15.4 The Liver And Excretion

Question 1

Define excretion

Answer 1

The removal of the waste products (e.g. carbon dioxide, nitrogenous waste) of metabolism from the body.

These waste products are removed because they may be toxic and damage body cells if accumulated

Excreting waste products also maintains homeostasis by helping keep the levels of certain substances in the blood fairly constant

Question 2

Role of the liver

Answer 2

To break down metabolic waste products and other substances that can be harmful, like drugs and alcohol
They are broken down into less harmful waste products that can then be excreted

Question 3

Role of the hepatic artery

Answer 3

Supplies the liver with oxygenated blood from the heart, so the liver has a good supply of oxygen for respiration, providing plenty of energy

Question 4

The role of the hepatic vein

Answer 4

Takes deoxygenated blood away from the liver

Question 5

Role of the hepatic portal vein

Answer 5

Brings blood from the duodenum and ileum (parts of the small intestine), so its rich in the products of digestion
This means any ingested harmful substances are filtered and broken down straight away

Question 6

Role of the bile duct

Answer 6

Takes bile (a substance produced by the liver to emulsify fats - providing a larger surface area for enzymes to break down the products) to the gall bladder to be stored

Question 7

Describe the structure of the liver

Answer 7

• the liver is made up of liver lobules - cylindrical structures made of cells called hepatocytes that are arranged in rows
• each lobule has a central vein in the middle that connects to the hepatic vein
• many branches of the hepatic artery, hepatic portal vein, bile duct are also found connected to each lobule

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 lobules connect up to form the hepatic vein

Question 8

The role of Kupffer cells
Role of bile duct

Answer 8

They act as macrophages, ingesting foreign particles and helping to protect against disease. Cells that are attached to the walls of the sinusoids
They are remove bacteria and break down old red blood cells

Hepatocytes secret bile into canaliculi. The bile duct is connected to the central vein by tubes called canaliculi

Question 9

Describe and explain deamination

Answer 9

One of the liver’s most important roles is getting rid of excess amino acids produced by eating and digesting protein. Amino acids contain nitrogen, and nitrogenous substances can’t usually be stored by the body. This means excess amino acids can be damaging to the body, so they must be used by the body (e.g. to make proteins) or be broken down and excreted.

Here’s how amino acids are broken down are broken down in the liver:

1) the nitrogen containing groups are removed from any excess amino acids forming ammonia and organic acids - this process is called deamination
amino acids —> ammonia + organic acids

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

Question 10

Describe and explain the ornithine cycle

Answer 10

Ammonia is too toxic for mammals to excrete directly so its combined with CO2 in the ornithine cycle to create urea and water
Ammonia + carbon dioxide —> urea + water

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

Question 11

Describe detoxification

Answer 11

The liver also breaks down other harmful substances, like alcohol, drugs, paracetamol and unwanted hormones. They’re broken down into less harmful compounds that can be excreted from the body.

E.g. alcohol (ethanol):
- alcohol is a toxic substance that can damage cells
- its broken down in the liver into ethanal, which is broken down into a less harmful substance called acetic acid
- excess alcohol over a long period can lead to cirrhosis of the liver - this is when cells of liver die and scar tissue blocks blood flow

Question 12

Describe the role of the liver in glycogen storage

Answer 12

The body needs glucose for energy
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 glucose is needed for energy