Excretion of Nitrogenous Waste
WALT
• To recap types of nitrogenous waste
• To identify the ultrastructure of the kidney
• To explain ultrafiltration
Some definitions..
• Excretion is the process by which waste
products of the organism’s metabolism
are eliminated from the body.
• Secretion is the production by the cells
substances useful to the body.
• Egestion is the removal of undigested
food & other substances which have
never been involved in metabolism.
Excretory Products
There are three main waste products of
nitrogenous metabolism:
• Ammonia
• Urea
• Uric acid
Ammonia
• Ammonia is derived from the breakdown
of proteins and nucleic acids in the body.
• It is very toxic and is never allowed to
accumulate within the body tissues or
fluids.
• It is extremely soluble and diffuses readily
across cell membranes.
• It is the main excretory product of marine
invertebrates & all fresh water animals.
Uric Acid
• The synthesis of uric acid spends more
energy than urea, but it is advantageous
to many animals.
• It is insoluble, non-toxic and requires
very little water for its removal from the
body -- a suitable product for animals in
desert places.
Urea
Carbon dioxide + ammonia 
urea + water
CO2 +
2NH3  (NH2) 2CO + H2O
• Urea is produced by the ornithine cycle with
expenditure of some energy.
• It is much less toxic than ammonia, less water is
needed for its elimination because the tissues
can tolerate higher concentrations of it.
The Mammalian Kidney
• The kidney has two main functions:
• It removes metabolic waste from the body
through the process of excretion as urine
• It regulates the water and ion content of
the blood. This keeps the water potential
of the blood constant.
Position of the urinary organs
Kidney Structure
Kidney Cross Section
pelvis
medulla
cortex
Renal
cortex
Nephron
tubule
Renal medulla
Collecting duct
The functions of the kidney
• Ultrafiltration – filtering blood unbder
pressure
• Selective reabsorption – reabsorbing the
useful substances
• Production of an ion gradient in the
medulla – to allow production of hypertonic
urine if necessary
• Adjustment of the water and ion content of
the blood – to maintain homestasis
The Structure of the kidney
• Each kidney is made of millions of tiny
tubules called nephrons, each of which
consists of a Bowman's capsule and a
uriniferous tubule joining to a collecting
duct.
• A branch of the renal artery enters the
Bowman's capsule as the glomerulus, then
splits up into capillaries which spread out
and wrap around the tubule. Finally, these
capillaries join up to the renal vein.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Bowman's
capsule
Proximal
convoluted tubule
Distal
convoluted
tubule
Peritubular
capillaries
Fig. 49.17(TE Art)
Glomerulus
Renal cortex
Ascending
limb of loop
of Henle
Renal medulla
Descending
limb of loop
of Henle
Loop of Henle
Collecting
duct
To ureter
Vasa recta
Ultrafiltration
• Blood reaching the glomerulus is under
high pressure because:
• The afferent arteriole is wider than the
efferent arteriole
• The heart has a high pumping force
Ultrafiltration
• This pressure results in small molecules
from the blood being squeezed out
through the capillary walls and into the
capsular space of the Bowmans Capsule
Ultrafiltration
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. Bowman's
49.18(TE
Art)
capsule
Glomerulus
Filtration
Reabsorption to blood
Secretion from
blood
Renal tubule
Excretion
Cells of Bowman's capsule
• There are inner podocytes and outer
squamous epithelium
• The glomerular filtrate contains water,
amino acids, glucose, minerals and urea;
blood cells & proteins are too large to pass
through
• Ultrafiltration is a passive process and
selection of substances passing from the
blood into the glomerular filtrate is made
entirely according to the relative molecular
mass
Cells of Bowman's capsule
• The walls of the glomerulus have 3 layers:
• The inner layer of flat endothelial cells that
make the capillary walls.
• The basement membrane of the capillaries
• The podocytes of the Bowmans capsule:
Podocyte
Excretion of Nitrogenous Waste
Have we met our objectives?
• To recap types of nitrogenous waste
• To identify the ultrastructure of the kidney
• To explain ultrafiltration