OCR A2 Biology F214 - Excretion

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Excretion Revision notes
Excretion ­ The removal of metabolic waste from the body, such as excess, un-useful or toxic products.
Hepatic Vein ­ Takes away deoxygenated blood from the liver to the inferior vena cava.
Hepatic Artery ­ Brings oxygenated blood into the liver.
Hepatic Portal Vein ­ Brings nutrients from the small intestine.
Osmoregulation ­ The control of water levels and salts in the body which must be kept within strict limits.
Respiratory Acidosis ­ A condition causes by a large change in the blood pH.
Hepatocytes ­ Liver cells that radiate out from the centre of the lobule. They have microvilli and are responsible for
protein synthesis, carbohydrate storage, cholesterol synthesis and detoxification.
Sinusoids ­ Channels between hepatocytes that carry blood to the central vein. These are lined with macrophages to
destroy bacteria arriving from the intestine. These cells are sometimes known as Kupffer cells and are responsible for
the formation of bilirubin from haemoglobin breakdown.
Bile Canaliculi ­ Channels along which bile is produced by the hepatocytes before being moved in to the bile duct.
Deamination ­ Removing the potentially toxic amino group from the amino acid, forming keto acid (used in respiration)
and ammonia.
Ornithine Cycle ­ Converting the very soluble & toxic ammonia into less soluble & toxic Urea.
Ultrafiltration ­ Fluid from the glomerulus is forced into the Bowman's capsule by the hydrostatic pressure.
Podocytes ­ Gaps in the Bowman's capsule which allows fluid to pass through them easily.
ADH ­ The hormone made in the hypothalamus and released in the pituitary gland that acts on the collecting duct in the
kidneys to increase absorption of water into the blood.
Carbon Dioxide
Excess CO2 is toxic; it can cause respiratory acidosis as the CO2 combines with haemoglobin to form
carbaminohaemoglobin, which has a lower affinity for oxygen. (H+ concentration increases, pH decreases) symptoms
of Respiratory Acidosis include difficulty breathing, headache, drowsiness, confusion and rapid heart rate.
CO2 dissolves in the blood plasma. Once dissolved it can combine with water to produce carbonic acid:
CO2 + H2O H2CO3
The Carbonic acid dissociates to release hydrogen ions:
H2CO3 H+ + HCO3-
The hydrogen ions lower the pH and make the blood more acidic. Proteins in the blood act as buffers to resist the change
in pH. If the change is small then extra hydrogen ions are detected by the respiratory centre in the medulla oblongata of
the brain. This causes an increase in breathing rate to help remove the excess carbon dioxide. If pH drops lower than
7.35, then respiratory acidosis occurs.
Nitrogenous Compounds
The body cannot store proteins or amino acids. However, amino acids contain almost as much energy as carbohydrates.
Therefore it would be wasteful to simply excrete excess amino acids. Instead they are transported to the liver and the
potentially toxic amino group is removed (deamination). The amino group initially forms the very soluble and highly
toxic compound, ammonia. This is converted to a less soluble and less toxic compound called urea, which can be
transported to the kidneys for excretion. The remaining Keto acid can be used directly in respiration to release its energy
or it may be converted to a carbohydrate or fat for storage.
Liver structure

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The liver is the only organ with a second blood supply ­ the hepatic portal vein carries absorbed products of
digestion from the gut.
Oxygenated blood from the heart travels from the Aorta via the Hepatic Artery into the liver. This supplies the
oxygen that is essential for aerobic respiration. The liver cells are very active as they carry out many metabolic
processes. Many of these require ATP, so it is important that there is a good supply of oxygen.…read more

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Amino acid + oxygen Keto acid + Urea
Ornithine cycle ­ ammonia is combined with CO2 to form urea
The ammonia is very soluble and highly toxic. It must be converted to a less toxic form very quickly. The ammonia is
combined with carbon dioxide to produce urea. This occurs in the Ornithine cycle. Urea is less soluble and less toxic than
ammonia. It can be passed back into the blood and transported around the body to the kidneys.…read more

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Fatty deposits in hepatocyte cells
Normal structure of lobules is lost
Scar/fibrous tissue is laid down
Nodules form
Liver becomes hard and shrunken
Disruption to blood flow ­ blood bypasses sinusoids
Loss of liver function e.g. deamination doesn't take place, ammonia will accumulate.
Kidney structure
The cortex is the outer region of the kidney, the medulla the inner region and the pelvis is in the centre which leads
into the ureter.
The nephron is the functional unit.…read more

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Collecting Duct
Composition of fluid is changed by Selective re-absorption
In the PCT the fluid is altered by the reabsorption of all the sugars, most salts and some water.
In total about 85% of fluid is reabsorbed here.
In the descending limb of the loop of Henle, the water potential of the fluid is decreased by the addition of salts
and the removal of water.
In the ascending limb the water potential is increased as salts are removed by active transport.…read more

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Amino Acids
Inorganic ions (sodium, chloride and potassium).
In the capillary, blood cells and protein are left. The presence of proteins means that the blood has very low (negative)
water potential. This low water potential ensures that some of the fluid is retained in the blood containing water and
some of the substances above. This means that it is easier to reabsorb the water at a later stage.…read more

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The arrangement of the Loop of Henle is known as a hairpin countercurrent multiplier system. The overall effect of this
arrangement is to increase the efficiency of salt transfer from the ascending limb to the descending limb. This causes a
build-up of salt concentration in the surrounding tissue fluid.
The removal of ions from the ascending limb means that at the top of the ascending limb the urine is dilute. Water may
then be reabsorbed from the urine in the distal tubules and collecting duct.…read more

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Kidney failure
Kidney failure is the inability to (i) remove excess water urea and salts and (ii) regulate water and salt levels.
The most common causes are:
diabetes mellitus
Injury to the back
It can be treated by dialysis ­ the most common treatment for kidney failure. It removes wastes, excess fluid and salt
from the blood by passing the blood over a dialysis membrane.…read more

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As a result all the antibodies carrying a blue bead and attached to hCG are held in one place forming a blue
line. There is always one control blue line to use for comparison; a second blue line indicates pregnancy.
Testing for anabolic steroids
Anabolic steroids increase protein synthesis within cells. This results in the build-up of cells tissue, especially in the
muscles. Non-medical uses for anabolic steroids are controversial because they can give advantage in competitive sports
and they have dangerous side effects.…read more


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