SLIDE-exc

1. Comparative physiology of excretion, kidney, urine formation, urine
concentration, waste elimination, micturition, regulation of water balance
(osmoregulation), blood volume, blood pressure, electrolyte balance, acid-base
balance.
Excretory system

2. Four excretory organs
1. lungs
2. liver
3. skin
4. kidneys
EXCRETORY ORGAN #1 = THE LUNGS
Cellular respiration occurs in every living cell in your body. It is the reaction that
provides energy (in the form of ATP molecules) for cellular activities. If
respiration stops, the cell no longer has energy for cellular activities & the cell
dies.
As respiration occurs carbon dioxide is produced as a waste product. As the
carbon dioxide accumulates in body cells, it eventually diffuses out of the cells &
into the bloodstream, which eventually circulates to the lungs.
And here, in the alveoli of the lungs, carbon dioxide diffuses from the blood, into
the lung tissue, and then leaves the body every time we exhale. We should note
that some water vapor also exits the body during exhalation.

3. EXCRETORY ORGAN #2 = THE LIVER

4. EXCRETORY ORGAN #3 = THE SKIN
SWEAT is a mixture of three metabolic wastes: WATER, SALTS, & UREA
So as you sweat, your body accomplishes two things:
1) sweating has a cooling effect on the body, and 2) metabolic wastes are excreted.
So, how does the sweat form?
the sweat gland is a tubular structure tangled with capillaries (the smallest of blood
vessels). This close association of tubes allows wastes (namely water, salts & urea)
to diffuse from the blood & into the sweat gland.
And then, when body temperature rises, the fluid (sweat) is released from the
gland, travels through the tube (duct), & reaches the skin surface through
openings called pores. This is evaporated and body temp is reduced by cooling

5. oil glands,
hair follicles,
fatty layers,
nerves, and
sweat glands.

6. Urine and sweat have same composition but different concentration
EXCRETORY ORGANS #4 = THE URINARY SYSTEM
Kidneys filter blood
Renal artery
Renal vein
Three of the four major metabolic wastes produced
by the body are filtered from the blood by the
kidneys. They are water, salts, & urea and the 4th,
carbon dioxide, is excreted by the lungs

7. A passive biological system that
1. removes excess, unnecessary or dangerous materials from an
organism,
2. help maintain homeostasis within the organism
3. prevent damage to the body.
4. responsible for the elimination of the waste products of metabolism
as well as other liquid and gaseous wastes.
The excretory system gets rid of waste called urine
Basic structural and functional Unit of kidneys are known as NEPHRON
1. regulate the concentration of water and soluble substances like sodium
salts by filtering the blood,
2. reabsorbing what is needed and excreting the rest as urine
3. eliminates wastes from the body,
4. regulates blood volume and
5. blood pressure,
6. controls levels of electrolytes and metabolites
7. regulates blood pH.
1million microscopic

8. Layers of kidney
1. Cortex: lots of nephrons, filtering
layer
2. Medulla: collecting layer; take wastes
to pelvis
3. Pelvis: all collecting tubes come
together and connect with ureter

9. THE NEPHRON - the structural units of the Kidney
Afferent
arteriole
Efferent
arteriole
network
Henle’s Loop
PCT DCT

10. Glomeruli: cluster of capillaries which bring blood
Bowmann’s capsule: c shaped, urea, salts, water, glucose,& others pass from
the blood into the nephron
Loop of Henle: filtrate passes thro and useful substances are reabsorbed
into surrounding capillary network which transport and clean blood via heart
180 liters of filtrate is produced each day, but only 1.5 liters of urine.
So as you can see, most materials that initially enter the nephron are
reabsorbed, leaving only the urea, salts, & some water in the tubule. These
metabolic wastes form urine, which is transported to the urinary bladder by
the collecting tubule ----> ureter -----> U. bladder -----> excreted out

11. Kidney functions
Functions of kidney
1. Excretion of metabolic wastes and foreign chemicals
2. Regulation of water and electrolyte balace
3. ‘’ of body fluid osmolarity and electrolyte conc
4. ‘’ of arterial pressure
5. “ of acid base balance
6. Secretion, metabolism and excretion of hormones
7. Gluconeogenesis (generation of glucose from non-carbohydrate carbon substrates such as lactate,
glycerol, and glucogenic amino acids)
1. Waste products
Urea (aa metabolism)
Creatinine (muscle creatine)
Uric acid (from nucleic acids)
End prod of Hb (bilirubin)
Metabolites of various hormones
Toxins, pesticides, drugs, food additives,
Foreign chemicals
2. Water and electrolyte balance
3. Body fluid osmolarity and electrolyte conc
Intake > excretion-----> incr
Intake < excretion-----> decr
30----> 300mEq/L intake of NaCl
Excretion within 2 days of 300mEq/day
Blood Na incr in ECF, trigger hormonal changes
To balance and excretion of Na incr till normal
Na, Cl, K, Ca, H, Mg, PO4

12. 4. Regulation of arterial pressure
By excreting variable amt of Na and
H2O
Secretion of vasoactive Renin leading to
formation of Angiotensin II
5. Regulation of acid base balance
Along with lungs and body fluid buffers
by excreting acids (H2SO4 or
H3PO4) or alkali
6. Secretion, metabolism and
excretion of hormones
Erythrocyte production generates
ERYTHROPOIETIN stimulated by
Hypoxia. (hemodialysis in kidney
disease patients)
Regulation of Vitamin D (Calcitrol)
production
7. Gluconeogenesis
Kidneys synthesize glu from aa
(glucogenic aa) and other in
prolonged fasting, starvation,
prolonged intense exercise, low
carbohydrate diets.

13. Water loss
Skin: 300-400ml/day
Respiratory tract: 300-400ml/day
Insensible water loss
Sweat: 100ml/day; hot weather: 1-2L/day
Faeces: 100ml/day
Kidneys: 0.5L/day (dehydrated) 1.4L/day (normal)
20L/day (over hydration)
OUTPUT
350
350
100
100
1400
2300 ml/day

14. Extracellular fluid Intracellular fluid
• 70kg adult: body water is 60% of
body weight i.e. 42L
• More age water content decr since
fat incr
• Vary due to age, obesity and gender
Body Fluid compartments
Interstitial
Fluid
(tissue spaces)
Plasma
Transcellular fluid (1-2L)
Synovial fluid
Peritoneal fluid
Pericardial fluid
Intraocular fluid
Cerebro spinal fluid
ICF
28L
Capillary memb
ISF
11L
Capillary memb
Plasma
3L
ECF
14L
output input
Kidneys
Lungs
Feaces
Sweat
Skin
Lymphatics

15. Blood volume: ECF (plasma fluid) + ICF (inside RBCs)
Average BV is (adult) 5L (7% of body weight)
Blood: 60% Plasma and 40% RBCs
BV = PVvv (plasma volume)
1-HC 1-(hematocrit (RBC))
Cations and Anions in ECF and ICF
ECF
ICF
Na+
K+
Mg++
Ca++
PO4
In Io
Cl-
HCO3-
Protein
50
100
150
50
100
mEq/L
In Io:inorganic ions

16. Plasma ISF
Protein-
Cations+
Anions-
Protein-
Cations+
Anions-
Non electrolytes (mg/dl (100ml)) of plasma

17. Urine Formation Result of
• Glomerular Filtration (GFR)
• Tubular Reabsorption by active transport
• Tubular Secretion by active transport
Urinary Excretion Rate:
Filtration Rate- Reabsorption Rate
+Secretion Rate
GFR: quantity of GF formed in each
kidney, each minute, in all nephrons,
of both kidneys
Normal person: 125ml/min
180L filtrate is processed per day
Normal plasma flow thro kidneys:
650 ml/min
Fraction of renal plasma that
becomes GF is filtration fraction

18. Measures for renal function
1. GFR: flow rate filtered thro kidneys
2. CrCl or CCr: Creatinine clearence rate: volume of blood plasma
that is cleared ofcreatinine per unit time (helps in assessing
GFR)
Calculation of GFR and CrCl can be done by estimating
Various substances in blood and urine (pathology)
Often : urea, creatinine and electrolytes are estimated
Level of protein in urine is also diagnostic marker. (??)
filtration fraction is the ratio of the GFR to renal plasma flow (RPF)
Filtration Fraction, FF = GFR/RPF

19. GFR
Volume of fluid filtered in glomerular capillaries into the bowmanns
capsule per unit time.
Calculated by measuring a chemical which is filtered actively but
neither absorbed nor secreted (INULIN, SINISTRIN).
Rate of excretion is directly proportional to rate of filtration of
water and solutes by GF.

20. A creatinine
B electrolytes
C amino acids and glucose
D organic acids, bases

21. Renal Artery
100mm Hg
60mm Hg
Glomerular
Hydrostatic
pressure
18 mm Hg
18 mm Hg
13 mm Hg
18 mm Hg
8 mm Hg
10mm Hg DCT
0mm Hg Coll.
Duct
Interstitial fluid: 6mm Hg
10 mm Hg

22. High pressure (afferent)
Small molecules forced
(Water
Glu
aa
NaCl
Urea)
High pressure filtration is called
ULTRAFILTRATION
Oncotic pressure, or colloid osmotic pressure, is a form of osmotic pressure
exerted by proteins in blood plasma that tends to pull water into circulatory
system

23. Glomerular filtration rate (GFR) describes
the flow rate of filtered fluid through the
kidney.
125 ml/min
1mg/ml
1ml/min

24. Afferent Arteriolar Vasodialator and Efferent Arteriolar Constriction
Factors that affect GFR; AUTOREGULATION
Low GFR
decr in Na and Cl at macula densa
Decr ionic conc
Arteriolar dilation
Incr blood flow in glomerulus
Incr G pressure
Incr G blood flow
Incr in GFR back to normal
Low Na and Cl at Macula densa
Low GFR
Excess reabsorption of Na and Cl
In ascending limb of Lof Henle
Reduced ion conc at MD
JGC release renin
Causes formation of AngiotensinII
Causes constriction of efferent arteriole
Incr pressure in Glo
GFR returns back to normal
JGC” juxtaglomerular complex;epithelial cells of DCT come in contact with
arterioles and are more dense (macula Densa). Smooth muscle cells of afferent
and efferent arteriole are also dense when come in contact with MD. These
cells are JGC
1 2

26. MACULA DENSA: DCT
• SENSITIVE TO NaCl concentration in DCT
Decr in NaCl conc
Signal from MD
• Decr resistance to blood
flow in afferent arterioles
• Incr glomerular hydrostatic
pressure
• GFR normal
1
• Incr RENIN release from
JGC of afferent and
efferent arterioles
2
Go back to slide 24 and read again

27. Reabsorption and Secretion
Materials are selectively absorbed
Mechanisms are basically active and passive transport
Primary AT
Na+ - K+ ATPase Pump
Secondary active transport (co transport,counter transport; no
energy)
Na+ moves Glu, aa, organic cpds, Cl, PO4, Ca++, Mg++, H+ along with it
Passive absorption of water: by diffusion
Passive absorption of Cl, ions, urea and other solutes

28. Reabsorption and Secretion
Absorption of different tubule segments
PCT: permeable to most solutes, Glu, aa, H+, ions, water, 65% is reabsorbed
(Thick) Descending limb of L of H: highly permeable to water
Thin segment (Descending) of L of H: highly permeable to water but
moderately only to urea, Na and other ions
Thin segment (Ascending) of L of H: highly impermeable to water
(Thick) Ascending limb of L of H: impermeable to water and urea but not
to NaCl
DCT: starts at JGC, Diluting Segment is impermeable to urea and water but
ions are absorbed, hence dilutes urine
Late distal tubule and CD: impermeable to urea, reabsorbtion of Na+
(aldosterone) in exchange for K+ (control of K+ in ECF)
Contain “intercalated cells” secrete H+ by AT--contribute to acidification
of urine
LDCT and CD are permeable to water in presence of ADH and impermeable
in absence of ADH (dilution of urine)

30. ADH
Excess
Water absorbed
CD permeable
Conc urine
Water conserved
Low
CD impermable
Water not absorbed
Dilute Urine
No water conservation
H+
Acid base
Balance
urea

31. ADH or anti diuretic hormone or vasopressin
• Synthesized in hypothalamus
• Release by posterior pitutary
• Peptide hormone
• Role in homeostasis, by the regulation of water, glucose, and
salts in the blood.
• Incr water reabsorption by incr permeability of water in CD
(aquaporin channels incr)
Body dehydrated
ADH released ------------------ at high conc raises blood pressure
Kidneys conserve water
Conc urine
Causes vasoconstriction
Incr pressure

32. Renin–angiotensin-aldosterone system (RAAS)
• hormone system that regulates blood pressure and water (fluid) balance.
1. Blood volume low or drop in BP
JGC release RENIN in blood
Plasma renin causes
(in LIVER)Angiotensinogen ------> angiotensin I------> angiotensin II
(vasoconstrictor)
Causes vasoconstriction
Incr blood pressure
2. Decr in NaCl conc (sensed by MD of DCT activates RAAS)
Adrenal cortex (zona glomerulosa) ----------------> aldosterone
Incr reabsorption of Na and water into blood
Blood vol incr and blood pressure also incr
ACE
ACE: angiotensin converting enzyme present in lungs

33. Renin–angiotensin-aldosterone system (RAAS)
• If RAAS is too active
• BP will be very high
• Drugs are known to control hypertension, heart faliure, Diabetes,
kidney failure
• Inhibitors of ACE
• ARB (angiotensin receptor blockers)
• Renin inhibitors (reduce BP)

35. Reabsorption of water
Glomerular filtrate 125 65%
Loop of Henle: 45 15%
DCT : 25 10%
CD : 12 9.3%
Urine : 1 0.7%
ml/min %
Concentration of Solutes in different Tubular Segments
Concentration of Solute: Degree of reabsorption of solute vs reabsorption of water
More % of water reabsorbed: conc Urine
More % of solutes absorbed: dilute urine
Tubular fluid conc
Plasma conc of a subst
>1, If Greater than 1 ratio slowly then more
water reabsorbed than solute [H2O > solutes]
<1, If less than 1 ratio slowly, more solute
reabsorbed than water [solutes > H2O]

36. 1
2
5
10
20
50
100
0.5
0.2
0.1
0.05
0.02
Tubular
fluid/plasma
concentration
PCT L OF H DCT CD
aa prt
Glu
Cl
K and Na
K Cl
Na
HCO3
Less
reabsorbed
More
reabsorbed
more solute
more water

37. Hormonal Control of Tubular Reabsorption
Aldosterone zona glomerulosa cells Adrenal cortex
Regulator of sodium resorption and K secretion
Acts on cortical collecting duct
Increases NaCl and H2O reapsorption and K+ secretion
Addisons Disease
Less aldosterone
Loss of Na and Accumulation of K+
Conn’s Syndrome
More Aldosterone
Na retention and K+ depletion
Angiotensin II PCT, thick Ascending loop, DCT, CD NaCl and water
Reabsorprtion
H+ Secretion
ADH DCT, CD Incr H2O reabsorb
Parathyroid DCT, Thick As LoH decr PO4 reabsorp and
Hormone (PTH) incr Ca and Mg reabsorption

38. Osmolarity of ECF and Na Conc
Excess water -----> body fluid osmolarity reduces-----> dilute urine---> 50mOsm/L
Less water---------> body fluid osmolarity incr-----> conc urine---> 1200-1400mOsm/L
Kidneys vary conc of solutes and water in response to body’s challenges
But solutes like Na and K excretion rate is not
changed, hence reabsorbed and conserved
HENCE ABILITY TO REGULATE WATER EXCRETION INDEPENDENT OF SOLUTE EXCRETION
NECESSARY FOR SURVIVAL WHEN FLUID INTAKE IS MINIMAL

39. ADH (Vasopressin) CONTROLS URINE CONC: feedback mech for
plasma osmolarity and sodim conc by altering renal excertion of water
independent of solute excretion
NORMAL OSMOLARITY OF BODY FLUIDS
ABOVE NORMAL
(solutes conc in body incr)
ADH
Incr Permeability of
DCT and CD to water
Large amt of water reabsorbed
Decr urine volume and conc
urine, but no change in
solute excretion
BELOW NORMAL
(solutes conc in body decr)
Decr Permeability of
DCT and CD to water
(impermeable)
No water reabsorbed
Dilute urine in large amounts
20L/day
50mOsm/L
Incr
Secr
Decr
Secr

40. 0
800
400
Osmolarity
mOsmoles/L
Urine osmolarity
Plasma osmolarity
0
6
4
2
Urine flow rate
ml/min
Urine solute
Excretion
mOsmoles/min
1.2
0
0.6
0
60 120 180
Drink 1.0L water Diuresis: Incr production of urine
Plasma solute
osmolarity does
not decrease
drastically on
ingestion of
large amount of
water

41. OSMOLARITY CHANGES IN DIFFERENT TUBULAR SEGMENTS
PCT: Fluid is isosmotic to plasma (300mOsmoles/L)
Ascending LOH: Fluid is Dilute due to absorption of NaCl but no water is
absorbed despite ADH being present or not
Osmolarity decr to 100mosmole/L till DCT (hyposmotic fluid)
DCT and CD: further diluting of fluid and incr absorbtion of NaCl
In absence of ADH, impermeable to water-----> dilute urine (50mosmole/L)
HENCE CONTINUE ABSORBING SOLUTES BUT NO WATER
RESULTS IN DILUTE URINE

42. Solute Gradient and Water Conservation: CONCENTRATED URINE
Urine more concentrated than plasma: essential for survival
Water can be lost by lungs, GI tract, evaporation and perspiration
and excretion by kidneys
To maintain homeostasis kidneys decr volume and conc of urine
And continue to excrete solutes thereby conserving water by
water reabsorption
Urine becomes 1200-1400mosmoles/L ie. 4-5X that of plasma
Desert animals can do so by concetrating to 10,000 mosmole/L
Like desert mouse (can survive without water for days) or marine
animals also excrete conc urine
Minimal volume of urine that must be excreted out is 0.5L/day

43. Solute Gradient and Water Conservation: CONCENTRATED URINE
High conc of ADH and hyperosmotic renal Medulla
1. High level of ADH incr permeability and more water is absorbed
2. High osmolarity of renal medulla interstitial fluid provides osmotic
gradient necessary for water reabsorbtion in presence of ADH
Renal medulla is normally hyperosmotic;
•when ADH rises;
•permeability incr; water is reabsorbed (osmosis) into interstitial fluid;
•carried by vasa recta back to blood
COUNTERCURRENT MECHANISM
Due to special anatomical arrangement of LOH and vasa recta
LOH and VR go into the renal medulla; also the CD

44. HYPEROSMOTIC URINE
The important anatomical feature is the long hairpin construction of the
loops of the nephron (of Henle) and their associated capillaries, the vasa
recta, which provide the "countercurrents."
The filtrate in the nephron's tubule and the blood flowing in and down
and then up and out of the medulla run counter to each and in close
proximity to each other.
The collecting ducts bring a smaller volume of the tubular fluid, the
filtrate now having the chemical characteristics of urine, back down
from the cortex through the medulla to the renal papillae.
The collecting ducts also exhibit countercurrent flow arrangements with
the renal tubules and vasa recta.

46. Salt (sodium chloride) and urea active transport out of the ascending limb of (Henle's)
loop of the nephron alters the composition of the medullary interstitial fluid (ISF) and in
turn the medullary interstitial fluid (ISF) alters the composition of the fluid in the
descending limb of the loop of the nephron and in the collecting ducts.
The loop of Henle acts as the "countercurrent multiplier." The driving force for the
countercurrent system is the active transport mechanism of salt (sodium chloride) and
urea in the ascending limb of the loop. This active transport removes solutes from the
tubular fluid and transfers it to the medullary ISF

47. The impermeability of the tubular epithelium to water prevents water from following the
solute, so the osmotic concentration of the tubular fluid is reduced and the concentration
of the medullary ISF is raised. It is this initial, small, horizontal gradient (approximately
200 milliosmoles, see circles in the figures) across the tubular wall which is "multiplied"
vertically along the length of the loop by countercurrent flow.
The hyperosmotic concentration of the medullary ISF then causes water to move out of the
descending limb, thereby progressively raising the concentration of the remaining filtrate
in the tubule as it flows downward toward the tip of the loop.

48. Na+ and Cl- ions diffuse from the medullary ISF down their concentration gradients back
into the filtrate in the descending limb. This movement also serves to increase the
osmotic concentration of the filtrate.
After the filtrate flows around the bend and starts back up in the ascending limb, Na+
and Cl- ions, but little water, are removed and the filtrate becomes more and more dilute
as it approaches the distal tubule in the cortex. Thus Na+ and Cl- ions are trapped and
recycled in the medulla.

50. The distal tubules in the cortex and the cortical collecting ducts receive the hypo-
osmotic urine from the loop of the nephron (of Henle) and, in the presence of ADH =
antidiuretic hormone = vasopressin, transport the excess water into the cortical ISF.
(See the figure above on the left.)
Thus the solute free water, formed by salt reabsorption in the medulla, is returned to
the systemic circulation and a much smaller volume of isosmotic tubular fluid reenters
the medulla via the collecting ducts.
Note that the water reabsorbed in the cortex, from the distal tubule and cortical
collecting ducts, dilutes the systemic blood, reducing its osmotic concentration.

51. The target of the final effect of the countercurrent mechanism is the medullary
collecting ducts.
As the urine flows down the medullary collecting ducts, it comes into contact through the
duct's epithelium with the hyperosmotic medullary ISF.
In the presence of ADH, water is reabsorbed in excess of solute and the urine becomes
increasingly concentrated as it approaches the renal papilla.
This is the final effect of the countercurrent process, the return of water to the
circulation and the osmotic concentration of the urine

57. Osmoregulation
OSMOREGULATORS
Tightly regulate their body osmolarity,
which always stays constant, and are
more common in the animal kingdom.
Osmoregulators actively control salt
concentrations despite the salt
concentrations in the environment.
An example is freshwater fish.
OSMOCONFORMERS
Match their body osmolarity to
their environment.
It can be either active or passive.
Most marine invertebrates are
osmoconformers, although their
ionic composition may be different
from that of seawater and Marine
fishes.
Most fish are STENOHALINE (narrow range), which means they are
restricted to either salt or fresh water and cannot survive in water with a
different salt concentration than they are adapted to.
However, some fish show a tremendous ability to effectively osmoregulate
across a broad range of salinities; fish with this ability are known as
EURYHALINE (broad range) species, e.g. Salmon.

58. OSMOREGULATORS
Freshwater fishes
Active uptake of salt with mitochondria rich cells
Water diffuses in
Hence excretion of dilute urine to expel excess water

59. OSMOCONFORMERS
Marine fishes (Marine sharks and Chondrichthayans)
A marine fish has an internal osmotic concentration lower than that of the
surrounding seawater,
so it tends to lose water and gain salt.
It actively excretes salt out from the gills.
Urine is hypertonic

60. OSMOCONFORMERS
Marine fishes (Marine sharks and Chondrichthayans)
Marine organisms produce osmoregulatory solutes
Trimethylamine Oxide (TMAO) which protects proteins from damage by urea
TMAO incrs the osmoticity of internal environment of fishes and make them
hyper osmotic to seawater, with more than 1000mosmole/L, hence water
enters by osmosis or by food and excreted as conc urine.
ANHYDROBIOSIS: temporary living without water eg. Tardigrades
Trehalose helps to survive

61. Osmoregulation in protists and animals
Amoeba make use of CONTRACTILE VACUOLES to collect excretory
waste, such as ammonia, from the intracellular fluid by both diffusion and
active transport. As osmotic action pushes water from the environment into
the cytoplasm, the vacuole moves to the surface and disposes the contents
into the environment.
KIDNEYS play a very large role in human osmoregulation, regulating the
amount of water in urine waste. With the help of hormones such as
antidiuretic hormone, aldosterone, and angiotensin II, the human body can
increase the permeability of the collecting ducts in the kidney to reabsorb
water and prevent it from being excreted.
A major way animals have evolved to osmoregulate is by controlling the
amount of water excreted through the excretory system.

62. Nitrogenous Waste Products of Animals show their Phylogeny
Ammonia is a toxic by-product of protein metabolism mostly its is the excreted
form in fishes and is generally converted to less toxic substances after it is
produced then excreted;
mammals convert ammonia to urea,
whereas birds and reptiles form uric acid to be excreted with other wastes via
their cloacas (least toxic).
Nucleic acids+ proteins
Break down
NH3 (toxic)
Urea
Or
Uric Acid
ATP

63. Ammonia Urea Uric Acid
Very toxic and hence
needs to be diluted
Mostly by aquatic
species, fishes excrete
ammonium thro gills; in
freshwater fishes
ammonia is exchanged
for Na+ thro gills
Produced in Terrestrial
animals with no access to
water and marine fishes (bony
fishes) since ammonia needs
to be diluted
Liver produces urea and
combines NH3 with CO2
Circulatory system carries it
to kidneys
100,000 less toxic than
NH3,animals require less
water to excrete urea
Disadv: have to spend energy
when convert NH3 to urea.
Can switch mode of living and
excrete NH3 or urea. Eg.
Tadpoles (excrete NH3) and
frogs excrete (urea)
Non toxic waste but
insoluble in water
unlike urea and NH3.
Hence is excreted as
a semi solid paste
with minimum water
loss.
Adv: for animals with
little access to water
but energetically
production is more
expensive

64. Nitrogenous Waste Products of Animals show their Phylogeny
1. Type of waste eliminated show access to water and hence different
excretory wastes. Eg. To minimise loss of water less toxic urea and non
toxic uric acid is produced
2. Mode of reproduction: amphibians produce shellless eggs which can
diffuse soluble wastes which can further be carried by mother’s blood
and excreted. However, in birds and reptiles a hard shell only allows
permeability to gases but not to liquids. Hence insoluble wastes would
accumulate. If ammonia or urea (toxic if conc incr) were to be deposited
then it would be highly toxic hence the evolution of uric acid whose
deposition would not be a problem due to its non toxicity.
3. Type of habitat also determines the type of waste: eg. Aquatic turtles
excrete ammonia and urea but land turtles (in desert) excrete urea.
4. Change in environmental conditions can also cause a change in type of
waste. ie.land tortoises shift from urea to uric acid when temp incr
5. Since excretion is related to energy expenditure and budget hence type
of food eaten is directly related to type of waste. Endotherms need
energy at high rates hence thy must eat more food than ectotherms and
hence must produce more N2 wastes per unit volume. Predators get more
nutrition from meat (dietary protein) hence excrete more N2 waste than
those who consume more lipids and carbohydrates.