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What is excretion?

The removal of metabolic waste (by-products and toxic substances produced in excess by cellular processes) from the body.


What are the 2 main metabolic waste products excreted by the body?

- Carbon dioxide.
- Nitrogenous compounds (including urea).


Where is carbon dioxide produced and excreted from the body?

- Carbon dioxide is produced by every living cell as a by-product of respiration.
- Carbon dioxide diffuses out of cells and into the bloodstream where most is transported to the lungs as hydrogencarbonate ions and diffuses into alveoli from which it is removed by expiration.


Where is urea produced and excreted from the body?

- Urea is produced in the liver as a by-product of the deamination of amino acids.
- It dissolves in blood plasma and carried from the liver to the kidneys where a majority is removed through filtration, and leaves the body dissolved in urine.


Why is carbon dioxide toxic to the body?

- Most carbon dioxide is carried in the blood as hydrogencarbonate ions. However, H+ ions are also produced as a result of the dissociation of carbonic acid which competes with oxygen to binds with haemoglobin to form haemoglobinic acid. A larger amount of carbon dioxide in blood severely decreases oxygen affinity and carrying capacity of blood, which may result is some cells suffering from oxygen starvation.
- Carbon dioxide binds directly with haemoglobin to form carbaminohaemoglobin, which also has lower oxygen affinity.
- Respiratory acidosis is a result of too high a concentration of H+ ions in the blood which breaks the buffering capacity of blood. This may result in higher breathing rates in an attempt to remove carbon dioxide; but if ineffective, a range of symptoms including headaches, drowsiness and confusion may result. This is usually caused by conditions affecting the lungs including COPD, pneumonia and blockages to the airways.


Why is an excess of nitrogenous compounds toxic to the body?

- Excess amino acids in the body cannot be stored as the amine group is toxic.
- The amine groups on amino acids are removed in the process of deamination. This creates ammonia as a waste product, which is highly soluble and toxic. Ammonia is then converted to urea in the liver, which is less toxic and soluble and can be transported safely in the blood.


What is the arrangement of vessels in the liver?

- Oxygenated blood from the heart is directly pumped to the liver via the hepatic artery. This supplies the hepatocytes (which are active and require ATP for metabolism) with oxygen for respiration.
- Deoxygenated blood from the small intestines enters the liver via the hepatic portal vein. It contains every substance that has been absorbed by the small intestines into the bloodstream in unregulated amounts, even toxic substances.
- Deoxygenated blood leaves the liver via the hepatic vein which contains food substances in correct concentrations and waste products removed. This vessel eventually joins the inferior vena cava.
- The bile duct connects the liver to the gall bladder, and is used to carry bile made in the liver by hepatocytes to the gall bladder to be stored for later use; when it is secreted into the duodenum to emulsify fat.


What is the principle behind the arrangement of cells in the liver?

To ensure best possible contact with blood vessels.


What is the structural arrangement of hepatocytes in the liver?

1. Hepatoctes are divided into lobes which are subdivided into lobules.
2. Hepatic artery and portal vein divide into smaller branches which run parallel to the lobules. These are called inter-lobular vessels.
3. These haver further branches at intervals that enter the lobules.
4. Blood from the hepatic artery and portal vein mix in chambers called sinusoids, which is lined by hepatocytes.
5. Sinusoids empty into the intra-lobular vessels which run through the middle of every lobule, and are branches of the hepatic vein. Eventually, all intra-lobular vessels join to form the hepatic vein, which leaves the liver.
6. In addition to sinusoids, chambers called bile canaliculi collect bile produced by the hepatocytes and drains into branches of the bile duct.


What are the structural features of hepatocytes?

- All hepatocyes are unspecialised and are thus all have very similar appearances.
- They have many microvilli on the surface to increase surface area for secretion and absorption of substances.
- Very dense cytoplasm with specialised amounts of certain organelles, specialised to perform a range of metabolic functions (protein synthesis, carbohydrate transformation and storage, detoxification, production of bile/lipids/salts.


What are the roles Kupffer cells?

- They are located in the sinusoild.
- They are specialised macrophages responsible for breaking down old erythrocytes.
- Waste product of this process is bilirubin, which is responsible for the brown colour of faeces.


What are the functions of the liver?

1. Control of substance levels in body including: Blood glucose, amino acid, lipid...
2. Synthesis of substances in the body including: Foetal red blood cells, bile, plasma proteins, cholesterol...
3. Storage of substances in the body including: Vitamins (A,D and B12), iron, glycogen...
4. Detoxification of alcohol and drugs.
5. Breakdown of hormones.
6. Breakdown of old erythrocytes.


What is the process of forming urea?

1. Amino acids are deaminated to form ammonia and keto acids (respired to form energy).
2. Ammonia is converted to urea via the ornithine cycle.


What is the process of deamination?

1. Amine group removed from amino acid via reaction with oxygen to form ammonia and keto acid.
2. Keto acid can be respired directly to produce energy.


What is the benefit of converting ammonia to urea?

- Ammonia is very toxic to the body whereas urea is much less toxic.
- Ammonia is much more soluble than urea so is harder to remove.
- Urea can be safely transported in the bloodstream and removed by the kidneys whereas ammonia cannot.


What is the ornithine cycle?

- The process by which ammonia combines with CO2 to form urea.
- 2NH3 + CO2 --> CO(NH2)2 + H2O.


What are the stages of the ornithine cycle?

1. NH3 + CO2 + Ornithine --> Citrulline + H2O.
2. Citrulline + NH3 --> Arginine + H2O.
3. Arginine + H2O --> Urea + Ornithine.
4. Cycle repeats.


What role does the liver play in detoxification?

- The liver is responsible for the detoxification of many toxic substances either taken into the body (alcohol, drugs), or produced by the body (hydrogen peroxide).
- Methods of detoxification include oxidation, reduction, methylation and/or a series of catabolic/anabolic reactions.
- Hepatocytes contain a number of enzymes used in the process of detoxification, including catalase for breaking down hydrogen peroxide.


What is the process of detoxifying ethanol?

1. Ethanol is oxidised by the enzyme ethanol dehydrogenase to form ethanal.
2. Ethanal is further oxidised by the enzyme ethanal dehydrogenase to form ethanoic acid.
3. Ethanoic acid combines with coenzyme A which can be respired.
4. Hydrogen atoms lost during both oxidation processes combine with coenzyme NAD to form reduced NAD.


What may be a problem with excess alcohol consumption?

NAD is required in the process of respiring fatty acids to release energy. When too much alcohol is consumed, a large amount of NAD is converted to reduced NAD and there may not be sufficient NAD left to respire fatty acids. As a result, the fatty acids are converted back into lipids and stored in hepatocytes, which causes a condition called fatty liver, a precursor to hepatitis and cirrhosis.