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Higher Human Biology
Chapter 23
Removal of materials from the
• Excretion is the elimination of waste
products from metabolism
• Carbon dioxide is a waste product
produced during respiration
• Its removal from the lungs is an example
of excretion.
Role of the lungs
• Cells produce carbon dioxide during aerobic
• It diffuses to blood plasma
• CO2 forms bicarbonate ions
• These ions enter red blood cells in the pulmonary
• Carbonic acid is formed
• An enzyme then breaks down the acid to release
• This then diffuses out of the blood and into the
Diffusion in the alveoli
Role of the liver
Regulates the level of glucose in the blood
De-amination of some proteins
Excess glucose stored as glycogen
Some plasma proteins are synthesised
Liver maintains a stable internal
• Liver provides cells with optimum conditions
Detoxification of toxic materials
Chemical alteration
Chemical breakdown
Chemical attachment
Food preservatives attached to a. acids by liver
Rendered inactive
Converted by liver into Acetyl CoA
Acts as molecular label
Products excreted in bile or through kidney
Acetyl CoA used in respiration
Recognised as waste by kidney and excreted
Detoxification of toxic materials
• Foreign particles for example bacterial
cells are removed by macrophages which
line the liver’s blood vessels
Removal & excretion of bilirubin
• When red blood cells reach
the end of their 120 day life
span, they are destroyed by
macrophage cells in the liver,
bone marrow and spleen
• Haemoglobin is broken by
these cells into a yellow
pigment called bilirubin which
is released into the blood
giving plasma its faint yellow
Removal & excretion of bilirubin
• It is in this conjugated form that bilirubin is added
to bile and becomes bile pigment.
• When bile pigment passes into the small
intestine, bile salts aid digestion by emulsifying
• However, bile pigment (conjugated bilirubin)
does not perform a useful role in digestion. Its
release in bile is a form of excretion.
• In the gut, bilirubin is converted by bacteria to
the brown pigment that gives faeces their
characteristic colour.
• Accumulation of bilirubin
in the bloodstream
• Liver suffers a disease
which prevents its cells
from absorbing bilirubin
• Bile duct becomes
• Excessively high rate of
red blood cells occurs
Production of urea
• De-amination of amino acids in the liver
• Amino acids broken down to form
ammonia and an organic acid
• Organic acid may be pyruvic or one of
Kreb’s cycle intermediates
• It can then enter the respiratory pathway.
• De-amination needs
• It produces an organic
• It produces ammonia
which goes to the
ornithine cycle.
Urea production
• During the conversion of ammonia into urea, two
molecules of ammonia and one molecule of carbon
dioxide combine to form one molecule of urea and one of
water. Assisting this process there is a cyclical
conversion of ornithine into citrulline, arginine, and then
back to ornithine again.
Ornithine cycle
• This shows how
ammonia is converted
to urea
• Ornithine is constantly
Structure of the Nephron
distal convoluted tubule
proximal convoluted tubule
Loop of Henle
Role of the Kidneys
Role of the Kidneys