Module 5: Excretion.

Question 1

What is excretion? Give an example.

Answer 1

Excretion is the removal of the waste products of metabolism from the body.

For example, carbon dioxide is excreted by the lungs, when we breathe out and nitrogenous waste is excreted in the urine. These substances would cause tissue damage if allowed to build up, so excretion maintains homeostasis by keeping substances within a healthy range.

Question 2

Describe the structure and function of the liver.

Answer 2

It breaks down metabolic waste products and other substances that can be harmful such as drugs and alcohol.

• The hepatic artery supplies the liver with oxygenated blood from the heart, so the liver has a good supply of oxygen for respiration, providing plenty of energy.
• The hepatic vein takes deoxygenated blood away from the liver.
• The hepatic portal vein brings blood from the duodenum and ileum (parts of the small intestine), so it is rich in the products of digestion. This means any ingested harmful substances are filtered out and broken down straight away.
• The bile duct takes bile (a substance produced by the liver to emulsify fats) to the gall bladder to be stored.

Question 3

Describe the histology of the liver

Answer 3

• It is made up of liver lobules which are cylindrical structures made of cells called hepatocytes that are arranged in rows radiating out from the centre.
• Each lobule has a central vein in the middle that connects to the hepatic vein. Many branches of the hepatic artery, hepatic portal vein and bile duct are also found connected to each lobule.

Question 4

Explain how excretion takes place in the liver.

Answer 4

• The hepatic artery and the hepatic portal vein are connected to the central vein by capillaries called sinusoids.
• Blood runs through the sinusoids, past the hepatocytes that remove harmful substances and oxygen from the blood.
• The harmful substances are broken down by the hepatocytes into less harmful substances that then re-enter the blood.
• The blood runs to the central vein, and the central veins from all the lobules connect up to form the hepatic vein.
• Cells called Kupffer cells are also attached to the walls of the sinusoids. (they are a type of white blood cells that carries out phagocytosis). They remove bacteria and break down old red blood cells.
• Hepatocytes produce bile and secrete it into tubes called bile canaliculi. These tubes drain into bile ducts. The bile ducts from all the lobules eventually connect up and leave the liver.

Question 5

What would you see when you examine a liver tissue under a microscope?

Answer 5

• Large white circular shape is the central vein.
• White spaces are the sinusoids
• The red dots are the nuclei
• Cells that radiate out from the central vein are hepatocytes.

Question 6

Why does deamination in the liver occur?

Answer 6

There is excess amino acids produced by eating and digesting protein. Amino acids contain nitrogen, which cant be stored by the body.

This means excess amino acids can be damaging to the body, so they are used by the body to make proteins or to be broken down and excreted.

Question 7

Describe the process of how excess amino acids are broken down in the liver.

Answer 7

1) First, the nitrogen-containing amino groups (-NH2) are removed from any excess amino acids, forming ammonia (NH3) and organic acids- this process is called deamination.
Amino acids ——-(-deamination)——-> ammonia + organic acids.

2) The organic acids can be respired to give ATP or converted to carbohydrate and stored as glycogen.

3) ammonia is too toxic for mammals to excrete directly, so it is combined with CO2 in the ornithine cycle to create urea and water

Ammonia + CO2 ———-> urea + water

4) The urea is released from the liver into the blood and then the kidneys filter the blood and remove the urea as urine, which is excreted from the body.

Question 8

What is the ornithine cycle?

Answer 8

Ammonia is too toxic for mammals to excrete directly, so it is combined with CO2 in the ornithine cycle to create urea and water

Question 9

What role does the liver play in detoxification?

Answer 9

The liver breaks down harmful substances, like alcohol, drugs and unwanted hormones.

They are broken down into less harmful compounds that can then be excreted from the body - detoxification.

Question 10

Give 3 examples of harmful products the liver breaks down and how?

Answer 10

Alcohol - it is a toxic substance that can damage cells. It is broken down by the liver into ethanal, which is then broken down into a less harmful substances called acetic acid. Excess alcohol over a long time can lead to cirrhosis of the liver - this is when the cells of the liver die and scar tissue blocks blood flow.

Paracetamol - Broken down in the liver. Excess paracetamol in the blood can lead to liver and kidney failure.

Insulin - Is a hormone that controls blood glucose concentration. It is broken down as excess insulin can cause problems with blood sugar levels.

Question 11

Describe the role of the liver into glycogen storage.

Answer 11

The liver converts excess glucose in the blood to glycogen in a process called glycogenesis.

The glycogen is then stored as granules in the liver cells until the glucose is needed for energy.