Chapter 17
Physiology of the Kidneys
Dr. David Washington
Glomerular Filtration
A person with a cardiac output of 5,600
ml/min would have a flow rate through the
kidneys of about 1,200 ml/min, or 21% of
the C.O.
1,200
(renal fraction =
= 21%)
5,600
o
The glomeruli filter particles up to 80 A,
therefore; proteins and blood cells are not
filterable.
Measurement of GFR
(Glomerular Filtration Rate)
GFR =
Uv
p
U = concentration of substance in urine
P = concentration of substance in plasma
V = volume of urine
example (inulin)
U = 125 mg/ml
P = 1 mg/ml
V = 1ml/min
therefore,
Measurement of GFR
(Glomerular Filtration Rate)
U = 125 mg/ml
P = 1 mg/ml
V = 1ml/min therefore,
GFR
125 mg/ml X 1 ml/min
=
1 mg/ml
GFR = 125 ml/min
That is; 125 ml of plasma had to be filtered
to get 125 mg of inulin in the urine.
Counter-Current Mechanism
Na+ K+
(Urine Formation)
Na
+
H 2O
H 2O
Cortex
Glucose
Amino acid
320
400
Outer Medula
H 2O
Inner Medulla
Diffusion
passive
active
300
400
600
200
400
Na+
800
600
800
Na+
1000
800
1000
+
Na
600
1200
Highly permeable
to Na
Impermeable
to H2O
K+
Counter-Current Mechanism
(Urine Formation)
Cortex
Outer Medula
Inner Medulla
Net action in loop of Henle - H2O reabsorption
Net action at distal tubule:
a. ADH - increases reabsorption
b. Aldosterone - increases Na reabsorption
Countercurrent Multiplier Mechanism
Proximal Tubule
Tubular lumen
Peritubular fluid
Na+
+
K
70%
H2O
Glucose
Amino acids
Glucose
Countercurrent Multiplier Mechanism
15%
Loop of Henle
(Descending limb)
H2O
Na+
14%
(Ascending limb)
Na+
membrane
x impermeable
H2O
Distal Tubule
Na+
K+
H2O
Diabetes insipidus
Autoregulation of Glomerular
Filtration Rate (J-G apparatus)
Bowman’s capsule
Glomerulus
Juxtaglomerular cells
Macula densa cells
Afferent arteriole
Efferent
arteriole
Distal
tubule
Renin-Angiotensin-Aldosterone Pathway
Angiotensinogen
Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu-Leu-Val-Tyr-Ser
14
Renin
Angiotensin I
Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu
Ang.I
10
Converting enzyme
(in lungs)
Angiotensin II
Asp-Arg-Val-Tyr-Ile-His-Pro-Phe
8
Vasoconstriction
Aldosterone
Compensation for Low Sodium
Low Na+ Intake
Na+ retention in blood
Low plasma Na+
concentration
Na+ reabsorption in
distal tubules
Hypothalamus
Aldosterone
Posterior pituitary
Adrenal cortex
ADH
Angiotensin II
Water reabsorption in
collecting ducts
Renin
Urine Vol
Blood Vol
Juxtaglomerular
apparratus
Sympathetic nerve activity
Acids, Bases and Buffers
H
H
O
OH-
H+
+
H
H
OH-
H
O
H
H
O
H-O-H
H2O
H+ + OHhydroxide
Acids, Bases and Buffers
HCL
CL+ H
H
+ O
H
H
O
CLH H
H+
CL- O
H
H
HCL
Acid in H2O
H+ + CL-
Acids, Bases and Buffers
NaOH
+
+
H
H
H H
H H
O
- O
OH
+
Na
H H
OH- O
NAOH
Base in H2O
Na+ + OH-
Bicarbonate Buffer System
Contains mixture for converting strong
acids + bases to weak acids and bases.
Example: (H2CO3 and NaHCO3)
a) HCL + NaHCO3 H2Co3 + NaCL
sodium bicarbonate
carbonic
acid
b) NaOH + H2Co3
NaHCO3 + H20
pk - the ph at which the concentration of the
two portions of the buffer are equal.
Buffer power - ability to maintain a ph
following the addition of acid or base
Bicarbonate Buffer System
Buffer Curve:
100
25
75
50
50
pk
6.1
75
25
4
5
6
ph
7
8
NaHCO3
Base added
Acid added
H2CO3
0
Extracellular Fluid pH
Henderson-Hasselbalch Equarion
(pH caused by the concentration of CO2 in
blood)

HA
H + + A- 
1) H2 CO3
H++ HCO3-
carbonic
acid
bicarbonate ion
Extracellular Fluid pH
(H+)x(HCO3-)
2) KA =
(H2CO3)
(H+)x(HCO3-)
3) KA =
(CO3)
Since [H2CO3] in
extra-cellular fluids is an
express of [CO2], we
write --- equation 3.
(HCO3-)
+
4) log KA = log (H ) + log
(CO2)
-)
+
(HCO
3
5) - log(H ) = -logKA + log
(CO2)
Extracellular Fluid pH
6) pH = pKA +
(HCO3-)
(CO2)
7) pH =p KA + log
(HCO3-)
0.03 x Pco2
(solubility
coefficient
in mM/L)
pKA= 6.1; [HCO3-] = 24mM/L in arterial blood
 pH = 6.1 + log 24
0.03 x 40
= 6.1 + log 20
= 6.1 + 1.3
= 7.4