Option D3 Liver Functions Flashcards
The Liver
a lobed organ located below the diaphragm that functions to regulate the chemical composition of blood
- It receives oxygenated blood via the hepatic artery, which is used to sustain liver cells (hepatocytes)
- It also receives nutrient rich blood from the gut via the portal vein
- Deoxygenated blood is transported from the liver via the hepatic vein
Functions of the liver
- It is responsible for the storage and controlled release of key nutrients (e.g. glycogen, cholesterol, triglycerides)
- It is responsible for the detoxification of potentially harmful ingested substances (e.g. amino acids, medications, alcohol)
- It produces plasma proteins that function to maintain sustainable osmotic conditions within the bloodstream
- It is responsible for the breakdown of red blood cells and the production of bile salts
Hepatic lobules of the liver
- Each lobule is surrounded by branches of the hepatic artery (provide oxygen) and the portal vein (provide nutrients)
- These vessels drain into capillary-like structures called sinusoids, which exchange materials directly with the hepatocytes
- The sinusoids drain into a central vein, which feeds deoxygenated blood into the hepatic vein
- Hepatocytes also produce bile, which is transported by vessels called canaliculi to bile ducts, which surround the lobule
Sinusoids
Sinusoids are a type of small blood vessel found in the liver that perform a similar function to capillaries (material exchange)
- Sinusoids have increased permeability, allowing larger molecules (e.g. plasma proteins) to enter and leave the bloodstream
Sinusoids and the liver
The increased permeability of sinusoids is important for liver function and is due to a number of structural features:
- The surrounding diaphragm (basement membrane) is incomplete or discontinuous in sinusoids (but not in capillaries)
- The endothelial layer contains large intercellular gaps and fewer tight junctions (allowing for the passage of larger molecules)
Liver regulating levels of nutrients in the bloodstream
- Nutrients absorbed by the small intestine are transported by the hepatic portal vein to the liver for metabolism
- The liver converts these nutrients into forms that can be stored or used and mediates their transport to various tissues
- Nutrients stored within the liver include glycogen, iron, vitamin A and vitamin D
Carbohydrate metabolism
- Excess glucose in the bloodstream (e.g. after meals) is taken up by the liver and stored as glycogen
- When blood glucose levels drop, the liver breaks down glycogen into glucose and exports it to body tissues
- When hepatic glycogen reserves become exhausted, the liver synthesizes glucose from other sources (e.g. fats)
- These metabolic processes are coordinated by the pancreatic hormones – insulin and glucagon
Protein metabolism
- The body can not store amino acids, meaning they must be broken down when in excess
- Amino acid breakdown releases an amine group (NH2), which cannot be used by the body and is potentially toxic
- The liver is responsible for the removal of the amine group (deamination) and its conversion into a harmless product
- The amine group is converted into urea by the liver, which is excreted within urine by the kidneys
- The liver can also synthesize non-essential amino acids from surplus stock (via transamination)
Fat metabolism
- The liver is the major site for converting excess carbohydrates and proteins into fatty acids and triglycerides
- It is also responsible for the synthesis of large quantities of phospholipids and cholesterol
- These compounds are then stored by the liver or exported to cells by different types of lipoproteins
- Low density lipoprotein (LDL) transports cholesterol to cells, for use in the cell membrane and in steroid synthesis
- High density lipoprotein (HDL) transports excess cholesterol from cells back to the liver (for storage or conversion)
- LDL is considered ‘bad’ as it raises blood cholesterol levels, while HDL lowers cholesterol levels and is therefore ‘good’
- Surplus cholesterol is converted by the liver into bile salts, which can be eliminated from the body via the bowels
Liver function
Acts on drugs and toxins that have entered the bloodstream.
- Many toxic compounds are fat soluble, making it difficult to excrete
- Compounds are converted into less harmful, more soluble forms
Chemical pathways of detoxification
First, toxins are converted into less harmful chemicals by oxidation, reduction and hydrolysis reactions
- Reactions are mediated by a group of enzymes (cytochrome P450)
- Conversions produce damaging free radicals, which are neutralized by antioxidants within the liver
Then, the converted chemical is then attached to another substance via a conjugation reaction
- Renders the compound even less harmful, also functioning to make it water soluble
- Water soluble compounds can be excreted from the body within urine by the kidneys
Plasma proteins
Proteins present in the blood plasma produced by the liver (except for immunoglobulins).
- Produced by the rough ER in hepatocytes and exported into the blood via the Golgi complex
Types of plasma proteins
- Albumins: regulate the osmotic pressure of the blood (moderate the osmotic pressure of body fluids)
- Globulins: participate in the immune system (i.e. immunoglobulins) and act as transport proteins
- Fibrinogens: involved in the clotting process (can form an insoluble fibrin clot)
- Other: various roles, including enzyme neutralization, etc.
Red blood cells (erythrocytes)
Possess minimal organelles and no nucleus in order to carry more hemoglobin.
- Have a short lifespans and must be constantly replaced
- Is broken down and recycled in the liver
Kupffer cells
Specialized phagocytes within the liver that engulf red blood cells and break them down.
- Hemoglobin broken down into globin and iron-containing heme groups
- Globin digested by peptidases to produce amino acids (recycled or metabolized by the liver)
- Heme groups broken down into iron and bilirubin (bile pigment)
