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Chapter 25 Urinary system Lecture 16 Part 2: Regulation of Urine Osmolarity and Volume

Marieb’s Human Anatomy and Physiology Ninth Edition Marieb w Hoehn. Chapter 25 Urinary system Lecture 16 Part 2: Regulation of Urine Osmolarity and Volume. Where have we been; we are we going?. Renal corpuscle – provides the raw materials to the nephron for processing

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Chapter 25 Urinary system Lecture 16 Part 2: Regulation of Urine Osmolarity and Volume

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  1. Marieb’s Human Anatomy and Physiology Ninth Edition Marieb w Hoehn Chapter 25 Urinary system Lecture 16 Part 2: Regulation of Urine Osmolarity and Volume

  2. Where have we been; we are we going? Renal corpuscle – provides the raw materials to the nephron for processing PCT – reclaims those substances the body can use DCT – gets rid of those substances the body doesn’t want or need; reabsorb some more Na+, Ca2+ 300 mOsm/L *Note osmolarity of fluid in PCT Collecting duct – provides the OPTION of reclaiming H2O or letting it pass out of the body Obligatory H2O reabsorption…

  3. Overview of Facultative H2O Reabsorption • Note that outflow of water from collecting duct is dependent upon the osmotic gradient in the medulla Figure from: Hole’s Human A&P, 12th edition, 2010 Under influence of ADH Increasing concentration → (Facultative water reabsorption) Urea

  4. The Loop of Henle (Nephron Loop) Crucial renal function is to keep the body fluids at about 300 mOsm (osmolarity of blood plasma) by varying the concentration of urine The mechanism shown is called the “countercurrent multiplier” Increasing concentration → Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 SO HOW DOES THIS HELP?

  5. The Countercurrent Multiplier • Recall that all movement of H2O occurs passively by osmosis • We would like some mechanism to concentrate urine • Excrete more H2O when body fluids are tending to become hypotonic (more dilute) • Excrete less H2O when body fluids are tending to become hypertonic (more concentrated) • Utilizes two factors • Hypertonicity of the peritubular fluid, established by the countercurrent multiplier • Variable permeability of the collecting ducts to H2O depending upon levels of ADH • We will use the phrase, “Water follows salt (solute) when it can”

  6. Influence of ADH on H2O Reabsorption • Note that amount of water pulled out of collecting duct is dependent upon the osmotic gradient in the medulla that was established by the countercurrent multiplier. Under influence of ADH Figure from: Hole’s Human A&P, 12th edition, 2010 (Facultative water reabsorption) Urea

  7. Vasa Recta of Juxtamedullary Nephrons Figure from: Hole’s Human A&P, 12th edition, 2010 Recall that the vasa recta is present in juxtamedullary nephron loops (which give the kidneys the ability to produce a concentrated urine) • The vasa recta functions to • Deliver blood to medullary cells • Return reabsorbed solutes and water in the medulla to the general circulation without disrupting the medullary concentration gradient

  8. Urea and Uric Acid Excretion • Urea • product of amino acid catabolism (deamination) • plasma concentration reflects the amount or protein in diet • enters renal tubules through glomerular filtration • 50% reabsorbed • rest is excreted • Uric Acid • product of nucleic acid metabolism • enters renal tubules through glomerular filtration • 100% of filtered uric acid is reabsorbed • 10% secreted and excreted

  9. Summary of Events in the Nephron/Collecting Duct • Filtrate produced • Reabsorption of 65% of filtrate • Obligatory water reabsorption • Reabsorption of Na+ and Cl- by active transport • 5,6. Facultative reabsorption of water • 7. Absorption of solutes and water by vasa recta (Aldosterone) (Aldosterone)

  10. Diuretics A diuretic promotes the loss of water in the urine • Osmotic diuretics,e.g., mannitol, glucose • Drugs that block Na+/Cl- transport in PCT and DCT, e.g., hydrochlorothiazide • High-ceiling/loop diuretics that reduce gradient along nephron loop, e.g., furosemide (Lasix) • Aldosterone-blocking agents, e.g., spironolactone (K+ sparing), natriuretic peptides • ACE inhibitors, e.g., Captopril • Drugs with diuretic side-effects, e.g., alcohol (how?), caffeine

  11. Review • Regulation of urine concentration and volume • Results from a combination of • Countercurrent multiplier in loop of Henle • Responsiveness of the DCT and collecting ducts to ADH and aldosterone • Is critical to homeostasis • Urine composition • Is variable • Depends upon both diet and activity • Consists of mostly water plus • Creatinine • Urea • Uric acid • Traces of amino acids • Electrolytes

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