15.4 Excretion, Homeostasis and the Liver

What is the definition of excretion?

The removal of waste products from metabolic reactions

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What is the consequence of waste products accumulating in the body?

Waste products become toxic

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What are the three waste products produced in metabolic reactions?

Carbon dioxide, bile pigments (bilirubin) and nitrogenous waste products (e.g. urea)

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Carbon dioxide

One of the waste products of cellular respiration which is excreted from the lungs

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Bile pigments (bilirubin)

Formed from the breakdown of haemoglobin from old red blood cells in the liver. They are excreted in the bile from the liver into the small intestine via gall bladder and bile duct (they colour faeces)

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Nitrogenous waste products (urea)

Formed from the breakdown of excess amino acids by the liver. All mammals produce urea as their nitrogenous waste. Fish produce ammonia while birds and insects produce uric acid. Urea is excreted by the kidneys in the urine

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Features of the Liver

One of the major body organs involved in homeostasis, reddish-brown organ, makes up 5% of the total body mass, largest internal organ in the body, located just below the diaphragm and made up of several lobes (each lobe = 100,000 hexagonal lobules)

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Features of the Liver (2)

Very fast growing and damaged areas are generally regenerate very quickly and has a very rich blood supply (about 1 dm3 of blood flows through it every minute)

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Features of the Liver (3)

oxygenated blood is supplied to the liver via the hepatic artery and removed from the liver via hepatic vein, the liver is supplied with blood by a second vessel (hepatic portal vein)

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What is the definition of the hepatic portal vein?

A blood vessel which carries blood loaded with products of digestin straight from the intestines to the liver (starting point of many metabolic activities of the liver). Up to 75% of the blood flowing through liver comes via the hepatic portal vein

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What are hepatocytes?

Liver cells which have large nuclei, a prominent Golgi apparatus and lots of mitochrondria (they are metabolically active cells). They divide and replicate (even if 65% of the liver is lost, it will regenerate in a matter of months)

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What happens at the sinusoids?

Blood from the hepatic artery and the hepatic portal vein mix (sinusoids are also surrounded by hepatocytes)

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What type of cells are within sinusoids (apart from hepatocytes)?

Kupffer cells: they act as resident macrophages of the liver, ingesting foreign particles and helping to protect against disease

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What is secreted from hepatocytes?

Bile is secreted from the breakdown of the blood into canaliculi and from these the bile drains into the bile ductules which take it to the gall bladder

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Functions of the Liver

Around 500 different metabolic pathways are linked to the liver (many play a role in homeostasis)

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Carbohydrate Metabolism (high levels of blood glucose)

Hepatocytes control blood glucose levels by their interaction with insulin and glucagon. When blood glucose levels rise, insulin levels rise and stimulate hepatocytes to convert glucose into glycogen (100g is stored in the liver)

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Carbohydrate Metabolism (low levels of blood glucose)

When blood glucose levels start to fall, hepatocytes convert glycogen back into glucose under the influence of the hormone glucagon

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Liver's involvement in deamination

Liver plays a vital role in protein metabolism where hepatocytes synthesise most of the plasma proteins

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What is transamination?

Conversion of one amino acid into another (this is important becuse the diet does not alway contain the required balance of amino acids but transamination can overcome the problems this might cause)

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What is deamination?

The removal of an amino group from a molecule

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Other features of deamination

Any excess ingested protein would be excreted and wasted if it were not for hepatocytes (they remove the amino group and convert amino acids into ammonia (toxic) and then to urea

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Features of urea

Toxic in high concentrations but not in concentrations normally found in the blood, urea is excreted via kidneys and the remained of the amino acid can be fed into cellular respiration or converted into lipids for storage

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Deamination of excess amino acids

Ammonia prodcued in deamination of proteins is converted into urea in a set of enzyme-controlled reactions (ornithin cycle). Removing the amino group from amino acids and converting it to ammonia is less toxic and more manageable

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Detoxification

Apart from urea, many other metabolic pathways produce potentially poisonous substances. The liver is the site of detoxification of substances e.g. drugs, alcohol, breakdown of hydrogen peroxide

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Hepatocytes contain which enzymes?

Hepatocytes contain the enzyme alcohol dehydrogenase (ethanol to ethanal). Ethanal can be converted to ethanoate (used to build up fatty acids/used in respiration). Hepatocytes also contain catalase to break down hydrogen peroxide

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15.4 Excretion, Homeostasis and the Liver

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