The kidneys remove waste from the blood through three processes: ultrafiltration, selective reabsorption, and secretion. During ultrafiltration, small molecules are filtered from the blood into the nephron. Most of these filtered molecules, along with water, are reabsorbed back into the bloodstream. Additional molecules are secreted from the blood into the nephron. The kidneys regulate water balance and remove waste such as urea, maintaining homeostasis. When glucose or proteins appear in the urine, it can indicate issues with the pancreas or kidneys, respectively.

1. EXCRETION
UNIT 1

2. INTRODUCTION TO EXCRETION
• Excretion is the removal of the waste products of metabolism.
• Excretion is necessary to prevent the build –up of toxic waste and also because
of the constraints of space for storage
• Plants have no need for an excretory system as most of their waste products are
gaseous and they diffuse though stomata, also because of their autotrophic way
of feeding, they have less nitrogenous waste which are stored in various organs
until they lose the organ
• In animals , the main excretory product is nitrogenous waste which is toxic
• In animals, the elimination of excretory products is tied to the maintenance of
the correct balance between water and solutes in the body, thus excretion and
the homeostatic process of osmoregulation are bound together

3. SOME EXCRETORY PRODUCTS
PRODUCT SOURCE
oxygen Photosynthesis in autotrophs
Carbon dioxide Aerobic respiration and alcohol
fermentation
water Aerobic respiration and condensation
ions Nutrient metabolism
Bile salts Lipid metabolism in the liver
Bile pigments Breakdown of haemoglobin in the liver
Tannins and other organic
acids
Protein metabolism in plants
Urea, ammonia and uric
acid
Protein and nucleic acid metabolism in
animals

4. FORMATION OF UREA IN THE LIVER

5. THE URINARY SYSTEM

6. STRUCTURE OF A NEPHRON
Vasa Recta

7. FUNCTIONS OF THE KIDNEYS
Excretion -the kidneys are the main organs of excretion – they carry out three
separate but related processes:
• Ultra filtration –filtration under high pressure and at the molecular level- this takes
place as substances are forced out of the glomerulus and into the Bowman’s
Capsule
• Selective Reabsorption – desirable ions and nutrients and most of the water are
recaptured from the filtrate and taken back into blood
• Secretion – as the filtrate travels along the nephron, additional substances are added
to the filtrate from blood.
Osmoregulation – the kidneys are largely responsible for maintaining the correct
balance of water and solutes by ensuring most of the water from the filtrate is
absorbed and making adjustments based on the neds of the body fluids

8. MICROGRAPH OF KIDNEY - CORTEX

9. MICROGRAPH – KIDNEY MEDULLA

10. ULTRAFILTRATI
ON
A high pressure is created by:
• The afferent arteriole is wider than the
efferent
• The glomerulus is made of a tangle of
capillaries
• The capillaries themselves are very
narrow
The high pressure forces most of the
plasma into the Bowman’s Capsule but
blood cells, platelets and large plasma
proteins are too large to pass through.

11. THE FILTRATION MEMBRANE
During ultra-filtration, substances
squeeze through filtration pores in
the capillary walls and then pass
through a continuous structure
called the basement membrane,
which is made of collagen and
glycoproteins. The basement
membrane acts as the fine filter
and keeps out molecules above
69,000 RMM.
Some cells of the Bowman’s
capsule have foot-like extensions
and are called podocytes – these
act as the coarse filter

12. STRUCTURE OF THE PCT
The PCT has the following adaptations for
reabsorption:
• The walls are thick as there are a lot of
metabolic activities
• Microvilli forming a brush border on the
inner side which projects into the lumen
the lumen –this increases surface area,
creating more space for transporting
proteins to move substances from the
filtrate into the cells
• In-foldings on the outer wall also to
increase surface area for substances to
move from the cells back into blood
• Numerous mitochondria to operate
pumps
• Vesicles for endocytosis
• Tight junctions so substances must pass
through the membrane, which exercises

13. SELECTIVE REABSORPTION IN PCT
Absorption from lumen into cells:
• Glucose, amino acids, some ions, vitamins and hormones enter by co-transport
(symport) because of Na+ pumps which pump Na+ ions into the lumen, allowing those
substances to move along the gradient created.
• Water moves by osmosis as the pumping out of NA+ ions lowers the water potential in
the filtrate
• Fat soluble substances diffuse through the membrane, as they are non-polar
• Some urea also diffuse along their concentration gradient as they are relatively small
molecules
• Small plasma proteins enter by pinocytosis
Absorption into capillaries from cells:
On the outer membrane facing the capillaries, are Na+/K+ pumps , which pump out 3 Na+
ions and pump in 2K+ ions. This creates an electrochemical gradient, along which
substances move by facilitated diffusion into interstitial space and then into the capillaries

14. SECRETION
The arrows going into the tubule
indicate substances that are
secreted out of blood and into
the tubules, from which they will
be excreted. These include
drugs, toxins, ammonium,
potassium and hydrogen ions, so
the purpose of secretion is to:
• Maintain a pH balance
• Get rid of toxins
• Get rid of substances that
were not filtered
Movement can be passive or
passive e.g. ammonia diffuses
while H+ K+ , creatinine ,
histamine and penicillin are
actively transported

15. LOOP OF HENLE AND THE COUNTER-CURRENT
MULTIPLIER
The loop of Henle concentrates urine by bringing about a high salt concentration in the
medulla, which causes a vigorous osmotic outflow of water from the DCT and collecting ducts.
To do this, the Loop employs the counter-current mechanism, in which salts are actively
pumped out of the ascending limb , lowering the salt concentration relative to the descending
limb.
The overall effect is multiplied by the length of the loop, hence animals that need to conserve a
lot of water have a very long Loop of Henle and a deep medulla in their kidneys. The longer the
Loop of Henle, the greater the counter-current multiplier effect.
The capillaries in the medulla which make up the vasa recta, have the same loop shape and
therefore also operate on a counter current system, ensuring that the high concentration of salt
in the medulla is not dissipated.

16. URINE CONCENTRATION IN THE NEPHRON
Glomerular filtrate enters the tubule at the
same osmolarity of blood (300mOsm/L)
The following take place:
• Na+ ions pumped out of ascending limb,
Cl- follow
• Water moves out of descending limb by
osmosis
• Water carried away by vasa recta
• Filtrate becomes more concentrated as it
descends
• Most concentrated at the hairpin bend
• Becomes less concentrated as it ascends
due to the pumping out of ions
• At the top of the ascending limb, the
filtrate is at its most dilute
• Permeability of tubule to water determined
by amount of ADH (determined by
concentration of blood)
• As it travels through the DCT and
collecting ducts through the salty medulla,
water is vigorously drawn out and urine is
Note the thickness of the ascending limb-because
of high metabolic activity, also impermeability to

17. OSMOREGULATION
ADH is actually produced in the
pituitary gland but is stored and
released by the posterior pituitary
gland in response to changes in
the osmotic pressure of blood.

18. COMPONENTS OF BLOOD
Not filtered –
remain in blood
Filtered into
tubule
(RMM <69,000)
Reabsorbed from
tubule back into
blood
Secreted from
blood into tubule
Excreted as urine
Erythrocytes
Leucocytes
Platelets
Plasma proteins
e.g. albumin,
globulin,
fibrinogen,
prothrombin
Water
Salts
Glucose
Amino acids
Urea
Vitamins
Hormones
Small plasma
proteins
Bile salts and
pigments
SOME
• Water
• Salts
• Urea
• Vitamins
• Hormones
• Small proteins
ALL
• Glucose
• Amino acids
NH3
Ca2+
K+
H+
Creatinine
Histamine
penicillin
Water
Urea
Salts
Bile pigments
Bile salts all
secreted
substances

19. TESTING BLOOD GLUCOSE LEVEL
All glucose should be reabsorbed, so
there should be none in urine, however
the presence of glucose in urine is more
likely to be a problem with the pancreas
rather than the kidneys.
Blood glucose level is homeostatically
controlled at about 80mg/100cm3.
If a person has a level that is
significantly higher than this, it is likely
that they are suffering from diabetes
and they are either not producing
enough insulin or are not able to use it
well and it is also likely that there will
be some glucose in their urine, as even
properly functioning kidneys cannot
reabsorb more than a certain amount of

20. PROTEINURIA
Proteins should not filter into nephrons as they are too
large and therefore should never be found in urine.
Proteinuria is therefore a sign of poor kidney functioning
and can range from simple to serious conditions
including:
• Glomerulonephritis – inflammation of the kidneys
• Immune disorders e.g. lupus
• Cancer–multiple myeloma or kidney cancer
• Cardiovascular disease
• Abnormal blood pressure
• Trauma or stress
• Dehydration
• Blood thinners
• Congestive heart failure