1. CHAPTER 49 EXCRETORY SYSTEMS AND SALT AND WATER BALANCE
Prepared by
Brenda Leady, University of Toledo
Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Body water
Water is…
Major portion of an animal’s body mass
Solvent for chemical reaction
Transport vehicle
Dehydration compromises the circulatory system and regulation of body temperature

3. Electrolytes Salts dissociate in solution into charged ions
Electrolyte balance important
Imbalance can alter membrane potential or disrupt cellular activities

4. Water moves between adjacent body compartments by osmosis down an osmotic gradient
Changes in salt concentration in one compartment will lead to changes in fluid distribution between compartments
Shrinking or swelling cells can rupture plasma membranes leading to cell death

5. Obligatory exchanges
Many vital processes have the potential to disturb salt and water balance
Obligatory exchanges are required as part of respiration or elimination of wastes

6. Nitrogenous wastes Product of protein and nucleic acid degradation
Toxic at high concentrations
3 forms
Ammonia and ammonium ions
Urea
Uric acid

7. Ammonia (NH3) and ammonium ions (NH4+)
Most toxic of nitrogenous wastes
Aquatic animals can excrete it as soon as it forms
Some terrestrial snails and crustaceans secrete it as a gas
Chief advantage is that energy not required for production

8. Urea
All mammals, some marine fishes, some reptiles, some terrestrial invertebrates
Less toxic so it doesn’t need as large a volume of water for excretion
Can tolerate some urea accumulation
Drawback is conversion of ammonia to urea requires ATP

9. Uric acid Birds, insects, and most reptiles Less toxic than ammonia
More energetically costly to make from ammonia
Balanced against water conserved by excreting semisolid, partly dried precipitate

11. Exchanges during respiration
Associated with significant water loss
Small, active animals with higher metabolic rates and higher breathing rates have the potential for even greater water loss
Aquatic animals moving water over gills can lose salt and water

12. Osmolarity Number of dissolved solute molecules/ Liter
150 mM NaCl solution = 300 mosm/L
One Na+ and one Cl- made when salt dissociates
Internal fluid osmolarity of most fish and other vertebrates around mosm/L
Freshwater less than 25 mosm/L
Seawater 1000 mosm/L

13. Freshwater fish Gain water and lose salt when ventilating gills
Kidneys produce copious dilute urine
Specialized gill epithelial cells transport Na+ and Cl- from water into fish’s capillaries

14. Saltwater fish Gain salts and lose water across gills
Produce very little urine
Drink seawater to replace water lost
Expend energy to transport excess salt out of body through gill epithelial cells

15. Exchanges during ingestion
Food contains minerals and water
Unusable parts of food are excreted as solid waste with accompanying water and salt loss
Marine reptiles and birds ingest seawater when consuming prey or drinking – salt glands used to excrete excess salt

17. Body temperature Endotherms use water to cool off
Sweating and panting use the evaporation of water to draw heat out of the body
Sweat is hypo-osmotic to blood
Fluid left behind in body has lower volume and higher salt concentration

18. Metabolism
Water is generated as a by-product of ATP production – “metabolic water”
Some animals rely on this for all or nearly all of their water requirements
In other animals, this water is excess that must be eliminated

19. Cade and Colleagues at the University of Florida Discovered Why Athletes’ Performance Wanes on Hot Days
Symptoms from practicing on hot days due to water loss and electrolyte imbalance
Hypothesized that the best way to maintain salt and water homeostasis was to restore to the body what was lost – drink a solution resembling sweat
Analyzed the composition of sweat and develop a drink with similar components
Gatorade®

21. Regulate or conform
Osmoregulators maintain constant internal salt concentrations and osmolarities
Drink or excrete as needed to maintain 300 mosm/L
All terrestrial animals, freshwater animals, and many marine animals
Requires considerable expenditure of energy

22. Osmoconformers match osmolarity of blood and other fluids to seawater at 1000 mosm/L
Most marine invertebrates and some vertebrates
Less tendency to gain or lose water across skin or gills
Expend less energy to compensate for water gain
Generally limited to marine environment
Total amount of salt and organic compounds produces an osmolarity similar to seawater, even though salt concentration similar to osmoregulators
Body fluids less salty than seawater (like osmoregulators) so they tend to gain salt
Eliminate excess salt using rectal gland

23. Excretory organs Filtration Reabsorption Secretion
An organ acts like a filter to remove water and small solutes from blood while leaving behind blood cells and large solutes
Produces filtrate
Reabsorption
Material in filtrate recaptured and returned to blood
Secretion
Supplements solutes removed by filtration

25. Protonephridia Simplest filtration mechanism in invertebrates
Flatworms
Series of branching tubules filters fluids from body cavity using beating of ciliated cells (flame cells)
Solutes reabsorped
Excess water and some wastes emptied through openings in body wall called nephridiopores
Osmoregulatory – nitrogenous wastes diffuse out of body

27. Metanephridial system
Annelids
Pairs located in each body segment
Tubular network beginning in funnel-like structure called nephrostome
Collect coelomic fluid containing nitrogenous wastes
Na+, Cl- and others reabsorbed along tubule
Nitrogenous wastes excreted through nephridiopores in body wall

29. Malpighian tubules Insects do not use filtration
Cells lining tubules actively transport and secrete potassium ions and uric acid from hemolymph into lumen
Creates osmotic gradient drawing water and solutes into tubule
Moves to hindgut where water and solutes reabsorbed
Nitrogenous wastes and others excreted together with feces through anus

31. Kidney
Vertebrates
Specialized tubules composed of epithelial cells actively transporting sodium and other ions for salt and water homeostasis and nitrogenous waste elimination

32. Diet can influence kidney function
Sporadic, high protein meals have high nitrogenous waste production rates which require high rate of blood filtration in kidney
Polar bears don’t eat for months yet continue to make nitrogenous wastes – unique ability to recycle nitrogenous wastes into protein
Vampire bat consumes large volumes of water with blood – water must be quickly excreted while retaining nutrients

33. Urinary system Kidney Ureters Urinary bladder Urethra Renal cortex
(a) Human urinary
system
Kidney
Renal cortex
Renal medulla
Inner region
Outer region
Ureters
Urinary bladder
Urethra

34. Nephron Functional unit of the kidney
As many as several million in each kidney
Consists of
Renal corpuscle forms filtrate
Tubule performs secretion and reabsorption
Tubule empties into collecting duct

36. Renal corpuscle Glomerulus Bowman’s capsule encloses Bowman’s space
Cluster of interconnected, fenestrated capillaries
Supplied by afferent arteriole
Drained by efferent arteriole
Podocytes form filtration slits (physical barriers)
Bowman’s capsule encloses Bowman’s space

38. Tubular part Continuous with Bowman’s capsule
Epithelial cells differ in structure and function along length
Proximal convoluted tubule
Loop of Henle
Descending goes down into medulla
Ascending comes up out of medulla
Distal convoluted tubule
Then into collecting duct
Tubule surrounded by peritubular capillaries near junction of cortex and medulla and vasa recta capillaries in the medulla

39. Filtration
As blood flows through glomerulus, about 20% of plasma leaves capillaries and filters into Bowman’s space
Proteins and blood cells remain in plasma
Glomerular filtrate
Glomerular filtration Rate (GFR) is rate of filtrate production

40. Reabsorption in proximal tubule
Filtrate is around 300 mosm/L, similar to blood
Water and solutes are reabsorped into peritubular capillaries to return to body
Depending on solute 2/3 to all of it is reabsorbed – Na+, K+, Cl-, HCO3-, and organic molecules like glucose and amino acids
Solutes and water reabsorped in proportion to each other
Volume and composition change dramatically
Osmolarity remains the same – 300mosm/L

41. Reabsorption in Loop of Henle
Descending loop permeable to water but not solutes
Water leaves by osmosis because surrounding fluid hyperosmotic
Fluid increases in osmolarity
Ascending loop (thick segment) not permeable to water and actively transports salts out
Fluid decreases in osmolarity
Countercurrent multiplier because energy is used to multiply the gradient created in the medulla

43. Gradient created is used by collecting duct to determine final urine volume
Animals that must tightly regulate body water stores (desert animals) have very long loops of Henle and large osmotic gradients
Freshwater fish have no loop of Henle at all – need to excrete as much water as possible

44. Blood flowing through vasa recta in medulla designed not to remove gradient
Hairpin loops of vasa recta run parallel to loops of Henle and medullary collecting ducts
Minimizes excessive loss of solutes from interstitial fluid by diffusion

45. Hormones
Aldosterone
Acts on distal convoluted tubule cells to stimulate active transport of 3 molecules of Na+ out of tubule (reabsorption) for every 2 molecules of K + brought into tubule (secretion)
Water from tubule lumen follows Na+ by osmosis into blood

47. Antidiuretic hormone (ADH)
Acts to increase the number of aquaporins (water channels) in the collecting duct membranes
Collecting ducts travel through hyperosmotic medulla
Higher levels of ADH increase the number of aquaporins allowing water to leave the duct and urine volume decreases

48. Loop sequences highly conserved among different species
Agre and Colleagues Discovered the Mechanism of Water Movement Across Biological Membranes
Discovered in early 1990’s
Proteins with 6 transmembrane helical domain and 2 short loops in the membrane (form water channel)
Loop sequences highly conserved among different species
Domains have sites that can be modified by enzymes
Opening and closing of channels may be gated by stimuli like ion channels

50. Transport maximum (Tm)
Reabsorption will return solutes to the blood
Binding sites for transport can become saturated at high levels
Solutes not reabsorped are lost in the urine
Vitamin C is reabsorped unless plasma values are so high that transporters can’t reabsorb any more – then excess vitamin C lost in the urine

51. Impact on public health
Symptoms of renal malfunction similar regardless of cause
All stem from uremia
Potentially toxic waste products build up in the blood
Increased K+ in blood can cause heart and nerve function problems
Kidneys able to perform well with only 10% of nephrons functioning

52. Hemodialysis
Small solutes diffuse out of patient’s blood into solution in dialyzer until equilibrium reached
Uses artificial countercurrent exchange system to increase efficiency

53. Kidney transplants Rejection of donor kidney is a potential problem
Many people do not receive transplants because need exceeds donors
A person can donate a kidney and still function normally with 1 kidney