The excretory system removes waste from the body through various organs. The kidneys are the primary organs of excretion, removing nitrogenous wastes such as urea and excess water and salts. The kidneys contain nephrons, which filter the blood to produce urine. Urine passes from the nephrons to the bladder through the ureters for storage and later excretion through the urethra. Homeostasis is maintained as the kidneys regulate water balance and the concentration of substances in the blood and tissues.

1. EXCRETORY
SYSTEM

2. EXCRETION IS IMPORTANT FOR
HOMEOSTASIS

3. EXCRETION:
Is the name given to
the removal from the
body of:
a) waste products of
its chemical reactions
b) The excess of
water and salts taken
in with the diet
c) Spent hormones
d) Drugs and foreign
substances
SECRETION:
Production and
release of
USEFUL
substances in the
body
(saliva/hormones)

4. WATER
 INTRACELLULAR : inside the cells
(30 litres)
 INTERCELLULAR: tissue fluid
(9litres)
 PLASMA (3 litres)

5. EXCRETION
 Metabolic processes in cells may
produce toxic compounds. The two
most significant are:
1) Carbon dioxide: which dissolves to
form a weakly acidic solution in
blood and tissue fluid
2) Urea
Both of them can denature enzymes.

6. Excretory organs:
 Lungs: they supply the body with O2but
they also get rid of CO2.
 Kidneys: the kidneys remove urea and
nitrogenous waste from the blood. They
also expel excess of water, salts,
hormones and drugs. (expelled with the
faeces)

7.  Liver: it excretes bile pigments, bilirubin.
Bilirubin, the breakdown product of
haemoglobin, is secreted into the small
intestine and it gives the colour to the faeces
 Skin: the skin loses incidental water, salts
and urea when you sweat. However,
sweating is a response to a rise in
temperature and not to a change in blood
composition.

8. How urea is made

9. Excretory System

10.  The kidneys are two fairly solid, oval
structures.
KIDNEYS
 They are located on each side of the spine.
 They are red brown, enclosed in a
transparent membrane attached to the back
of the abdominal cavity.

11. Renal artery: branches off from the aorta and brings
oxygenated blood to the kidneys.
Renal vein: takes deoxygenated blood away from the
kidneys to the vena cava.
Ureter: is a tube which runs from each kidney to the
bladder in the lower part of the abdomen.
Urinary bladder is the organ that collects urine excreted by
the kidneys before disposal by urination. Urine enters the
bladder via the ureters and exits via the urethra.
Urethra: is a tube that connects the urinary bladder to the
genitals for removal out of the body.
The external urethral sphincter is a striated muscle that
allows voluntary control over urination.

12. A kidney cut by its length:
CORTEX: it is the dark outer region.
MEDULLA: lighter inner zone
PELVIS: where the ureter joins the kidney

13.  CORTEX: It contains the bowman’s capsules in
which the ultra filtration of blood takes place.
 MEDULLA: it contains loops of Henle and
collecting ducts.

14. NEPHRON
 Nephron is the basic
structural and
functional unit of the
kidney.
 There are up to 4
million nephrons in a
kidney.
 It’s a single glomerulus
with its renal capsule,
renal tubule and blood
capillaries.

15.  Its main function is to regulate the
concentration of water and soluble
substances like sodium salts by filtering
the blood, reabsorbing what is needed
and excreting the rest as urine.
 A nephron eliminates wastes from the
body, regulates blood volume and blood
pressure, controls levels of electrolytes
and metabolites, and regulates blood ph.

16. Branch of
renal artery
First coiled
tubule
Bowman
capsule
Collecting
duct
urine
NEPHRON:
Loop of
Henle

18. Kidneys tubules:
 Branch of renal artery: (WIDE): blood with high
urea concentration is delivered to the kidneys
and gets to the GLOMERULUS (NARROW)
 Renal capsule: filtration of blood under high
pressure to remove toxic urea. Unfortunately
useful glucose, amino acid, salts and water also
leave the blood. RED CELLS/PROTEINS/LIPIDS
stay

19.  First coiled tubule: here the useful solutes,
glucose, amino acids and some minerals are
selectively reabsorbed into the blood. (process
of absorbing back the substances needed by the
body) Up to 90% of water is reabsorbed here.
 Collecting duct: kidney can reabsorb water
from here and return it to the blood according
to the body’s demands.

20. Urine: mainly water with
concentrated urea and
excess salts.
 Remember: urea is produced
in the liver but excreted by
the kidneys.
 Revise how urea is made by
liver cells by un excess of
amino acids.

21. So…
 The nitrogenous waste products, excess
salts and water continue down the renal
tubule into the pelvis of the kidney. From
here the fluid, now called urine, passes
down the ureter to the bladder.

22. How toxic waste from cellular respiration
(catabolism) is excreted
 1- Blood + waste come to
kidneys by renal artery.
 2-Filtration of waste in the
nephron.
 3-Reabsorption of some
useful products that could
have been filtered, to the
capillaries surrounding the
nephron.
 4-Waste is concentrating as
urine.
 5-Urine goes to the
-collecting duct
- ureter
- bladder
- and goes out by urethra

23.  holds 300-350 ml of urine.
• As urine accumulates, the wall of
the bladder thins as it stretches,
store larger amounts of urine
without a significant rise in
internal pressure.
When the bladder reaches around 25% of its
working volume  urge to urinate starts
(Easy to resist)
Eventually, the bladder will fill to the point
where the urge to urinate becomes
overwhelming

24. How can our body keep the
percentage of water in our body
around 60/70%?
 60 – 70 % of the weight of our body is
water (around 40 l of water).
 If it is very hot and we sweat, we lose
some of our body water composition.
 If we lose water, we can suffer
dangerous dehydration
 We have to” save” water and we must
not lose more water through urine.
 The urine will be more concentrated
(water is reabsorbed)
 If we drink a lot of water, we have an
extra supply of water (that we don’t
need).
 Our urine will be more diluted (water
will not be reabsorbed in kidneys).

25. Water balance and
Osmoregulation
 we gain water food/drink
 we lose water by *evaporation (skin/lungs)
* urination
* defecation
*Keep the concentration of the body
fluids
*adjust the concentration of
BLOOD too dilute water is absorbed
too concentrated water is
absorbed
Stimulates HYPOTHALAMUS (a “THIRST” centre in the brain )
stimulates PITUITARY GLAND secretes ADH blood
Blood kidneys kidney tubule absorb MORE water
+concentrated
urine

26.  Negative feedback
happens when the
response to a given
action generates an
effect that inhibits that
action.
 In a variety of
processes: blood
pressure control,
glycemic control,
muscle contraction,

27.  Composition of the
TISSUE FLUID (its
concentration, acidity
and temperature) in the
body is adjusted all the
time to keep it STEADY
TISSUE FLUID
What is HOMEOSTASIS, Sheldon?
Supplies or removes the substances to or from the cells
If it is TOO DILUTE cells will take up too much water (by
OSMOSIS) and the tissues become swollen
If it is TOO CONCENTRATED, it will withdraw (=extraer) water
from the cells (by OSMOSIS) and the body dehydrates

28. Organs that contribute to Homeostasis
KIDNEYS
•Remove substances that might poison
the enzymes
•Control the level of SALTS, WATER,
ACIDS (ures/uric acid) in the blood
LIVER •Regulates the level of GLUCOSE (it affects
the brain cells) and amino acids
Lungs
•Keep the concentration of Oxygen and Carbon
dioxide in the blood for the cell’s reactions
(respiration)
Control by the

29. Dialysis Machine
Kidney failure can be
the result of
*A drop in blood pressure (recovery is
spontaneous/more than
2 weeks leads to failure)
*Disease of the kidneys with 1 kidney we can
survive/ if both kidneys fail Dialysis machine
It consists of a cellulose tube coiled up in a water
bath

30. * is kept at body temperature and it is constantly
changed as unwanted blood solutes accumulate
in it.
* is a solution of salts and sugar of the correct
composition only substances above this
concentration (UREA, URIC ACID, EXCESS
SALTS) are removed
Submicroscopic pores in the dyalisis
tubing allow small molecules to leak
out into the water bath (= FILTRATION
process in the glomerulus)
Has to spend 2/3 nights a week in the diayalisis machine
Water bath

31. KIDNEY
TRANSPLANT
Better solution
2 problems: a. to find donors (a close relative/healthy person
who dies in an
accident)
b. REJECTION body produces Lymphocytes
which attack and destroy the new
organ
Overcome by
•Choosing tissues similar to the
patient’s
•Using immunosuppressive drugs

32. Some
IGCSE
questions
Sandra Brinkhoff
http://sandrabrinkhoff.artspan.com/large-view/Medical%20Illustration/96153---7624/Mixed%20Media.html