The Importance of
Excreting Wastes
Why Pee?
• The body runs chemical reactions
necessary for life. The products of these
reactions tend to be useful to the body but
some are not – like carbon dioxide in
cellular respiration.
• These byproducts that are not used must
be removed from the cells/body through
excretion.
Protein Problems
• Proteins are the working molecules and they are
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an absolute necessity for the body.
When proteins are broken down the amine
group (NH2) is removed in a process called
deamination. These must be excreted as they
will form ammonia which is toxic to the body.
The ammonia is combined with carbon dioxide
by the liver to form urea.
Nucleic acids breakdown to give a similar
product – uric acid.
What Else is Excreted?
• See Table 1 on page 342. This table shows
what gets excreted and who gets rid of it.
• Ammonia, Urea and Uric Acid  Kidneys
• Carbon dioxide  Lungs
• Lactic Acid  Liver
• Solid Waste (Feces)  Large Intestine
The Urinary System
How to Get Urine Out
• Cells dump their waste into the blood – kind of
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like a sewer system. The blood travels through
the renal arteries to the kidneys. (Renal =
Kidneys)
The kidneys are the major filtration organ of the
body. They filter the blood and remove waste
products.
The kidneys have three parts:
– The renal cortex – the outer layer of the kidney.
– The renal medulla – the middle area of the kidney
which houses the nephrons.
– The renal pelvis – the collecting area for urine.
Nephrons
• The nephrons are the actual filtration units of
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the kidneys. They are microscopic and each
kidney has over a million of them.
The nephrons are housed in the renal medulla.
Nephrons are composed of several parts –
glomerulus, Bowman’s capsule, proximal tubule,
loop of Henle, distal tubule and the collecting
duct.
What Happens in the Nephron?
• Blood with waste in it is carried by the afferent
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arteriole to the glomerulus.
Dissolved matter – both useful and waste
products are sent from the glomerulus to the
Bowman’s capsule. Blood stays in circulation.
The good and bad stuff absorbed by the
Bowman’s capsule is separated as it travels
through the proximal tubule and the loop of
Henle – the good stuff is reabsorbed into the
blood while the bad stuff stays in the nephron.
After the Nephron?
• The bad stuff leaves the nephron through the distal
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tubule and enters the collecting duct.
The collecting duct leads to the renal pelvis where it
collects and is ready to leave the kidney.
Ureters extend down from the kidney to the urinary
bladder and the urine travels down these tubes.
Urine collects in the urinary bladder that has cells that
are capable of stretching. When stretched far enough a
nervous impulse is sent to the brain that says “ya gotta
go!”.
Urine exits the urinary bladder and the body through a
tube called the urethra.
Formation of Urine
The Process of Peeing
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Urination is the result of three steps or
processes…Filtration, Reabsorption & Secretion.
1. Filtration – Stuff is removed from blood to see what
we will get rid of – it is the glomerulus and Bowman’s
capsule part.
2. Reabsorption – The good stuff we need is brought
back into the blood from the nephron and into the
capillaries.
3. Secretion – Taking the bad stuff from the nephron,
mixing it with water and expelling it from the body.
Interesting Information
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Once you get to 600ml – you go – like it or not!
Only 1ml of every 120ml that is filtered by nephron is
turned into urine. The other 119ml is reabsorbed into
the blood.
The body will reabsorb all the good stuff until it hits a
threshold level for that substance – after that, good or
not, it is going to go out in the urine.
A dialysis machine does the same job as the kidneys
and nephrons only it takes several hours a day to do
so. This is a time consuming, and emotionally draining,
daily activity for people with kidney disease/failure
FIN