Tuesday, 28 December 2010

On the Liver...

If you are anything like me, you’d like to say a big thank you to your liver. I like my fatty chocolates and meat, and I love my drinks too! So why are we thanking our livers?

Well, hepatocytes (or liver cells) help us stay alive while we binge on these awesome (unhealthy) food. Its functions include –  [CSSBDD]

Control of blood glucose levels, amino acid levels, lipid levels (so you can store or release all the loveliness you eat or save)

Synthesis of bile (so you can digest all your fats!), plasma proteins, cholesterol

Storage of Vitamins A, D, B12, glycogen.

Breakdown of hormones

Destruction of Red Blood Cells (RBCs)

And most importantly (for me, anyway)

Detoxification of alcohol and drugs

The body understands how special the liver is for it has its own quirks. It has the Hepatic Portal Vein which is unusual because it is a blood vessel with capillaries on both ends and it carries blood from the digestive system to the liver. All your digested glucose and of course toxic substances like alcohol first filter through the liver before being set out into the wild i.e. the blood system which will lead to your heart!

Since, it receives blood from both the heart (Hepatic Artery) and the digestive system (Hepatic Portal Vein), it is said to have a dual blood supply.

Two vessels lead things out of the liver. The Hepatic Vein carries deoxygenated blood to the vena cava and then to the heart and the Bile Duct transports bile to the Gall Bladder where it is stored until needed to aid digestion of fats in the small intestine (chocolate anyone?) 

Now, go have some water! 

The structural wonders of the liver

The liver is divided into several lobes that divide into cylindrical lobules.

The interlobular vessels are smaller vessels divided from the hepatic artery and the portal vein that run between or parallel to the cylindrical lobules. At this point they have not entered the lobule!

At intervals, these branches enter the lobules and the mixed fluids pass through the sinusoids which are vessels closer to the liver cells. The smaller diffusion distance allows for a faster diffusion rate.

The sinusoids empty into the intra-lobular vessels which are branches of the hepatic artery and therefore lead blood OUT of the liver. Can you remember what the interlobular vessels were?

Inside the sinusoids are Kupferr Cells which are specialised macrophages (one of the defense troops of our body that primarily engulf and digest the bad guys). These cells are involved in the breakdown and recycling of old red blood cells.

Bilirubin (the brown pigment in faeces or poop) is a waste product of this breakdown.

Structural features of Hepatocytes

  1. Cuboidal shape with microvilli: Increase surface area and rate of diffusion
  2. Dense cytoplasm: Needs many proteins, enzymes and organelles for its many metabolic functions.
How to deal with Excess Protein!

As citizens of developed countries, we probably eat far too much protein – much more than the 40-60 grams required daily. When digested into amino acids, they cannot be stored but have a lot of chemical potential energy.

What can you do?

You remove the amine group from the amino acid in a process called deamination.

Don't worry too much about balancing!

So deaminating amino acids will give you:

  1. Keto Acids – these can be used directly in respiration to release energy.
  2. Ammonia – highly toxic and soluble, this poses another problem

How to deal with Ammonia!

You convert it to its less soluble and toxic little brother Urea by combining it with carbon dioxide. This process is called the ornithine cycle. Doing this allows Urea to be transported safely in the blood to the kidneys where it can be excreted.

The ornithine cycle occurs partly in the cytosol and partly in the mitochondria since its need energy from ATP.

How to deal with alcohol and drugs?

Toxins can be rendered harmless by oxidation, reduction or combination with another molecule. Hepatocytes also contain many enzymes that can be used for these processes.

Ethanol is broken down by ethanol dehydrogenase to form ethanal. (+2H)

Ethanal is broken down by ethanal dehydrogenase to form ethanoate. (+2H)

Ethanoate combines with coenzyme A to form Acetyl Co-enzyme A (used in respiration)

Released H combine with co-enzyme NAD to form reduced NAD.

The problem with a lower amount of NAD is…

NAD is needed to oxidise and breakdown fatty acids.

Lower amounts of it mean a larger proportion of fatty acids are converted back to lipids and stored in the liver.

  NAD fatty acids converted back to lipids and stored in the hepatocytes. 

This results in a ‘fatty liver’ which leads to alcohol related hepatitis and cirrhosis.

Final question about hepatocytes…

Why do they have large numbers of mitochondria and ribosomes?

Mitochondria provide energy from ATP for active processes such as protein synthesis, bulk transport (endo and exocytosis), mitosis and the ornithine cycle. The ribosomes allow the production of enzymes and proteins required for these processes and others in the liver. 

Images from:

Thank you very much! :) I earn nothing from doing this!