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1.2.1 Excretion
· The removal of metabolic waste (by-products and unwanted substances) from the body.
· Carbon dioxide (produced from respiration + then breathed out) and Nitrogen-containing compounds (produced from
excess amino acids + broken down by deamination + removed in the urine) are excreted in large amounts.
Why Must These Substances Be Removed?
· Excess carbon dioxide is toxic:
­ CO2 is transported as hydrogencarbonate ions which compete with oxygen for space on the haemoglobin. This reduced the oxygen transport.
­ CO2 + haemoglobin = carbaminohaemoglobin ­ lower affinity for oxygen than haemoglobin.
­ Excess CO2 can cause respiratory acidosis:
· CO2 dissolved in blood plasma to produce carbonic acid.
· Carbonic acid dissociates to release hydrogen ions.
· This makes blood more acidic.
· Causes slow/difficult breathing, headaches, confusion, rapid heart rate + changes in blood pressure.
· The body cannot store excess proteins/amino acids ­ wasteful to excrete amino acids (contain a lot of energy).
· They are transported to the liver and amino group removed (deamination) to produce ammonia.
· Ammonia is converted to urea + excreted.
· Remaining keto acid can be used in respiration or stored as a carbohydrate or fat.…read more

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1.2.2 The Liver
Structure of The Liver
· Liver cells = Hepatocytes - carry out lots of metabolic processes (needs a good supply of blood).
· Liver is supplied with blood from 2 sources:
­ OXYGENATED blood from the HEART = aorta -> hepatic artery
­ DEOXYGENATED blood from the digestive system = enters via the hepatic portal vein.
· Blood leaves the liver via the hepatic vein -> vena cava -> normal circulation.
· Bile duct carries bile from liver to gall bladder + stored until needed to help digestion of fats in small intestines.
· Kupffer cells are specialised macrophages. They
breakdown and recycle old red blood cells
­ this process produces bilirubin which is excreted
as bile + in faeces.…read more

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1.2.3 Functions of The Liver
· The liver carries out a wide range of functions, including:
­ CONTROL OF: blood glucose levels, amino acid levels, lipid levels.
­ SYNTHESIS OF: red blood cells, bile, plasma proteins, cholesterol.
­ STORAGE OF: vitamins, iron, glycogen.
­ DETOXIFICATION OF: alcohol, drugs.
­ BREAKDOWN OF: hormones.
­ DESTRUCTION OF: red blood cells.
Formation of Urea
· Urea is an excretory product formed from the breakdown of excess amino acids.
· Deamination produces ammonia (very soluble and toxic - cannot accumulate) and a keto acid (enter respiration and release
· Ammonia has to be converted to a less toxic form very quickly.
· It is combined with CO2 to produce urea ­ the ornithine cycle.
· Urea can be passed into the blood to travel to the kidneys, filtered out of the blood and made into urine, released from the
· Ornithine Cycle:
­ ammonia + carbon dioxide -> urea + water
· Detoxification is the conversion of toxic molecules to less/non toxic molecules.
· Liver cells contain many enzymes that detoxify molecules such as catalase (converts hydrogen peroxide to oxygen + water).
· Alcohol is broken down in liver cells by ethanol dehydrogenase to ethanal.
· Ethanal is broken down by ethanal dehydrogenase to ethanoate (acetate).
· Ethanoate + coenzyme A -> acetyl coenzyme A (enters respiration).
· The hydrogen atoms released in this process combine to NAD = reduced NAD.
· NAD used to oxidise + break down fatty acids. If this does not happen, they are stored in the liver as lipids = fatty liver.…read more

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1.2.4 The Kidney
· The role of the kidney is to remove waste products from the blood and to produce urine.
The Nephron
Medulla · About 1 million tiny tubules (nephrons) in each kidney.
How Does the Composition of the Fluid Change?
· The fluid is altered by the reabsorption of sugars, salts and water in the proximal convoluted tubule ­ about 85% of fluid is
reabsorbed here.
· Water potential of fluid changes in the ascending and descending loop of Henle.
· Water potential decreased in collecting duct due to removal of water.…read more

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1.2.5 Formation of Urine
· Blood flow: afferent arteriole -> glomerulus -> Bowman's capsule.
· The barrier between the glomerulus and the Bowman's capsule lumen consists of three layers:
­ ENDOTHELIUM OF CAPILLARIES = narrow gaps between cells that blood plasma can pass through.
­ BASEMENT MEMBRANE = fine mesh of fibres which filter the blood. Only small molecules can pass through.
­ EPITHELIAL CELLS (PODOCYTES) = podocytes have many finger-like projections which ensure there are no gaps between the cells.
· Blood plasma containing WATER, AMINO ACIDS, GLUCOSE, UREA + INORGANIC IONS is put under pressure from the capillary
-> Bowman's Capsule.
· Blood cells and proteins are left in the capillary.
Selective Reabsorption
· As fluid moves along the nephron, substances are removed from the fluid + reabsorbed into the blood (mostly in the
proximal convoluted tubule).
· GLUCOSE, AMINO ACIDS, SALTS and WATER are reabsorbed.
· Sodium ions actively transported out of cells into tissue fluid.
· The cells lining the proximal convoluted tubule are specialised to achieve this reabsorption:
­ Membrane is folded to increase surface area for reabsorption to occur.
­ Membrane contains co-transporter proteins (facilitated diffusion causes sodium ions, glucose + amino acids to enter cells).
­ Membrane contains sodium-potassium pumps (remove sodium ions from the cell cytoplasm).
­ Cell cytoplasm has many mitochondria.
­ Water enters the cells and is then reabsorbed into the blood by osmosis.…read more

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