The Urinary system

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					The Urinary system
• Functions of the kidneys
• Filter blood plasma, separate wastes from the
  useful chemicals, eliminate the wastes and
  return the rest to the bloodstream
• Regulate blood volume and pressure by
  eliminating or conserving water as necessary
• Regulate the osmolarity of the body fluids by
  controlling the relative amounts of water and
  solutes eliminated
• The kidneys secrete renin which activates
  hormonal mechanisms that control blood
  pressure and electrolyte balance
• The kidneys secrete the hormone
  erythropoietin which controls the red blood
  cell count and the oxygen carrying capacity of
  the blood
• They function with the lungs to control the
  partial pressure of carbon dioxide and the acid-
  base balance of the body fluids
• The kidneys contribute to calcium homeostasis
  through their role in synthesizing calcitriol
• They detoxify free radicals and drugs with the
  use of peroxisomes
• In times of starvation the kidneys carry out
• Nitrogenous wastes
• A waste is any substance that is useless to the
• A metabolic waste is a waste substance that is
  produced by the body
• Nitrogenous wastes are extremely toxic –
  proteins are broken down to amino acids
  which eventually produces ammonia -
  ammonia is very toxic and it is converted by
  the liver to urea – this makes up 50% of
  nitrogenous waste
• Other nitrogenous waste includes uric acid
  which is produced by the catabolism of nucleic
  acids and creatinine which is produced by the
  breakdown of creatine phosphate
• The level of nitrogenous waste is expressed as
  blood urea nitrogen (BUN) and is usually 10 –
  20 mg.dL
• Azotemia
• Uremia
• Excretion
• The respiratory system excretes carbon
  dioxide, other gases and water
• The integumentary system excretes water,
  lactic acid and urea in sweat
• The digestive tract secretes food residue ,
  water, salts, carbon dioxide, lipids, bile,
• The urinary system excretes a broad variety of
  metabolic wastes, toxins, drugs hormones salts
  hydrogen ions and water
• Anatomy
• Position of the kidneys
• Lie against the dorsal abdominal wall at the
  level of vertebrae T12 – L3
• Rib 12 crosses the middle of the kidney
• The right kidney is lower than the left because
  of the right lobe of the liver
• The kidneys are retroperitoneal
• The adrenals lie on top of the kidneys
•   Nephrons
•   Hilum
•   Renal fascia
•   Adipose capsule
•   Renal capsule
•   Renal sinus
•   Renal cortex
•   Renal medulla
•   Renal pyramids
•   Renal papilla
•   Minor calyx
•   Major calyx
•   Renal pelvis
• Circulation
•   The renal corpuscle
•   Glomerular (Bowman’s) capsule
•   Podocytes
•   Glomerular filtrate
•   Capsular space
•   Vascular pole
•   Urinary pole
• The Renal tubule – leads away from the
  glomerular capsule and ends at the tip of a
  medullary pyramid
• Divided into 4 major regions – Proximal
  convoluted tubule, nephron loop, distal
  convoluted tubule and collecting duct
• Proximal convoluted tubule(PCT)
• Arises from the glomerular capsule
• The longest and the most coiled of the 4
• Simple cuboidal epithelium with prominent
• Nephron loop of Henle
• Descending limb
• Ascending limb
• Thick segment with simple cuboidal
• Thin segment with simple squamous
  epithelium – very permeable to water
•   Distal convoluted tubule(DCT)
•   Shorter and less convoluted than the PCT
•   Cuboidal epithelium
•   The DCT is the end of the nephron

• Collecting duct – DCTs of several nephrons
  drain into a straight tubule called the
  collecting duct which passes down to the
  medulla – several collecting ducts form a
  papillary duct - about 30 of these drain into
  the minor calyx
• Flow of urine – glomerular capsule – proximal
  convoluted tubule – nephron loop – distal
  convoluted tubule – collecting duct – papillary
  duct – minor calyx – major calyx – renal pelvis
  – ureter – urinary bladder - urethra
• Cortical and Juxtamedullary nephrons
• Cortical nephrons are close to the kidney
  surface – short nephron loops that dip only
  slightly into the medulla
• Juxtamedullary nephrons are close to the
  medulla and very long loops that extend to the
  apex of the renal pyramid
• 15% of the nephrons are juxtamedullary but
  they are almost totally responsible for the
  salinity gradient that enables the body to
  conserve water
• The filtration membrane – 3 barriers
• Fenestrated endothelium of the capillary –
  filtration pores
• The basement membrane – proteoglycan gel
• The podocyte of the glomerular capsule has
  pedicels which has filtration slits
• Filtration pressure
• Glomerular filtration rate – the amount of
  filtrate formed per minute by the two kidneys
• For every 1 mm Hg of NFP the kidneys
  produce 12.5 mL of filtate per minute – this
  value is called the net filtration coefficient kf
• GFR = NFP x Kf = 10 x 12.5 = 125mL /min
• The filtration coefficient is about 10% lower in
  women than in men
• In the reference woman the GFR is about
  105mL/min and in the male it is about
• This would be about 180L/day in males and
  about 150L/day in females
• Only a small portion of this is excreted in the
• An average adult reabsorbs about 99% of the
  filtrate and excretes about 1-2 L of urine /day
• Regulation of glomerular filtration

• Renal autoregulation – myogenic mechanism
  and tubuloglomerular feedback

• Myogenic mechanism – smooth muscle
  contracts when stretched – when arterial
  pressure rises it stretches the afferent arterioles
  which makes them contract and decreases the
  blood flow into the glomerulus – when blood
  pressure falls the opposite happens and blood
  flow is increased into the glomerulus
• Tubuloglomerular feedback
• Involves the macula densa located at the
  beginning of the distal convoluted tubule and
• The juxtaglomerular cells located around the
  afferent arteriole and to some extent around
  the efferent arteriole
• Sympathetic control of GFR
• Sympathetic nerves innervate the renal blood
• With exercise or acute situations like shock the
  sympathetic nervous system and adrenal
  epinephrine constrict the afferent arterioles and
  reduce blood flow and filtration
• The Renin –Angiotensin Mechanism
• When the systemic blood pressure drops
  sympathetic nervous system stimulates the JG
  cells to secrete renin
• This results in the production of angiotensin II
  which has the following effects
• Stimulates widespread vasoconstriction which
  raises the MAP throughout the body
• Constricts both the afferent and efferent
  arterioles which reduces GFR and water loss
• Stimulates NaCl and water reabsorption by the
  proximal convoluted tubule
• Stimulates the adrenal cortex to secrete
  aldosterone which promotes sodium and water
  reabsorption by the distal convoluted tubule
  and the collecting duct
• Stimulates the secretion of ADH which
  promotes water reabsorption
• Stimulates the sense of thirst which encourages
  water intake
• Tubular reabsorption and secretion
• The proximal convoluted tubule
• Tubular reabsorption is the process of
  reclaiming water and solutes from tubular fluid
  and returning them to the blood
• The PCT reabsorbs a wider variety of
  chemicals than any other part of the nephron
• Two routes of reabsorption – transcellular or
• Uptake by peritubular capillaries –
  mechanisms are by osmosis and solvent drag –
  three factors promote osmosis
• High interstitial (3rd space) fluid hydrostatic
• Low hydrostatic pressure in the peritubular
• High COP in the capillary blood
• The transport Maximum – there is a limit to
  the amount of solute the renal tubule can
• The maximum rate of reabsorption is the Tm
• The Tm for glucose is 320 mg/min
• When that is exceeded glucose will appear in
  the urine and this is referred to as glucosuria
• This is a classic sign of diabetes
• Tubular secretion – process where the renal
  tubule extracts chemicals from the capillary
  blood and secretes them into the tubular fluid
• Tubular secretion has two main purposes
• Waste removal – urea, uric acid, bile acids,
  ammonia, prostaglandins, catecholamines, and
  creatinine – also clears penicillin, morphine
  and aspirin and other drugs
• Acid-base balance – tubular secretion of
  hydrogen and bicarbonate ions
• The nephron loop
• The primary function of the nephron loop is to
  generate a salinity gradient in the medulla that
  enables the collecting duct to reabsorb water
  and concentrate the urine
• The distal convoluted tubule – affected by
  hormones – the principal cells in the DCT have
  hormone receptors
• Aldosterone – salt and water retaining
• Atrial natriuretic peptide – antagonizes the
  angiotensin-aldosterone mechanism by
  inhibiting renin and aldosterone secretion
• ADH – promotes the reabsorption of water
  mostly in the collecting duct
• Parathyroid hormone Promotes calcium
• The collecting duct - begins in the cortex and
  receives water from numerous nephrons
• As it passes through the medulla it reabsorbs
  water and concentrates the urine
• Two facts enable the collecting ducts to
  reabsorb water – the osmolarity in the medulla
  is 4 times higher in the medulla than it is in the
  cortex – the medullary portion of the collecting
  duct is more permeable to water than to NaCl
• The ureters
• The renal pelvis funnels urine into the ureters
• The ureter is a muscular retroperitoneal tube
  that extends to the urinary bladder
• The ureters pass dorsal to the bladder and enter
  it from below
• The ureter has three layers – adventitia,
  muscularis and mucosa
• The muscularis has two layers of muscle and
  develops a third layer in the lower ureter
• The ureter has transitional epithelium that
  begins in the minor calyx of the kidney and
  extends to the bladder
• Voiding urine
• Between acts of urination it is important that
  the detrusor muscle relaxes and the urethral
  sphincters remain tightly closed
• This is achieved by sympathetic pathways that
  originate in the upper lumbar spinal cord
• Postganglionic fibers travel through the
  hypogastric nerve to the detrusor muscle and
  the internal sphincter – these fibers relax the
  detrusor and excite the internal sphincter
• Micturation is the act of urinating
• Micturation center in the pons

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