Excretion, a word often misunderstood, so lets begin with defining it in biological terms. Excretion is the removal of metabolic waste products. So, how is it different to egestion? Egestion is the removal of undigested waste products as they have not taken part in chemical reactions e.g faeces (us humans do not have the cellulose enzymes needed in order to break down fibre)
How are excess amino acids broken down?
Amino acids contain nitrogen in the amino group, if nitrogen is left to build up it is toxic, therefore it must be removed. The process to do so is deamination. Deamination involves removing the amino group (NH2) which then goes into the ornithine cycle. The remaining carboxyl group is a waste product from deamination, it can be used to make storage molecules or can enter the krebs cycle to be respired. Before NH2 goes into the ornithine cycle, it becomes ammonia (gains hydrogen) then ammonia combines with CO2 to make carbamoyl phosphate (in mitochondria of liver cells) to then go into the ornithine cycle in the cytoplasm. In the ornithine cycle, ATP is used and urea is produced to then be excreted. Urea combines with other waste products, e.g. salts, excess vitamins and water to become urine.
How does the liver remove harmful substances from the blood? E.g alcohol, drugs and hormone
The liver is one of the strongest organs in the body. It can handle a lot of damage and still function and it can repair itself. The liver breaks harmful substances down into less harmful substances, this process is known as detoxification. For example alcohol can be broken down by alcohol dehydrogenase (an enzyme) to an aldehyde (ethanol) then aldehyde dehydrogenase to an ester group (acetic acid.)
The liver has other roles, what are they?
The liver works in producing bile which is a substance that emulsifies and digests lipids. Additionally, it secretes substances like hormones (e.g. insulin.) A third role is the storage of glycogen, this helps control blood glucose levels making sure they are not too high, and if too low, receptors will signal that the glycogen needs to be broken down into glucose. It also stores fat soluble vitamins e.g A,D,E. Lastly, the liver is responsible for urea production.
There are various parts to the liver which are useful for you to know.
- Hepatocytes; liver cells
- Liver lobules- cylindrical structures made up of hepatocytes
- Hepatic artery; oxygenated blood taken to liver cells
- Hepatic vein; takes deoxygenated blood away from the liver cells
- Hepatic portal vein; takes blood from the duodenum and ileum (parts of small intestine) to
- Liver cells so they are rich in nutrients
- Bile duct; takes bile to the gall bladder to be stored, connected to bile canaliculus
- Sinosoids- capillaries which connect to the central vein
- Central vein- from each liver lobule they connect to form the hepatic vein
- Kupffer cells- Removing bacteria and old red blood cells (type of macrophage)
What is bilirubin? A pigment produced from the breakdown of haemoglobin
What is bililiverdin? A green pigment in bile
Ultrafiltration is the filtering of blood in the kidneys. Unwanted substances from the kidneys go through the connecting duct, through the pelvis of kidney, to the ureter into the bladder and is excreted from the urethra.
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