Download Renal Structure and Function

Renal Structure and Function
Introduction
•Main function of kidney
is excretion of waste
products (urea, uric
acid, creatinine, etc).
Other excretory organs
are ?
•As above function is
fulfilled, kidney also
extremely important in
maintaining
–fluid and electrolyte
homeostasis
–volume of the extracellular fluid volume
–Acid-base balance
–Blood pressure
Functional Background
•Kidneys receive ≈ ¼ of total
cardiac output even though small
 lots of capillaries
•Filter about 11X the extracellular
fluid volume/d. from plasma
•Produce ≈ 180 liters/d. of filtrate
•Filtrate very similar to plasma
except very little protein (protein in
urine = serious problem)
•99% reabsorbed as pass through
renal tubule
•Some substances secreted into
renal tubule
•Approximately 1.5 liters/d. of
filtrate is voided as urine
Gross Anatomy of the Kidney
•Each kidney consists
of
–an outer cortex
–Inner medulla
–Hollow pelvis 
empties into ureter
–Functional unit of the
kidney is the nephron (≈
1,300,000 nephrons per
kidney)
Structure of the Nephron
• Nephron
– Glomerulus
• Bowman’s capsule, Bowman’s space
– Proximal convoluted tubule
– Loop of Henle
• Descending limb
• Ascending limb
– Distal convoluted tubule
– Collecting duct
Diagram of nephron structure
Flow of
filtrate
Function of the Nephron: Glomerulus
–Site of filtration
–Each glomerulus consists
of a capillary network
surrounded membrane
called Bowman's capsule.
–Afferent arteriole carries
blood from renal artery into
glomerulular capillaries
–At distal end of
glomerulus, capillaries form
the efferent arteriole
through which blood leaves
the glomerulus.
Glomerular Filtration
• Fluid driven from the glomerular capillaries into
Bowman’s capsule by hydrodynamic force (blood
pressure)
Note: as blood pressure increases, volume of filtrate
increases  more filtrate formed = less blood
volume = lowered blood pressure
• Fluid crosses three layers that excludes large
molecules (especially proteins)
• Blood which passes on through to efferent
arteriole is thus high in protein plasma and thus a
higher oncotic pressure than normal
Note: oncotic pressure = osmotic pressure
contributed by large molecules such as plasma
proteins
Function of the Nephron: Proximal
Convoluted Tubule
• Epithelium is “leaky”  allows passive
movement of some ions (Na+, Cl-, glucose,
amino acids, HCO3-) and water out of
tubule
• 60-70% of filtered load reabsorbed in
proximal tubule
• Transport mechanisms (Na+/H+ exchanger
moves Na+ out of proximal tubule, water
follows; some Na+ is coupled to other
solutes (glucose, amino acids)
• Remaining 30-40% of filtrate, which is still
isosmotic with plasma, passes on to the
loop of Henle
Function of the Nephron: loop of Henle
• Consists of a descending and ascending portion
(which has both thin and thick segments)
• Descending portion  high permeability to water
allows leakage into tubular interstitium
• Increased oncotic pressure in peritubular capillaries
(coming from efferent arteriole) helps move water
from tubular interstitium back into capillaries 
filtrate becomes concentrated
• Ascending portion  active reabsorption of NaCl,
absorbs 20-30% of the Na+ in the tubule
• Reabsorption occurs by transport mechanism driven
catalyzed by Na+/K+-ATPase
• Volume of filtrate reduced by additional 5%
• Because NaCl is reabsorped filtrate becomes
hypotonic to plasma as enters distal convoluted
tubule
Water and NaCl reabsorption
in the loop of Henle
Function of the Nephron: Distal Convoluted
Tubule
• Active NaCl out of filtrate continues
• Active secretion of K+ (depending on
blood levels) and H+ (depending on
pH) into filtrate, driven by a Na+/K+
pump
• Regulation of Ca++ occurs here
• As filtrate leaves DCT it is isoosmotic
with plasma
• Enters collecting tubule/collecting
duct
Function of the Nephron: Collecting Tubule
and Collecting Duct
– Several distal tubules empty into each
collecting tubule; they join to form the
collecting duct
– Collecting duct: NaCl, K+ and water
reabsorption
– NaCl reabsorption, K+ secretion under control
of aldosterone
– Water under control of ADH
– Some H+ secretion occurs
– End result  highly concentrated urine
(mammals and birds only animals able to do
so) elimnates waste using as little water as
possible
Aldosterone
•
Has a threefold
action on Na+
reabsorption
1.
2.
3.
Rapid effect by
stimulation of an
Na+/H+ exchanger
containing an
aldosterone receptor
Delayed effect by
binding to
intracellular receptors
that direct synthesis
of a mediator protein
that activates sodium
channels
Long-term effect by
increasing the
number of basolateral
Na+ pumps
Review of reabsorption and secretion
in the nephron
Natriuretic peptides
•Involved with regulation of Na+ excretion in
distal tubule
•Release from heart atria when arterial pressure
is too high
•Causes water and salt diuresis by kidney,
reducing blood volume and eventually atrial
pressure
Acid-Base Balance
• Kidney’s help regulate H+ and bicarbonate
• Urine can be basic or acidic depending on
need
• Usually acidic to compensate for tendency
of blood pH to drop due to presence of
CO2 (which forms carbonic acid via
carbonic anhydrase-catalyzed reactions)
• Compensating mechanism is secretion of
H+ into tubular fluid and reabsorption of
bicarbonate
Potassium Balance
• Rapidly and narrowly regulated by
kidney
• Regulation important because small
changes in K+ levels can affect
function of many excitable tissues
(heart, brain, skeletal muscle
• Most filtered K+ is reabsorbed in
proximal tubule and loop of Henle
• Most secretion of excess K+ occurs in
collecting tubule
The End