Download The Urinary System

The Urinary System
• rids the body of
waste products.
Excretion
• Excretion—separation of wastes from body fluids and
the elimination of them
• Four body systems carry out excretion
– Respiratory system
• CO2, small amounts of other gases, and water
– Integumentary system
• Water, inorganic salts, lactic acid, urea in sweat
– Digestive system
• Water, salts, CO2, lipids, bile pigments, cholesterol, other
metabolic waste, and food residue
– Urinary system
• Many metabolic wastes, toxins, drugs, hormones, salts, H+, and
water
Functions of the Urinary System
• Regulation of extracellular fluid volume and BP
– the kidneys regulate body water content
• Regulation of blood osmolarity
• Maintenance of ion balance
– the kidneys regulate body Na+, K+ and Ca2+ levels
• Homeostatic regulation of pH
– the kidneys regulate body H+ and HCO3- levels
• Excretion of wastes
– the kidneys remove metabolic wastes and foreign
substances
• Production of hormones
– the kidneys secrete erythropoietin
– enzymes that the kidneys produce regulate the production
of hormones involved in blood pressure regulation and
Ca2+ balance
• Waste—any substance that is useless to
the body or present in excess of the
body’s needs
• Metabolic waste—waste substance
produced by the body
• Urea formation
– Proteins amino acids  NH2
removed  forms ammonia, liver
converts to urea
• Uric acid
– Product of nucleic acid catabolism
• Creatinine
– Product of creatine phosphate
catabolism
• Blood urea nitrogen (BUN)—expression
of the level of nitrogenous waste in the
blood
– Normal concentration of blood urea is
10 to 20 mg/dL
– Azotemia: elevated BUN
• Indicates renal insufficiency
Nitrogenous Wastes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
H
O
N
C
H
H2N
H
Ammonia
NH2
Urea
NH
O
H
C
HN
C
C
N
C
C
O
N
H
C
Uric acid
N
H
HN
O
C
N
CH3
CH2
O
Creatinine
Uremia: syndrome of
diarrhea, vomiting, dyspnea,
and cardiac arrhythmia
stemming from the toxicity
of nitrogenous waste
Treatment—
hemodialysis or organ
transplant
Retroperitoneal Position of the Kidney
Anterior
Small intestine
Stomach
Colon
Pancreas
Inferior vena cava
Aorta
L1
Ureter
Renal artery
and vein
Peritoneum
Spleen
Kidney
Hilum
Lumbar muscles
Posterior
Anatomy of the Urinary System
• Blood flows into the 2 kidneys by the renal arteries at the
medially facing concave surface of the kidney called the
hilus
– some of the plasma is filtered into tubes of simple
epithelium called renal tubules (the lumen of the renal
tubules is outside the body) which removes metabolic
wastes, toxins, drugs and substances in excess from the
body
• Blood that has been filtered, leaves the 2 kidneys by the
renal veins at the hilus
• The urine created by the renal tubules leaves each kidney
via a hollow tube called the ureter which pass the urine to
the urinary bladder
• As the bladder fills and stretches, reflexes coordinated by
the parasympathetic NS cause the bladder to contract which
voids the urine through the urethra
Renal Anatomy
• A coronal section shows 2 distinct layers: an outer
(superficial) cortex and an inner (deep) medulla
– these layers are formed by the organized arrangement of
nephrons which combine the renal tubules and renal
capillaries
– 80% of the 1 million nephrons found in each kidney are
called cortical nephrons since they are almost completely
contained within the cortex
– 20% of the nephrons dip down into the medulla are
called juxtamedullary nephrons
Gross Anatomy
• Shape and size
– About the size of a bar of bath soap
– Lateral surface is convex, and medial is concave with a slit, called the
hilum
• Receives renal nerves, blood vessels, lymphatics, and ureter
• Three protective connective tissue coverings
– Renal fascia immediately deep to parietal peritoneum
• Binds it to abdominal wall
– Perirenal fat capsule: cushions kidney and holds it into place
– Fibrous capsule encloses kidney protecting it from trauma and
infection
• Collagen fibers extend from fibrous capsule to renal fascia
• Still drop about 3 cm when going from lying down to standing up
Gross Anatomy of the Kidney
Fibrous capsule
Renal cortex
Renal medulla
Renal papilla
Renal pelvis
Major calyx
Minor calyx
Renal column
Renal pyramid
Ureter
Renal blood
vessels
23-12
Gross Anatomy
• Two zones of renal parenchyma (urine producing tissue)
– Outer renal cortex
– Inner renal medulla
• Renal columns—extensions of the cortex that project inward
toward sinus
• Renal pyramids—6 to 10 with broad base facing cortex and
renal papilla facing sinus
– Lobe of the kidney: one pyramid and its overlying cortex
– Minor calyx: cup that nestles the papilla of each pyramid;
collects it urine
– Major calyces: formed by convergence of two or three minor
calyces
– Renal pelvis: formed by convergence of two or three major
calyces
– Ureter: a tubular continuation of the pelvis that drains the urine
down to the urinary bladder
The Nephron
• Each kidney has about
1.2 million nephrons
• Each composed of two
principal parts
– Renal corpuscle:
filters the blood
plasma
– Renal tubule: long
coiled tube that
converts the filtrate
into urine
Tubular Elements of the Kidney
• Each nephron begins in the cortex with a hollow bowl-like
structure of simple squamous epithelial tissue called
Bowman’s capsule that surrounds the a small ball shaped
capillary bed called the glomerulus
– the endothelium of the glomerulus is fused to the
epithelium of Bowman’s capsule so that plasma filtering
out of the capillaries passes directly into he lumen of the
tubule
– combination of the glomerulus and Bowman’s capsule is
called the renal corpuscle
• After the plasma has been filtered into Bowman’s capsule it
will flow through the remaining segments of the renal
tubule where the composition of the tubular fluid will
change into urine
The Renal Corpuscle
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Key
Flow of blood
Flow of filtrate
Afferent
arteriole
Glomerular capsule:
Parietal layer
Capsular
space
Glomerulus
Blood
flow
Proximal
convoluted
tubule
Efferent
arteriole
Blood flow
Tubular Elements of the Kidney
• From Bowman’s capsule, the filtered fluid (filtrate) flows into the proximal
convoluted tubule, also located in the cortex and then into the loop of Henle,
a hairpin shaped segment that dips down (descending limb) into the medulla
and then back up (ascending limb) into the cortex
• Fluid from the ascending limb of the loop of Henle passes into the distal
tubule
• The distal tubule of up to 8 nephrons drains into a single larger tube called
the collecting duct
• Collecting ducts pass from the cortex through the medulla and drain urine
into the renal pelvis
• From the pelvis, the filtered and modified fluid (urine) flows into the ureter
Flow of fluid from the point where the glomerular filtrate is formed to
the point where urine leaves the body:
glomerular capsule → proximal convoluted tubule → nephron
loop → distal convoluted tubule → collecting duct → papillary duct
→ minor calyx → major calyx → renal pelvis → ureter → urinary
bladder → urethra
Vascular Elements of the Kidney
• The renal artery divides into smaller arteries and then into arterioles as
they branch from the hilus to cortex
• From the arterioles the arrangement of blood vessels turns into a
portal system
– blood flows from the afferent arteriole into a ball-like network of
capillaries called the glomerulus
– blood flowing out of the glomerulus flows into the efferent
arteriole and then into a second set of capillaries called the
peritubular capillaries which surround the tubule segments of the
nephron located in the cortex
• the peritubular capillaries of juxtamedullary nephrons
surrounding the loop of Henle and collecting duct (medulla) are
called vasa recta
• Finally the peritubular capillaries and vasa recta join to form venules
and small veins conducting blood out of the kidney through the renal
vein
• In the cortex,
peritubular capillaries
branch off of the
efferent arterioles
supplying the tissue
near the glomerulus,
the proximal and
distal convoluted
tubules
• In the medulla, the
efferent arterioles
give rise to the vasa
recta, supplying the
nephron loop portion
of the nephron
Cortical nephron
Juxtamedullary nephron
C
o
r
t
e
x
Afferent arteriole
Glomerulus
Efferent arteriole
PCT
Interlobular artery
DCT
Interlobular vein
Peritubular
capillaries
Corticomedullary
junction
Arcuate artery
Arcuate vein
Vasa recta
M
e
d
u
l
l
a
Collecting duct
Nephron loop
Unique Features of the Nephron
• An advantage of having 2 separate capillaries allows one capillary to
perform the process of filtration while the other capillary to performs
absorption
– the filtration of fluid and solutes out of the blood and into the
lumen of the renal tubule occurs at the renal corpuscle
– the (re)absorption of fluid and solutes from the tubule back into the
blood occurs at the peritubular capillaries
• Notice how the nephron twists and folds back on itself so that the final
part of the ascending limb of Henle passes between the afferent and
efferent arteriole
– this region is called the juxtaglomerular apparatus
– the proximity of the ascending limb and the arterioles allows for
paracrine communication which is a key feature of renal function
3 Basic Renal Functions
• Glomerular filtration
– the movement of fluid from blood into the lumen of the nephron
(takes place at the renal corpuscle)
• Tubular reabsorption
– moving substances in the filtrate from the lumen of the tubule back
into the blood flowing through peritubular capillaries
• Tubular secretion
– selectively removes molecules from the blood flowing through the
peritubular capillaries and adds them to the filtrate in the tubule
lumen
• The amount of solutes and water excreted in urine is determined by
the amount that is filtered + the amount that is secreted – the amount
that is reabsorbed
Glomerular Filtration
• In the glomerular capillaries, the hydrostatic pressure NEVER falls
below the osmotic pressure therefore fluid ONLY moves out of the
glomerulus into Bowman’s capsule (never the opposite)
– the rate that plasma exits the glomerulus and enters the Bowman’s
capsule is called the glomerular filtration rate (GFR) and
determines the rate of urine production by the kidneys
• 20% of the plasma that passes through the glomeruli (180 liters) is
filtered into the renal tubules daily
• Once the filtrate passes into the lumen of the nephron it becomes part
of the body’s external environment, thus anything that is filtered into
the nephron is destined for removal in the urine unless it is reabsorbed
back into the body
• Over 99% of the water and solutes that are filtered into
Bowman’s capsule is reabsorbed before it gets to the
bladder
– this leaves approximately 1.5 liters of urine to be
collected in the bladder per day
Tubular Reabsorption and Tubular Secretion
• As the filtrate moves through the segments of the renal
tubule, the processes of reabsosrption and secretion modify
not only the composition and volume of the fluid within the
tubule lumen converting it from what is essentially plasma
into urine, but also modifies the composition and volume of
plasma
• The epithelial cells of the renal tubule segments following
Bowman’s capsule have transporting proteins (channels,
primary and secondary active transporters) which
selectively move water and solutes between the blood and
the lumen of the tubule
Osmolarity of the Interstitial Fluid of the Kidneys
• The segments of the renal tubule and the renal vasculature is
surrounded by interstitial fluid of the kidney which in the cortex has a
total osmolarity (solute concentration) of 300 mOsm, which is
isosmotic with plasma
• However, the interstitial fluid of the medulla is hyperosmotic (more
solutes) compared to plasma as it increases from 300 mOsm at the
boundary between the cortex and medulla to a value of 1200 mOsm at
its deepest point
• The high amount of solutes in the interstitial fluid establish both solute
diffusion and osmotic gradients between the interstitial fluid and the
tubular fluid promoting tubular reabsorption
Osmolarity of the Interstitial Fluid of the Kidneys
Proximal Convoluted Tubule (PCT) (cortex)
• Very permeable to many solutes AND water due to the presence of
solute transporting proteins and aquaporins in the membranes of the
epithelial cells
– 70% of the glomerular filtrate is reabsorbed out of the renal tubule
into the interstitial fluid and then into the peritubular capillaries
– the reabsorption of solutes occurs mainly by primary and
secondary active transport processes
• all of the amino acids and glucose are reabsorbed
• most of the ions (Na+, K+, Cl-, HCO3-, Ca2+…) are reabsorbed
– the reabsorption of water follows the reabsorption of the solutes
via solvent drag which keeps the solute concentration of the filtrate
at 300 mOsm as it enters the descending limb of Henle (DLH)
• Two routes of reabsorption
– Transcellular route
• Substances pass through the cytoplasm of the PCT epithelial
cells and out their base
– Paracellular route
• Substances pass between PCT cells
• Junctions between epithelial cells are quite leaky and allow
significant amounts of water to pass through
• Solvent drag—water carries with it a variety of dissolved
solutes
Sodium reabsorption is the key to everything else
• Creates an osmotic and electrical gradient that drives the
reabsorption of water and other solutes
• Most abundant cation in filtrate
• Creates steep concentration gradient that favors its diffusion into
the epithelial cells
Proximal Tubule
peritubular
capillary
tubular fluid
amino acids,
glucose,
lactate, Pi
Na+
K+
K+
Na+
ClH2O
K+, H2O
glucose, amino
acids, lactate, Pi, ClNa+
ClH2O
K+, H2O
Descending Limb of Henle (cortex → medulla)
• Permeable ONLY to water due to the presence of aquaporins and the
absence of solute transporters in the membranes of the epithelial cells
• As the filtrate flows through the DLH, the higher solute concentration
of the surrounding interstitial fluid creates an osmotic gradient that
favors the diffusion of water out of the DLH into the interstitial fluid
and then into the vasa recta
– reabsorbs 15% of the water in the glomerular filtrate
– the solute concentration of the filtrate increases to 1200 mOsm as
it equilibrates with the surrounding interstitial fluid and enters the
ascending limb of Henle (ALH)
DLH
interstitial fluid
vasa recta
tubular fluid
solutes
solutes
H2O
solutes
solutes
solutes
H2O
solutes
H2O
H2O
H2O
H2O
solutes
solutes
Ascending Limb of Henle (medulla → cortex)
• Permeable ONLY to solutes (mainly Na+, K+ and Cl-) due to the
presence of solute transporters and the absence of aquaporins
• The osmolarity of the filtrate as it enters the ALH is 1200 mOsm
– As the filtrate flows through the ALH, the lower solute
concentration of the surrounding interstitial fluid creates a
diffusion gradient that favors the diffusion of solutes out of the
ALH into the interstitial fluid and then into the vasa recta
• reabsorbs 25% of the Na+, K+ and Cl- in the glomerular filtrate
• the solute concentration of the filtrate decreases as it
equilibrates with the surrounding interstitial fluid
Ascending Limb of Henle
• In the thin portion of the ascending limb, the reabsorption of Na+, K+
and Cl- is passive where solutes leave the filtrate by diffusion and
enter the surrounding interstitial fluid
• In the thick portion of the ascending limb, there is an aggressive
reabsorption of Na+, K+ and Cl- by active transport processes
– causes the filtrate to become dilute (hypotonic) compared to the
surrounding interstitial fluid
• at the end of the ALH (cortex) the filtrate osmolarity is 100
mOsm
Thick ALH
peritubular
capillary
tubular fluid
solutes
solutes
Cl-
solutes
K+
Na+
K+
solutes
K+
2 ClK+
2 ClK+
Na+
Na+
solutes
solutes
solutes
Countercurrent Multiplier of Nephron Loop
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1 More salt is continually
added by the PCT.
300
100
5 The more salt that
is pumped out of the
ascending limb, the
saltier the ECF is in
the renal medulla.
2 The higher the osmolarity
of the ECF, the more water
leaves the descending limb
by osmosis.
400
200
Na+
K+
Cl–
Na+
K+
Cl–
H2O
H2O
600
Na+
K+
Cl–
400
Na+
K+
Cl–
H2O
Na+
K+
Cl–
H2O
700
3 The more water that leaves
the descending limb, the
saltier the fluid is that
remains in the tubule.
900
H2O
23-37
1,200
Na+
K+
Cl–
4 The saltier the fluid in the
ascending limb, the more
salt the tubule pumps into
the ECF.
Distal Tubule (DT) and Collecting Duct (CD)
• Permeable to BOTH solutes AND water due to the presence of solute
transporting proteins and aquaporins in the membranes of the
epithelial cells
– the permeabilities can vary depending upon the levels of certain
hormones in circulation that bind to receptors on/in the epithelial
cells of the renal tubule
• change the number and/or activity of the membrane
transporting proteins
• As the filtrate flows through the DT and CD, the solute concentrations
of the surrounding interstitial fluid creates osmotic and diffusion
gradients that favors the diffusion of solutes and water out of these
segments into the interstitial fluid and then into the peritubular
capillaries and vasa recta
DT and CD
interstitial fluid
tubular fluid
peritubular
capillary/
vasa recta
solutes
solutes
K+
solutes
Na+
H2O
solutes
solutes
K+
Na+
K+
solutes
Na+
H2O
solutes
solutes
Water Reabsorption by the Collecting Duct
Tubular fluid
(300 mOsm/L)
Cortex
Medulla
Osmolarity of tissue fluid (mOsm/L)
300
600
H2O
H2O
900
Collecting
duct
H2O
H2O
1,200
H2O
Urine
(up to 1,200 mOsm/L)
Nephron
loop
• Collecting duct (CD) begins in the
cortex where it receives tubular
fluid from several nephrons
• As CD passes through the medulla,
it reabsorbs water and concentrates
urine up to four times
• Medullary portion of CD is more
permeable to water than to NaCl
• As urine passes through the
increasingly salty medulla, water
leaves by osmosis, concentrating
urine
Composition and Properties of Urine
• Urinalysis—the examination of the physical and chemical
properties of urine
• Appearance—clear, almost colorless to deep amber-yellow color
due to urochrome pigment from breakdown of hemoglobin
(RBCs); other colors from foods, drugs, or diseases
– Cloudiness or blood could suggest urinary tract infection,
trauma, or stones
– Pyuria: pus in the urine
– Hematuria: blood in urine due to urinary tract infection,
trauma, or kidney stones
• Odor—bacteria degrade urea to ammonia, some foods impart
aroma
Composition and Properties of Urine
• Specific gravity—compared to distilled water
– Density of urine ranges from 1.001 to1.028 g/mL
• Osmolarity (blood = 300 mOsm/L)
– Ranges from 50 mOsm/L to 1,200 mOsm/L in dehydrated
person
• pH—range: 4.5 to 8.2, usually 6.0 (mildly acidic)
• Chemical composition: 95% water, 5% solutes
– Normal to find: urea, NaCl, KCl, creatinine, uric acid,
phosphates, sulfates, traces of calcium, magnesium, and
sometimes bicarbonate, urochrome, and a trace of bilirubin
– Abnormal to find: glucose, free hemoglobin, albumin, ketones,
bile pigments
Urine Volume
•
•
•
•
Normal volume for average adult—1 to 2 L/day
Polyuria—output in excess of 2 L/day
Oliguria—output of less than 500 mL/day
Anuria—0 to 100 mL/day
– Low output from kidney disease, dehydration, circulatory
shock, prostate enlargement
– Low urine output of less than 400 mL/day, the body cannot
maintain a safe, low concentration of waste in the plasma
The Urinary Bladder
• Urinary bladder—muscular sac located on floor of pelvic cavity
– Inferior to peritoneum and posterior to pubic symphysis
• Three layers
– Parietal peritoneum
– Muscularis: detrusor muscle: three layers of smooth muscle
– Mucosa: transitional epithelium
• Rugae—conspicuous wrinkles in relaxed bladder Trigone—
smooth-surfaced triangular area marked with openings of ureters
and urethra
• Capacity—moderate fullness is 500 mL, maximum fullness is 700
to 800 mL
– Highly distensible
– As it fills, it expands superiorly, rugae flatten
– Epithelium thins from five or six layers to two or three
The Urethra
• Female urethra: 3 to 4 cm long
• Bound to anterior wall of vagina
• External urethral orifice
– Between vaginal orifice and clitoris
Ureter
Detrusor
muscle
Ureteral
openings
• Internal urethral sphincter
– Detrusor muscle thickening
Trigone
Internal urethral
sphincter
Urethra
– Smooth muscle under involuntary
control
• External urethral sphincter
Urogenital
diaphragm
External urethral
orifice
External urethral
sphincter
(a) Female
– Where the urethra passes through
the pelvic floor
– Skeletal muscle under voluntary
control
The Urethra
• Male urethra: 18 cm long
Ureter
Rugae
Detrusor
muscle
Ureteral
openings
• Three regions
– Prostatic urethra (2.5 cm)
Trigone
Internal urethral
sphincter
Prostate gland
Prostatic urethra
Urogenital
diaphragm
Membranous
urethra
Bulbourethral
gland
External urethral
sphincter
Spongy (penile)
urethra
• Passes through prostate gland
– Membranous urethra (0.5 cm)
• Passes through muscular floor of
pelvic cavity
– Spongy (penile) urethra (15 cm)
• Passes through penis in corpus
spongiosum
• Internal urethral sphincter
– Detrusor muscle thickening
Penis
• External urethral sphincter
– Part of skeletal muscle of pelvic floor
External urethral orifice
(b) Male
Involuntary micturition reflex
Stretch receptors detect filling
of bladder, transmit afferent
signals to spinal cord.
Signals return to bladder from
spinal cord segments S2 and S3
via parasympathetic fibers in
pelvic nerve.
Efferent signals excite
detrusor muscle.
Efferent signals relax internal
urethral sphincter. Urine is
involuntarily voided if not
inhibited by brain.
Voluntary control
For voluntary control, micturition
center in pons receives signals
from stretch receptors.
If it is timely to urinate, pons
returns signals to spinal
interneurons that excite detrusor
and relax internal urethral sphincter.
Urine is voided.
If it is untimely to urinate, signals
from pons excite spinal interneurons
that keep external urethral sphincter
contracted. Urine is retained in
bladder.
If it is timely to urinate, signals
from pons cease and external
urethral sphincter relaxes. Urine
is voided.
Neural Control of Micturition
To pons
From pons
5
6
7
Pelvic nerve
Sensory
fiber
Motor
fiber
Full
urinary bladder
Sacral segments
of spinal cord
S2
2
1
S3
3
Parasympathetic
ganglion in
bladder wall
Stretch receptors
Motor fibers to
detrusor muscle
Internal urethral
sphincter (involuntary)
External urethral
sphincter (voluntary)
4
Urethra
8
Somatic motor fiber
of pudendal nerve
S4
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Water, Sodium and Potassium Balance
• The kidneys function to regulate the water, Na+ and K+
amounts in the body
– imbalances in these substances will impact blood
pressure, membrane potentials, and cell volume
• The reabsorption of water and Na+ and the secretion of K+
at the DT and CD can be changed by certain hormones such
as Antidiuretic Hormone (ADH), aldosterone, and Atrial
Natriuretic Hormone (ANH)
– the secretion rates of these hormones can either increase
or decrease in response to an imbalance in the water, Na+
and K+ amounts in the body in order to reestablish
homeostasis
Water Balance
• Water balance is
achieved when input
equals output
• Changes in water input
and output force the
kidneys to alter urine
volume in order to
maintain water balance
– controlled by ADH
ADH Secretion
• Chemoreceptors in the hypothalamus monitor the ECF
solute (Na+) concentration
– an increase in ECF osmolarity (negative water balance)
stimulates ADH secretion from the posterior pituitary
gland
• caused by either an increase in solutes or a decrease in
water in the ECF
– a decrease in ECF osmolarity (positive water balance)
inhibits ADH secretion from the posterior pituitary gland
• caused by either a decrease in solutes or an increase in
water in the ECF
• A decrease in blood pressure can also stimulate the
secretion of ADH through a mechanism discussed later
ADH Action
• ADH targets the cells of the DT/CD
– binding of ADH to its receptor in tubular cells stimulates
the fusion of vesicles containing aquaporins with the
apical membrane, increasing the water permeability and
thus water reabsorption
• decreases the ECF osmolarity
• ADH stimulates the thirst center in the brain
– consumption of fluids decreases the ECF osmolarity
ADH
peritubular
capillary/
vasa recta
without ADH
H2O
H2O
H2O
H2O H O
2
ADH
interstitial fluid
tubular fluid
cAMP
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
with ADH
Aldosterone Secretion
• Chemoreceptors in the adrenal cortex (zona glomerulosa)
monitor the ECF solute (Na+) concentration
• Is secreted from the cortex of the adrenal gland in response
to any of the following stimuli:
– an increase in blood K+
– a decrease in blood Na+
• A decrease in blood pressure can also stimulate the
secretion of aldosterone through a mechanism discussed
later
Effects of K+ Imbalance
Aldosterone Action
• Aldosterone targets the cells of the DT/CD
– binding of aldosterone to its receptor in tubular cells
stimulates the transcription and translation of:
• the Na+, K+-ATPase which is inserted into the basal
membrane
• Na+ channels which are inserted into the apical
membrane
• K+ channels which are inserted into the apical
membrane
– the simultaneous functioning of these 3 proteins:
• increase Na+ reabsorption
• increase K+ secretion
Aldosterone
interstitial fluid
peritubular
capillary/
vasa recta
H2O
H2O
H2O
tubular fluid
K+
K+ +
Na
K+ +
Na
K+ +
Na
K+ +
Na
K+ +
Na
K+ +
Na
K+ +
Na
K+ +
Na
K+ +
Na
K+ +
Na
K+ +
Na
K+ +
Na
K+ +
Na
K+ +
Na
K+
Na+
without
aldosterone
Na+
K+
K+
K+ with
K+
Na+ aldosterone
Na+
Na+
Na+
H2O
H2O
H2O
Atrial Natriuretic Hormone (ANH)
• An elevated blood pressure increases venous return which
excessively stretches the wall of the right atrium
– causes the secretion of Atrial Natriuretic Hormone from
the cardiac myocytes of the right atrium
• ANH targets the cells of the CD
– binding of ANH to its receptor in tubular cells inhibits
Na+ channels responsible for Na+ reabsorption
• decreases the reabsorption of H2O as it follows the
Na+ to the bladder
– decreases blood volume and blood pressure
• ANH also causes vasodilation of arteries further decreasing
blood pressure
ANH
interstitial fluid
vasa recta
tubular fluid
K+
K+ +
Na
K+ +
Na
K+ +
Na
K+
K+
Na+
Na+
K+
K+ +
Na
K+ +
Na
K+ +
Na
K+ +
Na
K+
X
X
with ANH
Na+
Na+
H2O
H2O
without
ANH
H2O
H2O
• The JGA provides a way for the kidneys to monitor
and regulate blood pressure
Renal Control of Hypotension
• A decrease in systemic blood pressure will decrease the
glomerular hydrostatic pressure which will decrease GFR
– a decrease in GFR decreases the volume of filtrate in the
renal tubule
• decreases (slows down) the flow rate of the tubular
fluid through the renal tubule
• The slow moving tubular fluid is in the thick ALH longer
than normal resulting in an increased amount of Na+ that is
actively transported out of the ALH
– the reduced amount of Na+ remaining in the tubular fluid
as it flows out of the ALH, stimulates the macula densa
cells of the DT (chemoreception)
Renal Control of Hypotension
• The stimulated macula densa cells secrete a signaling
molecule that diffuses to the afferent arteriole and stimulates
the juxtaglomerular (JG) or granular cells of the afferent
arteriole to secrete an enzyme called renin into circulation
– renin catalyzes the conversion of a plasma protein called
Angiotensinogen (inactive) to Angiotensin I (active)
• Angiotensin I circulates through the capillaries of the
lungs where the enzyme Angiotensin Converting
Enzyme (ACE) converts it to Angiotensin II (more
potent form)
Angiotensin II
• Angiotensin II increases BP by stimulating:
– vasoconstriction of arterioles
– thirst
• consumption of water
– increases plasma volume
– the secretion of aldosterone from the adrenal cortex
• increases Na+ retention at the kidneys
– the secretion of ADH from the posterior pituitary
• stimulates vasoconstriction of arteries
• increases water retention at the kidneys
• “ACE inhibitors” are a class of drugs prescribed to
individuals with hypertension
– decrease Angiotensin II levels
• decrease blood pressure
Renal Regulation of pH (acid/base balance)
• The kidneys respond to acidosis by:
– secreting excess H+ into the CD
• decreases H+ in the body
– reabsorbing HCO3- out of the CD into the vasa recta
• increases HCO3- in the body
– synthesizing “new” HCO3- which enters circulation
• increasing HCO3- in the body
• The kidneys respond to alkalosis by:
– reabsorbing H+
• increases H+ in the body
– decreasing the reabsorption of HCO3• decreases HCO3- in the body
Control of Body Ca2+ and PO42- Levels
• Parathyroid glands are 4 tiny glands embedded in the posterior aspect
of the thyroid
• Functional cells are called chief (or principal) cells
– synthesizes and secretes the hormone parathyroid hormone (PTH)
following a decrease in plasma Ca2+ levels
• Thyroid gland (parafollicular cells) secrete calcitonin following an
increase in plasma Ca2+ levels in children
– targets the osteoblasts in bone and stimulates the synthesis of bone
matrix
Parathyroid Glands
PTH
• Targets:
– osteoclasts in bone to
• break down bone matrix
– Ca2+ and PO42- moves into plasma (increasing
levels of both)
– the small intestine
• increase the absorption of calcium from the diet into
plasma
– kidneys to:
• increase the Ca2+ permeability in the DT/CD
– increase Ca2+ reabsorption
• increase plasma Ca2+ levels
• decrease PO42- permeability in the DT/CD
– decrease PO42- reabsorption
• maintain plasma PO42- levels