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 break down 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 “pushed” into the glomerulus
within Bowman’s capsule. Blood continues in
circulation through the efferent arteriole.
The good and bad stuff transported into the
glomerulus 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 (everything
except large plasma proteins, blood cells, and
platelets); this occurs at relatively high pressure in the
glomerulus and Bowman’s capsule.
2. Reabsorption – The good stuff we need to keep 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!
Luckily, only 1ml of every 120ml/minute 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.
Interesting Information
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A dialysis machine does the same job as the kidneys
and nephrons except that it can take up to 4 hours a
day, 3 times a week to do so. This is a time consuming
and emotionally draining activity for people with
kidney disease/failure.
There are two types of dialysis: hemodialysis and
peritoneal dialysis.
Peritoneal dialysis is appropriate if the patient is
otherwise relatively healthy. It requires only a simple
pumping device, and it is possible to do once a day,
generally overnight.
FIN