Human excretion is the process of removing excess water, waste materail and harmful substances from human body. Also excretion is the process of eliminationg waste products of metabolism and other non-useful materials. It is an essintial process in all formes of life. It also eliminate waste products such as water, carbondioxide and nitrogenous wates formed during catabolism.

1. EXCREATORY SYSTEM
Prepared By: Vipin Kumar Shukla
Assistant Lecturer

3. Introduction:
 Excretion is the process of elimination of unwanted
materials and waste products such as water, carbon
dioxide and nitrogenous wastes formed during
catabolism.
 The chief excretory organs of vertebrates are the
Kidneys.
 These are made up of millions of units, which are known
as the Uriniferous tubules or Nephrons or the renal
tubules.

4. Nitrogenous Waste Products in Vertebrates:
 The metabolic waste products
which are excreted by the
animals my be grouped under
the following two heads:
 Respiratory waste products.
 Nitrogenous waste products.

5. Respiratory Waste Products:
 The catabolic waste products of various types of food stuffs
are Co2 and water.
 In lower animals Co2 is eliminated directly into the
environment through the general body surface.
 In higher animals it is excreted with the expired air through
the lungs.
 Excess of water is eliminated in the form of urine and sweat.

6. Nitrogenous Waste Products:
 The nitrogenous waste products are
derived from the deamination of the
excess of amino acid taken in with
the diet and also from the
breakdown of animal proteins and
nucleic acids.
 Following are the specific
nitrogenous waste products excreted
by the animals:

7. Continued……
 Ammonia:
 It is formed by the deamination
of amino acids during protein
metabolism.
 Ammonia is highly toxic and its
rapid elimination is essential.
 Since it is highly soluble in
water & it is quickly eliminated
in surrounding water.

8. Urea:
 It is less toxic than ammonia.
 Most mammals, Fishes, Marine elasmobranches and
amphibians excrete their nitrogenous waste products in the
form of Urea.
 Formation of urea enables elimination of both ammonia and
Co2.
 Urea is formed of two molecules of ammonia and a molecule
of carbon dioxide.

9. Continued…..
 Its synthesis occurs in the hepatic cells through “
Ornithine Cycle or Krebs Ornithine cycle.

10. Uric acid:
 Insects, some snails, birds and reptiles with few
exceptions, excrete uric acid.
 Uric acid is excreted in solid form.
 Uric acid is insoluble in water.
 It crystallizes out form the fluid urine and water is
reabsorbed, thereby allowing conservation of water.

12. Amino-Acids:
 In certain animals like molluscus and echinoderms, the excess
of amino acids is removed as such without undergoing any
change.
 Other nitrogenous compounds:
 Allantoin and allantoic acid are other nitrogenous excretory
products.
 These are insoluble in water and are excreted during
embryonic development by amniotes with shelled eggs.

13. Continued……
 There are some other nitrogenous waste products such
as Guanine and Adenine from nucleic acid breakdown
and creatine from the creatine of muscles that are
excreted along with other nitrogenous waste products in
the urine.

14. Difference between Urea & Uric acid
excretion:

15. Mineral Ions:
 Sodium, Potassium,
Calcium, magnesium
and Chloride are the
essential mineral ions
of the animals.
 The excess of these
ions taken in with the
diet are excreted by
one means or the other.

16. Types of excretion:
 Products' of catabolism are excreted from all animals by
diffusion.
 The end products of oxidation of proteins, pyrimidines
and purines are nitrogen compounds chiefly ammonia.
 But ammonia is a toxic substance and needs either to be
removed as soon as it is formed or to be converted to
some less toxic substance.
 These substances are either “ Trimethylamine oxide,
urea or uric acid.

17. Continued…..
 Therefore, the nitrogenous excretory substances in
animals are ammonia, Trimethylamine, urea and uric
acid.
 Ammonia is very toxic and very much soluble in water.
 Urea is less toxic and comparatively less soluble in
water.
 Uric acid is less toxic than either ammonia or urea and is
almost insoluble in body fluids.
 Thus, secretion of particular type of nitrogenous bases
depend upon its environment and mode of life.

18. Ammonotelic(Marine Invertebrates)
 Most of the marine invertebrates excrete large
proportion of their nitrogen as Ammonia. such animals
are known as Ammonotelic.
 Ammonia is highly soluble in water and its
concentration in the body fluids of marine invertebrates
varies from (o.4mg to 4.8 mg per 100ml).
 Marine animals live in unlimited volume of water and
their body fluids are mostly isotonic or hypertonic to sea
water.

19. Continued…..
 Therefore, ammonia which is soluble in water, readily
diffuses out to their body from higher to low
concentrations and never reaches a dangerous
concentration.

21. Difference between Ammonotelic,
Ureotelic & Uricotelic:

22. Ureotelic (Marine Fishes)
 Marine fishes excrete urea not ammonia.
 These are known as Ureotelic.
 They cannot afford to excrete ammonia because of several
reasons.
 In, marine cartilaginous fishes the body fluids are to be
maintained hypertonic to the sea water.
 They retain (2 to 2.5 %) of urea in the blood and thus
maintained themselves hyper osmotic to their medium.

24. Ammonotelic (Freshwater Animals)
 Fresh water animals imbibe plenty of water and
moreover, being hypertonic to the surrounding water
these have the problem to get rid of excess of water.
 Fresh water invertebrates as well as almost all
freshwater fishes are Ammonotelic.

25. Ornithine-Arginine Cycle or Ornithine Cycle:
 During catabolism of proteins, amino acids, undergo
deamination and ammonia is liberated.
 Free ammonia is toxic to the body cells and need to be
eliminated from the body.
 In vertebrates toxic ammonia is converted into nontoxic
urea by the liver cells.
 The mechanism involved in the formation of urea
constitute “Ornithine – arginine cycle.

26. Continued….
 The cyclic chain of chemical reactions involved in the
Ornithine-arginine cycle leading to the formation of urea
can be summarized as under:
 An Important amino acid Ornithine occurs freely in the
liver cells. It forms the starting point in the chain of
Ornithine cycle.
 Ornithine combines with (NH3 & CO2) and forms
citrulline and releases one molecule of water.
 Citrulline combines with large quantities of (NH3) and
water to form arginine.

27. Continued……..
 Arginine is catalyzed by enzyme arginase present in the
liver. This breaks arginine into Ornithine and urea, thus
completing the cycle.
 The Ornithine once again combines with ammonia to
repeat the Ornithine cycle.
 Thus arginase plays an important role.
 Arginase is found in all the tissues in elasmobranches'
fishes and in liver cell of mammals.

28. Continued…..
 Ornithine cycle was described by Krebs and Henseleit
(1932).
 They demonstrated the action of arginase in urea
formation by using slices of rat liver.

30. Significance of Ornithine Arginine Cycle:
 Ammonia produced due to deamination of amino acids
is toxic to the body cells and less soluble in water.
 In unicellular and simple multicellular organisms, living
in water immediately eliminate ammonia by simple
diffusion into the surrounding water.
 But terrestrial animals having complex organization, can
neither afford to lose large amount of water, nor can
retain ammonia for long.

31. Continued……
 Therefore, formation of urea as excretory product is a
device of retaining nitrogenous wastes in the body till
these are excreted out of the body.
 Composition of water molecules:
 In Ornithine-Arginine cycle three ATP
molecules are consumed:

32. Continued…..
 In the formation of active Co2 = 1 ATP.
 In the synthesis of carbamyl phosphate =1 ATP.
 In the formation of Orginine-succinate = 1 ATP.
 Total= 3 ATP,s are formed.

33. Formation of Urine:
 Urine formation takes place in the kidneys. It involves
three process which are mentioned below:
 Ultra filtration.
 Tubular Re-absorption.
 Tubular secretion.

34. Continued:….
 Ultrafilteration of Glomerular Filtration:
 Bowman's capsules act as ultra filters and lie in close
contact with Glomerulus.
 As blood flows through Glomerular capillaries, water
and all the substances of plasma except blood cells and
plasma protein filter out of the blood into Bowman's
capsules.

35. Continued….
 Filtration occurs rapidly because of the following
reasons:
 Blood is separated from the cavity of Bowman's capsule
by two very thin membranes, the endothelial layer of the
blood capillaries and epithelial layer of Bowman's
capsule.
 The capillary walls have numerous fine pores of about
(50-100nm) in diameter.

36. Continued…..
 The epithelial cells of Bowman's capsule are specialized
and are called podocytes.
 The fine or minor processes of podocytes reach up to the
basement membrane of Glomerular capillaries and
establish a very close contact.
 Afferent arteriole is wider than the efferent arteriole.
Therefore, the amount of blood that enters the afferent
arteriole in a definite time is not fully drained out by
efferent arteriole.

37. Continued…..
 This increases the hydrostatic pressure in the capillary
network of Glomerulus.
 The effective filtration pressure (EFP) or pressure
gradient responsible for its filtration is the outcome of
the interaction of following pressures:

38. Continued…..
 This increases the hydrostatic pressure in the capillary
network of Glomerulus.
 The effective filtration pressure (EFP) or pressure
gradient responsible for its filtration is the outcome of
the interaction of following pressures:
 A) The Glomerular hydrostatic pressure or the
capillary pressure is the main driving force that tends
to move out of the glomeruli.
 It is exerted by blood while passing through Glomerulus.

39. Continued…..
 The capsular hydrostatic pressure (renal interstitial
pressure+ renal intratubular pressure) about (20mmHg)
and blood colloidal osmotic pressure about (30mmHg)
exert force against capillary pressure.
 Summary Filtration Pressure in Glomerulus:
 1.) GHP (Glomerular Hydrostatic Pressure) ; 75mmHg.
 2) BCOP (Blood Colloidal Osmotic Pressure): 30mmHg
 3) RIP (Renal Interstitial Pressure) : 10mmHg.
 4) Renal Intratubular Pressure): 10mmHg.

41. Continued…
 Net Filtration Pressure =
 75-(30+10+10 mmHg)
 (75-50mmHg).
 Therefore, the net filtration Pressure responsible for
filtration is about :
 75-(20+30)= +25mmHg.

42. Tubular or Selective Reabsorption:
 From, the Bowman's capsule, the Glomerular filtrate
courses through the tubule and passes into the collecting
duct.
 During this course its composition osmotic pressure and
PH change due to Reabsorption of water and many
solutes from it in the tubules.
 The process by which only useful substances are
reabsorbed from the Nephric filtrate in the blood
capillaries is called tubular or selective Reabsorption.

43. Continued….
 This filtration is called Ultrafiltration.
 As a result of Ultrafiltration almost all the substances
dissolved in plasma (Urea, Salts, Glucose, Creatine etc)
along with water except blood cells and colloids and
certain plasma and is called “ Nephric Filtrate or
Glomerular filtrate).
 Glomerular filtration rate in a normal adult is
about(120ml per minute).
 The average volume filtered from the plasma into
Bowman's capsule is about (190 liters per day).

44. Mechanism of tubular Reabsorption:
 There are two mechanism for tubular Reabsorption:
 Active reabsorption:
 Substances like glucose and amino acids . Which are of
considerable importance to the body are reabsorbed
actively.
 Active reabsorption is very rapid and continues even when
the concentration of the substance is far lower in the
glomerular filtrate than in the blood.

45. Passive reabsorption:
 Some substances like Urea, ammonia , creatinine, ketone
bodies etc., are reabsorbed from the tubules slowly by
diffusion.
 It continues so long as their concentration in the
glomerular filtrate exceeds their concentration in the
blood.
 Thus these substances are never reabsorbed totally from
the urine.
 Water is also reabsorbed by the passive process of
osmosis.

47. Proximal Convoluted Tubule (PCT)
 In the proximal convoluted tubule almost (100%) of glucose,
amino acids and vitamins C, about (70%) of (K+),nearly
(75%) of (Na+) and a large amount of (Ca2+) ions are
absorbed from the glomerular filtrate.
 Chloride ion reabsorbed by diffusion from the PCT.
 About (75%) of the water of the filtrate is also reabsorbed
here by osmosis during the reabsorption of the solutes.

48. Henle,s Loop:
 About (5%) of water in the glomerular filtrate is
reabsorbed by osmosis from the descending limb of
Henle,s loop due to higher osmotic pressure of the
medullary extracellular fluid maintained by Vasa rectae.
 The ascending limb is impermeable to water along its
entire length.
 But it actively absorbs the remaining (25%) of the
filtered (K+) and some amounts of Cl- ions.

49. Continued….
 So, the filtrate becomes dilute (hypotonic) than the
plasma as it follows through the ascending limb.
 Distal Convoluted Tubules (DCT) and collecting duct:
 The distal convoluted tubule, collecting tubule and
collecting duct actively reabsorb sodium from the filtrate
under the influence of the adrenal
hormone,aldestrone,which makes their walls permeable
to ions.

50. Continued…
 These actively absorbs some (Na+) from the filtrate and
in exchange excrete some (K+) ions in the urine.
 Some (Cl_) ions are also reabsorbed by diffusion.

51. Tubular Secretion:
 This process is of considerable importance in marine
fishes and desert amphibians.
 These animals lack Glomerulus and Bowman's capsule
in their nephrons.
 So, these animals form urine by secreting solutes such as
Urea, Creatinine and minerals ions into their tubules.

52. Continued….
 But, tubular secretion is of less importance in mammals.
 Most of the (K+) eliminated in the mammalian urine is
secreted by the distal convoluted tubule and collecting
ducts in exchange of the reabsorbed (Na+).
 The distal convoluted tubule and collecting tubule also
secrete uric acid and ammonia in the urine.

53. Continued….
 The volume of urine formed is far less than the
volume of glomerular filtrate and its
composition is also quite different from that of
the glomerular filtrate.