cause a decrease in what?
GFR
How is Na balance regulated?
by adjusting urine Na output to match Na intake
Na intake restricted–>gradually increasing Na reabsorption until lower Na output in urine is achieved
What happens to ECF volume when Na intake is reduced?
ECF volume contracts in an amount equivalent to the volume of urine needed to eliminate the excess Na isosmotically
An imbalance in hydrostatic and oncotic pressure across the capillary wall inducing a shift of fluid from intravascular space to extravascular space
Edema
How does the circulating volume change in edema and what does it do physiologically?
- decreases
- activates the renin-angiotensin-aldosterone system–>increase in sodium retention–>maintenance of the edema
All of the following are potential causes of what? CHF Liver Disease Nephrotic Syndrome Pulmonary Edema
Edema
How do diuretics work in treating edema?
they force an increased elimination of Na and water–>decrease in hydrostatic pressure and increase in oncotic pressure–>absorption of edematous fluid
Renal Handling equation
Excretion= Filtration - Reabsorption + Secretion
What is produced in the glomerulus?
an ultrafiltrate of the blood with similar solute concentration to the plasma (-protein)
Where is the proximal tubule located and what is its function?
- in the cortex
- reabsorbs 66% of filtered Na, 67% of K+ and most of filtered Bicarb
- This is a leaky epithelium so reabsorption is isosmotic and 66% of the filtered water is also reabsorbed
Where is the loop of henle located and what is its function?
- thin desc. and asc. in med, thick ascend, in medulla and cortex
- 25% of filtered Na, and 20% of K+ reabsorbed in thick asc limb by the Na/K/2Cl contransporter w/o water**–>drives the counter current multiplication of interstitial solute concentration
Where is the distal tubule located and what is its function?
- in the cortex
- reabsorption of 6% of filtered Na via Na/Cl cotransporter at luminal membrane and Na/K ATPase at basolateral membrane
- *Na/Cl cotransporter is target of thiazide diuretics**
- impermeable to water
- late distal tubule-last part of the nephron to control ion composition of the urine
Which hormone regulates Na reabsorption and K+secretion in the distal tubule? Water reabsorption?
- aldosterone
- ADH
Where is the collecting duct located and what is its function?
- outer cortex and inner medulla
- reabsorption of water induced by ADH
What is the filtration fraction?
fraction of plasma flowing through the glomeruli which is ultrafiltered to form tubular fluid (GFR/RPF)
~.20 (20%)
How is the fractional excretion of water calculated? (FEwater)
= urine flow rate (V)/GFR
-fraction of glomerular filtrate not reabsorbed from the tubular fluid
How can GFR be measured?
=Clearance of inulin= (Urine Inulin)(urine flow rate)/(Plasma inulin)
How can the fractional excretion of Na be calculated?
=(Urine Na X Plasma Cr) / (Urine Cr X Plasma Na)
When in water and Na balance, approximately how much of the filtered water and Na is in the urine?
1%
When in NEGATIVE water balance, approximately how much of the filtered water is in the urine?
<1%
When in POSITIVE water balance, approximately how much of the filtered water is in the urine?
> 1%
In which 2 areas of the nephron is K+ reabsorbtion/secretion NOT affected by dietary K+ intake? which areas are affected by dietary K+ intake?
- are NOT affected-proximal tubule and thick ascending loop of henle
- are affected- distal tubule and collecting duct
What happens in negative K+ balance?
- it is induced by a low K+ diet
- decreased secretion and increased reabsorption of K+
What happens in positive K+ balance?
- induced by normal or high K+ diet
- increased secretion and decreased reabsorprtion of K+
What is the effect of alkalosis on K+ secretion? Acidosis? Diuretic induced increase in tubular flow?
- alkalosis-increases K+ secretion
- acidosis- decreases K+ secretion
- Diuretic induced increase in tubular flow-increases K+ secretion
How does the level of Na+ affect the secretion of K+?
Very important point from lecture
- an increase in Na+ reabsorption in the late distal tubule and collecting duct is functionally linked to an increase in K+ secretion–>increase in K+ excretion
- *This is why patients taking loop diuretics are at risk for hypokalemia and instructed to increase their K+ intake**
What drives the Ca and Mg reabsorption across the tubular epithelium in the thick ascending loop of henle?
- K+ leaks back into tubule lumen after being reabsorbed by Na/K/2Cl cotransporter
- Cl- efflux across basolateral membrane
- ->Lumen + potential difference–>drives Ca and Mg reabsorption
The solute reabsorption in the thick and thin ascending loop of henle without water reabsorption leads to….
dilution of the tubular fluid and the capacity to make a dilute hyposmotic urine
What does the preservation of ECF volume depend on?
the ability of the kidney to concentrate or dilute the urine in response to ECF volume contraction or expansion
What does a hypertonic urine indicate?
the kidney is responding to ECF volume contraction by retaining water in excess of solutes (free water)
What does hypotonic urine indicate?
the kidney is responding to ECF volume expansion by excreting water in excess of solutes (free water clearance)
Does ADH increase or decrease in response to an INCREASE in plasma osmolarity?
ADH increases with increase osmolarity–>retention of water–>hypertonic urine (- free water clearance)
Does ADH increase or decrease in response to a DECREASE in plasma osmolarity?
ADH decreases with decrease in osmolarity–>elimination of free water by kidney–>hypotonic urine (+ free water clearance)
What is free water clearance (C h2o) defined as?
the rate at which the kidney can retain or eliminate water free of solutes
When C h2o (free water clearance) is equal to 0 is the urine hypotonic, isotonic, or hypertonic? >0?
=0-urine is isotonic
>0- urine is hypotonic
How does Aldosterone affect the late distal tubule?
1) diffuses into the cell from basolateral membrane
2) binds intracellular receptor
3) enters nucleus and induces transcription of mRNA for membrane transport proteins mediating Na reabsorption
4) increased Na reabsorption
