Biology 25: Human Biology

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Chapter 10

• Biology 25 Human Biology
• Prof. Gonsalves
• Los Angeles City College
• Loosely Based on Maders Human Biology,7th edition

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• Excretory (Urinary) System
• Components Kidneys, bladder, urethra, and
  associated ducts.
• Functions
• Kidneys remove nitrogen containing waste from
  blood.
• Urine with waste is temporarily stored in
  bladder.
• Excretion of urine.
• Homeostatic Role
• Regulates water levels in body.
• Removes excess water
• Helps conserve water

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Kidney Function

• Regulation of ECF through formation of urine.
  (primary function).
• Regulate volume of blood plasma.
• BP.
• Regulate concentration of waste products in the
  blood.
• Regulate concentration of electrolytes.
• Na, K, and HC03-.
• Regulate pH.
• Secrete erythropoietin.

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Micturition Reflex

• Actions of the internal urethral sphincter and
  the external urethral sphincter are regulated by
  reflex control center located in the spinal cord.
• Filling of the urinary bladder activates the
  parasympathetic neurons causing rhythmic
  contraction of the detrusor muscle and relaxation
  of the internal urethral sphincter.
• Voluntary control over the external urethral
  sphincter.

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Structure of the Urinary System

• 2 distinct regions
• Outer cortex
• Many capillaries.
• Medulla
• Renal pyramids separated by renal columns.
• Pyramid contains minor calyx which unite to form
  a major calyx.

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Nephron

• Functional unit of the kidney.
• Consists of
• Blood vessels
• vasa recta
• peritubular capillaries
• Urinary tubules
• PCT
• LH
• DCT
• CD

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Renal Blood Vessels

• Afferent arteriole
• Delivers blood into the glomeruli.
• Glomeruli
• Capillary network produces filtrate that enters
  the urinary tubules.
• Efferent arteriole
• Delivers blood from glomeruli to peritubular
  capillaries.

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Renal Blood Vessels

• Peritubular capillaries
• Deliver blood to vasa recta.
• Juxtamedullary nephrons.
• Deliver blood to veins.
• Cortical nephrons.

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Nephron Tubules

• Glomerular capsule
• Proximal convoluted tubule (PCT)
• Loop of Henle (LH)
• Distal convoluted tubule (DCT)
• Collecting duct (CD)

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Glomerular Capsule

• Bowmans capsule
• Surrounds the glomerulus.
• Location where glomerular filtration occurs.
• Passes into the urinary space.

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Proximal Convoluted Tubule

• PCT
• Single layer of cuboidal cells with millions of
  microvilli.
• Increase surface area.
• PCT functions
• Reabsorption.
• Secretion.

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Loop of Henle

• LH
• Descending limb
• H20 reabsorption.
• Ascending limb
• Active transport of Na.
• Impermeable to H20.

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Distal Convoluted Tubule

• DCT
• Few microvilli.
• Functions
• Secretion.
• Reabsorption.

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Collecting Duct

• CD
• Receives fluid from the DCT of several nephrons.
• Passes through renal pyramid into minor calyx.
• Functions
• Reabsorption.
• Secretion.
• H20 reabsorption influenced by ADH.

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Type of Nephrons

• Cortical nephron
• Osmolarity of 300 mOsm/l.
• Involved in solute reabsorption.
• Juxtamedullary nephron
• Important in the ability to produce a
  concentrated urine.

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Glomerular Filtration Membrane

• Basement membrane
• Filtrate must pass through the basement membrane
• Thin glycoprotein layer.
• Negatively charged.
• Podocytes
• Foot pedicels form small filtration slits.
• Negatively charged.

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Glomerular Filtration

• Mechanism of producing ultrafiltrate under
  hydrostatic pressure.
• GFR (glomerular filtration rate)
• Volume of filtrate produced by both kidneys each
  minute.
• Averages 180 L/day.

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Juxtaglomerular Apparatus

• Region in each nephron where the afferent
  arteriole comes in contact the the thick
  ascending limb of the loop.
• Granular cells
• Secrete renin.
• Converts angiotensinogen to angiotensin I.
• Initiates the renin-angiotensin-aldosterone
  system.
• Negative feedback.
• Macula densa
• Inhibit renin secretion when blood Na in
  blood increases.

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Regulation of GFR

• Vasoconstriction or dilation of afferent
  arterioles affects the rate of blood flow to the
  glomerulus.
• Affects GFR.
• Mechanisms to regulate GFR
• Sympathetic Nervous System.
• Autoregulation.
• Hormonal
• Renin-angiotension.

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Renal Acid-Base Regulation

• Kidneys help regulate blood pH by excreting H
  and reabsorbing HC03-.
• Most of the H secretion occurs across the walls
  of the PCT in exchange for Na.
• Antiport mechanism.
• Normal urine normally is slightly acidic because
  the kidneys reabsorb almost all HC03- and excrete
  H.
• Returns blood pH back to normal range.

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Reabsorption of HCO3-

• Apical membranes are impermeable to HCO3-.
• Reabsorption is indirect.
• HCO3- combines with H to form H2C0-, which is
  catalyzed by carbonic anhydrase (ca) located in
  the apical cell membrane of PCT.
• As C02 increases in the filtrate, ca forms
  H2C03.
• H2C03 dissociates to HCO3- and H.
• Shunt HCO3- generated within tubule cell to
  peritubular capillary.

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Urinary Buffers

• Nephron cannot produce a urine pH lt 4.5.
• IN order to excrete more H, the acid must be
  buffered.
• H secreted into the urine tubule and combines
  with HPO4-2 or NH3.
• HPO4-2 H H2PO4-2
• NH3 H NH4

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Glucose and Amino Acid Reabsorption

• Filtered glucose and amino acids are normally
  reabsorbed by the nephrons.
• Carrier mediated transport
• Saturation.
• Exhibit Tm.
• Renal transport threshold
• Minimum plasma of a substance that results in
  excretion of that substance in the urine.

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Electrolyte Balance

• Kidneys regulate Na, K, H, Cl-, HC03-, PO4-2.
• Control of Na important in regulation of blood
  volume and pressure.
• Control of plasma of K important in proper
  function of cardiac and skeletal muscles.
• Match ingestion with urinary excretion.

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Role of Aldosterone

• 90 K reabsorbed in early part of the nephron.
• When aldosterone is absent, no K is excreted in
  the urine.
• Final K controlled in CD by aldosterone.
• High K or low Na stimulates the secretion
  of aldosterone.
• Only means by which K is secreted.

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Na Reabsorption

• In the absence of aldosterone, 80 remaining Na
  is reabsorbed.
• 2 is excreted (30 g/day).
• Final Na controlled in CD by aldosterone.

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ANP

• Produced by atria due to stretching of walls.
• Antagonist to aldosterone.
• Increases Na excretion.

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Na, K, and H Relationship

• Na reabsorption in CD creates electrical
  gradient for K secretion.
• Plasma K indirectly affects H.
• When ECF H increases, H moves into the cell,
  causing K to diffuse into the ECF.
• In severe acidosis, H is secreted at the expense
  of K.

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Diuretics

• Increase urine volume excreted.
• Increase the proportion of glomerular filtrate
  that is excreted as urine.
• Loop diuretics
• Inhibit NaCl transport out of the ascending limb
  of the LH.
• Thiazide diuretics
• Inhibit NaCl reabsorption in the DCT.
• Ca inhibitors
• Prevent H20 reabsorption when HC0s- is
  reabsorbed.
• Osmotic diuretics
• Increase osmotic pressure of filtrate.

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