The Urinary System

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The Urinary System Taking The Piss !!!!
Dr Usama ALAlami
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Introduction
Nitrogenous waste products produced form
degradation of proteins and nucleic acids are
removed by the kidneys.
Kidney Function
1) Maintain water balance in the body
2) Control the type and amount of ions in the body
3) Maintain acid-base balance
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Overview Of Structure Function
Paired organs kidneys (filter blood to produce
urine)
Two tubes (ureters) convey urine to urinary
bladder
Urethra extending to external orifice
Kidneys are highly selective filtering devices
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Basic Mechanism Of Filtration
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Tissue Fluid
Derived from blood plasma
Water and low molecular weight substances
No blood cells or plasma proteins
Kidneys process tissue fluid by retaining useful
substances which are returned to the blood stream
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Nephron
Functional unit of the kidney
Each kidney contains approximately a million or
more nephrons (miniature filtering devices)
Form the bulk of the kidney tissue
Packed tightly together and coursed by blood and
lymph vessels and nerves
Urine is the collective product of all the
nephrons of the kidney
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Microscopic Structure Of The Nephron
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Microscopic Structure Of The Nephron
Each nephron consists of
a Vascular component
b Tubular component
a Vascular Component
The dominant portion of the vascular component is
the GLOMERULUS
Glomerulus Ball-like tuft of capillaries through
which filtration takes place
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b Tubular Component
Four components
_at_ Bowmans capsule
_at_ Proximal convoluted tubule
_at_ Loop of Henle
_at_ Distal convoluted tubule
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Bowmans Capsule
Double-walled structure
Inner surface consists of podocytes (podo foot)
(octopus-like cells)
Inner wall closely encircles and opposes the
endothelial membranes of the glomerular
capillaries
Narrow openings between podocytes Filtration
Slits
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b Tubular Component-Continued
Outer wall of Bowmans capsule is continuous with
the Proximal convoluted tubule
Proximal convoluted tubule winds extensively and
then straightens to form a thin U-shaped region
Lop of Henle
Ascending part of the loop of Henle joins a thick
tubule Distal convoluted tubule
Distal convoluted tubules of many nephrons empty
into the Collecting tubule or duct
Collecting tubule merge into larger vessels
Papillary ducts
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Blood Supply To The Nephron
Aorta gives rise to right and left renal arteries
The artery branches to form Afferent arterioles
(Afferent leading to)
Afferent arterioles deliver blood to the
glomerular capillaries
Afferent arterioles leave the glomerulus to form
Efferent arterioles (Efferent leading from)
Diameter of the Efferent arterioles is less than
that of the Afferent arterioles
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At the glomerular capillaries, no oxygen or
nutrients are extracted to be used by the kidney
Efferent arterioles subdivide to form peritubular
capillaries
Peritubular capillaries function
1) Supply renal tissue with blood
2) Exchange between tubular system and blood
during conversion of filtered fluid into urine
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The Kidneys
Bean-shaped organs on either side of vertebral
column
Outer cortex and inner medulla
Outer cortex contains glomeruli and most of the
proximal and distal convoluted tubules
Medulla contains 6-18 cone-shaped renal pyramids
Renal pyramids contain loops Of Henle, collecting
tubules and papillary ducts
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Renal Processes
Three basic processes involved in formation of
urine
1 Glomerular Filtration
2 Tubular Reabsorption
3 Tubular Secretion
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1 Glomerular Filtration
Filtration of protein-free plasma through
glomerular capillaries in the Bowmans
capsule
Average 180 litres daily
Average plasma volume in an individual 3 litres
Therefore, entire plasma volume is filtered 60
times daily
Three physical forces are involved in glomerular
filtration
a) Glomerular capillary blood pressure
b) Plasma colloid osmotic pressure
c) Bowmans capsule hydrostatic pressure
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a) Glomerular Capillary Blood Pressure
Fluid pressure exerted by blood within
glomerular capillaries
Approximately 55 mm Hg

• Depends on
• Contraction of the heart
• Resistance to blood flow by afferent and efferent
  arterioles

Ultrafiltration occurs in other capillaries
throughout the body
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However, glomerular filtration differs in two
ways
1 Glomerular capillaries are more permeable
2 Filtration occurs throughout the entire
length of the capillaries
Glomerular capillary blood pressure favours
filtration
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b) Plasma Colloid Osmotic Pressure
Plasma proteins cannot be filtered
Therefore, present in glomerular capillaries but
absent in Bowmans capsule
Accordingly, the concentration of water is higher
in Bowmans capsule than in glomerular
capillaries
Water moves by osmosis into glomerulus ? oppose
glomerular filtration
Approximately 30 mm Hg
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c) Bowmans Capsule Hydrostatic Pressure
Pressure due to fluid (hydrostatic) in Bowmans
capsule
Approximately 15 mm Hg
Push fluid out of Bowmans capsule
Therefore, oppose filtration of fluid from the
glomerulus
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Net Filtration Pressure
Net difference in pressure that favours
filtration
10 mm Hg
Forces fluid from blood through glomerular
membrane into Bowmans capsule
Glomerular Filtration Rate (GFR)
Actual rate of Filtration
Depends on
1) Net filtration pressure
2) Glomerular surface area
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Alteration In GFR
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Alteration In GFR
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Glomerular Filtration Rate (GFR)
Examples mentioned above are of changes to plasma
colloid osmotic pressure in altered states
disease, thereby altering the GFR
GFR can however be controlled under normal
circumstances by controlling glomerular capillary
blood pressure
Glomerular capillary blood pressure can be
controlled by
_at_ Autoregulation
_at_ Extrinsic sympathetic control
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Extrinsic Sympathetic Control of GFR
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Kidneys Share Of Cardiac Output
20-25 of cardiac output goes to the cleaners
(kidneys) (1.1 litres/min)
Only due to this large volume of blood are the
kidneys able to regulate electrolyte composition
of the internal environment
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2 Tubular Reabsorption
Essential materials that were filtered and
returned to the blood
Several mechanisms
a) Simple diffusion
b) Osmosis
c) Active transport requiring energy
Tubular reabsorption takes place primarily in the
proximal convoluted tubule
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Cells of the proximal convoluted tubule possess
microvilli on their luminal surfaces
This increasers surface area for reabsorption
(50-60 m2 for all proximal convoluted tubules of
both kidneys)
90 of sodium ions are reabsorbed by active
transport
This requires energy in the form of ATP
Negatively charged chloride ions and bicarbonate
follow by electrical attraction
The increase in the concentration of ions in the
tissue fluid ? water movement by osmosis from
renal tubule into the tissue fluid
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Potassium, calcium, amino acids, glucose and
vitamins are reabsorbed
Different substances are reabsorbed from the
proximal convoluted tubule at varying rates
Some substances are reabsorbed completely under
normal dietary conditions (e.g. glucose)
However, after consumption of a large chocolate
cake, glucose levels in the blood go sky high
Upper limit of reabsorption is exceeded
Glucose appears in the urine (Glucosuria)
Glucosuria symptom of diabetes mellitus
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Urea Reabsorption
Urea Chief nitrogenous waste product
Water reabsorption from the filtrate ? increase
in urea concentrations in the renal tubule
Urea starts to diffuse through tubular cells into
the tissue fluid and finally to the peritubular
capillaries
Luckily, only 40-60 of urea in the glomerular
filtrate is reabsorbed, the rest is excreted in
urine
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3 Tubular Secretion
Transport of substances from tissue fluid
surrounding tubule into the lumen of the tubules
Passive or active transport
Most of these substances derived from blood in
the peritubular capillaries
Some from substances reabsorbed earlier
Major site is distal convoluted tubule
Essential for acid-base balance
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Tubular secretion is essential for eliminating
hydrogen ions (H)
To prevent urine becoming acidic by the hydrogen
ions, tubular cells produce ammonia (NH3)
Ammonia diffuses into renal tubule and combines
with hydrogen ions to form ammonium ion (NH4)
Tubular secretion regulates potassium levels ?
influences nerve and muscle excitability
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Urine Concentration
Concentration of urine is due to countercurrent
multiplication mechanism by the loop of Henle
Ascending Limb Of Loop Of Henle
a) Carries fluid out of medulla and into the
distal convoluted tubule
b) Actively transports sodium chloride out of the
tubular lumen and into the surrounding tissue
fluid
c) Impermeable to water
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Descending Limb Of Loop Of Henle
a) Carries fluid from proximal convoluted tubule
into the depths of the medulla
b) Highly permeable to water
c) Does not actively extrude sodium
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Urine Concentration-Continued
As sodium chloride leaves the ascending portion
and water starts to leave the descending portion,
tissue fluid becomes hyperosmotic
Collecting tubule passes through this
hyperosmotic region
Water is drawn by osmosis from the fluid within
the collecting tubule
NET RESULT URINE BECOME CONCENTRATED
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Renin, Angiotensin Alsosterone
Juxtaglomerular cells (JG cells) in the walls of
afferent arterioles
JG cells maintain blood pressure
(baroreceptors)
JG cells secrete an enzyme called renin
Renin converts angiotensinogen to angiotensin I
and later to angiotensin II
Angiotensin II elevates blood pressure by
1) Smooth muscle contraction of arteriole walls
2) Stimulate the secretion of the hormone,
aldosterone from the cortex of the adrenal
medulla
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Mechanism Of Aldosterone Action
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Antidiuretic Hormone (ADH)
Amount of water withdrawn from distal convoluted
tubule and collecting tubule depends on their
permeability
This property is regulated by ADH (Vasopressin)

ADH secreted from posterior pituitary gland in
response to high concentration of solutes in the
blood
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Antidiuretic Hormone
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Antidiuretic Hormone Ethyl Alcohol
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References
Human physiology from cells to systems. LauraLee
Sherwood. Second Edition. West Publishing
Company. Chapter 14.