Excretion, Homeostasis & the Liver

This lesson covers: 

1. The role of the liver in excretion
2. How the liver breaks down excess amino acids
3. How the liver detoxifies harmful substances
4. How the liver stores glycogen
5. The structure of the liver

What is excretion?

The liver plays a crucial role in metabolism, which involves many chemical reactions. These reactions generate waste products, including CO₂ and nitrogenous substances, which can harm cells if they accumulate.

Excretion is the process of removing metabolic waste from cells. This is essential for maintaining normal metabolism and homeostasis. For instance, CO₂ is excreted by cells following respiration and is then removed from the body by the lungs.

Many metabolic waste products, like urea, are metabolised in and excreted from the liver cells.

Function of the liver in detoxification

The liver breaks down toxic substances such as alcohol, medications, hormones, and excess amino acids. This detoxification process converts these substances into less harmful compounds that cells can excrete.

The liver breaks down amino acids through these steps:

1. Amine groups are removed from amino acids by deamination, producing toxic ammonia and organic acids.
2. Organic acids are either used for ATP production or stored as glycogen.
3. Ammonia combines with CO₂ to form urea via the ornithine cycle, occurring partially in the mitochondria of liver cells.
4. Urea is then excreted from liver cells, enters the bloodstream, and is filtered out of the body via the kidneys as a part of urine.

Other substances detoxified by the liver include:

• Alcohol - The enzyme alcohol dehydrogenase breaks down ethanol to ethanal, which is then converted to ethanoate to prevent damage to cells.
• Hydrogen peroxide - The enzyme catalase splits hydrogen peroxide into oxygen and water to prevent cell damage.
• Paracetamol - This is broken down to prevent toxicity to the liver and kidneys.
• Insulin - This is metabolised to help regulate blood glucose concentration.

Function of the liver in regulating blood glucose

The liver also plays other crucial roles in managing blood glucose levels.

Functions of the liver in blood glucose regulation:

• Converting excess glucose into glycogen, a storage molecule.
• Storing glycogen granules within its cells.
• Releasing glucose into the bloodstream by breaking down glycogen when blood glucose levels fall.

Function of the liver in breaking down red blood cells

Haemoglobin from old red blood cells is broken down in hepatocytes (liver cells) into bile pigments.

These are then excreted from liver cells and transported by the bile duct to the gallbladder where they are stored before their removal from the body.

Structure of the liver

The liver's structure is complex, featuring several vital blood vessels and ducts.

The key large parts of the liver structure you need to know:

1. The hepatic artery supplies oxygenated blood.
2. The hepatic vein carries away deoxygenated blood towards the heart.
3. The hepatic portal vein brings nutrient-rich blood from the intestines.
4. The bile duct transports bile to the gallbladder.

Structure of liver lobules

The liver is composed of numerous lobules, that mostly consist of liver cells called hepatocytes. Hepatocytes have large nuclei, a prominent Golgi apparatus, and several mitochondria that help them carry out their many functions.

The key cells and tissues of the liver you need to know in each liver lobule:

1. Hepatocytes are arranged along channels called sinusoids.
2. The sinusoids are where oxygen-rich blood from the hepatic artery mixes with blood rich in the products of digestion from the hepatic portal vein.
3. A branch of the hepatic vein is located in the centre of each lobule to remove deoxygenated blood.
4. Kupffer cells ingest pathogens and other foreign particles, helping to protect against disease.
5. A channel separate from the sinusoids, called the bile canaliculus, links to a branch of the bile duct.