1. CHAPTER 51 LECTURE SLIDES
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2. Osmotic Regulation and the Urinary System
Chapter 51

3. Osmolarity and Osmotic Balance
Water in a multicellular body distributed between
Intracellular compartment
Extracellular compartment
Most vertebrates maintain homeostasis for
Total solute concentration of their extracellular fluids
Concentration of specific inorganic ions

4. Osmolarity and Osmotic Balance
Important ions
Sodium (Na+) is the major cation in extracellular fluids
Chloride (Cl–) is the major anion
Divalent cations, calcium (Ca2+) and magnesium (Mg2+), the monovalent cation K+, as well as other ions, also have important functions and are maintained at constant levels

5. Extracellular compartment
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External environment
Animal body
H2O (Sweat)
Solutes and H2O
Intracellular
compartment
Intracellular
compartment
Solutes and H2O
Extracellular compartment
(including blood)
CO2 and H2O
CO2 and H2O
Solutes and H2O O2
Solutes and H2O O2
Food and H2O
Solutes and H2O
Solutes and H2O
Urine (excess H2O)
Waste

6. Osmolarity and Osmotic Balance
Osmotic pressure
Measure of a solution’s tendency to take in water by osmosis
Osmolarity
Number of osmotically active moles of solute per liter of solution
Tonicity
Measure of a solution’s ability to change the volume of a cell by osmosis
Solutions may be hypertonic, hypotonic, or isotonic

7. Osmolarity and Osmotic Balance
Osmoconformers
Organisms that are in osmotic equilibrium with their environment
Among the vertebrates, only the primitive hagfish are strict osmoconformers
Sharks and relatives (cartilaginous fish) are also isotonic
All other vertebrates are osmoregulators
Maintain a relatively constant blood osmolarity despite different concentrations in their environment

8. Osmolarity and Osmotic Balance
Freshwater vertebrates
Hypertonic to their environment
Have adapted to prevent water from entering their bodies, and to actively transport ions back into their bodies
Marine vertebrates
Hypotonic to their environment
Have adapted to retain water by drinking seawater and eliminating the excess ions through kidneys and gills

9. Osmolarity and Osmotic Balance
Terrestrial vertebrates
Higher concentration of water than surrounding air
Tend to lose water by evaporation from skin and lungs
Urinary/osmoregulatory systems have evolved in these vertebrates that help them retain water

10. Osmoregulatory Organs
In many animals, removal of water or salts is coupled with removal of metabolic wastes through the excretory system
A variety of mechanisms have evolved to accomplish this
Single-celled protists and sponges use contractile vacuoles
Other multicellular animals have a system of excretory tubules to expel fluid and wastes

11. Osmoregulatory Organs
Invertebrates
Flatworms
Use protonephridia which branch into bulblike flame cells
Open to the outside of the body, but not to the inside
Earthworms
Use nephridia
Open both to the inside and outside of the body

14. Osmoregulatory Organs
Insects
Use Malpighian tubules
Extensions of the digestive tract
Waste molecules and K+ are secreted into tubules by active transport
Create an osmotic gradient that draws water into the tubules by osmosis
Most of the water and K+ is then reabsorbed into the open circulatory system through hindgut epithelium

16. Osmoregulatory Organs
Vertebrate kidneys
Create a tubular fluid by filtering the blood under pressure through the glomerulus
Filtrate contains many small molecules, in addition to water and waste products
Most of these molecules and water are reabsorbed into the blood
Selective reabsorption provides great flexibility
Waste products are eliminated from the body in the form of urine

17. Evolution of the Vertebrate Kidney
Made up of thousands of repeating units – nephrons
Although the same basic design has been retained in all vertebrate kidneys, a few modifications have occurred
All vertebrates can produce a urine that is isotonic or hypotonic to blood
Only birds and mammals can make a hypertonic urine

19. Evolution of the Vertebrate Kidney
Kidneys are thought to have evolved among the freshwater teleosts, or bony fishes
Body fluids are hypertonic with respect to surrounding water, causing two problems
Water enters body from environment
Fishes do not drink water and excrete large amounts of dilute urine
Solutes tend to leave the body
Reabsorb ions across nephrons
Actively transport ions across gills into blood

20. Evolution of the Vertebrate Kidney
In contrast, marine bony fishes have body fluids that are hypotonic to seawater
Water tends to leave their bodies by osmosis across their gills
Drink large amounts of seawater
Eliminate ions through gill surfaces and urine
Excrete urine isotonic to body fluids

21. Evolution of the Vertebrate Kidney

22. Evolution of the Vertebrate Kidney
Cartilaginous fish, including sharks and rays, reabsorb urea from the nephron tubules
Maintain a blood urea concentration that is 100 times higher than that of mammals
Makes blood isotonic to surrounding sea
These fishes do not need to drink seawater or remove large amounts of ions from their bodies

23. Evolution of the Vertebrate Kidney
Amphibian kidney is identical to that of freshwater fish
Kidneys of reptiles are very diverse
Marine reptiles drink seawater and excrete an isotonic urine
Eliminate excess salt via salt glands
Terrestrial reptiles reabsorb much of the salt and water in their nephron tubules
Don’t excrete urine, but empty it into cloaca

24. Evolution of the Vertebrate Kidney
Mammals and birds are the only vertebrates that can produce urine that is hypertonic to body fluids
Accomplished by the loop of Henle
Birds have relatively few or no nephrons with long loops, and so cannot produce urine as concentrated as that of mammals
Marine birds excrete excess salt from salt glands near the eyes

25. Evolution of the Vertebrate Kidney

26. Nitrogenous Wastes
When amino acids and nucleic acids are catabolized, they produce nitrogenous wastes that must be eliminated from the body
First step is deamination
Removal of the amino (―NH2) group
Combined with H+ to form ammonia (NH3) in the liver
Toxic to cells, and thus it is only safe in dilute concentrations

27. Nitrogenous Wastes
Bony fishes and amphibian tadpoles eliminate most of the ammonia by diffusion via gills
Elasmobranchs, adult amphibians, and mammals convert ammonia into urea, which is soluble in water
Birds, reptiles, and insects convert ammonia into the water-insoluble uric acid
Costs most energy, but saves most water

28. Nitrogenous Wastes
Mammals also produce uric acid, but from degradation of purines, not amino acids
Most have an enzyme called uricase, which convert uric acid into a more soluble derivative called allantoin
Humans lack this enzyme
Excessive accumulation of uric acid in joints causes gout

30. The Mammalian Kidney Each kidney receives blood from a renal artery
Produces urine from this blood
Urine drains from each kidney through a ureter into a urinary bladder
Urine is passed out of the body through the urethra

31. The Mammalian Kidney
Within the kidney, the mouth of the ureter flares open to form the renal pelvis
Receives urine from the renal tissue
Divided into an outer renal cortex and inner renal medulla

32. The Mammalian Kidney

33. The Mammalian Kidney The kidney has three basic functions Filtration
Fluid in the blood is filtered out of the glomerulus into the tubule system
Reabsorption
Selective movement of solutes out of the filtrate back into the blood via peritubular capillaries
Secretion
Movement of substances from the blood into the extracellular fluid, then into the filtrate in the tubular system

35. The Mammalian Kidney
Each kidney is made up of about 1 million functioning nephrons
Juxtamedullary nephrons have long loops that dip deeply into the medulla
Cortical nephrons have shorter loops
Blood is carried by an afferent arteriole to the glomerulus
Blood is filtered as it is forced through porous capillary walls

36. The Mammalian Kidney
Blood components that are not filtered drain into an efferent arteriole, which empties into peritubular capillaries
Vasa recta in juxtamedullary nephrons
Glomerular filtrate enters the first region of the nephron tubules – Bowman’s capsule
Goes into the proximal convoluted tubule
Then moves down the medulla and back up into cortex in the loop of Henle

37. The Mammalian Kidney
After leaving the loop, the fluid is delivered to a distal convoluted tubule in the cortex
Drains into a collecting duct
Merges with other collecting ducts to empty its contents, now called urine, into the renal pelvis

39. Reabsorption and Secretion
Approximately 2000 L of blood passes through the kidneys each day
180 L of water leaves the blood and enters the glomerular filtrate
Most of the water and dissolved solutes that enter the glomerular filtrate must be returned to the blood by reabsorption
Water is reabsorbed by the proximal convoluted tubule, descending loop of Henle, and collecting duct

40. Reabsorption and Secretion
Reabsorption of glucose and amino acids is driven by active transport and secondary active transport
Maximum rate of transport
Glucose remains in the urine of untreated diabetes mellitus patients
Secretion of waste products involves transport across capillary membranes and kidney tubules into the filtrate
Penicillin must be administered several times a day

41. Excretion
Major function of the kidney is elimination of a variety of potentially harmful substances that animals eat and drink
In addition, urine contains nitrogenous wastes, and may contain excess K+, H+, and other ions that are removed from blood
Kidneys are critically involved in maintaining acid–base balance of blood

42. Transport in the Nephron
Proximal convoluted tubule
Reabsorbs virtually all nutrient molecules in the filtrate, and two-thirds of the NaCl and water
Because NaCl and water are removed from the filtrate in proportionate amounts, the filtrate that remains in the tubule is still isotonic to the blood plasma

43. Transport in the Nephron
Loop of Henle
Creates a gradient of increasing osmolarity from the cortex to the medulla
Actively transports Na+, and Cl– follows from the ascending loop
Creates an osmotic gradient
Allows reabsorption of water from descending loop and collecting duct
Two limbs of the loop form a countercurrent multiplier system
Creates a hypertonic renal medulla

45. Counter Current Multiplier system – Loop of Henle

47. Transport in the Nephron
Distal convoluted tubule and collecting duct
Filtrate that enters is hypotonic
Hypertonic interstitial fluid of the renal medulla pulls water out of the collecting duct and into the surrounding blood vessels
Permeability controlled by antidiuretic hormone (ADH)
Kidneys also regulate electrolyte balance in the blood by reabsorption and secretion
K+, H+, and HCO3–

49. Hormones Control Osmoregulation
Kidneys maintain relatively constant levels of blood volume, pressure, and osmolarity
Also regulate the plasma K+ and Na+ concentrations and blood pH within narrow limits
These homeostatic functions of kidneys are coordinated primarily by hormones

50. Hormones Control Osmoregulation
Antidiuretic hormone (ADH)
Produced by the hypothalamus and secreted by the posterior pituitary gland
Stimulated by an increase in the osmolarity of blood
Causes walls of distal tubule and collecting ducts to become more permeable to water
Aquaporins
More ADH increases reabsorption of water
Makes a more concentrated urine

52. Hormones Control Osmoregulation
Aldosterone
Secreted by the adrenal cortex
Stimulated by low levels of Na+ in the blood
Causes distal convoluted tubule and collecting ducts to reabsorb Na+
Reabsorption of Cl– and water follows
Low levels of Na+ in the blood are accompanied by a decrease in blood volume
Renin-angiotensin-aldosterone system is activated

54. Hormones Control Osmoregulation
Atrial natriuretic hormone (ANH)
Opposes the action of aldosterone in promoting salt and water retention
Secreted by the right atrium of the heart in response to an increased blood volume
Promotes the excretion of salt and water in the urine and lowering blood volume