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EXCRETORY CHAPTER SYSTEMS AND SALT AND WATER BALANCE
Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1
Homeostasis critical in regulation of salt and water levels Changes in salt concentrations either intra- or extracellular can disrupt cell function
potential differences across membranes Altering cell volume
Altering
Salt concentrations and water volumes in different compartments related to each other osmosis depends on dissolved salts
2
Homeostasis is energy requiring Significant portion of energy devoted to salt and water balance
Environment
Climate Access
to drinking water
Water used to eliminate metabolic waste needs to be replenished
3
Body water
Water is...
Major
portion of an animal's body mass Solvent for biochemical reactions Transport vehicle brings O2 and nutrients; removes wastes
Dehydration water volume reduced below normal range
Sufficient drinking
water not available Water lost by evaporation (sweating, panting) Compromises the circulatory system and regulation of body temperature
4
Electrolytes
Salts dissociate in solution into charged ions Electrolyte balance important Imbalance can alter membrane potential or disrupt cellular activities
Cofactors
for enzyme activation Bone formation ECM formation Exocytosis Muscle contraction
5
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
6
Electrolytes move by different mechanisms Limited ability to move across plasma membranes
High
water solubility, poor lipid solubility
Ions move through
formed channels in membrane Active transport across epi cells requires energy
Protein
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Obligatory exchanges
Many vital processes have the potential to disturb salt and water balance
Obtaining O2, releasing CO2, metabolizing food, regulating temp, eliminating nitrogenous wastes Require additional energy to minimize effects
Obligatory exchanges are required as part of respiration or elimination of wastes
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Nitrogenous wastes
Product of protein and nucleic acid degradation Toxic at high concentrations 3 forms
Ammonia
and ammonium ions
Urea Uric acid
and purines
Eliminated via excretory organs ex. kidneys, gills Sharks, rays, skates can survive with high levels of nitrogenous wastes in blood and body fluid
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Ammonia (NH3) and ammonium ions (NH4+)
toxic of nitrogenous wastes disrupt pH, ion electrochemical gradients, redox reactions Aquatic animals can excrete it as soon as it forms
Most
Marine invertebrates across skin Freshwater fish gills and kidneys
Some
terrestrial snails and crustaceans secrete it as a gas less toxic, does not require water
Chief advantage energy not required for production
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Urea
All
mammals, most amphibians, some marine fishes, some reptiles, some terrestrial invertebrates Less toxic doesn't need as large a volume of water for excretion Can tolerate some urea accumulation in blood, tissues, storage organs (urinary bladder) Conserves water, no necessity for constant excretion, reduces toxicity Drawback is conversion of ammonia to urea requires moderate expenditure of ATP
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Uric
acid and purines
Birds,
insects, and most reptiles Less toxic than ammonia More energetically costly to make than urea Poorly soluble in water Balanced against water conserved by excreting semisolid, partly dried precipitate
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Exchanges during respiration
Terrestrial animals lose water with each exhalation Associated with significant water loss Small, active animals with higher metabolic rates and higher breathing rates have the potential for even greater water loss particularly in endotherms Aquatic animals moving water over gills can lose salt and water due to constant movement of water over gills and high gill surface area
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Osmolarity
Number
of dissolved solute molecules/ liter Measured in milliosmoles/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 225-400 mosm/L Freshwater less than 25 mosm/L
Could promote loss of salt from fish Also promotes movement of water into fish Water flows across gills and into underlying capillaries and blood stream
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Freshwater fish
Gain water and lose salt when ventilating gills Kidneys produce copious dilute urine up to 30% of body mass/day (equivalent to 25 l urine/day in average male) Specialized gill epithelial cells actively transport Na+ and Cl- from water into fish's capillaries
Rarely drink water except what they take in with food
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Other
freshwater animals (frogs, adult amphibians)
Body
surface permiable to water and used in gas exchange Gain water by osmosis Excrete copious dilute urine Epi cells of skin actively transport electrolytes from water to blood
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Saltwater fish
Seawater ~1000 mOsm/L Gain salts and lose water across gills Produce very little urine higher [salt]
than freshwater
fish Drink seawater to replace water lost Expend energy to transport excess salt out of body through gill epithelial cells
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Exchanges during ingestion
Food contains minerals and water Some plants are 95% water Type of food animal ingests determines salt and water content Unusable parts of food are excreted as solid waste with accompanying water and salt loss
Desert-dwelling
kangaroo rats produce feces that are almost dry
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Marine reptiles and birds ingest seawater when consuming prey or drinking (no access to fresh water)
Specialized
epi cells lining salt glands used to excrete excess salt Located around nostrils, mouth and eyes Salt moves from blood to interstitial fluid Actively transported by epi cells into tubules of salt gland Collect in central duct and excreted as highly concentrated solution
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Body temperature
Endotherms use water to cool off Sweating and panting use the evaporation of water to draw heat out of the body
Animal
loses salt and water
Sweat is hypoosmotic (more dilute) to blood
Fluid
left behind in body has lower volume and higher salt concentration
Other than sweating or panting little water lost across body surface of terrestrial vertebrates
Skin impermeable to water Amphibians exception Invertebrates rate of water
loss depends on whether animal is soft-bodied (worms) or covered in waterimpermeable cuticle (insects)
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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 desert dwellers In other animals, this water is excess that must be eliminated
Through
excretory organs Metabolism is ongoing and required excretion and retention of metabolic water an obligatory exchange
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Cade and Colleagues at the University of Florida Discovered Why Athletes' Performance Wanes on Hot Days
Early 1960s U. Fla. Football team practiced during summer in full uniform
Osmolarity
of body fluids increased as sweat produced to maintain body temp
Symptoms
No
urination for long periods after practice Increasing fatigue and susceptibility to muscle cramps Extreme seizures (uncontrolled activity of brain neurons)
Fatigue related to loss of water putting strain on circulatory system reducing blood flow to muscles and other organs Muscle cramps and seizures imbalance in extracellular electrolytes secreted in sweat especially Na+ and K+
Change
in electrical potential across muscle and nerve membranes causing spasms
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 (rung out the uniforms)
Flame spectrophotometer Contained Na+, K+, and Cl- -
a more dilute compared to
blood
Produced a drink similar to human sweat
Added
lemon flavor and sugar for energy
Fed it to players during and after practice Players fed the drink outperformed players fed water Drink effectively improved performance and reduced dehydration and its effects Called it Gatorade 1975- KC Chiefs (old AFL) first professional team to use drink
Effectiveness due to restoration of correct amounts of water and salt lost during exercise Rapidly absorbed because osmoloarity close to body fluids Newer "sports drinks" contain additional solutes (minerals, vitamins) and sugar may be hyperosmotic to body fluids
Initially
draw water into gut lumen slows down rate at which water from drink is absorbed
Drinking copious amounts of pure water prevents dehydration but will eventually reduce plasma salt concentration The "Gatorade bath", is credited to the New York Giants' Jim Burt who dumped the concoction over head coach Bill Parcells in 1985
Regulate or conform
Osmoregulators maintain constant internal salt concentrations and osmolarities
Drink
or excrete as needed to maintain 300 mosm/L (33% saltwater, 10X fresh water) All terrestrial animals, freshwater animals, and many marine animals Requires considerable expenditure of energy Many aquatic osmoregulators (ex. salmon) can migrate between fresh water and salt water
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Osmoconformers match osmolarity of blood and other fluids to seawater at 1000 mosm/L
Most
marine invertebrates and some vertebrates (sharks) Less tendency to gain or lose water across skin or gills Expend less energy to compensate for water gain Generally limited to marine environment Vertebrates have high concentration of uncharged molecules in extracellular fluids
Produces an osmolarity similar to seawater, even though salt concentration similar to osmoregulators Prevents excessive accumulation of ions in body
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Body fluids contain sugars, amino acids, metabolic waste products
Urea
TMAO
(trimethylamine oxide)
Total amount of salt and organics in extracellular fluid produces osmolarity similar to seawater but [salt] similar to osmoregulators Cannot tolerate high ion concentrations in body fluids
Proper
ion levels necessary for nerve and muscle function High [salt] disrupts 3-D structure of many proteins Body fluids less salty than seawater (like osmoregulators) so they tend to gain salt Eliminate excess salt using kidneys and rectal gland
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Stenohalines cannot survive wide changes in salinity of surroundings
Most
marine invertebrates and many types of
fish
Euryhalines tolerate certain changes in salinity of surroundings
fish (ex. salmon) Oysters, mussels Sudden changes in salinity results in changes in gene expression Many genes associated with ion transport in skin and other organs
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Migratory
Excretory organs
Regulation of salt, water, and waste levels in body fluid arose early in evolution Most contain tubular structures lined with epi cells Have ability to actively transport ions across membranes Waste secreted through tubes Ability to regulate rate of waste excretion and how much water is lost
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Filtration
An organ acts like a filter to remove water and small solutes from blood while leaving behind blood cells and large solutes Mammalian kidney plasma forced under pressure through capillaries into tubules Produces filtrate
Reabsorption
Some material in filtrate recaptured and returned to blood Formation of filtrate not selective necessary molecules (salts, sugars, amino acids) in filtrate Remaining part of filtrate excreted urine
Secretion
Supplements solutes removed by filtration Actively transported from interstitial fluid surrounding epi cells of tubules into lumen Way to eliminate toxic compounds from body
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Protonephridia
Simplest
filtration mechanism in invertebrates Flatworms Series of branching tubules filters fluids from body cavity using beating of ciliated cells located at ends of tubules cells) (flame Solutes reabsorped Excess water and some wastes emptied through openings in body wall called nephridiopores Most of the nitrogenous waste diffuses across body surface protonephridia primarily osmoregulatory
Urine hypoosmotic compared to body fluids Adaptation to fresh water
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Metanephridial system
Annelids
located in each body segment Tubular network beginning in funnel-like structure called nephrostome Open to body cavity and collect coelomic fluid containing nitrogenous wastes
Pairs
Pores exclude large solutes
Na+,
Cl- and others reabsorbed via active transport along tubule into capillaries Nitrogenous wastes excreted through nephridiopores in body wall Urine hypoosmotic
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Mollusks and crustaceans
Antennal
glands
Readsorption occurs along length of excretory system Most nutrients do not appear in urine Urine excreted via nephridiopores
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Malpighian tubules
Insects
do not use filtration Arise from midgut and extend into surrounding hemolymph Cells lining tubules actively transport K+, uric acid, and purines from hemolymph into lumen Creates osmotic gradient drawing water and solutes into tubule Moves to hindgut (intestine and rectum) where water and solutes reabsorbed Nitrogenous wastes and others excreted together with feces through anus Most terrestrial insects (except blood-suckers) excrete isoosmotic or hyperosmotic urine
Efficient readsorption of water adaptation to life in dry environment
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Kidney
Vertebrates
tubules composed of epithelial cells actively transporting sodium and other ions for salt and water homeostasis and nitrogenous waste elimination Process can be controlled by nerves and hormones Most are filtration kidneys
Specialized
Some marine fish have secretory kidneys
Filtration
controlled by mechanical forces
Hydrostatic pressure exerted by blood entering kidney capillaries
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Not all animals have same requirement for salt and water Environmental demands on animals predicted by examining kidney function
fish adapted for rapid largescale filtering of blood and production of dilute urine Amphibians permeable skin; absorb fresh water from environment; kidneys resemble fresh water fish Marine fish and desert animals kidney adapted to produce concentrated urine; helps conserve water
Freshwater
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Diet can influence kidney function
Sporadic,
high protein meals (mammals and insectivores) 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
Do not defecate or urinate during nonfeeding periods Break down fat and protein stores Unique ability to recycle nitrogenous wastes into protein
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Vampire
bats and other blood-suckers
large volumes of water with
Consume
blood Can increase weight almost 40% after blood meal Water must be quickly excreted while retaining nutrients
Urine
production begins a few minutes after feeding Urine produced at ~4 ml/kg body mass/minute (400X rate produced in human) Occurs only after blood meal other times kidneys produce concentrated urine to preserve water
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Mammalian urinary system
Kidney
In abdomen Renal cortex
Primary site of blood filtration
Primary site of readsorption Inner region Outer region
Renal medulla
Renal pelvis where urine collects Ureters Urinary bladder Urethra
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Nephron
Functional unit of the kidney As many as several million in each kidney Consists of
corpuscle forms filtrate Tubule performs secretion and reabsorption
Renal
Tubule empties into collecting duct
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Renal corpuscle
Glomerulus
of interconnected, fenestrated capillaries Supplied by under pressure afferent arteriole
Cluster
Protrudes into fluid filled space (Bowman's space) lying within Bowman's capsule
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Tubular part
Continuous with Bowman's capsule Single layer of epi cells on basement membrane Epithelial cells differ in structure and function along length (10-12 distinct segments) Proximal convoluted tubule drains Bowman's capsule Loop of Henle long hairpin loop
Descending goes down into medulla Ascending comes up out of medulla
Distal convoluted tubule Then into collecting duct Tubule surrounded by peritubular capillaries in the cortex and vasa recta capillaries in the medulla
Carry away reabsorbed solutes and water to bloodstream
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Filtration
As blood flows through glomerulus, about 20% of plasma leaves capillaries and filters into Bowman's space Remaining blood leaves glomerulus via efferent arteriole Proteins and blood cells remain in plasma fenestrations to small
Capillaries surrounded by podocytes that form slits acting as physical barrier to movement of large solutes and cells
Glomerular filtrate Glomerular filtration Rate (GFR) is rate of filtrate production
Increased by dilation of afferent arteriole
Excess water in body
Constriction of afferent arteriole reduces GFR
Loss of blood following injury minimize water loss to compensate for blood loss
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Reabsorption in proximal tubule
Depending on solute 2/3 to 100% reabsorbed Na+, K+, Cl-, HCO3-, and organic molecules like glucose and amino acids
Some diffuse through channels in tubular epi cells Others actively transported Organic molecules transported by transporters (ex. Na+/glucose symporter)
Most of water leaves by osmosis Solutes and water reabsorbed in proportion to each other Water and solutes enter peritubular capillaries and return to blood Filtrate leaving proximal tubule has reduced water and salts and no organic molecules Filtrate is around 300 mosm/L, similar to blood
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Reabsorption in Loop of Henle
Descending loop permeable to water but not solutes
Water leaves by osmosis because surrounding fluid hyperosmotic
Three sources of hyperosmolarity
1. 2. 3.
Initial upturn of thin segment of ascending limb permiable to Na+ and Cl-, but not to water Epi cells of thick segment actively transport Na+ and Clout of tubule into interstitial fluid of outer medulla Some urea diffuses out of lower ends of collecting ducts into inner medulla interstitial fluid
Fluid increases in osmolarity from 300 1200 mOsm/L
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Ascending loop (thick segment) not permeable to water and actively transports salts out
Fluid
decreases in osmolarity
Fluid increases in osmolarity second time during passage along collecting ducts Osmolarity of interstitial fluid increases from cortex to inner medulla
Allows
water to diffuse from descending limb along
length
Countercurrent exchange system
Energy
is used to power ion pumps to create necessary gradients (countercurrent multiplication system)
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Advantage to animal of loops of Henle:
Urine
volume can be reduced and concentrated by recapture of water along osmotic gradient
Desert animals have very long loops of Henle with very large osmotic gradients in medulla Freshwater fish do not have loops of Henle need to excrete as much water as possible
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Blood flowing through vasa recta capillaries in medulla designed not to remove gradient
Blood
entering these capillaries at 300 mOsm/L should have an influx of Na+ and Cl- and outflow of water remove gradient
Hairpin loops of vasa recta run parallel to loops of Henle and medullary collecting ducts Near top of loop of Henle blood in vas recta has normal osmolarity As blood flows into medulla Na+ and Cl- diffuse in and water diffuses out of capillaries
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After bend in loop is reached, blood flows up the ascending vessel loop where process is reversed Minimizes excessive loss of solutes from interstitial fluid by diffusion Salt and water being reabsorbed from loops of Henle and collecting ducts carried away in equivalent amounts by passive movement of water and solutes between compartments countercurrent gradient is maintained
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Tubule does not become permeable to water again until distal tubule Most of reabsorbed water and salts already returned to blood Fine tuning of urine composition mediated by antidiuretic hormone (ADH) and aldosterone Aldosterone
Acts
on basolateral surface of distal convoluted tubule cells to stimulate active transport of three molecules of Na+ out of tubule (reabsorption) for every two molecules of K + brought into tubule (secretion) Water from tubule lumen follows Na+ by osmosis into blood
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Final solute concentration determined in collecting ducts
Cells along collecting duct in inner medulla permeable to urea contributes to osmotic gradient Permeability of epi cells in collecting ducts to water can be regulated according to body's water needs Under control of ADH
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Acts
to increase the number of aquaporins (water channels) in the collecting duct membranes Collecting ducts travel through hyperosmotic medulla
Water moves into surrounding interstitial fluid entering vasa recta Urine becomes hyperosmotic compared to blood Ability of kidney to produce hyperosmotic urine major determinant of animal's ability to survive in limited water conditions
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When
body water plentiful ADH level in blood decreases
Decreases permeability of ducts to water Urine increases in volume and becomes more dilute
Higher
levels of ADH increase the number of aquaporins allowing water to leave the duct and urine volume decreases
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Agre and Colleagues Discovered the Mechanism of Water Movement Across Biological Membranes
Aquaporins discovered in early 1990's Subfamily of large family of membrane transport proteins Some aquaporins can transport other small molecules (glycerol, urea) Proteins with 6 transmembrane helical domains and 2 short loops in the membrane (form water channel)
Loop sequences highly conserved among different species Loops create a channel opening of about 30 pm (width of water molecule) Scattered along channel are positively charged Arg
H-bond
with water molecules facilitating single file movement through channel at ~109 molecules/sec
Sites within intracellular and extracellular domains can be modified by enzymes such as kinases Opening and closing of channels may be gated by stimuli like ion channels Promoter regions of some genes contain site recognized by transcriptional activators responsive to cAMP
cAMP generated by cells stimulated by ADH ADH stimulates transcription of one or more aquaporin genes Molecular basis for nephrogenic diabetes insipidus (NDI)
Inherited Patients
cannot produce concentrated urine and lose large amount of water Mutation in aquaporin gene results in misfolded protein
Impaired ability to enter plasma membrane Impaired ability to form channel core
Transport maximum (Tm)
Reabsorption will return solutes to the blood Limit to amount of material that can be transported within a given amount of time Binding sites on transport proteins can become saturated with ligands at high levels Occasionally molecules may reach concentration levels >Tm
Get
excreted in 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
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Impact on public health
Diseases affecting kidneys:
Diabetes, infections, allergies, congenital defects, kidney stones, tumors, toxic chemicals, obstruction in urethra or ureter
Symptoms of renal malfunction similar regardless of cause All stem from uremia urine in blood 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
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Hemodialysis
Small
solutes diffuse out of patient's blood into solution in dialyzer until equilibrium reached Uses artificial countercurrent exchange system to increase efficiency
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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
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