Maintaining the internal environment

1
Maintaining the internal environment

• Chapter 26

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How the Animal Body Maintains Homeostasis

• Homeostasis may be defined as the dynamic
  constancy of the internal environment.
• Conditions fluctuate continuously within narrow
  limits.

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How the Animal Body Maintains Homeostasis

• To maintain internal constancy, the vertebrate
  body uses
• Sensors that measure each condition of the
  internal environment.
• An integrating center that contains the set
  point, or proper value for a particular internal
  condition.
• Effectors, which are muscles or glands that can
  change the value of the condition back toward the
  set point.
• The activity of the effectors is influenced by
  the effects they produce in a negative feedback
  loop.

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How the Animal Body Maintains Homeostasis

• Regulating body temperature
• Humans, as well as other mammals and birds, are
  endothermic.
• This means that they can maintain relatively
  constant body temperature.
• Other vertebrates are ectothermic, meaning their
  body temperatures depend more or less on the
  environmental temperature.
• But they can modify their behavior to affect body
  temperature.

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How the Animal Body Maintains Homeostasis

• Regulating blood glucose
• Excess glucose is stored in the liver as glycogen
  under the influence of the hormone insulin, which
  is released from the pancreas.
• When glucose levels are low in the blood, the
  pancreas releases the hormone glucagon, which
  stimulates the liver to convert glycogen back to
  glucose.

6
Control of blood glucose levels
7
Regulating the Bodys Water Content

• Animals use various mechanisms for
  osmoregulation, the regulation of the bodys
  osmotic composition.
• This refers to how much water and salt the body
  contains.
• The proper operation of many vertebrate organ
  systems requires that the osmotic concentration
  of the blood be kept within narrow bounds.

8
Regulating the Bodys Water Content

• In many animals and single-celled organisms, the
  removal of water and salts from the body is
  coupled with the removal of metabolic wastes
  through the excretory system.

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Regulating the Bodys Water Content

• For example, protists, like Paramecium, employ
  contractile vacuoles.

10
Regulating the Bodys Water Content

• Flatworms employ a system of excretory tubules
  called protonephridia to expel fluids and wastes
  from the body.

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Regulating the Bodys Water Content

• Other invertebrates have a system of tubules that
  open both to the inside and to the outside of the
  body.
• In annelids, these tubules are called nephridia.

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Regulating the Bodys Water Content

• The excretory organs in insects are called
  Malpighian tubules, which are extensions of the
  digestive tract.

13
Regulating the Bodys Water Content

• Kidneys are the excretory organs in vertebrates.
• Kidneys create a tubular fluid by filtration.
• The filtrate contains many valuable nutrients in
  addition to waste products.
• Selective reabsorption ensures that these
  nutrients and water are reabsorbed into the
  blood, while wastes remain in the filtrate.

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Evolution of the Vertebrate Kidney

• The kidney is a complex organ made up of many
  repeating units called nephrons.
• Blood pressure forces the fluid in the blood
  through a capillary bed at the top of each
  nephron, called a glomerulus.
• The glomerulus excludes blood cells, proteins,
  and other large molecules from the filtrate.
• The remainder of the nephron tube reabsorbs
  anything else useful from the filtrate

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Basic organization of the vertebrate nephron
Proximal arm
Distal arm
Glomerulus
Neck
NaCl
Glucose
H2O
H2O
Amino acids
H2O
NaCl
H2O
H2O
Divalent ions
Intermediate segment
H2O
Collecting duct
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Evolution of the Vertebrate Kidney

• Only birds and mammals can reabsorb water from
  the glomerular filtrate to produce a urine that
  is hypertonic to (more concentrated than) blood.

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Evolution of the Vertebrate Kidney

• Kidneys are thought to have evolved first among
  the freshwater fish.
• The body fluids of a freshwater fish have a
  greater osmotic concentration than the
  surrounding water. So,
• Water tends to enter the body from the
  environment.
• Solutes tend to leave the body and enter the
  environment.

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Evolution of the Vertebrate Kidney

• Freshwater fish address these problems by
• Not drinking water.
• Excreting a large volume of dilute urine.
• Reabsorbing ions (mainly NaCl) from the nephron.
• Actively transporting NaCl across the gills from
  the surrounding water into the blood.

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Evolution of the Vertebrate Kidney

• Marine fish probably evolved from freshwater
  ancestors.
• Their bodies are hypotonic to the surrounding
  seawater. So,
• Water tends to leave their bodies through osmosis
  across the gills.
• They lose water in their urine.
• To compensate, marine fish drink lots of seawater
• They excrete isotonic urine.

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Evolution of the Vertebrate Kidney

• Elasmobranchs solve the osmotic problem posed by
  their seawater environment by reabsorbing urea
  from the nephron tubules.
• The blood is approximately isotonic to the
  surrounding sea.

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Evolution of the Vertebrate Kidney

• The amphibian kidney is like that of freshwater
  fish.
• Amphibians produce a very dilute urine and
  actively transport Na across their skin.
• The kidneys of terrestrial reptiles reabsorb much
  of the salt and water in the nephron tubules.
• Their urine is still hypotonic but they can
  absorb additional water in the cloaca

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Evolution of the Vertebrate Kidney

• Because mammals and birds can produce hypertonic
  urine, they can excrete their waste products in a
  small volume of water.
• The kidneys of some mammals are even more
  extremely efficient at conserving water.
• The kidneys of the kangaroo rat are so efficient
  it never has to drink water it can obtain all
  the water it needs from its food and aerobic cell
  respiration.

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Evolution of the Vertebrate Kidney

• Birds have relatively few or no nephrons with
  long loops.
• At most, they can only reabsorb enough water to
  produce a urine that is about twice the
  concentration of their blood
• Marine birds solve the problem of water loss by
  drinking sea water and excreting excess salt
  through salt glands near the eyes.

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The Mammalian Kidney

• In mammals, each kidney receives blood from a
  renal artery, and it is from this blood that
  urine is produced.
• Urine drains from each kidney through a ureter.
• The ureters carry urine to a urinary bladder.
• Urine passes out of the body through the urethra.

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The Mammalian Kidney

• Within the kidney, the mouth of the ureter flares
  open to form a funnel-like renal pelvis.
• The renal tissue is divided into
• An outer renal cortex
• An inner renal medulla

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The Mammalian Kidney

• The mammalian kidney is comprised of roughly 1
  million nephrons, each of which is composed of
  three regions
• Filter
• The filtration device at the top of each nephron
  is called the Bowmans capsule which receives
  filtrate from the glomerular capillaries.
• Tube
• The Bowmans capsule is connected to a long renal
  tubule, which includes the Loop of Henle, that
  acts as a reabsorption device.
• Duct
• The renal tubule empties into a collecting duct
  that operates as a water conservation device.

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The Mammalian Kidney

• There are five steps involved in the formation of
  urine in the kidney
• Pressure filtration
• Reabsorption of water
• Selective reabsorption
• Tubular secretion
• Further reabsorption of water

http//youtu.be/TzwPmz5V6Xg
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Eliminating Nitrogenous Wastes

• Amino acids and nucleic acids are
  nitrogen-containing molecules.
• When animals metabolize these substances, they
  produce nitrogen-containing by-products, called
  nitrogenous wastes, that must be eliminated by
  the body.

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Eliminating Nitrogenous Wastes

• The first step in the metabolism of amino acids
  and nucleic acids is the removal of the amino
  (NH2) group.
• This group is then combined with H to form
  ammonia (NH3).
• This takes place in the liver.

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Eliminating Nitrogenous Wastes

• Ammonia is quite toxic and is safe only in very
  dilute concentrations.
• Fish and tadpoles, ammonia can be directly
  eliminated across the gills or excreted in dilute
  urine.
• In sharks, adult amphibians, and mammals, the
  nitrogenous waste is eliminated as urea, which is
  less toxic.
• Reptiles, birds, and insects excrete nitrogenous
  wastes in the form of uric acid, which can be
  excreted with very little water.

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Nitrogenous wastes
Amino acids and nucleic acids
1
Catabolism
Ammonia by-product
3
4
Converted to urea
Converted to uric acid
2
Eliminated directly
O
Uric acid
Urea
NH2
H
Ammonia
N
O
C
HN
NH3
NH2
O
N
N
O
H
H
Most fish
Mammals, some others
Reptiles and birds