Renal Structure and Function

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Renal Structure and Function
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Introduction

• Main function of kidney is excretion of waste
  products (urea, uric acid, creatinine, etc).
  Other excretory organs are ?
• As above function is fulfilled, kidney also
  extremely important in maintaining
• fluid and electrolyte homeostasis
• volume of the extra-cellular fluid volume
• Acid-base balance
• Blood pressure

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Functional Background

• Kidneys receive ¼ of total cardiac output even
  though small ? lots of capillaries
• Filter about 11X the extracellular fluid
  volume/d. from plasma
• Produce 180 liters/d. of filtrate
• Filtrate very similar to plasma except very
  little protein (protein in urine serious
  problem)
• 99 reabsorbed as pass through renal tubule
• Some substances secreted into renal tubule
• Approximately 1.5 liters/d. of filtrate is voided
  as urine

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Gross Anatomy of the Kidney

• Each kidney consists of
• an outer cortex
• Inner medulla
• Hollow pelvis ? empties into ureter
• Functional unit of the kidney is the nephron (
  1,300,000 nephrons per kidney)

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Structure of the Nephron

• Nephron
• Glomerulus
• Bowmans capsule, Bowmans space
• Proximal convoluted tubule
• Loop of Henle
• Descending limb
• Ascending limb
• Distal convoluted tubule
• Collecting duct

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Diagram of nephron structure
Flow of filtrate
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Function of the Nephron Glomerulus

• Site of filtration
• Each glomerulus consists of a capillary network
  surrounded membrane called Bowman's capsule.
• Afferent arteriole carries blood from renal
  artery into glomerulular capillaries
• At distal end of glomerulus, capillaries form the
  efferent arteriole through which blood leaves the
  glomerulus.

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Glomerular Filtration

• Fluid driven from the glomerular capillaries into
  Bowmans capsule by hydrodynamic force (blood
  pressure)
• Note as blood pressure increases, volume of
  filtrate increases ? more filtrate formed less
  blood volume lowered blood pressure
• Fluid crosses three layers that excludes large
  molecules (especially proteins)
• Blood which passes on through to efferent
  arteriole is thus high in protein plasma and thus
  a higher oncotic pressure than normal
• Note oncotic pressure osmotic pressure
  contributed by large molecules such as plasma
  proteins

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Function of the Nephron Proximal Convoluted
Tubule

• Epithelium is leaky ? allows passive movement
  of some ions (Na, Cl-, glucose, amino acids,
  HCO3-) and water out of tubule
• 60-70 of filtered load reabsorbed in proximal
  tubule
• Transport mechanisms (Na/H exchanger moves Na
  out of proximal tubule, water follows some Na
  is coupled to other solutes (glucose, amino
  acids)
• Remaining 30-40 of filtrate, which is still
  isosmotic with plasma, passes on to the loop of
  Henle

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Function of the Nephron loop of Henle

• Consists of a descending and ascending portion
  (which has both thin and thick segments)
• Descending portion ? high permeability to water
  allows leakage into tubular interstitium
• Increased oncotic pressure in peritubular
  capillaries (coming from efferent arteriole)
  helps move water from tubular interstitium back
  into capillaries ? filtrate becomes concentrated
• Ascending portion ? active reabsorption of NaCl,
  absorbs 20-30 of the Na in the tubule
• Reabsorption occurs by transport mechanism driven
  catalyzed by Na/K-ATPase
• Volume of filtrate reduced by additional 5
• Because NaCl is reabsorped filtrate becomes
  hypotonic to plasma as enters distal convoluted
  tubule

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Water and NaCl reabsorption in the loop of Henle
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Function of the Nephron Distal Convoluted Tubule

• Active NaCl out of filtrate continues
• Active secretion of K (depending on blood
  levels) and H (depending on pH) into filtrate,
  driven by a Na/K pump
• Regulation of Ca occurs here
• As filtrate leaves DCT it is isoosmotic with
  plasma
• Enters collecting tubule/collecting duct

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Function of the Nephron Collecting Tubule and
Collecting Duct

• Several distal tubules empty into each collecting
  tubule they join to form the collecting duct
• Collecting duct NaCl, K and water reabsorption
• NaCl reabsorption, K secretion under control of
  aldosterone
• Water under control of ADH
• Some H secretion occurs
• End result ? highly concentrated urine (mammals
  and birds only animals able to do so) elimnates
  waste using as little water as possible

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Aldosterone

• Has a threefold action on Na reabsorption
• Rapid effect by stimulation of an Na/H
  exchanger containing an aldosterone receptor
• Delayed effect by binding to intracellular
  receptors that direct synthesis of a mediator
  protein that activates sodium channels
• Long-term effect by increasing the number of
  basolateral Na pumps

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Review of reabsorption and secretion in the
nephron
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Natriuretic peptides

• Involved with regulation of Na excretion in
  distal tubule
• Release from heart atria when arterial pressure
  is too high
• Causes water and salt diuresis by kidney,
  reducing blood volume and eventually atrial
  pressure

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Acid-Base Balance

• Kidneys help regulate H and bicarbonate
• Urine can be basic or acidic depending on need
• Usually acidic to compensate for tendency of
  blood pH to drop due to presence of CO2 (which
  forms carbonic acid via carbonic
  anhydrase-catalyzed reactions)
• Compensating mechanism is secretion of H into
  tubular fluid and reabsorption of bicarbonate

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Potassium Balance

• Rapidly and narrowly regulated by kidney
• Regulation important because small changes in K
  levels can affect function of many excitable
  tissues (heart, brain, skeletal muscle
• Most filtered K is reabsorbed in proximal tubule
  and loop of Henle
• Most secretion of excess K occurs in collecting
  tubule

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The End