The document summarizes key processes involved in homeostasis and excretion in the human body. It discusses how the kidneys, lungs, skin, and colon help maintain homeostasis by removing waste through excretion and osmoregulation. The kidneys play a central role by filtering the blood to remove nitrogenous wastes and regulating water balance. Through selective reabsorption and tubular excretion, the kidneys are able to regulate water and electrolyte levels in the blood and produce concentrated urine when fluid levels are low in the body.

1. Module One: Life Processes in
Plants and Animals
Paper One
Sian Ferguson

2. • Homeostasis is the process of maintaining the ideal internal
conditions (i.e. correct temperature, right amount of water and
glucose & other solutes) for the body to work at it’s optimum.
• Excretion is the process of removing metabolic waste products
and other toxins.
• Osmoregulation is maintaining the correct balance between
water and solutes.
• Excretion, which includes osmoregulation, is thus extremely
important in maintaining homeostasis.
• Secretion is the release of useful substances, e.g.
hormones, from the body. Thus, it is not excretion. Egestion, i.e.
defecation, is also not excretion.

3. • Certain waste products would become highly toxic if they
were to accumulate. This could damage tissues.
• An excess of water could also lead to a number of
complications.
• Thus, the waste products must be removed – they continually
move into the bloodstream, which carries them to the
excretory organs.

4. Raw Materials
(Food and 02)
Useful Materials
Useless Materials
Egested (faeces) Metabolised
Metabolic Waste
Useful Products Products (urea and CO2)
Excreted

5. • There are four main excretory organs:
– Lungs
– Colon
– Skin
– Kidneys
• The liver is not an excretory organ, but produces many
products which are excreted elsewhere. Toxins and drugs as
well as alcohol, is broken down in the liver. Hence, an excess
of smoking, medication and alcohol is extremely harmful to
the liver.

6. • The carbon dioxide released from cellular respiration is
carried to the lungs in the blood. It then diffuses across the
respiratory membrane and is exhaled.
• A small amount of heat and water is excreted this way.
• Bile pigments, from the break down of haemoglobin, and
cholesterol are synthesised in the liver.
• They pass into the small intestine as bile and are finally
excreted in the faeces as bile salts, from the colon.
• Mucus and bacteria are too excreted through the colon.

7. • Sweat, which is excreted through the skin, contains
water, salts and some urea.
• As the water in the sweat is excreted, heat is lost and the
body is cooled.
• Sweat is a form of excretion as it rids the body of waste, as
well as a form of secretion as it maintains the body
temperature.

8. • When amino acids and nucleic acids are broken down,
nitrogenous wastes are released as ammonia, urea, uric acid
and creatinine. Ammonia is toxic if it accumulates and is
therefore converted to less toxic urea in the liver.
• The following substances are made in the liver and excreted
by the kidneys:
– Urea, the main nitrogenous waste compound secreted. It is formed by
the breakdown of excess amino acids in the process of deamination.
– Uric acid, the nitrogenous end product of nucleic acid metabolism.
– Creatinine is formed from creatinine phosphate, found in the muscle
cells.
– Non-nitrogenous waste, e.g. CO2, excess water, ions, hormones,
poisons and drugs.

9. • The main function of the urinary system is to maintain
homeostasis by regulating the volume and concentration of
body fluids. It filters and reabsorbs certain materials from the
blood.
• The urinary system is made up of the following parts:
A) Two kidneys
B) A bladder
C) An urethra

10. • The kidneys aid in:
– excretion as they filter waste out of the blood
– regulate the water and salt balance in the body.
– One kidney – the right kidney – is slightly lower than the left as it is
pushed down by the liver, which is larger on the right.
• Each kidney contains:
– A renal artery (a branch of the abdominal aorta), which carries waste
products to the kidney and supplies the kidneys with oxygen and
nutrients.
– A renal vein that contains the purest blood in the body. It carries CO2
to the inferior vena cava.
– Ureter that carries urine from the kidney to the bladder.

11. • Stores urine
• Is stimulated by impulses from a motor nerve, to contract to expel
the urine.
• Has sphincter muscles at the base to control the flow of urine
• Carries urine from the bladder to outside the body

13. Aorta
Adrenal Gland Carries oxygenated blood,
Regulates Salt food and waste from the
heart
Renal Artery
Renal Vein Carries blood from body to
Carries blood from kidneys kidneys
to inferior vena cava
Kidney
Excretory and
Inferior vena cava osmoregulatory organ.
Carries deoxygenated blood Ureter
and other substances back Carries urine from
to the heart kidneys to bladder
Motor Nerve
Stimulates bladder nerve
Bladder
Stores urine
Urethra
Carries urine from
bladder to outside the
body

14. • Found in abdominal cavity below the diaphragm, near the
posterior, on either side of the vertebral column.
• Kept in position with connective tissue, the peritoneum, as
well as renal blood vessels. They are wedged in with other
organs.
• Externally they are bean-shaped, dark red and the size of a
large bar of soap.
• The inner, concave border is called the hilum.
• Surrounded by three layers of protective tissue:
– A tough, fibrous renal capsule on the surface protects them from
disease.
– A middle layer of adipose tissue cushions them against blows.
– An outer layer of fibrous connective tissue, the renal fascia, anchors the
kidneys to surrounding structures.

15. Nephron
Renal Capsule
Protects kidney
Papilla
Tips of each pyramid, fits into calyx
Renal artery
Carries blood to kidneys
Renal vein
Carries blood fromkidneys
Pyramid
Made up of collecting ducts
Calyx
Collects urine from collecting ducts
Medulla
Made up of pyramids
Ureter
Carries urine from pelvis to
bladder for storage

16. The kidney needs to have a constant supply of blood in order to
control the composition of body fluids.
• The renal artery, a branch from the aorta, enters the kidney
at the hilum. It supplies blood rich in nitrogenous
waste, oxygen and nutrients.
• The renal artery branches and spreads through the
medulla, between the pyramids, to the cortex.
• In the cortex, they branch into afferent arterioles, carrying
blood to the Bowman’s capsule.
• In the capsule, the arteriole divides to form a tuft of loose
capillaries around the renal tubule called the glomerulus.

17. • The capillaries unite to form the efferent arteriole which is
narrower than the afferent arteriole.
• The efferent arteriole divides again to form a mass of
capillaries around the renal tubule, called the peritubular
capillaries.
• In this way a portal system is formed.
• Capillaries join to form venules and then larger veins which
eventually form the renal vein.
• The renal vein carries purified, deoxygenated blood to the
inferior vena cava, and then to the heart.

20. • A passive, non selective process.
• Fluids and solutes are forced through the glomerular
membrane by hydrostatic pressure.
• The glomerular filtrate has the same composition as blood,
without the blood cells and plasma proteins. These are too
large to fit through the glomerular membrane.

21. This is the process by which substances are reabsorbed.
• Carrier molecules on the microvilli join up with certain
molecules from the filtrate and actively transport them
through the epithelial cells to the blood.
• Energy from ATP is used to join the molecule to the
carrier molecule. The following are actively reabsorbed:
– All organic nutrients such as glucose, amino acids and water
soluble vitamins are completely reabsorbed.
– Sodium ions and fat soluble vitamins are selectively
reabsorbed, according to the needs of the body.

22. Passive Reabsorption
• 65% of water is passively reabsorbed by
osmosis.
• Passive no energy needed
• Chloride ions passively follow paths of sodium
ions.

23. Tubular Excretion
• Urine formation may occur mostly through
filtration-reabsorption, a process called
tubular excretion is also involved.
• Takes place in:
- Proximal tubules
- Distal tubules
• Tubular excretion is reabsorption in reverse

24. Tubule Excretion
• Cells of tubules remove certain molecules and
ions from blood
• Then deposit these into the filtrate within the
tubules such as:
- Both Hydrogen ions and potassium ions secreted
in to filtrate. As each secreted, sodium ions
reabsorbed by blood.
- More creatine & uric acid
- Drugs, preservatives & colourants actively
excreted.

25. Osmoregulation
• Kidneys continuously regulating the chemical
composition of blood within narrow limits
Homeostasis is maintained
• The control of water and solute content in
body is called osmoregulation and is largely
brought about by kidneys

26. How is water balance
regulated?
• When fluid intake high – kidneys excrete
dilute urine (saving salt and excreting water)
• Fluid intake low – kidney conserves water by
forming concentrated urine.

27. How does Henle’s
loop conserve water?
• Role is to create a very high SALT
CONCENTRATION in tissue fluid in medulla
area of kidney.

28. What is the final
outcome?
• Water actively conserved & passed back in to
blood and not lost in urine.
• So less urine formed – concentrated and dark
in colour.

29. What happens to the
urine now?
• Filtrate that finally flows from duct can be
called urine as it is the fluid the body does not
want.
• From collecting ducts  passes drop by drop
through papillae  renal calyx  renal pelvis
for transport  bladder which stores until 
expelled out urethra
• Urine propelled by peristaltic movements of
smooth muscular walls

30. What is the final
composition of urine?
• 96% water
• Salts (mainly sodium chloride) 1.5%
• 2% Urea
• Small quantity = drugs, colourants, homones
preservatives.

31. Key functions of
• Homeostasis
kidney?
1. Excretion of nitrogenous waste = urea + small
amounts uric acid + creatine + ammonium ions
to prevent toxic substances.
2. With help of skin osmoregulation by selecting
water and salts according to body's needs.
3. Maintain pH of body fluid  excrete more or
less hydrogen ions.
4. Maintain electrolytic(salts) balance of body
fluids by selectively reabsorbing or secreting
ions