Unit 15: Urinary System

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Unit 15 Urinary System
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Urinary System

• The urinary system functions to keep the body in
  homeostasis by controlling the composition and
  volume of the blood.
• This is accomplished by removing water and
  solutes from the blood and then selectively
  restoring them.
• The excessive water, ions and solutes including
  toxins are excreted in the form of urine

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• The urinary system is composed of 4 different
  organs 2 kidneys, 2 ureters, 1 bladder and 1
  urethra

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Other functions

• Regulates blood pressure by producing and
  excreting the enzyme renin which regulates blood
  volume by water retention
• Producing and secreting the hormone
  Erythropoiten, which stimulates erythropoiesis
• Regulate blood pH ( acidity)
• Activate vitamin D

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Nephron Functional unit of the kidney

• There are over 1 million nephrons that process
  and filter the blood, selectively reabsorbing the
  filtrate or allowing it to become urine.
• A nephron consist of a renal tubule and a
  capillary network called a glomerulus
• The end of the renal tubule expands into a cup
  shaped capsule which encloses the glomerulus ( it
  is called the glomerular capsule) (Bowmans
  capsule)

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• Together the glomerulus and its capsule are
  called the renal corpuscle
• The glomerular capillaries are fenestrated ( very
  porous) allowing virtually all substances except
  blood cells and large proteins to pass from the
  capillaries into the glomerular capsule

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• The glomerular capsule surrounding these
  fenestrated capillaries consists of an outer
  (parietal) layer of simple squamous epithelium
  and a visceral layer ( next to the glomerulus)
  made of epithelial cells called podocytes ( the
  arms of these octopus like cells are called
  pedicles and they wrap around the fenestrated
  capillaries leaving openings called filtration
  slits, which control the openings to the pores in
  the capillaries)

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Proximal convoluted tubules

• First section of tubule that connects to Bowmans
  capsule
• composed of simple cuboidal epithelium with
  microvilli ( fuzzy)

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Ascending and descending loop of Henle

• Ascending loop has simple cuboidal epithelium
• Descending loop has simple squamous epithelium

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Distal convoluted tubules

• Shorter and has fewer microvilli than PCT (so
  not fuzzy)

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Types of nephrons classified according to
position in kidney

• 1. Juxtamedullary nephron
• originated in inner 1/3 of the cortex ,
• have long loops of Henle that extend deep into
  the medulla
• 2. Cortical nephrons
• Originated in outer 2/3 of cortex
• Very short loops of Henle which do not extend
  deep into the medulla

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Collecting ducts and urine collection

• Collecting ducts drain fluid from several distal
  convoluted tubules
• Collecting ducts merge to make about 30 papillary
  ducts which take urine to the renal papilla
• The urine passes through the papillary ducts in
  pyramid to minor calyx then to major calyx then
  to renal pelvic then to ureters

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So Urine flow is as follows

• Glomerular capsule
• to PCT
  in nephron
• to loop of Henley
• ( descending then ascending)
• to DCT
• To collecting tube
• To papillary duct
• To minor calyx
• To major calyx
• To renal pelvis

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• Out kidney
• to ureter
• To bladder
• To urethra
• Out body

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Blood supply to the kidneys

• Renal artery brings blood to kidney
• Segmental artery branches of renal artery
  supplying segments in plevis
• Interlobar artery takes blood up renal columns
• Arcuate artery takes blood across base of
  pyramid
• Interlobular artery takes blood up into cortex

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• Afferent arteriole microscopic vessel taking
  blood to glomerulus
• Glomerulus capillary bed in Bowmans capsule
• Efferent arteriole arteriole with a smaller
  diameter that takes blood away from glomerulus

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• Peritubular capillaries takes blood from
  efferent arterioles to interlobular vein
• Veins in the kidney corresponds to arteries in
  kidneys

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Kidneys

• Is retroperitoneal behind the parietal
  peritoneum) attached to the posterior body wall
  right below the 12th thoracic vertebrae ( lowest
  rib)
• Renal Ptosis kidney drops due to inadequate
  support
• This can cause a kink in the ureter
• Occurs most often in old, thin, or anorexic people

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Kidney functions

• Remove nitrogen waste
• Regulate pH
• Excrete excess metabolic substances
• Maintain water balance
• Help regulate blood pressure
• Secrete erythropoietin to stimulate RBC
  production
• Participates in synthesizing vitamin D

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Anatomy of Kidney

• 3 layers of tissue surround, support and protect
  each kidney
• renal capsule inner most layer, primary barrier
  to infection
• Adipose capsule fatty tissue which holds kidney
  in place and protects it against trauma (
  cushions it)
• Renal fascia thin fibrous connective tissue
  which anchors kidney to body wall

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Internal anatomy

• Cortex outer layer
• Medulla inner layer contains the renal pyramids
• Renal pelvis where major calyx empties into and
  ureter enters kidney

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Ureter

• A slender tube running retroperitoneally ( 10 t0
  12 inches) from kidney to bladder
• The ureters enter the bladder at its base in such
  a manner that during the pressure of urination,
  the ureters are compressed and closed preventing
  backflow of urine to the kidneys
• This also prevents the spread of bladder
  infections
• Works via wave-like contractions that carry urine
  through ureters ( peristalysis)

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Bladder

• Retroperitoneal
• Storage sac with maximum capacity of 700 to 800
  ml
• Located posterior to pubic symphysis and anterior
  to rectum
• It is a collapsible hollow sac composed of
  transitional epithelium ( stretch changing from
  cuboid to squamous shape)

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Urethra

• Tube draining from floor of bladder to outside
• Since female urethras (1 ½ inch) are shorter
  than males (8 inches), they are more prone to
  bladder infections
• Has 2 sphincter muscles
• Internal urethral sphincter involuntary
• External urethral sphincter voluntary

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Micturition ( urination)

• Involves both the involuntary and voluntary
  nervous systems
• There are 2 sphincter muscles that control the
  opening form bladder to urethra
• The internal ( involuntary) open when bladder is
  ¼ to ½ full) the external is voluntary and under
  conscious control
• Lack of control is called incontinence
• In infants it is normal.. Since the voluntary
  nerve does not develop until around 12 to 18
  months
• In adults it is due to nerve/muscle damage or
  unconsciousness, or overriding physical/emotional
  stress

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A few Conditions

• Nephritis inflammation of kidney tissue
• Cystitis inflammation of the bladder
• Enuresis ( or nocturia) bed wetting
• Retention failure to void
• Suppression or anuria failure the kidneys to
  make urine or daily output of less than 50 ml
• Renal calculi kidney stones, crystals of slats
  solidify not insoluble stones common components
  are calcium oxalate, uric acid and calcium
  phosphate

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Fluid and Electrolytes

• Depending on age, sex, health our bodies are
  about 50 to 75 water
• Intracellular fluid (IFC) inside cells 2/3 of
  total body fluids
• Extracellular fluids (ECF) 1/3 of total body
  fluid and is outside the cells

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Types of Extracellular fluids (ECF)

• Plasma within blood vessels
• Interstitial fluids between cells
• Other lymph, gastric juices, tears, synovial
  fluids
• Water serves as the universal solvent in which a
  variety of solutes are dissolved
• This internal environment bathes our cells and
  caries dissolved nutrients, gases, waste, etc..

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The solutes are broadly classified as

1. Electrolytes chemical substances such as salts,
   acids and bases, which when they dissolve, they
   dissociate ( break up) into ions ( charged
   particles)
2. Non-electrolytes chemical substances which do
   not dissociate or form ions when dissolved ex
   glucose, lipids, some proteins

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• To control the water balance in our bodies, we
  simply control the number of solutes
• The most important solutes for controlling
  osmosis are electrolytes.
• They dissociate when dissolved creating more
  particles

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3 General electrolyte functions

1. Many are essential minerals
2. They control osmosis ( between body compartments)
3. They help maintain the proper pH (acid/base
   balance)

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Examples of electrolytes

1. Sodium most abundant extracellular ion.
   Involved in verve transmission, muscle
   contraction and in fluid/electrolyte balance
2. Chloride primarily extracellular and involved in
   osmotic regulation
3. Potassium most abundant cation in the
   intracellular fluids. Key to maintaining fluid
   volume of cells and to controlling pH

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Examples of electrolytes

1. Calcium extracellular electrolyte. A component
   of bone, blood clotting and muscle and nerve
   impulses
2. Phosphate intracellular electrolytes. A bone
   component, necessary part of nucleic acids
   including the p in ATP

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Osmotic Regulation

• Is the maintenance of normal water and salt
  balance in body fluids
• Water enters animal bodies in various ways
  depending on the animal
• The water is lost through evaporation, feces and
  excretion
• To be fluid balance, the amount of water that
  exits the body must equal the amount that enters

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Physiology

• The work of the urinary system is done by the
  nephrons other parts are simply passage ways and
  storage areas ( system fillters 45 gallons of
  blood per day, the plasma is filtered and
  removed, and replaced 60 times a day)

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3 steps of nephron urine formation

• Glomerular filtration The fluids and solutes (
  plasma) are force out of the glomerular
  capillaries by hydrostatic pressure (blood
  pressure).
• This is a non-selective process where by the
  plasma is forced through a filtration membrane (
  consists of fenestrated capillary endothelium,
  basement membrane, and filtration slits of
  visceral wall of glomerular capsule)

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Pressure involved in Glomerular filtration

• Glomerular (blood) hydrostatic pressure
  (G.H.P.)
• Means blood pressure entering glomerulus
• Is kept high because the afferent arteriole is
  much bigger than the efferent arteriole
• Pressure moves fluid out of glomerulus at a force
  of 60mmHg

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2 forces oppose the G.H.P

• 1. Capsular hydrostatic pressure (C.H.P)
• Resistant pressure due to the fluids already in
  the glomerular capsule
• Is about 20 mmhg
• 2. Blood osmotic pressure (B.O.P)
• Since blood remains more concentrated than the
  filtrate, the water tries to move back into the
  gloverular capillaries at a pressure of 30 mmHg

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Net Filtration Pressure

• The net result of the three above pressures
• N.F.P. G.H.P - C.H.P - B.O.P 10
• 60 mmHg - 20 mmHg - 30 mmHg
• G.F.R ( golmerular filtration rate)
• The amount of filtrate flowing out of all the
  renal corpuscles ( both kidneys) per minute
• Normal 125 ml/min (or 48 gallons per day)
• 99 of this filtrate is reabsorbed back into the
  boold ( leaving 1 ½ liters of urine)

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3 steps of nephron urine formation

• Tubular reabsorption
• The movement of filtrate from the renal tubules
  back into the blood vessels
• Needed substances are removed from the filtrate
  and returned to the peritubular capillary system
• Reabsorption occurs both passively and actively

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• The permeability of the epithelial cells of the
  distal and collecting renal tubules is controlled
  by ADH
• so urine concentration is controlled by ADH which
  increases permeability and thus reabsorption
• The proximal renal tubules absorb 80 of filtrate
  that is absorbed
• Certain substances are not reabsorbed usually
  because the body does not need them

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steps of nephron urine formation

• Tubular secretion
• Chemicals still in the blood which are not needed
  by the body are discharged inot the urine by
  tubular secretion
• Important in
• Eliminating substances to large to pass thru
  pores
• Eliminating undesirable or excess substance which
  have been reabsorbed
• Rid body of potasium ions
• Control blood pH

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Urinary System
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