Download The Urinary System

The Urinary System
Dr. Ahmed Al-Dwairi
Department of Physiology
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Functions of the Urinary System
 Contributing to homeostasis
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Controlling electrolyte and water balance of the ECF, plus urinary
output.
If the ECF has an excess of water or electrolytes, the kidneys eliminate
the excess. If there is a deficiency of these substances, the kidneys can
reduce the loss of these from the body.
Maintaining the proper osmolarity of body fluids
Maintaining proper plasma volume
Helping to maintain proper acid-base balance
Excreting wastes of body metabolism
Excreting many foreign compounds
Producing erythropoietin and renin
Converting vitamin D to an active form
 Gluconeogenesis
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Excretion of Metabolic Waste Products
• Urea (from protein metabolism)
• Uric acid (from nucleic acid metabolism)
• Creatinine (from muscle metabolism)
Secretion, Metabolism, and Excretion of Hormones
Hormones produced in the kidney
• Renal erythropoetic factor
• 1,25 dihydroxycholecalciferol (Vitamin D)
• Renin
Hormones metabolized and excreted by the kidney
• Most peptide hormones (e.g. insulin, angiotensin II,
etc.)
Body fluid regulation
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Guyton and Hall, Medical physiology 11th edition.
• Appearance
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Clear = normal
Cloudy = ? Infection
If sediment = kidney disease
Dark = ?blood, ?bilirubin,
?concentrated
• Color
– Urochrome pigment = yellow
• comes from breakdown of
hemoglobin
– Concentration
• More Concentrated = Deeper
Yellow
– Change of Color From:
• Meds
– Vitamin = yellow
• Diseases
– Blood = red-brown
– Liver = Orange
• Foods
– Rhubarb = red-brown
• Odor
– Normal = ammonia-like smell
• from breakdown of urea
– Unpleasant = ? infection
• Quantity
– Average per 24 hours = 1500 cc
• 60 cc per hour
• GFR = 125 cc/min
– Thus, 7500 cc/ hour
• Urine Made Per Hour = 60 cc
• GFR, Per Hour = 7500 cc
– KEY: 1 % of filtered urine remains
urine; 99 % becomes reabsorbed
back into blood
– Oliguria = 100 - 400 cc per day
– Anuria = less than 100 cc per day
– Polyuria = diabetes, nerves, diuretics
• Specific Gravity
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Determines Concentration
Compares Test Liquid to H2O
Normal = 1010 - 1030
In many kidney diseases, one
loses the ability to concentrate
urine
• Protein
– OK to have a Trace in the urine
– Benign Conditions:
• exercise
• exposure to cold
•  protein consumption
– Generally Means Kidney Disease
• Glucose
• pH
– Determines Acidity or Alkalinity
– Normal = 6.0
– Range = 4.5 - 8.0
• Acidity example = diabetes
• Alkaline example = UTI
– Will only be in urine if exceed Renal
Threshold.
• Ketone
– Ketones are products of Fat
Metabolism
– If cant breakdown Sugars for energy,
the body will begin using Fat
– Seen in:
• Uncontrolled Diabetes
• Starvation
• Hi-Fat Diet
Physiological Anatomy
Kidneys– The functional units of the system
Ureters
Urinary Bladder Conducting & Storage
components
Urethra
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Physiological anatomy:
• 1. General organization :
– 150 gm, size of a clenched fist.
– Consists of capsule, outer cortex, and inner
medulla.
– Medulla (renal pyramids)  papilla  renal pelvis
 ureter  bladder  urethra
• 2. Renal blood supply :
– 22% C.O  1100 ml/min .
– Renal artery  interlobar arteries  arcuate
arteries  interlobular arteries  afferent
arterioles  glomerular capillaries  efferent
arterioles  peritubular capillaries  venous
system .
Physiologic Anatomy
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Each kidney is supplied by a renal
artery and renal vein. The kidney acts
on the blood plasma flowing through it.
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As urine is formed, it drains into the
renal pelvis and is channeled into the
ureter.
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The urine is stored in the urinary
bladder. It is emptied periodically
through the urethra.
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The urethra serves the urinary and
reproductive tracts in the male.
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Guyton and Hall, Medical physiology 11 th edition.
The nephron; the functional unit of the
kidney
 Nephron is the smallest unit that can
perform all the functions of the
kidney. Each kidney has about one
million nephrons.
 Kidney layers:
• Cortex: the outer layer, looks
glomerular.
• Medulla: the inner layer made up of
striated triangles; the renal
pyramids.
 The nephrons are arranged through
the cortex and medulla of the kidney.
 Each nephron consists of:
i.
Vascular component
ii. Tubular component.
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i. The vascular component; the
dominant portion of the nephron
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ii. The tubular component; a hollow
tube with different regions
• Renal corpuscle – the glomerulus and
its Bowman’s capsule
Juxtaglomerular Apparatus
 Combined vascular/tubular microscopic
structure in the kidney regulating the function
of each nephron
 Between the vascular pole of the renal
arterioles and the returning distal convoluted
tubule of the same nephron
 Cellular components of the apparatus:
 Macula densa of the distal convoluted
tubule,
• Senses any increase in sodium
chloride concentration in the distal
tubule of the kidney and secretes a
locally active (paracrine)
vasopressor which acts on the
adjacent afferent arteriole
 Smooth muscle cells of the afferent
arteriole
 Juxtaglomerular cells (granular cells),
Secrete renin
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Types of Nephrons
1. Cortical nephron
 Glomeruli in outer cortex & short loops of
Henle that extend only short distance into
medulla
 Majority of nephrons
 Cortical interstitial fluid 300 mOsmolar
2. Juxtamedullary nephron
 Glomeruli in inner part of cortex & long loops
of Henle which extend deeply into medulla
 Blood flow through vasa recta in medulla is
slow
 This nephron maintains osmolality in
addition to filtering blood and maintaining
acid-base balance
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Basic Processes of the Nephron
 Glomerular filtration is the first process.
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protein-free plasma is filtered from the glomerulus into
the Bowman’s capsule. Blood cells are not normally
filtered. Normally about 20 % of the plasma is filtered.
Glomerular filtrate is produced at the rate of 125 ml per
minute (180 liters per day).
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The 80% of the plasma not filtered passes into the
efferent arteriole and through the peritubular
capillaries.
 Tubular reabsorption,
filtered substances move
from the inside of the tubular part of the nephron into
the blood of the peritubular capillaries. The
reabsorption rates of most substances are very high.
(of the 180 liters filtered, 178.5 liters are reabsorbed)
 Tubular secretion is a selective process by which
substances from the peritubular capillaries enter the
lumen of the nephron tubule.
 Urine excretion results from these three processes
= GF – TR + TS
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Renal Handling
of
Different Substances
Figure 26-10
Glomerular Filtration
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Renal Handling of Water and Solutes
Filtration
Reabsorption
Excretion
Water
(liters/day)
180
179
Sodium
(mmol/day)
25,560
25,410
Glucose
(gm/day)
180
180
0
Creatinine
(gm/day)
1.8
0
1.8
1
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Glomerular Filtration
GFR = 125 ml/min = 180 liters/day
• Plasma volume is filtered 60 times per day
• Glomerular filtrate composition is about the
same as plasma, except for large proteins
• Filtration fraction (GFR/Renal Plasma Flow)
= 0.2 (i.e., 20% of plasma is filtered)
Nephron Filtration Membrane
1. The wall of the glomerular capillaries, which is a single
layer of flattened endothelial cells. It is perforated by many
large pores that make it over 100 times more permeable to H2O
and solutes than capillaries elsewhere in the body.
2. The basement membrane, which is an acellular (lacking
cells) gelatinous layer.
3. The inner layer of Bowman’s capsule, which consists of
podocytes, octopus-like cells that encircle the glomerular tuft.
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The filtration membrane
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Guyton and Hall, Medical physiology 11 th edition.
Why huge GFR?
• 1) Most waste product are removed rapidly
• 2) Allow all body fluids to be filtered and
processed by kidneys many times each day .
 60 times/day
 high GFR provides precise & rapid control
over body fluids (volume, composition)
Effects of size and electrical charge of dextran on
filterability by glomerular capillaries.
Figure 26-12
Glomerular Filtration Forces
No active transport mechanisms or local
energy expenditure are involved in in moving
fluid across the glomerular membrane into
Bowman’s capsule.
Passive physical forces, with the same
principles of fluid dynamics are involved in
filtration:
1. The glomerular capillary hydrostatic
pressure is the result of the blood pressure
pushing on the inside of the capillary wall
(e.g., 60 mm Hg).
•
2.
The plasma-colloid osmotic pressure is
due to the retention of plasma proteins in the
blood of the glomerulus.
•
3.
It is the major force that induces glomerular
filtration. Depends contraction of the heart, and
afferent/efferent arteriolar resistance.
The concentration of water is higher in the
glomerulus. Water tends to return to the blood in
the glomerulus by osmosis (32 mm Hg).
The capsule hydrostatic pressure tending
to move fluid from the Bowman’s capsule into
the glomerulus (18 mm Hg).
• Net filtration pressure
 PG – PB – VTG + VTB
 60 – 18 – 32 + 0 = 10 mmHg
• K= 12.5 ml/min/mmHg
Glomerular Filtration Rate
(GFR)
 Glomerular Filtration Rate (GFR): the total amount of filtrate formed per
minute by the kidneys
 GFR regulation is mainly due to changes in the glomerular capillary blood
pressure; a higher arterial blood pressure supplying the glomerulus can
increase the GFR.
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Factors that affect GFR
1.
↓ Glomerular capillary filtration coefficient
↓ GFR
e.g.: chronic uncontrolled hypertension, DM
2.
↑ Bowman's capsule hydrostatic pressure
↓ GFR
e.g.: stones
Glomerular Capillary Filtration Coefficient (Kf)
• Kf = hydraulic conductivity
x
surface area
• Normally it is not highly variable
• Disease that can reduce Kf and GFR
- chronic hypertension
- obesity/diabetes mellitus
- glomerulonephritis
3. ↑ Glomerular capillary colloid osmotic
pressure ↓ GFR:
Averages 32 mmHg and this pressure is
determined by:
1. Arterial plasma colloid osmotic pressure
2. Fraction of plasma filtered (filtration Fraction)
 increased by: ↑GFR or ↓renal plasma flow
4. ↑ Glomerular capillary hydrostatic pressure 
↑ GFR :
Important in physiological regulation of GFR
this pressure is determined by:
1. Arterial pressure
2. Afferent arteriolar resistance
3. Efferent arteriolar resistance this has biphasic
effect:
Moderate ↑resistance  slight ↑ GFR
Severe constriction ↓ GFR
Effect of change in afferent arteriolar resistance or
efferent arteriolar resistance on glomerular filtration
rate and renal blood flow.
Regulation of GFR and renal blood flow
 Changes in the GFR primarily result from changes in glomerular
capillary blood pressure. Changes in plasma colloid osmotic pressure
and Bowman’s capsule hydrostatic pressure are not subject to
regulation and do not vary much under normal conditions.
 Uncontrolled shifts in the GFR can lead to fluid and electrolyte
imbalances.
 Three basic mechanisms to keep relatively constant GFR despite
changing mean arterial pressure
1. Autoregulation
2. Tubuloglomerular feedback mechanism
3. ANS and Hormones.
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Renal blood flow control
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•
•
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Autoregulation of GFR and Renal blood flow :
GFR remains constant if the Arterial pressure
ranges from 75 to 160 mmHg.
Normally filtration= 180 /day, reabs.= 178.5/day
 urine=1.5 /day
Without autoregulation if pressure ↑ by 25% 
GFR = 225 L/day If reabsorption is constant
urine= 46.5 L/day.
30 folds increase in urine formation depletes the
body
Renal blood flow:
1100 ml/min 22% c.o
large fraction of O2 is consumed by
the kidneys due to the high rate of
active sodium reabsorption by the
tubules
Regulation of GFR
1. Autoregulation; myogenic, altering the
caliber of the afferent arterioles due to
stretch.
•
If the GFR rises by increased arterial
pressure, the afferent arterioles
constrict. This lowers the GFR.
•
If the GFR decreases, the afferent
arterioles dilate. This increases the
GFR.
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Renal blood flow control (cont.)
Sympathetic N.S :
 Strong activation↓GFR and flow
 Moderate activation little effect
 Sympathetic nervous system plays an important role on
adjusting glomerular blood flow, while parasympathetic
nervous system does not have any influence on the
kidneys.
 Sympathetic innervation to afferent arterioles, 
vasoconstriction
 Stimulated when blood pressure drops to reduce GFR and
conserve fluid volume.
 Stimulation of sympathetic nerves increases the release of
renin by the kidneys
Renal blood flow control(cont.)
Hormones and autacoids:
 Norepinephrine , epinephrine, endothelin
constrict renal blood vessels and decreases GFR
 Angiotensin II constricts efferent arterioles
↑GFR - ↑ Na, H2O reabsorption.
 Endothelial - Derived Nitric oxide decreases
resistance and ↑ GFR
 Prostaglandins, bradykinin cause vasodilatation
Regulation of GFR
Tubuloglomerular
feedback mechanism.
sensing changes in flow in the
nephron’s tubular component.
• The cells of the macula densa
monitor NaCl concentration in
the fluid moving into the distal
convoluted tubule.
– If GFR increases, then NaCl
movement also increases
– Macula densa cells send a
paracrine message causing the
afferent arteriole to contract,
decreasing GFR and NaCl
movement
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• Autoregulation and glomerulotubular
balance try to maintain a constant GFR
 these processes are not 100% effective
so ↑BP will always lead to ↑GFR (pr.
Diuresis or pr. Natriuresis) .
Baroreceptor
reflex
influence on
the GFR in
long-term
regulation of
arterial blood
pressure
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