Ch 19: The Kidneys
Functional unit of kidneys: ??
4 basic processes of urinary system:
Filtration
Reabsorption
Secretion
Excretion
Functions of Kidneys
Homeostatic regulation of
 H2O & solute concentration
– ECF volume
– osmolarity, ion concentration
 pH (acid-base balance)
Further functions
 Excretion of wastes & foreign substances
 Hormone and enzyme production etc.
Four Processes of Urinary System

Filtration, Reabsorption, Secretion,
Excretion
 Related by equation:
Figs 19-2/3
E=F-R+S
180 L / day filtered, >99% reabsorbed,
1.5 L / day excreted
Peritubular capillaries
Efferent
arteriole
Distal
tubule
S
R
Glomerulus
F
Afferent
arteriole
R S
Bowman’s
capsule
Proximal
tubule
R
S
R
KEY
F
= Filtration: blood to lumen
R
= Reabsorption: lumen to blood
S
= Secretion: blood to lumen
E
= Excretion: lumen to external
environment
R
Loop
of
Henle
To
renal
vein
Collecting
duct
E
To bladder and
external environment
1) Filtration
= Movement of fluid from blood to lumen
of nephron (rel. nonspecific process)
Once in lumen – consider it outside body
Composition of filtrate?
Fig 19-4
Filtration: Passage across 3 Barriers
1.
Capillary endothelium
2.
Basal lamina (= BM)
3.
Bowman’s capsule
epithelium (visceral layer)
Some small molecules (Ca2+,
low m.w. fatty acids) bind to
plasma proteins  ?
Fig 19-4
Cause of Filtration
Three types of pressures are at work:
Hydrostatic pressure in capillaries
(see exchange in tissues)
Osm. Pin caps > Osm. P in Bowman’s
capsule
Hydrostatic fluid P from presence of
fluid in Bowman’s capsule
Fig 19-6
Net (?) driving pressure: ~ 10 mmHg
Filtration
Pressure
Net driving pressure = ?
Glomerular Filtration Rate =
GFR
Describes filtration efficiency:
Amount of fluid filtered per unit of time
Average GFR ?
influenced by
 Net filtration pressure
 Available surface area
Fig 19-5
GFR closely regulated
to remain constant over range of BPs
(80 - 180 mm Hg)
Goal is to control blood flow though afferent
and efferent arterioles – via
1. Reflex regulation  NS & Hormones
(e.g.: angiotensin II and prostaglandins)
2. Autoregulation  (myogenic &
tubuloglomerular feedback)
Autoregulation via
Tubuloglomerular Feedback
GFR 
Macula densa cells:
release paracrines
juxtaglomerular cells
(smooth muscle fibers from
afferent arteriole):
contract
GFR 
Fig 19-10
Distal tubule Efferent arteriole Bowman’s capsule Glomerulus
1 GFR increases.
Proximal
tubule
Macula
densa
2 Flow through tubule increases.
4
1
5
Afferent
arteriole
Granular
cells
3 Flow past macula densa
increases.
3
2
4 Paracrine diffuses from macula
densa to afferent arteriole.
5 Afferent arteriole constricts.
Resistance in afferent
arteriole increases.
Collecting
duct
Loop
of
Henle
Hydrostatic pressure
in glomerulus decreases.
GFR decreases.
Tubular Reabsorption
Highly selective
and variable
Amount of filtrate / day?
Urine production / day?
% reabsorbed?
Fig 19-5
Mostly transepithelial transport
(examples: Sodium and glucose)
Reabsorption may be active (Na+,
glucose) or passive (urea)
Figs 19-11(!!!) and 13
1 Na+ is reabsorbed
by active transport.
Filtrate is
similar to
interstitial fluid.
2 Electrochemical
gradient drives
anion reabsorption.
1 Na+
2
Anions
3 Water moves by
osmosis, following
solute reabsorption.
3 H2O
4 K+, Ca2+,
urea
Tubule lumen
Tubular
epithelium
Extracellular fluid
4 Concentrations of
other solutes increase
as fluid volume in lumen
decreases. Permeable
solutes are reabsorbed
by diffusion.
Saturation of Renal Transport
Same 3 characteristics as discussed in
mediated transport
Transport maximum determined by

Saturation  Renal Threshold
Fig 19-14
 Specificity
 Competition
Example: Glucose (Fig 19-15)
Secretion
2nd route of entry into tubules for selected
molecules
Mostly transepithelial transport (analogous
to reabsorption). Depends mostly on
membrane transport systems (usually 2o
active transport)
Provides mechanism for rapid removal of
substances (most important for H+, K+, foreign
organic ions and drugs such as penicillin etc.)
Excretion = Urine Output
Excretion of excess ions, H2O, toxins, "foreign
molecules“ "nitrogenous waste“ (NH4+ , urea)
Depends on F, R, S (formula ?)
Direct measurement of F, R, S impossible 
infer from blood & urine analysis
Kidneys clean or “clear” plasma of certain
substances
For any substance: Clearance = plasma volume
completely cleared of that substance per minute
Clinical Importance of GFR and Clearance
GFR is indicator for overall kidney function
Clearance → non-invasive way to measure
GFR (inulin and creatinine)
Substance is filtered and reabsorbed but not
secreted  clearance rate < GFR
Substance is filtered and secreted but not
reabsorbed  clearance rate > GFR
Micturition Reflex
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
1
Stretch receptors fire.
2
3
Parasympathetic neurons fire.
Motor neurons stop firing.
Smooth muscle contracts.
Internal sphincter passively
pulled open. External
sphincter relaxes.
(b) Micturition
Stretch
receptors
Higher CNS
input may
facilitate or
inhibit reflex.
Sensory neuron
1
Parasympathetic
neuron
2
3
+
–
Motor neuron
Internal
sphincter
External
sphincter
3
2
Tonic
discharge
inhibited
Renal Failure & Artificial Kidney
Symptoms when < 25%
functional nephrons
due to:
1. Kidney infections
2. Chemical poisoning (lead,
paint-thinner) etc.
Hemodialysis:
3/week 4-8h/session
Alternative:
CAPD
Continuous
Ambulatory
Peritoneal
Dialysis
The End