artery
Afferent
arteriole
Glomerular
capillary
Efferent
arteriole
Renal Physiology 2
Office hours/help after lecture
Peritubular
capillary
Bowman’s
space
vein
tubule
Basic Renal Processing
Steps in urine formation
• General strategy
– Everything but cells and protein gets filtered
into Bowman’s space (called glomerular
filtrate)
– Flitrate has the same substances at same
concentrations as plasma
– Further along tubule, substances are added
(secretion) or removed (reabsorption)
1
1. Filtration
2. Secretion
3. Reabsorption
4. Excretion (urine)
2
3
4
1
Amount excreted =
amount filtered + amount secreted – amount
reabsorbed
1. Filtered and secreted;
not reabsorbed
3. Filtered and
completely reabsorbed
2. Filtered and some
reabsorbed
Glomerular Filtration
• For each substance in plasma
there is some combination of filtration,
reabsorption and secretion
• For many substances (eg, Na+, Ca++,
water) these processes are under
physiological control
2
Capillary Fenestra allow movement
from plasma to Bowman’s space
Glomerular Filtration
• A bulk flow process
πGC
PGC
PBs
• Glomerular Filtration Pressure
•
πBS
Glomerular Filtration Rate (GFR)
– Function of:
GFP= ((PGC-PBS)-(πGC- πBS))
1. Filtration pressure
2. Permeability of substance
– GFR = filtration pressure x Kf
Usually 0
GFP=(60-15)-(29)
= 16 mmHg
•
•
Kf is the filtration coefficient
70 kg person
– GFR water = 180 L/day (125 ml/min)
Positive pressure indicates filtration
3
Role of hydrostatic pressure in controlling GFP
1. PBC – primarily a function of the tubules & ureters;
movement of fluid through the system keeps PBC
low
2. PGC – a function of blood volume in glomerular
capillary
–
–
–
–
–
Afferent and efferent arterioles may be regulated
independently
Function of Reff / Raff
Constriction of afferent ↓flow into glomerular capillary
therefore ↓ PGC
Constriction of efferent ↓ flow out of glomerular capillary
therefore ↑ PGC
Opposites for dilation
Assessing kidney function by
measuring plasma clearance & GFR
To estimate plasma clearance
Total entering=
[plasma] x renal blood flow
plasma clearancesubstance X =
[urine]substance x urine vol
[plasma]substance
Amount filtered =
[plasma] x GFR
If substance is completely filtered and not reabsorbed nor secreted
plasma clearance = GFR
Amount in urine =
[urine] x urine volume
4
Inulin is a small sugar that is completely
filtered and is not reabsorbed or secreted
1. Inject a small amount of inulin into blood
and measure [inulin]plasma
2. Measure [inulin]urine and urine volume
Inulin excretion = 0.1 L/h x 300 mg/L
= 30 mg/h
Plasma clearanceinulin =
[inulin]plasma= 4mg/L
Urine volume = 0.1 L/hour
[inulin]urine = 300 mg/L
300 x 0.1
4
Plasma clearanceinulin = 7.5 L/h = GFR
Steps in urine formation
For any substance:
• If plasma clearance <GFR there must be
reabsorption
• If plasma clearance >GFR there must be
secretion
1
1. Filtration
2. Secretion
3. Reabsorption
4. Excretion (urine)
2
3
4
5
Distal tubule
Tubular Reabsorption
180 L of glomerular filtrate, only 1 L of urine
→99% of all water is reabsorbed
1800g/day of NaCl in glomerular filtrate, only
10g/day in urine
→ 99% of NaCl is reabsorbed
Glucose → 100% reabsorption
Collecting duct
Proximal tubule
Descending LofH
Ascending LofH - thick
Ascending LofH - thin
paracellular
capillary
Tubular fluid
transcellular
Tight junction
Na+
Na+
K+
ATP
K+
ATP
Na+
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• Na+,K+ pump establishes Na+ gradient
• In the proximal tubule Na+ transport
• Water movement is secondary to Na+
transport
– Antiport with H+
– Symport with glucose
Na+
1. Na+
K+
2a. ↓osmolarity
Na+
Na+
H+
Na+
glucose
K+
ATP
ATP
2b. ↑osmolarity
H2O
H2O
3. osmosis
4. Bulk flow
glucose
• Na+,K+ pump establishes Na+ gradient
• In the distal tubule Na+ transport
– Symport with Cl-
Na+
Na+
ClK+
ATP
H20
Na+
Proximal tubule
High
Active transport
Descending LofH
High
Low
Ascending LofH
Low
Active transport
Distal Tubule &
Collecting Duct
Variable Active transport
due to
hormones
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Formation of urine
Excretion of dilute urine occurs with low
levels of ADH/vasopressin
100
300
1. Dilute
2. Concentrated
300
150
300
80
H2O
375
375
400
Active transport
of solutes
350
H2O
500
700
500
700
65
Low permeability
to water
• To form dilute urine:
– Reabsorb solute from tubular fluid, while
leaving water in the distal tubules.
– Occurs only in the absence of
ADH/vasopressin
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