Renal Flashcards
(162 cards)
1
Q
What are the functions of the kidney?
A
Regulate blood pressure, regulate red blood cell production, regulate water and electrolyte balance, excrete metabolic waste products and foreign chemicals, regulate body fluid osmolality and electrolyte concentrations, endocrine gland, acid-base balance, gluconeogenesis
2
Q
How much of the cardiac output do the kidneys receive?
A
22%
3
Q
What is the functional unit of the kidney?
A
The nephron
4
Q
Each kidney contains up to how many nephrons?
A
A million
5
Q
At what age do the kidneys stop regenerating new nephrons?
A
40
6
Q
What are the juxtamedullary nephrons and what percentage do they make up?
A
have glomeruli deep in the renal cortex near the medulla, 20-30%
7
Q
What are cortical nephrons and what percentage do they make up?
A
have glomeruli located in the outer cortex, 70-80%
8
Q
Describe the flow through the nephron.
A
Blood enters kidney via renal artery -> arteries -> afferent arterioles-> glomerulus -> efferent arterioles -> small veins -> renal vein
9
Q
What is the importance of the vast recta?
A
Important for urine concentration - osmotic exchangers for the production
10
Q
What are the steps for urine formation?
A
Glomerular filtration, reabsorption, secretion (then excretion removes)
11
Q
What is the “formula” for excretion rate?
A
Filtration- reabsorption + secretion
12
Q
Approximately how much is excreted per day?
A
1L
13
Q
Glomerular filtration rate depends on what?
A
Rate of blood flow
14
Q
What is the filtration fraction and its formulate?
A
Fraction of renal plasma flow that is filtered (20%), FF = GFR/RPF
15
Q
What is GFR usually?
A
125 mL/min = 180 L/day
16
Q
How many times a day is plasma volume filtered a day?
A
60
17
Q
What are the three layers of the glomerular capillary membrane?
A
endothelium, basement membrane, podocytes (epithelial cells)
18
Q
Which layer of the glomerular capillary membrane forms foot processes with slit pores to allow passage?
A
podocytes
19
Q
What can happen if podocytes get injured?
A
Become misshapen and can allow proteins to exit and can cause proteinuria
20
Q
Negatively charged molecules are filtered less or more easily than positively charged ones?
A
less easily
21
Q
Where does the negative charge across the glomerulus arise from?
A
proteoglycans on the glycocalyx of the glomerular epithelium (is negative = repel other negatives)
22
Q
Finding protein in urine for patients in hypertension can be early detection for what?
A
hypertensive renal disease
23
Q
Finding protein in urine for patients in diabetes can be early detection for what?
A
diabetic nephropathy
24
Q
Finding protein in urine for patients in pregnancy can be early detection for what?
A
gestational proteinuric hypertension
25
What are the determinants of GFR?
hydraulic conductance and net filtration pressure
26
What is hydraulic a measure of?
hydraulic conductivity and surface area of glomerular capillaries = 4.2 ml/min/100g
27
Diseases such as uncontrolled HTN and DM have what effect on hydraulic conductance?
reduce it by increasing the thickness of the glomerular capillary basement membrane
28
What is net filtration pressure?
net filtration pressure = glomerular hydrostatic pressure - Bowman's capsule pressure - Glomerular oncotic pressure
29
What is the formula for GFR?
Kf x net filtration pressure
30
Which pressures favor filtration? which oppose filtration?
Pgc = favors, Pbs and oncotic pressure oppose filtration
31
What determines the oncotic pressure in the glomerular capillaries?
protein concentration
32
What happens when oncotic pressure reaches net ultrafiltration pressure?
becomes 0 and glomerular filtration stops = filtration equilibrium
33
Most of filtration occurs in the first _ of the capillary.
1/3
34
Increases in plasma protein concentration have what effect on oncotic pressure in the glomerular capillaries?
increase the pressure = decrease net ultrafiltration pressure and GFR
35
Decreases in plasma protein concentration have what effect on oncotic pressure in the glomerular capillaries?
decrease the pressure = increase net ultrafiltration pressure and GFR
36
What change in Bowman's capsule hydrostatic pressure causes decreased GFR?
increase
37
Increasing glomerular capillary hydrostatic pressure has what effect on GFR?
increases
38
What determines capillary hydrostatic pressure?
arterial pressure, afferent arteriolar resistance, efferent arteriolar resistance
39
Increasing the pressure on outside of the afferent end of the capillary has what effect on the inside?
decreases Pg (decreases RPF and GFR) due to decreased blood flow = reduced filtration rate
40
Decreasing the pressure on the efferent end of the arterial has what effect on the in inside pressure?
increases Pg = decreased RPF but increased GFR (blood is restricted from exiting = increase net filtration rate)
41
Name an example of things that constrict the efferent arteriole.
low levels of AGT II
42
Name a couple examples of things that constrict the afferent arteriole.
SNS and high levels of AGT II
43
True/False: Even if efferent constriction is severe, GFR remains/increases.
False, if it is severe, the rise in Glom colloid oncotic pressure exceeds the rise in hydrostatic capillary pressure, eventually leading to a decrease in GFR
44
How does renal disease /DM/HTN cause a decrease in GFR?
by decreasing Kf
45
How does urinary obstruction cause a decrease in GFR?
increase hydrostatic pressure in the Bowman's capsule
46
How does a decrease in renal blood flow or an increase in plasma proteins cause a decrease in GFR?
by increasing Glom colloid oncotic pressure
47
How does a decrease in arterial pressure cause a decrease in GFR?
by decreasing glom hydrostatic pressure
48
How does a decrease in AGT II (using ACE inhibitors) cause a decrease in GFR?
by decreasing resistance to the efferent arterioles = decrease Glom hydrostatic pressure
49
How does an increase in SMS activity/vasoconstrictor hormones cause a decrease in GFR?
by increasing the resistance of the afferent arterioles leading to a decrease in glom hydrostatic pressure
50
What is the formula for RBF?
change in pressure/resistance
51
What pressures are calculated in the RBF formula?
difference between renal artery pressure and renal vein pressure
52
Resistance in the RBF formula is the sum of what?
total renal vascular resistance = Ra + Re + Rv
53
What is the relationship between RBF and Oxygen?
Kidneys receive more O2 than needed due to their high blood flow - a large fraction of the renal O2 consumption is related to renal tubular sodium reabsorption (in turn related to GFR and rate of Na+ filtration
54
What are the types of neurohumoral control of GFR and RBF?
SNS/catecholamines, AGT II, Prostaglandins, Endothelial-derived Nitric Oxide (EDRF), endothelin
55
Describe the SNS control of GFR and RBF and name an example of cause.
produces vasoconstriction by alpha1 receptors which increase afferent and efferent arteriole resistance which decrease RBF and GFR, severe hemorrhage
56
Describe the AGT II control of GFR and RBF and name an example of cause.
vasoconstriction of both arterioles = increase resistance = decreases RBF but since efferent arterioles are more sensitive, low levels of AGTII increases GFR and high levels of it decrease GFR due to constriction of both
57
Describe the prostaglandin control of GFR and RBF and name an example of a cause.
vasodilate both arterioles increasing both rates, blockade of prostaglandin synthesis due to ex: NSAIDs, pt w/heart failure, cirrhosis
58
Describe the EDRF control of GFR and RBF and name an example of a cause.
decrease resistance of arterioles which increases GFR and RBF, Ex: endothelial dysfunction (atherosclerosis)
59
Describe the endothelin control of GFR and RBF and name an example of a cause.
vasoconstricts = increase resistance of the arterioles which decrease both, ex: acute renal failure, hepatorenal syndrome
60
What are the steps of local control (autoregulation) of GFR and RBF?
myogenic mechanism, macula densa feedback, AGT II
61
What two things are important for regulation of urine volume?
Good autoregulation + adaptive increase in tubular reabsorption
62
Describe the process of myogenic autoregulation.
increase in arterial pressure causes a stretch of the afferent arterioles which causes contraction of the smooth muscle walls, increasing resistance leading to opening of the stretch activated Ca2+ channels allowing entry of Ca2+ into the cell which increases tension, increasing vascular resistance which balances the increase in Pa and rates are kept constant
63
What is the role macula densa for tubuloglomerular feedback?
responds to the increase in delivery load by secreting a vasoactive substance that constricts afferent arterioles via a paracrine mechanism, returning the rates back to normal
64
Describe the process of macula densa feedback?
decrease GFR leads to NaCl delivery
65
Describe the process of AGT II for autoregulation.
decrease in arterial pressure causes a decrease in Glomerular hydrostatic pressure, leading to a decrease in GFR. This leads to a decrease in mascula densa NACL causing a decrease in afferent arteriolar resistance and increases renin secretion which stimulated the release of AGT II causing an increase in efferent arteriolar resistance to increase GFR back.
66
Name some other things that increase GFR.
fever, pyrogens, glucocortocoids, hyperglycemia, high protein diet
67
Name some other things that decrease GFR.
aging, low protein diet
68
When does tubular reabsorption occur?
when filtration exceeds excretion
69
When does tubular secretion occur?
When excretion exceeds filtration
70
The _ rate is the difference between filtration and excretion.
reabsorption
71
What is the formula to determine Filtration rate?
GFR x [X]p
72
What is the formula to determine excretion rate?
[X]u x urine flow (v)
73
The _ rate is the difference between excretion and filtration.
secretion
74
What type of transport is utilized for tubular reabsorption?
passive diffusion (solutes) and active transport
75
Name some examples of PAT thru the Tubular lumen.
Na+ K+ ATPase, H+ ATPase, H+ K+ ATPase, Ca2+ ATPase
76
What additional provisions exist for Na+ transport?
brush border (invaginations for multiplying surface area to increase the change of Na+ reabsorption.) and carrier proteins on the luminal surface that provide facilitated diffusion
77
Describe the process of Reabsorption of Na+.
1, diffusion across the luminal membrane by the gradient est. by Na+K ATPase
2. transported across the bilateral membrane by Na+K+ ATPase
3. reabsorbed from the IF into peritubular capillaries by ultrafiltration
78
Describe the process of reabsorption of glucose.
(via secondary active transport)
1. glucose moves from tubular fluid into the tubular cell (against an ELC gradient by Na+ K ATPase) on SGLT (Na+-glucose cotransporter) in luminal membrane. then glucose is transported into the cell into peritubular capillary blood by facilitated diffusion by GLUT1 and 2.
79
What is a consequence of facilitated diffusion?
has a transport maximum due to limited saturation of carriers, limited ATP, etc
80
What happens once the transport maximum is reached?
further increases in tubular load are not reabsorbed and therefore excreted
81
What is the normal reabsorption for glucose?
all
82
What is the Tmax for glucose?
375 mg/min
83
What is the normal filtered load of plasma glucose? (@100mg/dL)
125 mg/min
84
What is the filtered load of plasma glucose if the concentration increases to 200 mg/dL?
250 min
85
What is the threshold for glucose?
250 mg/min
86
Instead of a Tmax, what characteristic do passively transported substances show?
gradient time transport (= rate of transport is dependent on the ELC gradient, permeability of the membrane, and time that it takes to reach luminal membrane)
87
Reabsorption of water and solutes is coupled with the reabsorption of what?
Na+
88
Describe the process of Na+ Reabsorption Coupling.
Na reabsorption: directly increases lumen negative potential aiding in passive Cl- reabsorption, stimulates H20 reabsorption which increases Luminal [Cl-] to also aid in passive Cl- reabsorption, H20 reabsorption also increases luminal [urea] concentration, creating its passive reabsorption
89
How much of the filtered load of Na+ and water is reabsorbed by the proximal tubule?
65% - the proximal tubules cells have a high capacity for active and passive transport
90
Describe how the concentration of substances changes in the proximal tubule.
in the first half: Na+ is reabsorbed by co-transport with glucose, AA, and other solutes. In the 2nd half: little glucose and AA remain and now Cl- is reabsorbed with Na+
91
True/False: The decrease in the amount of Na+ along the proximal tubule decreases the concentration of it.
False, concentration remains relatively constant due to high permeability for water reabsorption to keep up with Na+ reabsorption
92
Describe the solute/water transport capabilities of the segments of the loop of Henle.
Thin segments are very permeable to H20 but thick ascending loop is not but is capable of active reabsorption of Na+, Cl-, and K (also secretes H+)
93
Movment of Na+ across the luminal membrane is mediated primarily by what?
1-Na+, 2-Cl-, 1K+ cotransporter
94
The 1-Na+, 2-Cl-, 1K+ cotransporter is the site of action of what?
loop diuretics
95
Name a few loop diuretics.
Furosemide, ethacrynic acid, bumetanide
96
How do loop diuretics work?
inhibiting the cotransporter to inhibit water reabsorption = increased secretion
97
What can occur due to loop diuretics?
back leak of K+ into the lumen (hypokalemia) creating a slight positive charge of +8 mV in the lumen (forcing cations [Mg2+, Ca2+] to diffuse thru the paracellular spaces)
98
Describe the absorption/secretion through the early distal tubule.
Functionallly similar to the thick ascending loop - not permeable to water, contains macula densa, active reabsorption of Na+, Cl-, K+, and Mg2+
99
What medication work to inhibit the Na+Cl- cotransporter in the distal tubule?
thiazide diuretics
100
Is the late distal tubule permeable to water?
depends on ADH
101
What is the late distal tubule not very permeable to?
urea
102
Secretion of K+ involves what two steps?
1. K+ enters the cell via Na+K+ ATPase
| 2. K+ then diffuses down concentration gradient across the membrane
103
What are the primary site of action for postassium-sparing diuretics?
principle cells
104
What other medications inhibit reabsorption/stimulate secretion?
Na+ channel blockers (Amiloride, Triamterene), Aldosterone antagonists (Spironolactone, Epierenone)
105
What cells play a major role in acid-base regulation?
intercalated cells
106
There are type A and B intercalated cells. Describe them,
Type A: secrete H+ by H+K+ ATPase, important in acidosis to secrete H+ and reabsorb HCO3-
Type B: secrete HCO3 and reabsorb H+, important in alkalosis
107
Describe the transport characteristics of the collecting duct.
permeability to water is controlled by ADH, permeable to urea, capable of secreting H+ against a large concentration gradient
108
What type of diuretics work for the proximal tubule?
osmotic diuretics
109
Name the things that regulate tubular reabsorption.
```
Glomerulotubular balance
Peritubular physical forces
Hormones
SNS
Arterial pressure
osmotic factors
```
110
What is glomeulotubular balance?
the total rate of reabsorption increases as filtered load increases, despite the constant GFR % of 65.
111
True/False: Anything that increases the Peritubular capillary pressure will decrease reasborption.
True, Increase = decrease, decrease = increase
112
Name some things that will decrease reabsorption into the peritubular capillary.
anything that increases the hydrostatic pressure of the peritubular capillary = decreased Afferent arteriole resistance, decreased efferent arteriole resistance, increased arterial pressure
113
Name some things that will increase reabsorption into the peritubular capillary.
increasing Kf,
anything that increases peritubular capillary colloid osmotic pressure: increasing the arterial plasma colloid osmotic pressure, increased FF
114
Describe the mechanism by which decreased peritubular capillary reabsorption decreases net reabsorption of solutes and water.
By increasing the amounts of solutes and water that leak back into the tubular lumen through gap junctions
115
What change in peritubular reabsorption would occur if arterial pressure was increased? decreased?
increase = decrease reabsorption
decrease = increase reabsorption
116
What change in peritubular reabsorption would occur if afferent arteriolar resistance was decreased? increased?
decrease = decrease reabsorption
increase = increase reabsorption
117
What change in peritubular reabsorption would occur if efferent arteriolar resistacne was decreased? increased?
decrease = decrease reabsorption
increase = increase reabsorption
118
What change in peritubular reabsorption would occur if arterial plasma colloid osmotic pressure was increased? decreased?
increase = increase reabsorption
decrease = decrease reabsorption
119
What change in peritubular reabsorption would occur if filtration fraction was increased? decreased?
Increase = increase reabsorption
Decrease = decrease reabsorption
120
What change in peritubular reabsorption would occur if if hydraulic conductance was increased? decreased?
Increase = increase reabsorption
Decrease = decrease reabsorption
121
Describe how aldosterone increases Na+ reabsorption/K+ secretion.
Created Na+ channels (by new synthesis), inserts them at the luminal membrane for NA+ reabsorption
122
Names some factors that increase aldosterone secretion.
AGT II, increased K+, Adrenocorticotropic hormone
123
Name some factors that decrease aldosterone secretion.
ANP, increased NA+ concentration (increased water)
124
What effect does K+sparing diuretics have on Na+ reabsorption?
inhibition
125
Describe how AGT II increases Na+ and water reabsorption.
by stimulation of aldosterone secretion,
directly increasing Na+ reabsorption,
and constricting the efferent arterioles (=decrease in peritubular capillary hydrostatic pressure decreasing RBF which increases FF, which increases peritubular colloid osmotic pressure)
126
Describe how ADH controls reabsorption.
increase H20 permeability and reabsorption via insertion of aquaporin2 channels into luminal membrane, allowing Na+ to travel with it for coupled reabsorption
127
Describe how ANP controls reabsorption.
secreted by the atria in response to an increase in ECF volume = decreased renin release = decreased AGTII release = decreased renal and Na+ reabsorption, along with decreased aldosterone which also decreases reabsorption.
This + GFR is increased leading to increased excretion of H20 and Na+
128
What response occurs to a high Na+ intake?
increased excretion
129
Describe the mechanism of how a high sodium intake responds by increasing excretion.
increasing intake = increased ECF volume and effective arterial blood volume which:
1. decreases SNS activity to dilate the afferent arterioles to decrease reabsorption into the proximal tubule
2. Increases ANP secretion which leads to the constriction of efferent arterioles to decrease reabsorption into the collecting ducts
(both 1&2 increase GFR)
3. decrease peritubular capillary colloid oncotic pressure to decrease reabsorption in the proximal tubule.
4. decrease the RAAS system which will decrease reabsorption into both the proximal tubule and collecting ducts
=which altogether will increase Na+ excretion
130
What response occurs to a low Na+ intake?
decreased excretion
131
Describe the mechanism of how a low sodium intake responds by decreasing excretion.
decreasing intake = decreased ECF volume and effective arterial blood volume which:
1. increases SNS activity to constrict the afferent arterioles to increase reabsorption into the proximal tubule
2. Decreases ANP secretion which leads to the dilation of efferent arterioles to increase reabsorption into the collecting ducts
(both 1&2 decrease GFR)
3. Increase peritubular capillary osmotic pressure which increases reabsorption into the proximal tubule.
4. increase the RAAS system which will increase reabsorption into both the proximal tubule and collecting ducts
=which altogether will decrease Na+ excretion
132
What effect does increasing the ECF volume have on reabsorption?
will dilute peritubular capillary osmotic pressure and inhibit proximal tubule Na+ reabsorption
133
What effect does decreasing ECF volume have on reabsorption?
will concentrate peritubular capillary osmotic pressure and stimulate proximal tubule Na+ reabsorption
134
Briefly describe the RAAS system response.
RAAS is activated in response to decreased arterial pressure which stimulates the release of AGTII to stimulate Na+ reabsorption in the proximal tubule AND aldosterone which stimulates its reabsorption into the late distal tubule and collecting duct.
135
A shift of K+ out of the cells produces an increase in the blood [K+] which is called what?
hyperkalemia
136
A shift of K+ into the cells produces a decrease in blood [K+] which is called what?
hypokalemia
137
What effect does insulin have on the K+ balance?
stimulates K+ uptake into cells by increasing the activity of Na+ K+ ATPase - this action ensures that ingested K+ does not remain in the ECF and produce hyperkalemia
138
What effect does deficiency of insulin (such as in DM1) have?
decreases K+ uptake and produces hyperkalemia
139
Describe the mechanism behind the H+K+ exchanger for acid-base balance.
H+ must enter or leave the cell and in order to preserve electroneutrality, it either needs to be accompanied by an anion or exchanged for another cation = when it is the latter = K+.
140
What effect does alkalemia have on the H+K+ exchange?
decreased [H+] in the blood which causes H+ to leave the cell and K+ to enter = hypokalemia
141
What effect does acidemia have on the H+K+ exchange?
increases [H+] in the blood which causes H+ to enter the cell and K+ to leave = hyperkalemia
142
What effects does hyperosmolarity have on K+ balance?
causes a shift of K+ out of the cell due to the increased osmolarity of the ECF
143
What effect does cell lysis have on the K+ balance?
releases a large amount of K+ from the ICF and produces hyperkalemia
144
What are some examples of cell lysis?
burns, rhabdomyolysis (breakdown of skeletal muscle), destruction of malignant cells during chemo.
145
What effect does exercise have on the K+ balance?
Exercise causes a shift of K+ out of the cells from depletion of ATP stores, usually small but in a person being treated with a B2-adrenergic antagonist or impaired renal function, strenuous exercise can cause hyperkalemia
146
What cells function to secrete K+?
principle cells
147
Name some factors that increase K+ secretion,
High K+ diet, hyperaldosteronism, alkalosis, thiazide diuretics, loop diuretics, luminal anions
148
Name some factors that decrease K+ secretion.
Low K+ diet, hypoaldosteronism, acidosis, K+ sparing diuretics
149
At what value is body fluid osmolarity regulated to?
290 mOsm/L (300 for simplicity)
150
Describe the response to water deprivation.
1. if body water is not replaced by drinking water, then plasma osmolarity increases
2. This increase stimulated osmoreceptors in hypothalamus
3. these osmoreceptors stimulate thirst (which drives drinking behavior) and stimulates ADH secretion
4. Secretion of ADH increases water permeability
5. which increases water reabsorption (as urine osmolarity increases, urine volume decreases)
6. increased reabsorption = more water returned to body fluids and plasma osmolarity can be restored if both drinking and this occur
151
Describe the response to water drinking.
1. intake is distributed throughout the body fluids, will dilute and cause a decrease in plasma osmolarity
2. this inhibits osmoreceptors in the hypothalamus
3. decreases thirst and suppresses water drinking and inhibits secretion of ADH
4. Circulating levels of ADH are reduced and less is delivered to the kidneys, decreasing water permeability of the principal cells.
5. decrease permeability = decreased reabsorption = excretion of what is not reabsorbed, decreasing urine osmolarity and increasing urine vol.
6. less reabsorption = less returned to circulation which that + decreased drinking behavior will restore normal values
152
What is the role of the corticopapillary osmotic gradient?
gradient of osmolarity in the IF of the kidney from the cortex to the papilla (300 @ cortex, 1200 @ papilla)
153
What factors contribute to the corticopapillary osmotic gradient?
the countercurrent multiplyer and urea recycling
154
Countercurrent multiplication is the function of what part of the kidney?
loop of Henle
155
What the loop of Henle's role in formation of the corticopapillary osmotic gradient?
deposits NaCl in IF of the deeper portions of the kidney
156
Countercurrent multiplication will build up the gradient in two steps, what are they?
single effect and flow of tubular fluid
157
Describe the single effect step of countercurrent multiplication.
in the thick ascending limb, NaCl is reabsorbed via the Na+K+2Cl- cotransporter. Water is not reabsorbed which dilutes the tubular fluid. NaCl is transported out, enters the IF, increasing its osmolarity. Water will flow out of the descending limb until osmolarity increases to a level of the adjacent IF.
158
What increases the activity of the Na+ K+ 2Cl- cotransporter?
ADH = enhance of the single effect
159
Describe the flow of tubular fluid step of countercurrent multiplication.
new fluid enters the descending limb from the proximal tubule which pushes existing fluid down and will equilibrate with the IF space
160
So altogether, briefly describe the mechanism of countercurrent multiplication.
1. Single effect: NaCl is reabsorbed out of the ascending limb and deposited in the IF and water is left behind = increase in IF Osm to 400 mOsm/L and the fluid in the ascending limb dilutes to 200. Fluid in IF will equilibrate with IF.
2. Flow of fluid: new fluid w/ 300 mOsm/L enters and the older fluid is pushed down.
3. Single effect: NaCl is reabsorbed out and water remains, increasing osmolarity, adding to the gradient - ~150 mOsm/L
4. Flow of fluid: New fluid @ 300 mOsm/L enters and pushes down existing and the gradient is now larger.
* these steps are repeated until the full corticopapillary gradient is established.
161
describe the process of urea recycling.
In the cortical and outer medullary collecting ducts, ADH increases water permeability which causes water reabsorption, Urea remains behind causing an increase in its concentration.
In the inner medullary collecting ducts, ADH increases water permeability AND increases the transporter for FD of urea (UT1). Due to the elevated conentration, urea diffuses down its gradietn into the IF (otherwise would just be recycled into the inner medulla to be readded to the corticopapillary osmotic gradient)
162
Briefly desceibe how countercurrent exchange in the vasa recta preserves the corticopapillary osmotic gradient.
blood entering @ 300 mOsm/L flows down descending limb, exposed to IF which increases its osmolarity. Small solutes (NaCl, urea) diffuse into the limb and water diffuses out , allowing blood in that limb to equillibrate with the IF ~600. At the bend of the vasa recta, osmolarity is at its peak (1200) equal to the tip of the papilla. In the ascending limb, the opposite occurs: small solutes flow diffuse out and water in and it equilibrates down to 600 again. Blood leaving will be slightly higher than the blood that came in ~ 325 for some recycling to occur to keep the gradient
