Excretion and the Kidneys

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Excretion and the Kidneys
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Excretion

• Excretion is the removal of metabolic waste from
  the cells. Metabolic waste is the unwanted
  material that is formed as a result of the bodies
  metabolism. That is the large number of chemical
  reactions that occur in the cells, tissues and
  organs. The waste products of metabolism are
  frequently toxic and so must be removed from the
  body.

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Deamination and Urea Synthesis

• In the liver deamination is the process which
  breaks down excess amino acids in to ammonia and
  keto acids. Ammonia is still very toxic so it is
  converted to urea by the process called urea
  synthesis. Urea is less toxic than ammonia and
  so can travel in the blood, but it must be got
  rid of quickly because it can still have ill
  effects. Urea is then transported by the blood
  from the liver to the Kidneys where it forms part
  of urine.

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The Kidney

• The Kidney has two main functions, it removes
  metabolic waste from the body through the process
  of excretion and it regulates the water and ion
  content in the blood. The excretion is of a
  dilute solution called urine which contains urea,
  mineral ions, water and other foreign chemicals
  from the blood. The two kidneys have a very
  extensive blood supply and the whole blood supply
  passes through the kidneys every five minutes
  ensuring that the waste materials dont build up.
  The renal artery carries blood to the kidneys
  and the renal vein carries blood away from the
  kidneys. The most important part of the Kidneys
  is the nephron.

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The Nephron

• The Nephron is a the kidney tubule and there are
  thousands of nephrons in each kidney. Within the
  nephron there are
• Renal Capsule, proximal convoluted tubule, loop
  of Henle, distal convoluted tubule and the
  collecting duct. Each part of the nephron has a
  different role to play in filtering the blood to
  rid it of toxins and in the overall kidney waste
  and water control. The stages are
  ultrafiltration, selective reabsorption,
  production of an iron gradient in the medulla and
  adjustment of the water and ion gradients.

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Ultrafiltration
Ultrafiltration occurs in the renal or bowman's
capsule. The renal artery which brings blood to
the kidney is split up into numerous arterioles,
each feeding a nephron. The arterioles split
into numerous capillaries which form a knot
called a glomerulus. This is surrounded by the
bowman's capsule. The arteriole leading into the
glomerulus is wider than the one leading out so
there is a high blood pressure in the capillaries
of the glomerulus. This pressure forces plasma
out of the blood by ultrafiltration. Once the
blood is filtered in this way only blood cells
and the large plasma proteins remain in the blood
and continue in to the proximal convoluted tubule.

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Selective Reabsorption

• Selective reabsorption occurs in the proximal
  convoluted tubule. This is the longest and
  widest part of the nephron. In this part over
  80 of the filtrate is reabsorbed into the blood.
  Ensuring all useful material is returned to the
  blood. All glucose, amino acids and 85 of
  mineral ions are reabsorbed by active transport.
  Small proteins are reabsorbed. 80 of water is
  absorbed back into the blood by osmosis. The
  proximal convulated tubule cells have many
  mitochondria to provide ATP for active transport
  and mirovilli to increase surface area for
  absorption.

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Loop of Henle

• The loop of henle creates the conditions for the
  final reabsorption of water in the collecting
  duct by creating an ion gradient. In the loop of
  henle salt is added to the filtrate in the
  descending limb. The cells surrounding are
  impermeable to ions. Due to water moving from a
  less negative to a more negative water potential,
  water comes into the filtrate. In the ascending
  limb the walls are impermeable to water so the
  water remains and the ions leave creating an ion
  gradient in the medulla.