Chapter 16: Urinary System and Excretion

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Chapter 16 Urinary System and Excretion
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Urinary System

• Urinary Organs
• The urinary system consists of the kidneys,
  ureters, urinary bladder, and urethra.
• The bean-shaped kidneys are at the back of the
  abdominal wall beneath the peritoneum, protected
  by the lower rib cage.
• The renal artery and renal vein along with
  ureters exit the kidney at the hilum.

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• The kidneys produce urine which is conducted by
  two muscular tubes called ureters to the urinary
  bladder where it is stored before being released
  through the urethra.
• Two urethral sphincters control the release of
  urine.
• In females, the urethra is 4 cm long in males,
  the urethra is 20 cm long and conveys both urine
  and sperm during ejaculation.

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The urinary system
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Urination

• As the bladder fills with urine, sensory
  impulses travel to the spinal cord where motor
  nerve impulses return and cause the bladder to
  contract and sphincters to relax.
• With maturation, the brain controls this reflex
  and delays urination, the release of urine, until
  a suitable time.

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Urination
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Functions of the Urinary System

• Excretion refers to the elimination of metabolic
  wastes that were cell metabolites this is the
  function of the urinary system.
• Kidneys play a role in homeostasis of the blood
  by excreting metabolic wastes, and by maintaining
  the normal water-salt and acid-base balances of
  blood.

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Excretion of Metabolic Wastes

• Kidneys excrete nitrogenous wastes, including
  urea, uric acid, and creatinine.
• Urea is a by-product of amino acid metabolism.
• The metabolic breakdown of creatine phosphate in
  muscles releases creatinine.
• Uric acid is produced from breakdown of
  nucleotides.
• Collection of uric acid in joints causes gout.

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Maintenance of Water-Salt Balance

• Kidneys maintain the water-salt balance of the
  body which, in turn, regulates blood pressure.
• Salts, such as NaCl, in the blood cause osmosis
  into the blood the more salts, the greater the
  blood volume and also blood pressure.
• Kidneys also maintain correct levels of
  potassium, bicarbonate, and calcium ions in blood.

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Maintenance of Acid-Base Balance

• The kidneys regulate the acid-base balance of the
  blood.
• Kidneys help keep the blood pH within normal
  limits by excreting hydrogen ions (H) and
  reabsorbing bicarbonate ions (HCO3-) as needed.
• Urine usually has a pH of 6 or lower because our
  diet often contains acidic foods.

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Secretion of Hormones

• Kidneys secrete or activate several hormones
• They secrete the hormone erythropoietin to
  stimulate red blood cell production,
• They activate vitamin D to the hormone calcitriol
  needed for calcium reabsorption during digestion,
  and
• They release renin, a substance that leads to the
  secretion of aldosterone.

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Kidney Structure

• The kidneys filter wastes from the blood, and
  thus the renal arteries branch extensively into
  smaller arteries and then arterioles inside each
  kidney.
• Many venules unite to form small veins, which
  merge to become the renal vein.

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Blood supply in a kidney
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• There are three regions to a kidney an outer
  renal cortex, an inner renal medulla, and a
  central space called the renal pelvis.
• Microscopically, each contains over one million
  nephrons.
• The nephrons produce urine which flows into a
  collecting duct several collecting ducts merge
  and drain urine into the renal pelvis.

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Gross anatomy of a kidney
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Anatomy of a Nephron

• Each nephron has its own blood supply.
• An afferent arteriole approaches the glomerular
  capsule and divides to become the glomerulus, a
  knot of capillaries.
• The efferent arteriole leaves the capsule and
  branches into the peritubular capillary network.

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Nephron anatomy
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Parts of a Nephron

• The closed end of the nephron is a cuplike
  glomerular capsule.
• Spaces between podocytes of the glomerular
  capsule allow small molecules to enter the from
  the glomerulus via glomerular filtration.
• The cuboidal epithelial cells of the proximal
  convoluted tubule have many mitochondria and
  microvilli to carry out active transport
  (following passive transport) from the tubule to
  blood.

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Proximal convoluted tubule
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• The descending loop of the nephron allows water
  to leave and the ascending portion extrudes salt.
  
• The cuboidal epithelial cells of the distal
  convoluted tubule have numerous mitochondria but
  lack microvilli.
• They carry out active transport from the blood to
  the tubule or tubular secretion.
• Collecting ducts gather in the renal medulla and
  form the renal pyramids.

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Urine Formation

• Glomerular Filtration
• During glomerular filtration, small molecules
  including water, wastes, and nutrients are forced
  from the blood inside the glomerulus to the
  inside of the glomerular capsule.
• Blood cells, platelets, and large proteins do not
  move across.
• About 180 liters of water are filtered daily.

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Reabsorption from Nephrons
Substance Amount Filtered Amount Excreted Reabsorp-tion ()
Water, L 180 1.8 99.0
Sodium, g 630 3.2 99.5
Glucose, g 180 0.0 100.0
Urea, g 54 30.0 44.0
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Tubular Reabsorption

• During tubular reabsorption, certain nutrients,
  water and some urea moves from the proximal
  convoluted tubule into the blood of the
  peritubular capillary network.
• Tubular reabsorption is a selective process
  because only molecules recognized by carrier
  molecules are actively reabsorbed.
• The rate of this process is limited by the number
  of carriers.

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Tubular Secretion

• During tubular secretion, specific substances
  such as hydrogen ions, creatinine, and drugs such
  as penicillin move from the blood into the distal
  convoluted tubule.
• In the end, urine contains substances that have
  undergone glomerular filtration but have not been
  reabsorbed, and substances that have undergone
  tubular secretion.

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Steps in urine formation
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Maintaining Water-Salt Balance

• The kidneys maintain the water-salt balance of
  the blood within normal limits.
• By doing so, they also maintain blood volume and
  blood pressure.
• Most of the water and salt (NaCl) present in the
  filtrate is reabsorbed across the wall of the
  proximal convoluted tubule.

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Reabsorption of Water

• Salt passively diffuses out of the lower portion
  of the ascending limb of the loop the upper
  thick portion actively extrudes salt into the
  tissue of the outer renal medulla.
• Water is reabsorbed by osmosis from all parts of
  the tubule.
• The ascending limb of loop of the nephron
  establishes an osmotic gradient that draws water
  from the descending limb of the nephron and the
  collecting duct.

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• The permeability of the collecting duct is under
  the control of antidiuretic hormone (ADH).
• Diuresis is an increase in urine flow and
  antidiuresis is a decrease.
• When ADH is present, more water is reabsorbed,
  blood volume and blood pressure rise, and there
  is a decreased amount of urine.
• If there is insufficient water intake, the
  posterior pituitary releases ADH, causing more
  water to be reabsorbed with a decreased urine
  output.

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Reabsorption of water
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Reabsorption of Salt

• Kidneys regulate salt balance by controlling
  excretion and reabsorption of ions.
• Two hormones, aldosterone and atrial natriuretic
  hormone (ANH), control the kidneys reabsorption
  of sodium (Na).
• When the juxtaglomerular apparatus detects low
  blood volume, it secretes renin that eventually
  results in the adrenal cortex releasing
  aldosterone that restores blood volume and
  pressure through reabsorption of sodium ions.

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Juxtaglomerular apparatus
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• Reabsorption of salt increases blood volume and
  pressure because more water is also reabsorbed.
• ANH is secreted by the atria of the heart when
  cardiac cells are stretched by increased blood
  volume.
• ANH inhibits secretion of renin the resulting
  excretion of sodium also causes excretion of
  water and blood volume drops.

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Diuretics

• Diuretics are chemicals that lower blood pressure
  by increasing urine output.
• Alcohol inhibits secretion of ADH dehydration
  after drinking may contribute to the effects of a
  hangover.
• Caffeine increases the glomerular filtration rate
  and decreases tubular reabsorption of sodium.
• Diuretic drugs inhibit active transport of Na so
  a decrease in water reabsorption follows.

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Maintaining Acid-Base Balance

• Kidneys rid the body of acidic and basic
  substances.
• If the blood is acidic, hydrogen ions (H) are
  excreted and bicarbonate ions (HCO3-) are
  reabsorbed.
• If the blood is basic, H are not excreted and
  HCO3- are not reabsorbed.
• Breathing also ties up H when carbon dioxide is
  exhaled.

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Acid-base balance
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Chapter Summary

• The urinary system has organs specialized to
  produce, store, and rid the body of urine.
• Kidneys excrete nitrogenous wastes and maintain
  the water-salt and the acid-base balance of the
  blood within normal limits.

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• Kidneys have a macroscopic anatomy and a
  microscopic anatomy.
• Urine is produced by many microscopic tubules
  called nephrons.
• Urine formation is a multistep process.
• Kidneys are under hormonal control as they
  regulate the water-salt balance of blood.
• Kidneys excrete hydrogen ions and reabsorb
  bicarbonate ions to regulate the pH of blood.