Transcript Anatomy of the Kidney, GFR and RBF

Anatomy of the Kidney, GFR and
RBF
Learning Objectives
• Know the basic anatomy of the kidney and nephron.
• Know how urine is transported to the bladder and the
process of micturition.
• Understand the relationship between filtration,
reabsorption, secretion and excretion.
• Understand the determinates of the glomerular
filtration rate (GFR).
• Know how afferent and efferent arteriolar resistances
influence GFR and RBF.
• Know how the juxtamedullary complex autoregulates
GFR and RBF.
Functions of the Kidneys
• Excrete metabolic waste products, foreign
chemicals, drugs and hormone metabolites.
• Regulate H2O and electrolyte balances.
• Regulate arterial pressure.
• Regulate acid-base balance.
• Release erythropoietin for erythrocyte
production.
• Produce vitamin D – important in Ca2+ regulation.
• Regulate glucose synthesis.
Gross Anatomy of the Kidney
Renal Anatomy
Nephron Anatomy
Anatomy of the Urinary Bladder
Transport of Urine
from Kidney to
Bladder
•Stretching of the renal
calyces induces peristaltic
contractions that spread
down the ureter and force
urine into the bladder.
•The ureter is constricted at
the entrance to the bladder
due to the basal tone of the
detrusor muscle. The
constriction prevents
backflow of urine.
•Peristaltic contractions in
the ureter are enhanced by
parasympathetic stimulation
and inhibited by
sympathetic stimulation.
Micturition
• Micturition is the process of emptying the
urinary bladder.
- As the bladder fills, stretch receptors initiate
the micturition reflex. This causes contraction
of the bladder, whose strength progressively
increases.
- Once the micturition reflex is stronger than
the voluntary control of the external sphincter,
urination occurs.
Innervation and Anatomy
of the Urinary Bladder
Pelvic Nerves
•Sensory nerves detect the
degree of stretch in the
bladder wall.
•Parasympathetic motor
nerves then cause
contraction of the bladder.
•Somatic nerves (pedendal)
that control the voluntary
skeletal muscle of the
external sphincter.
•Sympathetic nerves
stimulate the bladder neck
and external sphincter during
filling (contraction). This is
inhibited during micturition.
Micturition Reflex
•As pressure rises, stretch
receptors begin to induce
contractions via the
parasympathetic nerves.
•As the bladder fills, this
reflex become more
frequent and stronger.
•When the bladder
contains a lot of urine, a
signal is also sent that
inhibits the pedendal
nerves.
•If this last signal is more
powerful than voluntary
constriction of the external
sphincter, urination occurs.
Voluntary Urination
• Contract abdominal muscles to increase
pressure in bladder.
• This excites the micturition reflex and inhibits
the external sphincter.
Factors of Urine Formation
Renal Handling of
Some Substances
A. Waste products
such as creatine.
B. Many electrolytes.
C. Nutritional
substances, such
as glucose and
amino acids.
D. Organic acids and
bases, some
foreign
compounds and
some drugs.
Purpose of Reabsorption
• Why filter, then reabsorb?
- Control
- High filtration rate makes it easy to remove
waste products.
- Reabsorption allows control of the body’s
electrolyte balance.
Volume of Filtered Plasma
• ~20% of plasma flowing through a glomerular
capillary is filtered.
• In the average adult human, the glomerular
filtration rate (GFR) for the kidney is 125
ml/min, or 180 L/day.
• Entire plasma volume is ~3 L; so the entire
plasma can be filtered ~60 times/day.
Glomerular Capillary
Membrane
•Similar to fluid flow at
other capillaries
except, that the pores
are larger.
•Negative charge on
the 3 layers of the
capillary wall keep
negatively charged
albumin from being
filtered.
Determinants of GFR
• As we discussed with other capillaries:
- Glomerular hydrostatic pressure (PG).
- Bowman’s capsule hydrostatic pressure (PB).
- Glomerular colloid osmotic pressure (PGC).
- Bowman’s capsule colloid osmotic pressure (PBC).
• In addition,
- Glomerular capillary filtration coefficient (Kf).
- Kf is the product of the hydraulic conductivity and the
surface area.
• GFR – Kf(PG – PB – PGC + PBC)
- PBC is normally considered to be 0.
Net Filtration Pressure (excludes Kf)
Capillary Filtration Coefficient
• Kf is the product of the hydraulic conductivity
and the surface area.
- Hydraulic conductivity is the ease at which H2O
flows through.
• Kf is not measured directly, but calculated
from Kf = GFR/Net filtration pressure
• Kf does not normally change
– IT’S A CONSTANT!!
Bowman’s Capsule Hydrostatic
Pressure
• Increases would decrease GFR.
• Normally not changed.
• Obstructing urinary outflow can increase the
hydrostatic pressure in Bowman’s capsule, as
may occur with kidney stones.
Glomerular Capillary
Osmotic Pressure
•PG changes during flow
through glomerular
capillary.
•PG is increased by
increasing filtration
fraction and arterial
osmotic pressure.
•Filtration fraction is
GFR/renal plasma flow.
•Increased renal plasma
flow decreases the
filtration fraction. This
causes a slower rise in the
PG.
•With a constant PG, an
increase of BF into the
glomerulus tends to
increase GFR and viceversa.
Glomerular Capillary Hydrostatic
Pressure
• Primary mechanism for physiological
regulation of GFR, with increase in glomerular
hydrostatic pressure, causing an increase in
GFR.
• 3 variables influencing glomerular hydrostatic
pressure:
1. Arterial pressure; however, this is buffered by
autoregulation (more later).
2. Afferent arteriolar resistance.
3. Efferent arteriolar resistance.
Anatomy Reminder
Changing Afferent and
Efferent Arterioles
Renal Blood Flow
• RBF exceeds the kidney’s metabolic need,
because of the need to filter the plasma.
• RBF is determined with Ohm’s Law, which you
already know.
• Autoregulation keeps GFR and RBF,
particularly GFR, fairly constant during
changes in arterial pressure.
Sympathetic and Humoral Control of
GFR and RBF
• Sympathetic nervous system constricts the
vessels. Little influence at moderate
stimulation. Important during severe
stimulation, such as brain ischemia.
• Norepinephrine and epinephrine – same as
above.
• NO provides a basal level of vasodilation.
• Angiotensin II – constricts the efferent
arteriole (more next lecture).
Autoregulation of GFR and RBF
What if GFR Increased with Arterial
Pressure?
GFR does not increase proportionally with arterial pressure
(autoregulation) and reabsorption does increase with GFR.
Autoregulation
• There is a feedback mechanism that ensures a
constant delivery of NaCl to the distal tubule.
• This feedback mechanism, called
tubuloglomerular feedback, is mediated by
the macula densa in the juxtaglomerular
complex.
Juxtaglomerular Complex