This document provides an overview of excretion and the urinary system prepared by Ms. Wong Fui Yen. It defines excretion and outlines the key excretory organs and their roles. It describes urine formation through ultrafiltration in the kidney nephrons and selective reabsorption. It also discusses osmoregulation through anti-diuretic hormone and kidney failure treatment using dialysis.

1. Prepared by, Ms Wong Fui Yen
Chapter 11 Excretion

2. Prepared by, Ms Wong Fui Yen
Learning Objectives:
(a) define excretion and explain the importance of removing
nitrogenous and other compounds from the body
(b) outline the function of kidney tubules with reference to ultra-
filtration and selective reabsorption in the production of
urine
(c) outline the role of anti-diuretic hormone (ADH) in the
regulation of osmotic concentration
(d) outline the mechanism of dialysis in the case of kidney
failure

3. Prepared by, Ms Wong Fui Yen
Excretion is the removal of toxic
materials and the waste products of
metabolism from the body.
Egestion is the removal of undigested
food matter which have never been
involved in the metabolic activities of
cells, from the alimentary canal.
Faeces is NOT a product of metabolism!

4. Prepared by, Ms Wong Fui Yen
Metabolic Waste Products:
Excretory product Organ Mode of excretion
Carbon dioxide Lungs Exhalation
Urea and excess
mineral salts
Kidneys
Skin
Urine
Sweat
Excess water Kidneys
Skin
Lungs
Urine
Sweat
Expired Air
Bile pigments
(from breakdown of
haemoglobin)
Liver Faeces

5. Prepared by, Ms Wong Fui Yen
Excretory Organs in Man
Lungs:
CO2, water vapour
Kidneys: excess water, excess salt,
Nitrogenous waste products (urea)
Liver: Bile pigmentsSkin: Urea, excess water, excess salt

6. Prepared by, Ms Wong Fui Yen
Ureter: thin tube
carries urine from
kidneys to bladder
Urinary bladder:
elastic, muscular
bag to store urine
temporarily.
Sphincter muscle
voluntarily relaxes
to release urine
from bladder
Urethra: passage of
urine out of body.
The Urinary System

7. Prepared by, Ms Wong Fui Yen

8. Prepared by, Ms Wong Fui Yen
Blood reaches the
kidney via the renal
artery, which
branches into
arterioles
Blood from each
arteriole flows into a
network of
capillaries
(glomerulus) in the
Bowman's capsule
Blood leaving the
glomerulus flows
through the
capillaries that run
along the kidney
tubules
Blood from the
capillaries enter the
venules, which lead
to the renal vein

9. Blood vessels
Renal artery
Renal vein
Ureter

10. Nephron- smallest unit of kidney
Prepared by, Ms Wong Fui Yen

11. Urine formation
Prepared by, Ms Wong Fui Yen
•Nitrogeneous wastes and excess mineral salts are removed from the body
via the urine
•The renal artery carries oxygenated blood, nutrients, and wastes to the
kidney while the renal vein carries filtered blood away from the kidney.
•The pathway of blood flow in the kidney is shown below:

12. Prepared by, Ms Wong Fui Yen
Urine Formation
Two stages:
1. Ultrafiltration
2. Selective Reabsorption
1. Ultrafiltration:
• Occurs in the Renal/Bowman's capsule.
• Blood enters kidney via the renal artery
• Renal artery divides to form a mass of arterioles, and
then capillaries which form a knot (glomerulus) in the
Bowman’s capsule
• Afferent arteriole leading into the glomerulus is wider
than the efferent arteriole that leads out, so creates a
high blood pressure

13. Prepared by, Ms Wong Fui Yen
Urine Formation
•This pressure forces plasma containing small
molecules to be filtered out of the blood.
• Red and white blood cells, large plasma proteins
remain in the blood capillaries and continue into
the proximal convoluted tubule.
•Glomerular filtrate: liquid that is filtered out contains
 glucose,
 amino acids,
 mineral salts,
 excess water
 nitrogenous waste (urea).

14. Prepared by, Ms Wong Fui Yen
2. Selective re-absorption:
Starts at the proximal convoluted tubule.
This is the longest and widest part of the nephron.
Over 80% of the filtrate is reabsorbed into the blood.
Filtrate flows down the nephron, all useful materials
are reabsorbed back into network of capillaries.
Examples such as glucose, amino acids, salts, water
are reabsorbed.
Excess water and salts, and metabolic waste
products pass from the tubules into the collecting
ducts and then into the renal pelvis as urine.
Urine Formation

15. Prepared by, Ms Wong Fui Yen
Loop of Henle: mainly water (by osmosis).
Distal tubule: remaining fluid with more urea,
excess useless substances e.g. sodium
chloride, uric acid, ammonia, creatinine,
water.
Collecting duct:
Urine collected, passed into renal pelvis,
leads to ureter.
Urine Formation

16. Blood containing
metabolic waste
products from
renal artery
Renal
capsule
(i) Ultrafiltration
(ii) Selective
Reabsorption
filtrate
Blood
capillaries
• amino acids
• glucose
• salts
• water
•
•amino acids
• glucose
• salts
• water
• urea
Excess water, excess
mineral salts and urea
removed through urine
Purified blood to
renal vein

17. Concentration of fluid in different regions of a nephron,
with and without presence of ADHPrepared by, Ms Wong Fui Yen

18. Prepared by, Ms Wong Fui Yen
Proximal convoluted reabsorption
Tubule
All of the glucose and amino
acids and most of the mineral
salts( by diffusion and active
transport )
Water ( by osmosis)
Loop of Henle reabsorption •Water (by osmosis)
•Sodium ions ( Na+) (by active
transport )
Distal convoluted reabsorption
tubule
•Water (by osmosis)
•Sodium ions (Na+) (by active
transport)
Collecting tubule reabsorption
/ duct
•Water (by osmosis)
Glomerular filtrate

19. Prepared by, Ms Wong Fui Yen

20. Osmoregulation
Prepared by, Ms Wong Fui Yen
•Osmoregulation is the control of water and solute levels in
the blood to maintain a constant water potential in the body.
•The water potential of blood (concentration of water and salts
in the plasma) must be maintained at a relatively constant
level. The water content is regulated via hormone vasopressin
(also known as anti-diuretic hormone, ADH)
•ADH is produced by the hypothalamus in the brain and is
released by the pituitary gland
•ADH increases water reabsorption of the kidneys by
increasing permeability of the distal convoluted tubule and
collecting duct to water.

21. Prepared by, Ms Wong Fui Yen
Kidney failure refers to the inability of the kidney to
function properly, causing accumulation of
nitrogenous waste in the blood.
Kidney failure may be caused of the following:
a) Diabetes
b) Hypertension
c) Excessive alcohol intake
d) Severe injury or bacterial infection
When both kidneys fail to function, the person will need
to get a kidney transplant, or undergo kidney
dialysis to get rid of wastes from patient’s blood.
Kidney Failure & Dialysis

22. Prepared by, Ms Wong Fui Yen
Kidney Dialysis Machine

23. Prepared by, Ms Wong Fui Yen
Larger molecules
(e.g. plates and
blood cells)
remain in the
tubing
The filtered blood
is returned to a vein
in the patient’s arm
artery
dialysis
tubing
pump
filtered
blood
fresh dialysis
fluid
dialysis
machine
dialysis
fluid
vein
1
2
3
6
4
5
23
Blood is drawn
from an artery in
the patient’s arm
1 Blood is pumped
through a tubing
to the dialysis
machine
2
The tubing is bathed
in a special dialysis
fluid and the tubing
is semi-permeable.
3
Small molecules
(e.g. urea) and
metabolic waste
products diffuse
out of the tubing
45
6

24. Prepared by, Ms Wong Fui Yen

25. Prepared by, Ms Wong Fui Yen
How does the kidney dialysis
machine work?
1. Patient’s blood is drawn from the artery and
allow to flow through the tubing in the
dialysis machine.
2. Tubing which represents the partially
permeable membrane is bathed in a
specially controlled dialysis fluid.
They only allow small molecules, like urea
and other waste products, to diffuse out of
the tubing.

26. Prepared by, Ms Wong Fui Yen
3. The direction of blood flow is opposite to
the flow of the dialysis fluid.
4. A concentration gradient is set up to allow
diffusion of waste products from patient’s
blood to the dialysis fluid.
5. The filtered blood is then returned to the
patient’s arm vein.

27. Prepared by, Ms Wong Fui Yen
Comparing the Blood Composition
& Dialysis Fluid
Partially permeable
membrane of tubing

28. Resources Online:
The Urinary System:
http://www.youtube.com/watch?v=aQZaNXN
roVY&feature=related
Kidney Dialysis:
http://www.kidneypatientguide.org.uk/
Prepared by, Ms Wong Fui Yen

29. Prepared by, Ms Wong Fui Yen
• Factors affecting composition of urine
•Diet:
- Protein rich diet more urea being formed as proteins are deaminated in the
liver
- More water intake more urine formed
-Salty foods more salt secreted in urine
•Diseases:
-Diabetes: Glucose in urea
-Kidney Stone: glomerulus becomes fully permeable  Red blood cells able to pass
through membrane, hence RBC found in urine
-Infection of kidney: White blood cells in urine
•Lifestyle:
-More exercise less water left in the body less water secreted in urine
•Climate:
-Cold weather  sweat less  more water left in the body  urine more water

30. Prepared by, Ms Wong Fui Yen
Why you shouldn’t drink the
salty sea water?
•Drinking sea water will increase
the concentration of dissolved
mineral salts in the blood, hence
causing water to diffuse from all
the body cells into the
bloodstream.
•This causes the cells to shrink
and malfunction.