The Renal System
Five Processes of Urinary System
1.
2.
3.
4.
5.
Filtration,
Reabsorption,
Secretion,
Excretion
Micturition
180 L / day filtered, >99% reabsorbed,
1.5 L/day excreted
1) Filtration
Movement of fluid from blood to lumen of nephron
1) Filtration: Passage across 3
Barriers
1.
Capillary endothelium is
fenestrated
2.
3.
Basal lamina
1.
Filters proteins
2.
Negative Charges
Bowman’s capsule epithelium
(visceral layer), including
podocytes
Cause of Filtration
Three types of pressures are at
work:
Hydrostatic pressure in capillaries (see
exchange in tissues)
Osm. Pcapillaries > Osm. P Bowman’s
capsule
Hydrostatic fluid P from presence of
fluid in Bowman’s capsule
Net driving pressure: ~ 10 mmHg
2) Tubular Reabsorption (99% of filtrate)
Mostly transepithelial transport
(examples: Sodium and glucose)
Reabsorption may be active (Na+,
glucose) or passive (urea)
2) Tubular Reabsorption (99% of filtrate)
Active Transport
 Na+ /K+ ATPase pumps
Passive
concentration and osmotic
gradients
Na+ Reabsorption in PCT
Apical: Na+
movement
down conc.
gradient.
Basolateral: Na+/K+
ATPase.
Na+ Linked Glucose Reabsorption
Basolateral:
Apical: Na+-glucose
cotransport
Glucose diffusion
down conc.
gradient
Saturation of Renal Transport
Saturation = Maximum rate of transport (tm)
Same 3 characteristics as
discussed in mediated transport
Transport maximum determined by

Saturation  Renal
Threshold

Specificity

Competition
Clinical Importance of GFR and
Clearance
GFR is indicator for overall kidney function
Clearance → non-invasive way to measure GFR

Inulin (research use)


Neither secreted nor reabsorbed
Creatinine (clinically useful)
If a substance is filtered and reabsorbed but not
secreted  clearance rate < GFR
If a substance is filtered and secreted but not
reabsorbed  clearance rate > GFR
Regulation of Blood Pressure via
Tubuloglomerular Feedback
Decrease blood pressure in
afferent arteriole
juxtaglomerular cells
release Renin
Renin
Angiotensinogen  Angiotensin I
ACE
Angiotensin I
 Angiotensin II
Aldosterone/ ADH secretion
3) Secretion
2nd route of entry (from ECF) into tubules for
selected molecules
Mostly transepithelial transport (analogous to
reabsorption). Depends mostly on active membrane
transport systems
Provides mechanism for rapid removal of
substances (most important for H+, K+, foreign organic ions
and drugs such as penicillin etc.)
4) Excretion = Urine Output
Excretion of excess ions, H2O, toxins, “foreign
molecules” “nitrogenous waste” (NH4+ , urea)
Depends on Filtration, Reabsorption, Secretion

E=F–R+S
Direct measurement of F, R, S impossible

infer from comparison of blood & urinalysis
For any substance: (Renal) Clearance =
plasma volume completely cleared of that
substance per minute

Typically expressed as ml/min
5. Micturition
Spinal cord integration: 2 simultaneous efferent signals
In infant just simple spinal reflex
Later: learned reflex under conscious control from higher brain
centers
Various subconscious factors affect reflex
Fig 19-18
Manneken Pis in Brussels
GFR = Glomerular Filtration Rate
Describes filtration efficiency: Amount of fluid filtered
per unit of time
Average GFR ~ 180 L/day!
Filtration Coefficient is influenced by
 Net filtration pressure
 Available surface area of glomerular capillaries
GFR is closely regulated to remain constant over range
of BPs
(80 - 180 mm Hg)
Goal is to control blood flow though both afferent and
efferent arterioles – via ?
Regulation of GFR
Several mechanisms provide close
control of GFR;

Filtration Pressure (BP)



Tubuloglomerular feedback


Hydrostatic, colloid
Resistance in afferent vs. efferent
arterioles
JG Apparatus
Hormones and ANS


Angiotensin II (vasoconstrictor)
Prostaglandins (vasodilator)
Passive Urea
Reabsorption
Na+ actively reabsorbed
H2O follows passively
 [urea]  passive
reabsorption
(diffusion into blood)
An increase in afferent arteriolar
pressure will result in all of the
following except
A.
B.
C.
D.
An increase in GFR
An increase in glomerular hydrostatic
pressure
An increase in the osmotic pressure of
the glomerular capillaries
A somewhat greater output of urine
Urine flows to the urinary bladder
from the kidney by means of the
A.
B.
C.
D.
E.
Urethra
Proximal tubule
Peritubular capillary
Ureter
Prostate
The fluid that enters the
glomerulus is essentially similar
to
A.
B.
C.
D.
E.
Serum
Blood
Sea water
Fresh water
Plasma
The driving force that pushes
fluid from the blood into
Bowman’s capsule is exerted
A.
B.
C.
D.
E.
By the heart
By the muscles lining the abdominal
cavity
By the urinary bladder
By the urethra
Gravity
The rate of reabsorption of water
from the collecting duct is
determined by the hormone
A.
B.
C.
D.
E.
Oxytocin
Cortisone
Antidiuretic hormone
Lactogenic hormone
Aldosterone
If the Tm for a particular amino acid is
120 mg/100 ml and the concentration
of that amino acid in the blood is 230
mg/100 ml, the amino acid
A.
B.
C.
D.
Will be actively secreted into the filtrate
Will be completely reabsorbed the tubule
cells
Will appear in the urine
Only a and c
Which of the following is not
reabsorbed by the proximal
convoluted tubule?
A.
B.
C.
D.
E.
Na+
K+
Glucose
Creatinine
Water
Alcohol acts as a diuretic
because it
A.
B.
C.
D.
Is not reabsorbed by the tubule cells
Increases the rate of glomerular filtration
Blocks the activity of ADH
Increases the secretion of Aldosterone
The kidneys are stimulated to
produce renin
A.
B.
C.
D.
When the peritubular capillaries are
dilated
When the pH of the urine decreases
By a decrease in the blood pressure
When the specific gravity of urine rises
above 1.10
The descending limb of the loop
of Henle
A.
B.
C.
D.
Is not permeable to water
Is freely permeable to sodium and urea
Contains fluid becoming more
concentrated as it moves down into the
medulla
Pulls water by osmosis into the lumen of
the tubule
Renal Failure & Artificial
Kidney
Symptoms when < 25%
functional nephrons
due to:
1.
Kidney infections
2.
Chemical poisoning (lead, paintthinner) etc.
Hemodialysis:
3/week 4-8h/session
Alternative: CAPD
Continuous
Ambulatory
Peritoneal
Dialysis