PP 3
Excretion in Humans
Define excretion

the removal from organisms of toxic materials, the
waste products of metabolism (chemical reactions in
cells including respiration) and substances (carbon
dioxide, urea and salts) in excess of requirements.
the function of the kidney:
removal of urea /excess water / the reabsorption of glucose
and some salts
Urea:
- Made in liver (from excess of proteins / a.a)
- A. a. are absorbed into blood and go via the hepatic portal
vein to the liver
- AA in liver:
1.
Pass through / pass into cells and cells use it to make
proteins
2.
Liver itself uses it (makes fibrinogen)
3.
Excess is broken down by the liver – changed to carbs or
fats for storage
•
This requires the N part of a.a. to be removed
*** Alcohol / drugs and hormones are also broken down in
liver
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-
-
Deamination – removal of Nitrogen from a.a.
- N containing part made into urea (nitrogen
excretory product)
Urea is carried away from liver in blood to kidneys
Kidney’s filtering system:
Plasma is filtered into kidney
Useful substance (glucose / ions (NaCl) are taken
back to blood (reabsorption)
Urea kept in kidney
Urine – urea in water.
 Too much water in body is excess – also removed in
urine
 If there is not enough water in body – only little
water will flow out as urine and the majority of it
will be taken back into blood
 Urine: water + salts + urea
- urea, uric acid, ammonia, hormones, dead blood
cells, proteins, salts and minerals, and toxins

Urine flows through here into
Bladder
Urine is released here
After it is left the bladder
Inside part
Outside part
Both contain tiny tube =
kidney tubules (start in
cortex as renal capsule
Kidney
Structure
Function
Renal capsule
•Blood is delivered here and flows through a capillaries network
•blood plasma is squeezed through wall of capillary wall of capsule
into the cavity inside the kidney tubule
• Blood plasma passes through filter
• It does not pass through: cells / big proteins
•Molecules that pass through: water, urea, glucose, ions : sodium &
chloride
Renal Tubule
• As fluid flows through tubule - anything useful is reabsorbed into
the blood (not lost from body)
•Water is removed from the tubule
Ureter
• collects excess water / salts / urea = urin
1. Blood vessels bring
blood into tubule
2. In the
renal
capsule
plasma
filtered
into
tubule
3. 1st part of tubule where
Glucose / water / salts are
Reabsorbed
5. Last
Portion
Where
Water
Can be
Reabsorbed if
Body need
4.
Blood
vessels
take
blood
away
from
tubules
Kidney Dialysis
Kidney stops working (infection / trauma / genetics)
 Kidney no longer can remove urea / excess water
 Feeling ill ->death
 2 options : Kidney Transplant or if possible Kidney
Dialysis
 Dialysis: filtering patients blood (like the kidney does)
using a Kidney Dialysis Machine by passing blood
through it.
- The process of diffusion of different molecules each
according to its own concentration gradient though a
partially permeable membrane

How dialysis works
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blood passes through tiny channels in dialysis
tubing in machine
Dialysis tubing allows only certain molecules though
because of the size of holes (very small – molecule
size) that the tubing is made of.
Smalls molecules can pass though – water /
urea/glucose can pass through
Large molecules like proteins can not

-
-
Dialysis fluid – the liquid outside the dialysis tubing
This made to have similar composition of healthy
human blood plasma (water / glucose / salts – but
no urea)
It is sterile (no germs)
Dialysis fluid flows in 1 direction in the machine
Patients blood passes in the opposite direction
Fluid and blood will diffuse down its concentration
gradient
Therefore:
 - No urea in dialysis fluid - urea in patients blood will
diffuse out of the patient and into the dialysis fluid
 If more water in blood than in dialysis fluid then water will
diffuse out of blood of patient
 If the opposite is true – then water will flow in from the
dialysis tubing into the patients blood (same for salt and
glucose)
 Problems:
- This needs to be done several times a week for several hours
- Can make people feel sick / must be careful of what they
eat /
 Positive:
- cheaper than transplant

How Dialysis works
Kidney transplant
Kidney taken from a donor and placed in a
recipient
 Problems:
- not enough kidneys for number of donors
- Must come from healthy donor
- Cells or donor kidney must be similar to recipient’s
- Immunosuppressant drugs are still needed
Positive: - if transplant is successful
– then kidney function is restored
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