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Maintaining the Internal Environment

Chapter 49

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Outline

• Need to Maintain Homeostasis• Antagonistic Effectors and Positive

Feedback• Osmolality and Osmotic Balance• Osmoregulatory Organs• Evolution of the Vertebrate Kidney• The Mammalian Kidney• Transport Processes in Mammalian Nephron• Ammonia, Urea, and Uric Acid• Hormones Control Homeostatic Functions

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Need to Maintain Homeostasis

• Homeostasis may be defined as dynamic constancy of the internal environment.

• Negative feedback loops– The vertebrate body must have sensors to

measure conditions of the internal environment.

information relayed to integrating centerWhen a deviation occurs, the

integrating center sends a message to increase or decrease the activity of particular effectors.

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Negative Feedback Loop

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Need to Maintain Homeostasis

• Regulating body temperature– Hypothalamus responds to increased

body temperature by promoting the dissipation of heat through sweating, dilation of blood vessels, and other mechanisms.

Coordinates a different set of responses such as shivering and blood vessel constriction for decreased body temperature.

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Need to Maintain Homeostasis

• Regulating blood glucose– Glucose levels are constantly monitored

by the islets of Langerhans in the pancreas.

When levels increase, the islets secrete insulin which stimulate blood glucose uptake.

In this case, the islets are the sensor and the integrating center.

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Antagonistic Effectors and Positive Feedback

• Antagonistic effectors– Increasing activity of one effector is

accompanied by decreasing activity of an antagonistic effector.

• Positive feedback loops– feedback loops that accentuate a

disturbance Deviations cause the effector to drive

the value of the controlled variable even farther from the set point.

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Positive Feedback During Childbirth

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Osmolality and Osmotic Balance

• Osmolality and osmotic pressure– Because the total solute concentration of a

solution determines its osmotic behavior, the total moles of solute per kilogram is expressed as osmolality of the solution.

– Osmotic pressure of a solution is a measure of its tendency to take in water by osmosis.

hypertonic, hypotonic, isotonic

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Osmolality and Osmotic Balance

• Osmoconformers and osmoregulators– osmoconformers - Osmolality of body fluid

is same as that of surroundings. no osmotic gradient

most marine invertebrates– osmoregulators - Maintain a relatively

constant blood osmolality despite different concentrations in the surrounding environment.

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Osmolality and Osmotic Balance

• Freshwater vertebrates are hypertonic to their environment.

– Water tends to enter their bodies. Must actively transport ions back into

their bodies.• Most marine vertebrates are hypotonic to

their environment.– in danger of losing water by osmosis

drink seawater and eliminate excess ions through kidneys and gills

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Osmoregulatory Organs

• In many animals, removal of water or salts is coupled with removal of metabolic wastes through the excretory system.

– flatworms - flame cells– earthworms - nephridia

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Osmoregulatory Organs

• Insects - Malpighian tubules– Create an excretory fluid by secreting K+

into tubules. Creates an osmotic gradient.

• Vertebrates - Kidneys create a fluid (urine) by filtration of the blood under pressure.

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Malpighian Tubules of Insects

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Evolution of the Vertebrate Kidney

• Kidney is made up of thousands of repeating units (nephrons), each with the structure of a bent tube.

– Blood pressure forces the fluid in blood past a filter, glomerulus, at the top of each nephron.

Water and small molecules pass through filter and into the nephron tube.

Sugars and ions are removed by active transport.

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Vertebrate Nephron Organization

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Evolution of the Vertebrate Kidney

• Freshwater fish– Body fluids have greater osmotic

concentration than surrounding water.– Water enters body from environment.

They do not drink water and excrete large amounts of dilute urine.

– Solutes tend to leave the body. reabsorb ions across nephron tubules

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Evolution of the Vertebrate Kidney

• Marine bony fish– Body fluids are hypotonic to surrounding

seawater. Water tends to leave body via osmosis.

drink large amounts of seawater Actively transport ions out of the

blood across the gill surfaces. Excrete urine isotonic to body

fluids.

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Evolution of the Vertebrate Kidney

• Cartilaginous fish– Reabsorb urea from nephron tubules and

maintain a blood urea concentration 100 times higher than that of mammals.

Blood is approximately isotonic to surrounding sea.

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Osmotic Problems

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Evolution of the Vertebrate Kidney

• Amphibians and reptiles– Amphibian kidney is identical to that of

freshwater fish.– Reptile kidneys are very diverse.

Marine species eliminate excess salt through salt glands.

Terrestrial reptiles reabsorb much of salt and water in nephron tubules.

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Evolution of the Vertebrate Kidney

• Mammals and birds– Only vertebrates able to produce urine with

a higher osmotic concentration than their body fluids.

Hypertonic urine accomplished by loop of Henle portion of the nephron.

– Birds have relatively few or no loops, and thus cannot produce urine as concentrated as that in mammals.

Marine birds excrete excess salt from salt glands.

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Ammonia, Urea, and Uric Acid

• When amino acids and nucleic acids are catabolized, they produce nitrogenous wastes that must be eliminated from the body.

– First step is the removal of the amino (-NH2) group and its combination with H+ to form ammonia (NH3) in the liver.

toxic to cells

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Ammonia, Urea, and Uric Acid

• Elasmobranchs, adult amphibians, and mammals eliminate nitrogenous wastes in the form of urea.

• Reptiles, birds, and insects excrete nitrogenous wastes in the form of uric acid.

– Most mammals have enzyme uricase which converts uric acid into a more soluble derivative, allantoin.

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Nitrogenous Wastes

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The Mammalian Kidney

• Each kidney receives blood from a renal artery, and produces urine.

– Urine drains from each kidney through a ureter which carries urine to urinary bladder.

– Within the kidney, mouth of ureter flares to form renal pelvis.

Divided into renal cortex and renal medulla.

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Urinary System of a Human Female

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The Mammalian Kidney

• Nephron structure and function– Blood is carried by an afferent arteriole to

the glomerulus. Blood is filtered as it is forced through

porous capillary walls.Glomerular filtrate enters Bowman’s

capsule. Moves to the proximal convoluted

tubule.

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The Mammalian Kidney

• Fluid then moves down the medulla and back into the cortex in a loop of Henle.

– After leaving the loop, the fluid is delivered to a distal convoluted tubule in the cortex that drains to a collecting duct.

merges with other collecting ducts to empty its contents into the renal pelvis

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Mammalian Kidney Nephron

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The Mammalian Kidney

• Reabsorption and secretion– Most of the water and dissolved solutes

that enter the glomerular filtrate must be returned to the blood.

– Reabsorption of glucose and amino acids, is driven by active transport carriers.

– Secretion of waste products involves transport across capillary membranes and kidney tubules.

• Excretion

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Transport Processes in the Mammalian Nephron

• Some mechanism is needed to create an osmotic gradient between the glomerular filtrate and the blood, allowing reabsorption.

• Proximal convoluted tubule– Approximately two-thirds of NaCl and

water filtered in Bowman’s capsule is immediately reabsorbed across the walls of the proximal convoluted tube.

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Transport Processes in the Mammalian Nephron

• Loop of Henle– Descending limb is permeable to water,

thus water leaves via osmosis.– Water loss in the descending limb multiples

concentration achieved at each loop.– Ascending limb actively extrudes N+ and Cl

follows.– NaCl pumped out of ascending limb is

trapped within surrounding interstitial fluid.– countercurrent multiplier system

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Transport Processes in the Mammalian Nephron

• Distal tubule and collecting duct– Permeability of the collecting duct to water

is adjusted by antidiuretic hormone (ADH - vasopressin).

Kidneys also regulate the balance of electrolytes in the blood by reabsorption and secretion.

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Reabsorption of Salt and Water

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Hormones Control Homeostatic Functions

• Antidiuretic hormone– Stimulates reabsorption of water by the

kidneys.

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Hormones Control Homeostatic Functions

• Aldosterone– Promotes reabsorption of NaCl and water

across the distal convoluted tubule and the secretion of K+ into the tubule.

• Atrial natriuretic hormone– decreases NaCl reabsorption

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Summary

• Need to Maintain Homeostasis• Antagonistic Effectors and Positive

Feedback• Osmolality and Osmotic Balance• Osmoregulatory Organs• Evolution of the Vertebrate Kidney• The Mammalian Kidney• Transport Processes in Mammalian Nephron• Ammonia, Urea, and Uric Acid• Hormones Control Homeostatic Functions

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