The liver plays a key role in metabolism and homeostasis. It receives a dual blood supply and filters waste from the blood to produce urea, which is excreted. Excess alcohol is broken down but can overload the liver, causing fatty deposits. The kidneys filter blood to produce urine via nephrons and selective reabsorption maintains electrolyte balance. Kidney failure requires dialysis or transplant. Pregnancy and drug tests analyze samples for target hormones or compounds.

1. Excretion
Chapter Content

2. The Liver
The liver needs a good blood supply in order for the
hepatocytes (liver cells) to carry out metabolic processes and
to be involved in homeostasis. There are two sources of blood
supply…
 Oxygenated blood from the heart enters via the hepatic
artery. This is oxygen is essential for aerobic respiration
and so ATP can be produced for the metabolic processes.
 Deoxygenated blood from the digestive system enters via
the hepatic portal vein. It is rich in digestion products.
Blood leaves via the hepatic vein to rejoin the vena cava.

3. The Bile duct carries bile from the liver to the gall bladder
where it is stored until it needs to aid the digestion of fats in the
small intestine.

4. Functions of the Liver
 Control of blood glucose, amino acid and lipid levels
 Synthesis of red blood cells in the fetus bile, plasma
proteins and cholesterol
 Storage of vitamins A, D, B12, iron and glycogen
 Detoxification of alcohol and drugs
 Breakdown of hormones
 Destruction of red blood cells

5. Urea Formation
A high intake of proteins results in excess amino acids, which
can’t be stored. Instead, these amino acids are converted to
urea by two processes…
Amino acid  Ammonia  Urea
Deamination Ornithine Cycle

6. The 2 Processes
 Deamination: this is the removal of the amine group from
the amino acid, which produces ammonia.
 Ornithine Cycle: the ammonia produced during
deamination is very toxic and highly soluble, so it must be
converted to urea.
Citrulline
Ornithine Arginine
NH3
H2O
Urea H2O
NH3 and CO2 H2O

7. The Equations
 Deamination:
 Ornithine Cycle:
2NH3 + CO2  CO (NH2)2 + H2O

8. Detoxification of Alcohol
Ethanol is oxidised to ethanal (catalysed by ethanol
dehydrogenase). Ethanal is dehydrogenated, and the two
hydrogen atoms are accepted by NAD which becomes
reduced. Ethanal is then oxidised to ethanoic acid/acetate
(catalysed by ethanal dehydrogenase). The ethanoic acid is
degydrogenated, and the two hydrogen atoms are accepted by
NAD, which becomes reduced. Ethanoic acid is then converted
to Acetyl Coenzyme A, which enters the Krebs Cycle in
respiration

9. Excess Alcohol
NAD is required to oxidise and break down fatty acids so
they can be used in respiration. If the liver has too much
alcohol to detoxify, it will have insufficient NAD to deal
with fatty acids. These fatty acids are then converted
back to lipids and stored in the hepatocytes which
enlarges the liver (a ‘fatty liver’). This is known as
cirrhosis.

10. The Kidney

11. The Nephron
How does the composition of fluid change?
1. Glucose is actively transported out of the tubule
2. Sodium ions enter the descending and leave via the
ascending limb
3. Urea concentration rises as water is removed from the
tubule
4. Water is reabsorbed from the tubule, which increases the
relative concentration of sodium ions and potassium ions
are then actively transported into the tubule to removed in
urine

12. Selective Reabsorption
As fluid moves along the nephron, substances are removed
from the fluid and reabsorbed into the blood. Most reabsorption
(85% of the filtrate) occurs at the proximal convoluted tubule
(PCT). All of the amino acids, and some salts and water are
reabsorbed here. The cells lining the PCT are specialised to
achieve this…
 Microvilli: a large surface area for contact with the filtrate
 The cell cytoplasm has many mitochondria to produce ATP
for the active transport
 The membrane has co-transporter proteins to transport
glucose or amino acids in association with sodium ions
(facilitated diffusion)

13. Ultrafiltration
Is filtration at molecular level at the basement membrane. Blood
flows into the glomerulus from the afferent arteriole. The
afferent arteriole is wider than the efferent arteriole, and this
difference in diameter ensures a high hydrostatic pressure. This
pressure pushes the fluid into the Bowmans capsule. The
endothelium has narrow gaps (fenestrations) for the blood
plasma to pass through. Also, the basement membrane
consists of a fine mesh of collagen fibres and glycoproteins
which act as a filter to prevent the passage of molecules with a
relative molecular mass of >69 000.

14. What is filtered out?
 Water
 Urea
 Amino acids
 Glucose
 Inorganic ions (Na, Cl, K)

15. What is left in the capillary?
 Blood cells
 Proteins
The presence of these proteins means the blood has a
low, very negative, water potential. This ensures some of
the fluid is retained in the blood which contains some of
the things normally filtered out. This also helps reabsorb
water.

16. Reabsorption
1. The sodium-potassium pumps remove sodium ions from the cells lining
the PCT, reducing the concentration of Na ions in the cell cytoplasm
2. Sodium ions (associated with glucose and amino acids) are transported
along the cell by facilitated diffusion
3. As the glucose and amino acid concentration rises inside the cell, these
substances can diffuse into the tissue fluid
4. From the tissue fluid, they diffuse into the blood are are carried away
5. The reabsorption of salts, glucose and amino acids reduces the water
potential in the cell and increases the water potential in the tubule fluid.
Water will thereby enter the cell and be reabsorbed back into the blood
by osmosis
6. Larger molecules will be reabsorbed by endocytosis

17. The Loop of Henle
The role of the loop of Henle is to create low, very negative,
water potential in the tissue of the medulla. This ensures even
more water can be reabsorbed from the fluid in the collecting
duct.
It consists of a descending limb (into the medulla) and an
ascending limb (out into the cortex). It allows salts to be
transferred from the ascending to the descending limb.

18. How is this achieved?
 The loop of Henle has a countercurrent mechanism (a tube that
turns back on itself). This increases the efficiency of the
exchange of salts.
 Sodium and chloride ions move by active transport and diffuse
out of the filtrate as it moves along the ascending limb
 This decreases the water potential of the tissue fluid in the tips of
the pyramids of the medulla
 The ions pass back into the descending limb and are recycled,
so they can be concentrated in the medulla
 The low water potential outside the collecting duct allows water
to leave the collecting duct by osmosis (if ADH is present)

19. The Collecting Duct
From the DCT, fluid flows into the collecting duct (the fluid
has a high water potential). The collecting duct carries the
fluid through the medulla to the pelvis. As the tubule fluid
passes down the collecting duct, water moves by osmosis
into the surrounding tissue fluid, it then enters the
capillaries by osmosis and is carried away.

20. Osmoregulation
Osmoregulation is done by the kidney and is the control and
regulation of the water potential of the blood and body fluids.
This is monitored by osmoreceptors in the hypothalamus of
the brain.
The walls of the collecting duct can be altered. They respond to
the levels of the antidiuretic hormone (ADH) which is released
from the posterior pituitary gland. Cells in the wall of the
collecting duct have receptors for ADH to bind to. Vesicles
(containing aquaporins) are inserted into the cell surface
membrane. This makes the walls permeable to water, so more
water is reabsorbed by osmosis resulting in less urine with a
low water potential being produced.

21. Kidney Failure
Common causes of kidney failure include diabetes,
hypertension, infection. Once the kidneys have failed, the body
can’t remove excess water or waste products, nor can thy
regulate water or salt level.
One treatment for kidney failure is dialysis. This is the use of a
partially permeable membrane to filter the blood. The waste,
excess fluid and salt pass over a dialysis membrane which
allows the exchange of substances between the blood and
dialysis. There are two types of dialysis…

22. Haemodialysis Peritoneal Dialysis (PM)
Blood from a vein is passed into a
machine that contains an artificial
dialysis machine. Heparin is added to
avoid clotting and any bubbles are
removed before the blood returns to
the body. This is usually carried out a
clinic 3x a week although some
patients carry it out at home
The filter is the body’s own
abdominal membrane (the
peritoneum). A surgeon implants a
permanent tube into the abdomen
and dialysis fluid is poured through
the tube. The fluid fills the space
between the organs abdomen wall.
After several hours, the fluid is
drained. This is usually carried out
several times a day, but the patient
can move around (ambulatory)

23. Kidney Transplant
The second form of treatment is to have a transplant. This
has many advantages and disadvantages…
Advantages Disadvantages
• No time consuming dialysis
• Diet is less limited
• Feel better physically
• Better quality of life
• Don’t see yourself as chronically ill
• Need immuno-suppresants for life
and this can cause a susceptibility
to anti infection
• Majory surgery under general
anaesthetic
• Surgery risks (infection, bleeding
etc.)
• Frequent checks for signs of
rejection

24. Pregnancy Testing
 The embryo secretes human chorionic
gonadotrophin (hCG). The pregnancy test stick is
marked with monoclonal antibodies which will only
bind to hCG. Any hCG in the urine will attach to an
antibody (they are tagged with blue beads). The hCG-
antibody complex moves up the strip until is sticks to a
band of immobilised enzymes. A blue line is formed
from the antibody-hCG complex being held in one
place (there is always a control line)

25. Anabolic Steroid Testing
Anabolic steroids increase protein synthesis so there is a
build up of cell tissue (especially in muscle cells). They
have a half-life of around 16hrs but can remain in the
blood for many days. Analysing one involves gas
chromatography. The sample is vaporised in the
presence of a gaseous solvent and passed down a long
tube lined with an absorbing agent. Each susbstance
dissolves differently in the gas and stays there for a
unique time (retention). Eventually, it comes out of the
gas and is absorbed onto the lining, creating a
chromatogram.