Animals must regulate the chemical composition of its body fluids by balancing the uptake and loss of water and fluids.

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Excretion

• Animals must regulate the chemical composition of
  its body fluids by balancing the uptake and loss
  of water and fluids.
• Management of the bodys water content and solute
  composition, osmoregulation, is largely based on
  controlling movements of solutes between internal
  fluids and the external environment.

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• Animals must also remove metabolic wastes before
  they accumulate to harmful levels
• Water
• Carbon dioxide
• Salts
• Bile pigments
• Nitrogenous waste
• Ammonia
• Urea
• Uric Acid
• Creatine

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Metabolic Wastes

• Because most metabolic wastes must be dissolved
  in water when they are removed from the body, the
  type and quantity of waste products may have a
  large impact on water balance.
• In general, the kinds of nitrogenous wastes
  excreted depend on an animals evolutionary
  history and habitat - especially water
  availability.
• The amount of nitrogenous waste produced is
  coupled to the energy budget and depends on how
  much and what kind of food an animals eats.

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• Animals that excrete nitrogenous wastes as
  ammonia need access to lots of water.
• This is because ammonia is very soluble but can
  only be tolerated at very low concentrations.
• Therefore, ammonia excretion is most common in
  aquatic species.
• Many invertebrates release ammonia across the
  whole body surface.
• In fish, most of the ammonia is lost as ammonium
  ions (NH4) at the gill epithelium.

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• Ammonia excretion is much less suitable for land
  animals and even many marine fishes and turtles.
• Because ammonia is so toxic, it can only be
  transported and excreted in large volumes of very
  dilute solutions.
• Most terrestrial animals and many marine
  organisms (which tend to lose water to their
  environment by osmosis) do not have access to
  sufficient water.
• Instead, mammals, most adult amphibians, and many
  marine fishes and turtles excrete mainly urea.

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• Urea is synthesized in the liver by combining
  ammonia with carbon dioxide and excreted by the
  kidneys.
• The main advantage of urea is its low toxicity,
  about 100,000 times less than that of ammonia.
• Urea can be transported and stored safely at high
  concentrations.
• This reduces the amount of water needed for
  nitrogen excretion when releasing a concentrated
  solution of urea rather than a dilute solution of
  ammonia.

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• The main disadvantage of urea is that animals
  must expend energy to produce it from ammonia.

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• Land snails, insects, birds, and many reptiles
  excrete uric acid as the main nitrogenous waste.
• Like urea, uric acid is relatively nontoxic.
• But unlike either ammonia or urea, uric acid is
  largely insoluble in water and can be excreted as
  a semisolid paste with very small water loss.
• While saving even more water than urea, it is
  even more energetically expensive to produce.

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Human Excretory Organs

1. Lungs
2. Skin
3. Liver
4. Kidneys (Urinary system)

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The Human Urinary System

• Mammals have a pair of bean-shaped kidneys.
• These are supplied with blood by a renal artery
  and a renal vein.
• Urine exits each kidney through a duct called the
  ureter, and both ureters drain to a common
  urinary bladder.
• During urination, urine is expelled from the
  urinary bladder through a tube called the
  urethra, which empties to the outside near the
  vagina in females or through the penis in males.
• Sphincter muscles near the junction of the
  urethra and the bladder control urination.

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Human Urinary System
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• The kidney has two distinct regions, an outer
  renal cortex and an inner renal medulla.
• Both regions are packed with microscopic
  excretory tubules, nephrons, and their associated
  blood vessels.
• Each human kidney packs about a million nephrons.

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Longitudinal Section of Kidney
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Nephron Functional Unit of Kidney

1. Bowmans capsule
2. Proximal convoluted tubule
3. Loop of Henle
4. Distal convoluted tubule
5. Collecting tubule

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Circulatory System Interface with Urinary System

1. Renal artery
2. Renal arterioles
3. Afferent renal arteriole
4. Glomerulus ? Bowmans Capsule
5. Efferent renal arteriole
6. Peritubular capillary network ? tubules
7. Renal venules
8. Renal vein

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Nephron
Peritubular capillary network
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Stages of Urine Formation

1. Pressure Filtration Glomerulus? Bowmans
   Capsule
2. Selective Reabsorption Proximal convoluted
   tubule and Loop of Henle? Peritubular capillary
   network
3. Tubular Secretion Peritubular capillary network?
   Distal convoluted tubule
4. Urine concentration Collecting tubule

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Pressure Filtration

• Filtration occurs as blood pressure forces fluid
  from the blood in the glomerulus into the lumen
  of Bowmans capsule.
• The porous capillaries, along with specialized
  capsule cells called podocytes, are permeable to
  water and small solutes but not to blood cells or
  large molecules such as plasma proteins.
• The filtrate in Bowmans capsule contains salt,
  glucose, vitamins, nitrogenous wastes, and other
  small molecules.

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Glomerulus and Bowmans Capsule
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Pressure Filtration
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Selective Reabsorption

• From Bowmans capsule, the filtrate passes
  through the proximal convoluted tubule and the
  loop of Henle, a hairpin turn with a descending
  limb and an ascending limb.
• Selective reabsorption of molecules out of
  filtrate in nephron into blood in peritubular
  capillary network occurs in the proximal tubule

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• Valuable nutrients, including glucose, amino
  acids, and vitamins are actively or passively
  absorbed from filtrate into blood.
• The epithelial cells actively transport Na out
  of the filtrate into the blood.
• This transfer of positive charge is balanced by
  the passive transport of Cl- out of the filtrate
  into the blood.
• As salt moves from the filtrate to the blood,
  water follows by osmosis.

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Selective Reabsorption in Proximal Tubule
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• The reabsorption of water continues as the
  filtrate moves into the descending limb of the
  loop of Henle.
• The membrane of the descending limb is freely
  permeable to water but not very permeable to salt
  and other small solutes.
• For water to move out of the tubule by osmosis,
  the interstitial tissue fluid bathing the tubule
  must be hypertonic to the filtrate.
• Because the osmolarity of the interstitial tissue
  fluid does become progressively greater from the
  outer cortex to the inner medulla, the filtrate
  moving within the descending loop of Henle
  continues to loose water.

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Selective Reabsorption in the Loop of Henle
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• In contrast to the descending limb, the membrane
  of the ascending limb is permeable to salt, not
  water.
• As filtrate ascends the thin segment of the
  ascending limb, NaCl diffuses out of the
  permeable tubule into the interstitial tissue
  fluid, increasing the osmolarity of the medulla.
• The active transport of salt from the filtrate
  into the interstitial tissue fluid continues in
  the thick segment of the ascending limb,
  increasing the osmolarity of the medulla
• This reinforces the water loss from the filtrate
  in the descending limb (counter-current effect)

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Selective Reabsorption in the Loop of Henle
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Tubular Secretion and Urine Concentration

• The distal tubule plays a key role in regulating
  the K and NaCl concentrations in body fluids by
  varying the amount of K that is secreted into
  the filtrate and the amount of NaCl reabsorbed
  from the filtrate.
• The distal tubule also contributes to pH
  regulation by controlled secretion of H and the
  reabsorption of bicarbonate (HCO3-).

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• As the collecting duct traverses the gradient of
  osmolarity in the kidney from cortex to medulla,
  the filtrate becomes increasingly concentrated as
  it loses more and more water by osmosis to the
  hypertonic interstitial tissue fluid.
• In the inner medulla, the collecting duct becomes
  permeable to urea, contributing to hypertonic
  interstitial tissue fluid and enabling the kidney
  to conserve water by excreting a hypertonic urine.

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Tubular Secretion and Urine Concentration
H
K
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• Summary of Urine Formation

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Summary of Urine Formation
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Hormones of the Kidney

• If blood pressure/ volume is too low
• Anti-diuretic Hormone (ADH)
• Renin, Angiotensin II, Aldosterone
• If blood pressure/ volume is too high
• Natriuretic Peptide Hormones

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Antidiuretic Hormone (ADH)

• One hormone important in regulating water balance
  is antidiuretic hormone (ADH).
• ADH is produced in hypothalamus of the brain and
  stored in and released from the pituitary gland,
  which lies just below the hypothalamus.
• Osmoreceptor cells in the hypothalamus monitor
  the osmolarity of the blood.

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Antidiuretic Hormone (ADH)
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No ADH Present - Collecting tubule is NOT
permeable to water and large volume of urine is
produced
Collecting tubule
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ADH Present - Collecting tubule is permeable to
water and a small volume of urine is produced
Collecting tubule
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Renin

• One of the functions of the kidney is to monitor
  blood pressure at the juxtaglomerular apparatus
  and take corrective action. If blood pressure
  should drop
• The juxtaglomerular apparatus of the kidney
  secretes the enzyme renin
• Renin catalyzes the conversion of the plasma
  protein angiotensinogen to angiotensin I
• Angiotensin converting enzyme (secreted by blood
  vessels) catalyzes the conversion of angiotensin
  I to angiotensin II

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• Angiotensin II
• constricts the walls of arterioles ? increase
  blood pressure
• stimulates the proximal convoluted tubules to
  reabsorb sodium ions ? water follows by osmosis ?
  increase blood pressure
• stimulates the adrenal cortex to release the
  hormone aldosterone
• aldosterone causes the kidneys to reclaim still
  more sodium ions? water follows by osmosis ?
  increase blood pressure
• increases the strength of the heartbeat
• stimulates the pituitary glands to release ADH

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Action of Renin, Angiotensin II, and Aldosterone
Angiotensin I
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Natriuretic Peptide Hormones

• In response to a rise in blood pressure, the
  heart releases two peptides
• A-type Natriuretic Peptide (ANP) This hormone of
  28 amino acids is released from stretched atria
  (hence the "A")
• B-type Natriuretic Peptide (BNP) This hormone (29
  amino acids) is released from the ventricles. (It
  was first discovered in brain tissue hence the
  "B")

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• Both hormones lower blood pressure by
• dilating arterioles
• inhibiting the secretion of renin and aldosterone
• inhibiting the reabsorption of sodium ions by the
  kidneys
• The latter two effects reduce the reabsorption of
  water by the kidneys, so the volume of urine
  increases as does the amount of sodium excreted
  in it. The net effect of these actions is to
  reduce blood pressure by reducing the volume of
  blood in the circulatory system