D.3 LIVER FUNCTIONS Flashcards
The liver
is 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)
t also receives nutrient rich blood from
the gut via the portal vein
Deoxygenated blood is transported from the liver via
the hepatic vein
The liver functions to
process the nutrients absorbed from the gut and hence regulates the body’s metabolic processes
- storage/controlled release of key nutrients
- detoxification
- plasma protein production
- breakdown of red blood cells
- the production of bile salts
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)
lobules 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 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)
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
carbohydrate metabolism
When blood glucose levels drop,
the liver breaks down glycogen into glucose and exports it to body tissues
carbohydrate metabolism
When hepatic glycogen reserves become exhausted
the liver synthesises glucose from other sources (e.g. fats)
carbohydrate metabolism
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
Protein Metabolism
A.A
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
Protein Metabolism
Amine group
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
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
Fat Metabolism
LDL
Low density lipoprotein (LDL) transports cholesterol to cells, for use in the cell membrane and in steroid synthesis
LDL is considered ‘bad’ as it raises blood cholesterol levels, while HDL lowers cholesterol levels and is therefore ‘good’
Fat Metabolism
HDL
High density lipoprotein (HDL) transports excess cholesterol from cells back to the liver (for storage or conversion)
Surplus cholesterol
is converted by the liver into bile salts, which can be eliminated from the body via the bowels
triglyceride pathway
triglycerides are transported to cells by VLDL from liver to chylomicrons (from intestine) for energy use of storage (adipose tissue)
cholesterol
transported to cells by LDL for use in plasma membranes and steroid synthesis, while excess cholesterol is moved from cells by HDL to be converted by liver into bile
drugs and toxin in the 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 by the liver
The detoxification of compounds by the liver typically involves two sets of chemical pathways:
- 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 are produced by
Endoplasmic reticulum and Golgi apparatus in hepatocytes
Plasma proteins are
proteins present in the blood plasma and are produced by the liver (except for immunoglobulins)
plasma proteins are exported to
exported into the blood via the Golgi complex
different types of plasma proteins
Albumins
Globulins
Fibrinogens
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)
RBC have a life span of X and thus
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)
Kupffer cells
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)
The released iron must be complexed within a protein in order to avoid
oxidation to a ferric state
Iron can be stored by the liver within
a protein shell of ferritin
Iron can be transported to the bone marrow
(where new haemoglobin is produced) within the protein transferrin
Jaundice is a condition caused by
an excess of bile pigment – bilirubin – within the body
Bilirubin is produced as part
the natural breakdown of haemoglobin by the liver
Normally, the liver conjugates this bilirubin to other chemicals and then secretes it in bile
When there is an excess of bilirubin
it may leak out into surrounding tissue fluids
Jaundice may be caused by
any condition which impairs the natural breakdown of red blood cells
Liver disease – impaired removal of bilirubin by the liver may cause levels to build within the body
Obstruction of the gall bladder – preventing the secretion of bile will cause bilirubin levels to accumulate
Damage to red blood cells – increased destruction of erythrocytes (e.g. anemia) will cause bilirubin levels to rise
consequences of jaundice
yellowish discoloration of the skin and whites of the eyes (sclera)
Other common symptoms include itchiness, paler than usual stools and darkened urine
Jaundice treatment
Other common symptoms include itchiness, paler than usual stools and darkened urine
