D .3 Liver Functions

describe the liver

• lobed organ

• located below the diaphragm

• functions to regulate the chemical composition of blood

where does the liver receive oxygenated blood from

what does this sustain

the hepatic artery

used to sustain liver cells (hepatocytes)

where does de-oxygenated blood from the liver go

hepatic vein

what does the liver receive from the portal vein

receives nutrient rich blood from the gut via the portal vein

function of the liver

process the nutrients absorbed from the gut and hence regulates the body’s metabolic processes

livers responsibilites

• 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

what is the liver composed of

what is their shape

smaller histological structures called lobules, which are roughly hexagonal in shape

what is each lobule is surrounded by

branches of the hepatic artery to provide oxygen and the portal vein to provide nutrients

what is a sinusoid

capillary-like structures from the veins which exchange materials directly with the hepatocytes

where does the blood in the sinusoid go to

what does this do

drain into a central vein, which feeds deoxygenated blood into the hepatic vein

what do hepatocytes produce and how is this transported

produce bile

• transported by vessels called canaliculi to bile ducts, which surround the lobule

what does the liver play an important role in

regulating the levels of nutrients in the bloodstream

explain how nutrients enter the liver and what happens to them

• 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 in the liver

• 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 in the liver

• 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 in the liver

• 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

what does the liver remove from the blood

toxins and detoxifies them

why are many toxins difficult for the body to excrete

what are they converted into

Many of these toxic compounds are fat soluble

• into less harmful and more soluble forms, which are then excreted from the body

describe the detoxification of compounds by the liver that typically involves two sets of chemical pathways

1. 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

2. 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

what are plasma proteins

what are they produced by

proteins present in the blood plasma and are produced by the liver (except for immunoglobulins)

• produced by the rough ER in hepatocytes and exported into the blood via the Golgi complex

name 3 plasma proteins and their function

• 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)

another name for a red blood cell

erythrocyte

what is the lifespan of an erythrocyte, why

~120 days so must constantly be replaced

possess minimal organelles and no nucleus in order to carry more haemoglobin

the liver and erythrocytes

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)

what are Kupffer cells and what do they do

specialised phagocytes within the liver which engulf red blood cells and break them down

explain the process between Kupffer cells and rbc

• 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)

what must happen to the released iron from the process between Kupffer cells and rbcs

must be complexed within a protein in order to avoid oxidation to a ferric state

what is jaundice caused by

an excess of bile pigment – bilirubin – within the body

what type of condition can cause jaundice, name 3

any condition which impairs the natural breakdown of red blood cells, including:

• 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

symptoms 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

how can jaundice be treated

by treating the underlying cause for the build up of bilirubin within the body