D3 - Liver Flashcards
Liver
lobed organ located below the diaphragm that functions to regulate the chemical composition of blood
Liver blood flow
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
Liver functions
t 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
The liver is composed of smaller histological structures called lobules, which are roughly hexagonal in shape
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
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)
Nutrient supply and conversion
The liver plays an important role in regulating the 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 synthesises 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 synthesise 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
Toxins in bloodstream
Many of these toxic compounds are fat soluble, making them difficult for the body to excrete
These compounds are converted into less harmful and more soluble forms, which are then excreted from the body
Detoxification
Toxins are converted into less harmful chemicals by oxidation, reduction and hydrolysis reactions
These reactions are mediated by a group of enzymes known as the cytochrome P450 enzyme group
These conversions produce damaging free radicals, which are neutralised by antioxidants within the liver
The converted chemical is then attached to another substance (e.g. cysteine) via a conjugation reaction
This renders the compound even less harmful and also functions to make it water soluble
The water soluble compounds can now be excreted from the body within urine by the kidneys
Plasma proteins
proteins present in the blood plasma and are produced by the liver (except for immunoglobulins)
The proteins are 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 (and hence moderate the osmotic pressure of body fluids)
Globulins participate in the immune system (i.e. immunoglobulins) and also act as transport proteins
Fibrinogens are involved in the clotting process (soluble fibrinogen can form an insoluble fibrin clot)
Low levels of other plasma proteins have various functions (e.g. α-1-antitrypsin neutralises digestive trypsin)
Erythrocyte recycling
In humans, red blood cells possess minimal organelles and no nucleus in order to carry more haemoglobin
Consequently, red blood cells have a short lifespan (~120 days) and must be constantly replaced
The liver is responsible for the break down of red blood cells and recycling of its components
These components are used to make either new red blood cells or other important compounds (e.g. bile)
Function of Kupfter cells
Kupffer cells are specialised phagocytes within the liver which engulf red blood cells and break them down
Kupffer cells break down haemoglobin into globin and iron-containing heme groups
Globin is digested by peptidases to produce amino acids (which are either recycled or metabolised by the liver)
Heme groups are broken down into iron and bilirubin (bile pigment)
