The Urinary System

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The Urinary System
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TABLE OF CONTENT 1) General Introduction 2)
Anatomy of Urinary System 3) Urine Formation 4)
Urine Storage and Elimination
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Composition of the Urinary System
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Functions of the Kidneys
1) filter blood plasma, separate wastes, return
useful materials to the blood, and eliminate the
wastes.
Toxic nitrogenous wastes - ammonia, urea, uric
acid, creatine, and creatinine
- cause diarrhea, vomiting, and cardiac
arrhythmia, convulsions, coma, and death.
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Functions of the Kidneys
1) filter blood plasma, separate wastes, return
useful materials to the blood, and eliminate the
wastes.
2) regulate blood volume and osmolarity.
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Functions of the Kidneys

• 3) produce hormones
• renin
• erythropoietin
• calcitrol

4) regulate acid-base balance of the body fluids.
5) detoxify superoxides, free radicals, and drugs.
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ANATOMY OF THE KIDNEYS
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- The kidneys lie along the posterior abdominal
wall
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• The medial surface of the kidney is concave with
  a hilum carrying renal nerves and blood vessels.

The renal parenchyma is divided into an outer
cortex and inner medulla.
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Extensions of the cortex (renal columns) project
toward the sinus, dividing the medulla into 6-10
renal pyramids. Each pyramid is conical with a
blunt point called the papilla facing the sinus.
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The papilla is nestled into a cup called a minor
calyx, which collects its urine. Two or three
minor calyces merge to form a major calyx. The
major calyces merge to form the renal pelvis.
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The Nephron
- The kidney contains 1.2 million nephrons, which
are the functional units of the kidney.
- A nephron consists of i. blood vessels
afferent arteriole glomerulus efferent
arteriole ii. renal tubules proximal
convoluted tubule loop of Henle distal
convoluted tubule
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The Nephron
glomerulus
proximal convoluted tubule
efferent arteriole
blood
distal convoluted tubule
blood
afferent arteriole
Loop of Henle
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The Nephron
- Most components of the nephron are within the
cortex.
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Nephrons are connected to renal artery/vein and
ureter.
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The glomerulus is enclosed in a two-layered
glomerular (Bowman's) capsule.
Proximal tubule
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URINE FORMATION
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The kidney produces urine through 4 steps.
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Glomerular Filtrate Tubular fluid Urine
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1) Glomerular Filtration
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The Filtration Membrane
From the plasma to the capsular space, fluid
passes through three barriers.
foot processes
fenestrated epithelium
basement membrane
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The Filtration Membrane
Almost any molecule smaller than 3 nm can pass
freely through the filtration membrane into the
capsular space.
These include Water, electrolytes, glucose,
amino acids, lipids, vitamins, and nitrogenous
wastes
Kidney infections and trauma commonly damage the
filtration membrane and allow plasma proteins or
blood cells to pass through.
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Blood cells
in urine
Plasma proteins
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Filtration Pressure Glomerular filtration
follows the same principles that govern
filtration in other capillaries.
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• Glomerular Filtration Rate (GFR)
• is the amount of filtrate formed per minute by
  the two kidneys combined.
• For the average adult male, GFR is about 125
  ml/min.
• This amounts to a rate of 180 L/day.
• An average of 99 of the filtrate is
  reabsorbed, so that only 1-2 L of urine per day
  is excreted.

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• GFR must be precisely controlled.
• If GFR is too high
• - increase in urine output
• - threat of dehydration and electrolyte
  depletion.
• If GFR is too low
• - insufficient excretion of wastes.
• c. The only way to adjust GFR from moment to
  moment is to change glomerular blood pressure.

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• Renal Autoregulation
• the ability of the kidneys to maintain a
  relatively stable GFR in spite of the changes
  (75 - 175 mmHg) in arterial blood pressure.

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The nephron has two ways to prevent drastic
changes in GFR when blood pressure rises 1)
Constriction of the afferent arteriole to reduce
blood flow into the glomerulus 2) Dilation of
the efferent arteriole to allow the blood to flow
out more easily.
Change in an opposite direction if blood pressure
falls
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• Mechanisms of Renal Autoregulation
• 1)    myogenic response
• 2)    tubuloglomerular feedback

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1)    myogenic response
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2)    tubuloglomerular feedback
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1) Glomerular Filtration 2) Tubular
Reabsorption 3) Tubular Secretion 4)
Concentrating Urine by Collecting Duct
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About 99 of Water and other useful small
molecules in the filtrate are normally reabsorbed
back into plasma by renal tubules.
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Reabsorption in Proximal Convoluted Tubules
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- The proximal convoluted tubule (PCT) is formed
by one layer of epithelial cells with long apical
microvilli. - PCT reabsorbs about 65 of the
glomerular filtrate and return it to the blood.
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Routes of Proximal Tubular Reabsorption
1) transcellular route 2) paracellular route
PCT
peritubular capillary
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Mechanisms of Proximal Tubular Reabsorption   1)
Solvent drag   2) Active transport of sodium. 3)
Secondary active transport of glucose, amino
acids, and other nutrients.  4) Secondary water
reabsorption via osmosis  5) Secondary ion
reabsorption via electrostatic attraction 6)
Endocytosis of large solutes
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Osmosis
Water moves from a compartment of low osmolarity
to the compartment of high osmolarity.
low osmolarity ( high H2O conc.)
H2O
high osmolarity ( low H2O conc.)
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• Solvent drag

Proteins stay

• - driven by high colloid osmotic pressure (COP)
  in the peritubular capillaries
• Water is reabsorbed by osmosis and carries all
  other solutes along.
• Both routes are involved.

H2O
Proteins
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2) Active transport of sodium Sodium pumps (Na-K
ATPase) in basolateral membranes transport sodium
out of the cells against its concentration
gradient using ATP.
Na
capillary
PCT cell
Tubular lumen
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There are also pumps for other ions
Ca
Ca
capillary
PCT cell
Tubular lumen
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3) Secondary active transport of glucose, amino
acids, and other nutrients
- Various cotransporters can carry both Na and
other solutes. For example, the sodium-dependent
glucose transporter (SDGT) can carry both Na and
glucose.
Na
Glucose
capillary
PCT cell
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3) Secondary active transport of glucose, amino
acids, and other nutrients
Amino acids and many other nutrients are
reabsorbed by their specific cotransporters with
sodium.
Na
amino acids
capillary
PCT cell
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4) Secondary water reabsorption via osmosis
Sodium reabsorption makes both intracellular
and extracellular fluid hypertonic to the tubular
fluid. Water follows sodium into the peritubular
capillaries.
Na
Na
H2O
capillary
PCT cell
Tubular lumen
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5) Secondary ion reabsorption via electrostatic
attraction
Negative ions tend to follow the positive sodium
ions by electrostatic attraction.  
Na
Na
Cl-
capillary
PCT cell
Tubular lumen
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6) Endocytosis of large solutes
The glomerulus filters a small amount of protein
from the blood. The PCT reclaims it by
endocytosis, hydrolzes it to amino acids, and
releases these to the ECF by facilitated
diffusion.
protein
amino acids
capillary
PCT cell
Tubular lumen
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• The Transport Maximum
• - There is a limit to the amount of solute that
  the renal tubule can reabsorb because there are
  limited numbers of transport proteins in the
  plasma membranes.
• If all the transporters are occupied as solute
  molecules pass through, some solute will remain
  in the tubular fluid and appear in the urine.

Example of diabetes
Na
Glucose
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high glucose in blood
high glucose in filtrate Exceeds Tm for glucose
Glucose in urine
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Reabsorption in the Nephron Loop
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- The primary purpose is to establish a high
extracellular osmotic concentration. - The
thick ascending limb reabsorbs solutes but is
impermeable to water. Thus, the tubular fluid
becomes very diluted while extracellular fluid
becomes very concentrated with solutes.
mOsm/L
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The high osmolarity enables the collecting duct
to concentrate the urine later.
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Reabsorption in Distal Convoluted Tubules
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• Fluid arriving in the DCT still contains about
  20 of the water and 10 of the salts of the
  glomerular filtrate.
• A distinguishing feature of these parts of the
  renal tubule is that they are subject to hormonal
  control.

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• Aldosterone
• secreted from adrenal gland in response to a ?
  Na or a ? K in blood
• to increase Na absorption and K secretion in
  the DCT and cortical portion of the collecting
  duct.
• helps to maintain blood volume and pressure.

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• Atrial Natriuretic Factor
• secreted by the atrial myocardium in response to
  high blood pressure.
• It inhibits sodium and water reabsorption,
  increases the output of both in the urine, and
  thus reduces blood volume and pressure.

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1) Glomerular Filtration 2) Tubular
Reabsorption 3) Tubular Secretion 4)
Concentrating Urine by Collecting Duct
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• Tubular Secretion
• Renal tubule extracts chemicals from the blood
  and secretes them into the tubular fluid.
• serves the purposes of waste removal and
  acid-base balance.

H
H
capillary
PCT cell
Tubular lumen
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1) Glomerular Filtration 2) Tubular
Reabsorption 3) Tubular Secretion 4)
Concentrating Urine by Collecting Duct
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• The collecting duct (CD) begins in the cortex,
  where it receives tubular fluid from numerous
  nephrons.
• CD reabsorbs water.

Cortex
collecting duct
urine
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1. Driving force The high osmolarity of
extracellular fluid generated by NaCl and urea,
provides the driving force for water
reabsorption. 2. Regulation The medullary
portion of the CD is not permeable to NaCl but
permeable to water, depending on ADH.
Cortex
medulla
mOsm/L
urine
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Control of Urine Concentration depends on the
body's state of hydration.
a. In a state of full hydration, antidiuretic
hormone (ADH) is not secreted and the CD
permeability to water is low, leaving the water
to be excreted.
Cortex
medulla
b. In a state of dehydration, ADH is secreted
the CD permeability to water increases. With the
increased reabsorption of water by osmosis, the
urine becomes more concentrated.
mOsm/L
urine
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No more reabsorption after tubular fluid leaving
CD
Cortex
medulla
urine
urine
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Urine Properties
Composition and Properties of Urine Fresh urine
is clear, containing no blood cells and little
proteins. If cloudy, it could indicate the
presence of bacteria, semen, blood, or menstrual
fluid.
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Urine Volume An average adult produces 1-2 L
of urine per day. a. Excessive urine output
is called polyuria. b. Scanty urine output is
oliguria. An output of less than 400 mL/day is
insufficient to excrete toxic wastes.
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• Diabetes
• is chronic polyuria resulting from various
  metabolic disorders, including Diabetes mellitus
  and Diabetes insipidus

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• Diabetes mellitus
• caused by either
• 1) deficiency of insulin (Type I) or
• 2) deficiency of insulin receptors (Type II).
• Diabetes mellitus features high glucose in the
  blood (hyperglycemia)

pancreatic ? cell
insulin receptors
insulin
cell
glucose
cell
glycogen
blood
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high glucose

• When glucose in tubular fluid exceeds the
  transport maximum (180 mg/100 ml), it appears in
  urine (glycosuria).
• Glucose in tubular fluid hinders water
  reabsorption by osmosis, causing polyuria.

high glucose in filtrate
Retain H2O by osmosis
high urine volume
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Diabetes insipidus - is caused by inadequeate
ADH secretion. - Due to the shortage of ADH,
water reabsorption in CD is compromised, leading
to polyuria.
? urine
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Diuresis refers to excretion of large amount
of urine. Natriuresis refers to enhanced
urinary excretion of sodium
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Diuretics - are chemicals that increase
urine volume. They are used for treating
hypertension and congestive heart failure because
they reduce overall fluid volume. - work by
either increasing glomerular filtration or
reducing tubular reabsorption. Caffeine falls
into the former category alcohol into the latter
(alcohol suppresses the release of ADH).
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Many diuretics produce osmotic diuresis by
inhibiting sodium reabsorption
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Renal Function Tests
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• Renal Clearance
• the volume of blood plasma from which a
  particular waste is removed in 1 minute.
• b. can be measured indirectly by measuring the
  waste concentration in blood and urine, and the
  urine volume.

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• Glomerular Filtration Rate
• a. Measuring GFR requires a substance that is not
  secreted or reabsorbed at all. Inulin, a polymer
  of fructose, is suitable.
• b. Inulin filtered by the glomeruli remains in
  the renal tubule and appears in the urine none
  is reabsorbed, and the tubule does not secrete
  it. For this solute, GFR is equal to the renal
  clearance.

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Hemodialysis
artificially clearing wastes from the blood
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1) Dialysis machine - efficient - inconvenient
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2) Continuous ambulatory peritoneal dialysis
(CAPD)
Dialysis fluid
- The peritoneal membrane is a natural dialysis
membrane - convenient - less efficient
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Urine Storage and Elimination
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The Ureters The ureters are muscular tubes
leading from the renal pelvis to the lower
bladder.
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The Urinary Bladder - is a muscular sac on the
floor of the pelvic cavity.
- is highly distensible and expands superiorly.
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The openings of the two ureters and the urethra
mark a triangular area called the trigone on the
bladder floor.
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The Urethra - conveys urine from the urinary
bladder to the outside of the body. Females
male 3-4 cm 18 cm
greater risk of urinary tract infections
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• The male urethra has three regions
• prostatic urethra
• 2) membranous urethra
• 3) penile urethra.

Difficulty in voiding urine with enlarged prostate
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In both sexes - internal urethral sphincter-
under involuntary control. - external urethral
sphincter - under voluntary control
internal urethral sphincter
external urethral sphincter
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Voiding Urine in infants micturition
reflex When the bladder contains about 200 ml of
urine, stretch receptors in the wall send
impulses to the spinal cord. Parasympathetic
signals return to stimulate contraction of the
bladder and relaxation of the internal urethral
sphincter.
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Voiding Urine in adults
2. Once voluntary control has developed, emptying
of the bladder is controlled predominantly by a
micturition center in the pons. This center
receives signals from stretch receptors and
integrates this information with cortical input
concerning the appropriateness of urinating at
the moment. It sends back impulses to stimulate
relaxation of the external sphincter.
Once voluntary control has developed, emptying of
the bladder is controlled predominantly by a
micturition center in the pons. This center
receives signals from stretch receptors and
integrates this information with cortical input
concerning the appropriateness of urinating at
the moment. It sends back impulses to stimulate
relaxation of the external sphincter.
Voluntary control
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SUMMARY 1) General Introduction 2) Anatomy of
Urinary System 3) Urine Formation 4) Urine
Storage and Elimination