Module 2: Excretion
F214 Communication,
homeostasis and energy
Learning Outcomes
• Define the term excretion.
• Explain the importance of removing
metabolic wastes, including carbon
dioxide and nitrogenous waste, from
the body.
Excretion
• Excretion is the removal of unwanted
products of metabolism
• Main products
– carbon dioxide
– urea
Carbon dioxide
• Produced in every living cell during
aerobic respiration
• Transported in the blood stream
Carbon Dioxide Transport
• CO2 carried in three ways
– 5% in solution in plasma as CO2
– 10% combines with amino groups in Hb
molecule (carbamino haemoglobin)
– 85% hydrogen carbonate ions
Carbon dioxide transport
• Transported in blood as hydrogen
carbonate ions
• Carbonic anhydrase catalyses the
reaction
CO2 +
H2O
H2CO3
Carbon Dioxide Transport
• Carbonic acid dissociates
H2CO3  H+ + HCO3• H+ ions associate with haemoglobin
(buffer)  Haemoglobinic acid (HHb)
Chloride Shift
• Build up HCO3- causes them to diffuse
out of RBC
• Inside membrane positively charged
• Cl- diffuse into RBC from plasma to
balance the electrical charge
What happens if CO2 is not
excreted?
• Carbon dioxide is toxic
– Hydrogen carbonate ions can reduce the
ability to transport oxygen
– Forms carbaminohaemoglobin which has
a lower affinity for oxygen than normal
haemoglobin
– Respiratory acidosis
• Drop in blood pH
– Headache, drowsiness, restlessness, tremor and
confusion
Learning Outcomes
• Describe the formation of urea in the
liver, including an outline of the
ornithine cycle.
Urea
•
Produced by the liver from excess amino
acids
•
Deamination
•
Urea formation
•
Transported from the liver to the kidneys in
the blood stream, where it is excreted
dissolved in water, as urine.
Amino acid + oxygen  keto acid + ammonia
Ammonia + carbon dioxide  urea + water
Urea Formation
• Amino Acid molecules contain a lot of
energy
• Urea is formed from two processes
– Deamination
– Ornithine cycle
Amino acid  ammonia  urea
deamination
Ornithine cycle
Deamination
• Amine group (NH2) of each amino
acid is removed
• Ammonia (NH3) is formed which is
highly soluble and highly toxic
• Keto acid can enter respiration to
release energy or stored as fat
deamination
Ornithine Cycle
• Ammonia is quickly converted into the
less soluble and less toxic urea CO(NH2)2
• The ornithine cycle requires the input
of energy from ATP
The ornithine cycle
Citruline
CO2
Ammonia
NH3
ATP
ornithine
ATP
ADP
ADP
arginine
Urea
CO(NH2)2
Learning Outcomes
• Describe, with the aid of diagrams and
photographs, the histology and gross
structure of the liver.
The liver and it’s associated
organs
Histology.
• The liver lies below
the diaphragm and
just to the right of
centre.
• Its is supplied by two
different blood
supplies, the hepatic
artery brings
oxygenated blood
from the aorta, the
hepatic portal vein
delivers blood from
the digestive tract.
Histology
• Blood is carried away by the hepatic
vein to the vena cava.
• 75% of the blood delivered to the liver
arrives via the hepatic portal vein at a
very low pressure.
Histology of the liver
• The liver is divided into two principal
lobes
• The lobes are subdivided into about
100,000 lobules.
Histology of liver
Liver
Lobules
Liver Lobules
lobules
• Each lobule is a six sided structure
consisting of specialised epithelial cells
called hepatocytes.
• The lobules are arranged in irregular,
branching, interconnected plates
around a central vein.
• The blood passes through large
endothelium-lined spaces called
sinusoids.
sinusoids
• Within the sinusoids there are fixed
phagocytes called Kupffer cells.
– The role of these is to destroy worn out red and
white blood cells, bacteria and foreign matter
arriving from the digestive tract.
• Blood flows from the hepatic portal vein and
hepatic artery towards the centre of the
lobule, where it enters a central vein leading
to the hepatic vein.
Liver Lobule
Liver Lobule
Bile Duct
• Running alongside the sinusoids are
bile canaliculi.
• These carry bile from the centre of the
lobule to the outside, where it enters a
branch of the bile duct.
• The bile ducts merge and form the
larger right and left hepatic ducts.
Learning Outcomes
• Describe the roles of the liver in
detoxification
Detoxification
• Detoxification can take place by the
oxidation, reduction or methylation of
toxins
• Detoxification of alcohol, antibiotics,
steroid hormones etc
• Example
catalase
Hydrogen peroxide water and oxygen
2H2O2

2H2O
+
O2
Detoxification of alcohol
• Alcohol is broken down into ethanoate
by the enzymes alcohol
dehydrogenase and aldehyde
dehydrogenase.
• Ethanoate enters the Krebs cycle and
is metabolised to produce ATP.
• Reduced NAD is produced
Detoxification of alcohol
Fatty Liver
• NAD is used to
oxidise fatty acids
in the liver
– If NAD is reduced it
can not oxidise the
fatty acids, which
accumulate and
are converted into
fat
– The fat is deposited
in the liver
Cirrhosis
• Damaged hepatocytes are replaced
by fibrous tissue
• Structure of blood supply is lost
– Blood flows from hepatic artery into
hepatic vein without flowing through
sinusoids
• Effects
– Increase in ammonia concentration
which can cause major damage to the
central nervous system
Pupil Activity
• Answer the question sheet on the liver
• Answer the past paper questions on
the liver
• Answer the stretch and challenge
question on page 41 of your textbook.
The Kidneys
Module 2: Excretion
Learning Outcomes
• Describe, with the aid of diagrams and
photographs, the histology and gross
structure of the kidney.
• Describe, with the aid of diagrams and
photographs, the detailed structure of
a nephron and its associated blood
vessels.
Position of the kidney and associated
structures in the human body
Kidney cut in half
The structure of the nephron
• Each kidney is
made up of
thousands of
tiny tubes called
nephrons
The
Nephron
• Bowman’s
capsule
• Proximal
convoluted
tubule
• Loop of Henle
• Distal
convoluted
tubule
• Collecting duct
Nephron
• Blood supply
Afferent arteriole
 glomerulus
 efferent
arteriole
Kidney Function
• Cortex
– Filtration is carried out by the nephrons.
– Dense capillary network which receives blood
from renal artery.
• Medulla
– Nephron extends across to form renal pyramids
• Pelvis
– Renal pyramids project into a central space
called the pelvis.
– Urine passes from the pelvis into the ureter
Learning Outcomes
• Describe and explain the production
of urine, with reference to the
processes of ultrafiltration and
selective reabsorption.
Revision
• On a sheet of A4 plain paper
– Draw and label the cross section of a
kidney
– Draw and label a nephron
• Swap your sheets over for marking – both are
to be marked out of 10
• What is your total out of 20?
The Nephron
1. Ultrafiltration
2. Selective reabsorption in the proximal
convoluted tubule
3. Reabsorption of water in the loop of
Henlé (counter current system)
4. Reabsorption in the distal convoluted
tubule and the collecting duct.
Ultrafiltration in the renal capsule
• The blood in the
capillaries is
separated from the
lumen of the renal
capsule by
– Endothelium
– basement
membrane
– Podocytes
• The endothelium of
the renal capsule
Podocytes
Ultrafiltration in the renal capsule
• Gaps allow most molecules to pass
through
• Basement membrane
– prevents large molecules e.g. plasma
proteins, from passing through.
Ultrafiltration
• High pressure maintained as afferent
arteriole is wider than efferent arteriole.
• Hydrostatic pressure builds up forcing
substances through endothelial pores,
across basement membrane and into
Bowman’s capsule.
• The fluid in the renal capsule is known
as glomerular filtrate
Comparison of blood and
glomerular filtrate
substance
Concentration in blood
plasma (g dm-3)
Concentration in
glomerular filtrate (g dm-3)
Water
900
900
Inorganic
ions
7.2
7.2
Urea
0.3
0.3
Uric acid
0.04
0.04
Glucose
1.0
1.0
Amino acids
0.5
0.5
proteins
80.0
0.05
selective reabsorption
• All glucose, amino acids, vitamins and many
sodium and chloride ions are reabsorbed
out of the proximal convoluted tubule and
back into the blood.
• The walls of the nephron are made up of
cuboidal epithelium with microvilli – give LSA
• Blood capillaries lie close to the outer
surface of tubule
Selective reabsorption
• Outer membranes of cells actively
transport sodium ions out of cytoplasm
(sodium potassium pump)
• Sodium ions diffuse down a
concentration gradient back into the
cytoplasm, passing through cotransporter proteins that transport
glucose or amino acids
Selective reabsorption
1. Sodium ions are actively transported
out of cells into the tissue fluid
2. Glucose or amino acids enter cells
with sodium ions by facilitated
diffusion
3. Glucose and amino acids diffuse into
the blood capillary
Selective reabsorption
• Reabsorption of salts, glucose and
amino acids
– reduces the water potential of cells
– Increases water potential in tubule
• Water is reabsorbed into the blood by
osmosis
Learning outcomes
• Explain, using water potential
terminology, the control of the water
content of the blood, with reference
to the roles of the kidney,
osmoreceptors in the hypothalamus
and the posterior pituitary gland.
Reabsorption of water
• The Loop of Henlé
– Function
• Create a high concentration of sodium ions and
chloride ions in the tissue fluid of the medulla
– Why?
• Allows water to be reabsorbed from the contents of the
nephron as they pass through collecting duct
– Survival advantage
• Very concentrated urine can be produced
• Conserves water and prevents dehydration
The loop of
Henlé
• Descending limb
– Water-permeable
• Ascending limb
– more permeable
to salts/less
permeable to
water.
The loop of Henlé
1. Upper part of ascending limb
• Active transport of sodium and chloride
ions out of the nephron
• Increases water potential of fluid inside
nephron
• Decreases water potential outside it
2. Descending limb
• Water moves down a water potential
gradient from nephron and into
surrounding tissues
The loop of Henlé
3. Lower part of ascending limb
•
•
•
Fluid is very concentrated
Sodium and chloride ions diffuse out into the
surrounding tissues
Counter-current system
–
–
Solute concentration at any part of the
ascending tubule is lower than that in the
descending limb.
Causes a build up of salt concentration in the
surrounding tissues
Distal convoluted tubule and
collecting duct
• DCT
– Sodium ions actively pumped out of
nephron into blood.
• 2nd part DCT
– acts as collecting duct
– regulating how much water passes into
the medulla (and urine concentration).
Reabsorption of water from the
collecting duct
Reabsorption of water in the
collecting duct
• Tissue fluid deep in the medulla has a
very low water potential
• Water moves down a water potential
gradient and enters the blood by
osmosis
Conservation of water
• The longer the loop of Henlé
• The lower the water potential can be
built up
• Greater the water potential gradient
• More water reabsorbed
• Smaller volume of concentrated urine
produced
Osmoregulation
• Learning outcomes
– Explain, using water potential terminology,
the control of the water content of the
blood, with reference to the roles of the
kidney, osmoreceptors in the
hypothalamus and the posterior pituitary
gland
Osmoregulation
• Osmoregulation is the control of water
content of the body
• It is an example of negative feedback
• Negative feedback
–
–
–
–
–
stimulus
Receptor
Message
Effector
response
Osmoregulation
• Osmoreceptors in the hypothalamus
are sensitive to the water potential of
the blood
• Cell bodies of neurosecretory produce
ADH (anti-diuretic hormone)
• ADH passes along the axon which
terminates in the posterior pituitary
gland
• ADH released into the blood
ADH
• Synthesised in the hypothalamus
• Secreted from the pituitary gland
• Target cells
– cells lining the collecting duct
– ADH attaches to cell surface membrane
– Vesicles containing aquaporins move to
the plasma membrane
– Forming channels that allow water
molecules to pass through
Learning Outcomes
• Describe how urine samples can be
used to test for pregnancy and detect
misuse of anabolic steroids.
Urine Sampling
• Urine contains many waste products of
metabolism
• This can be tested and used to
diagnose illness
• Examples
– Early diagnosis of pregnancy
– Evidence for the misuse of drugs
Pregnancy testing
• Most pregnancy test kits use
monoclonal antibodies to test for the
presence of HCG (human chorionic
gonadotrophin) in the urine.
• The antibodies bind with HCG.
How a dipstick works
• HCG-specific
antibodies are coated
with gold atoms
• Anti-body gold
complexes coat the
end of the dipstick
• Further up the dipstick
in the Patient Test Result
region are monoclonal
anti-bodies which bind
to the HCG-antibodygold complex.
How the dipstick works
Anabolic steroids
• Anabolic steroids
stimulate anabolic
reactions in the body
– Large molecules are
built up from smaller
ones
• Steroids are
– Derived from cholesterol
– Lipid soluble
– Increase protein
synthesis inside the cell
Anabolic Steroids
• Urine testing
– Gas chromatography to create a
chromatogram
– Test for presence of nandrolone
– Although difficult to ascertain what is an
abnormal level
Learning Outcomes
• Outline the problems that arise from
kidney failure and discuss the use of
renal dialysis and transplants for the
treatment of kidney failure.
Kidney failure
• Renal dialysis
– Haemodialysis
– Peritoneal dialysis
• Kidney transplant
– Dues to a shortage of donors scientists are
studying the possibility of a
xenotransplant.
Haemodialysis
Haemodialysis
• Blood from the patient’s vein is passed
through very small tubes made from a
partially permeable membrane.
• On the outside of the membrane,
dialysis fluid flows in the opposite
direction.
• The fluid has the water potential and
concentration of ions and glucose that
the patient’s blood should have.
Peritoneal dialysis
• Peritoneum is the layer of tissue that
lines the abdominal cavity.
• A catheter is inserted into the
peritoneum cavity
• Dialysis fluid is passed in and left there
• Fluid is drained off.