How the Liver Works
How the Liver Works
By Mr Ghaz, January 14, 2011
How the Liver Works
The liver has two vital roles: making (or processing new chemicals, and neutralizing poisons and wastes.
The organ stands four-square in the way of every drop of blood coming away from our intestines absorbed from the food we eat. In other words, blood can only get back to the heart and lungs from the stomach by first passing through a system of veins into the liver, known as the portal system.
The liver is the largest organ in the body, weighing between 1,36 and 1.81 kg (3 and 4 Lbs). It is tucked underneath the diaphragm, protected from damage by the lower ribs. There are two projecting parts, or Lobes, called the left and right lobes, the right being the largest, occupying the whole of the top of the abdomen’s right side. The left is smaller, reaching the midpoint of this left area. It is not usually possible to feel the liver, but when it enlarges – as a result of disease – it protrudes from behind the rib cage, and can then be left if the abdomen is pushed in.
As in any other part of the body, it is cells that do the real work, at a microscopic level, of maintaining life’s processes.
Medical science calls the ‘creative’ cells of the liver hepatocytes. They are specialized to handle the basic substances our bodies run on – proteins, carbohydrates and fats.
Protein processing: proteins are essential for renewal and creation of cells all over body, for the formation of hormones, the body’s chemical ‘messengers,’ and for making enzymes.
We eat protein in various forms, both vegetable and animal in origin, and from ‘raw’ proteins, the liver has to create protein acceptable to the body by first breaking them down, and then actually re-building them.
Put simply, this process, called synthesis, means raw proteins being taken or absorbed from the blood flowing through the portal veins into the surrounding hepatocytes, being synthesized by the liver’s enzymes, and then being handed back in their new form. Waste, however, does it return to the blood stream?
Carbohydrate processing: These are the large class of chemical substances made of three atoms - basic building blocks of all physical matter, carbon, hydrogen and oxygen.
They occur foods, and we need them for energy. Our muscles literally burn sugar, or sugar-a process assisted by oxygen. The liver plays a vital role in organizing this fuel into forms which can be used.
This it does by turning carbohydrates into two forms, closely skin to pure sugar. One is ‘instant energy’, a substance similar to glucose and called glycogen. Sugar shortage rapidly causes brain damage, and so the level of sugar in the blood must be precisely maintained, hence the need to store sugar for times of need, such as sudden exertion or starvation. Equally, if too much sugar is present in the blood, a hormone made by the liver can store the excess as glycogen.
Conversion of fats: fats are essential to the body, too they are turned by the liver into forms which can actually be built into or renew existing fatty tissue, typically the subcutaneous layer beneath the skin which acts as insulation and shock absorber. Fat, in addition, is a means of storing energy.
Waste disposal: lining the veins of the liver are highly specialized cells, called Cupper cells after the man who identified them, which ‘vacuum’ clean’ the blood of impurities, such as bacteria. These cells also weed out excess red blood cells manufactured (our bodies always over-produce these) and hand them across to the hepatocytes for processing.
From all the sources mentioned – blood itself, protein, fats but to a much lesser extent carbohydrates – by-produced during the rebuilding that goes on in the hepatocytes. Some, such as ammonia (produced during breakdown of protein), is poisonous, and the liver cells neutralize this, sending the harmless waste product urea back into the main circulation. Fat and blood waste products pass out as bile.
The same applies to actual poisons we consume – such as alcohol – and indeed medicines. If a drug is to be long-lasting in its effects, if needs either to be resistant to the liver’s enzymes or to by-pass the liver completely.
We need a continuous supply of glucose in the bloodstream to carry out all the body’s functions and to supply tissues with energy. When glucose intake is low – for example, during dieting – proteins and carbohydrates are broken down to make more glucose. However, because all our reserves of protein (mainly muscle) would queerly dwindle away, many tissues switch over to using the products of fat breakdown as an alternative source of fuel. These are known as ketones.
There are three types of ketones: two ketone bodies (aceto-acetic acid and betahydroxy-butyric acid) and acetone. Acetone, which is a waste product of fat breakdown, is produced at the same times as the ketone bodies, but has no useful function. Ketone bodies, on the other hand, are readily utilized as a source of energy.
When glucose is scarce, ketones are produced and transported into the liver, where ketone bodies are formed. The ketones are then released into the circulation, to be taken up and used for energy by the muscles, the heart, the brain and the many other tissues.
In Health and Disease
Ketones do not appear in the blood until several hours after a balanced meal. By the time most of us wake up in the morning, we are slightly ketotic: small amounts of ketones are present in the blood and urine. Much of the energy for an early morning jog would be supplied to the muscles by these ketones, which disappear from the bloodstream after a good break first.
During a weight-reducing diet, or more extreme food deprivation, a moderate amount of ketosis will occur.
Pregnant women in labor often become ketotic. However high levels of ketones in the blood may delay the progress of labor by interfering with the women’s ability to contract effectively, and so glucose is given by an intravenous drip to inhibit ketone formation.
When glucose is scarce, fatty tissue is broken down into fatty acids and carried in the bloodstream to the liver, where ketone bodies are formed.
The liver has a marvelous capacity to renew itself – a whole lobe cut away in an operation can be replaced in a few weeks. However, on rare occasions, destruction of liver cells out strips the rate of replacement, and this leads to acute liver failure.
The results of liver failure are easy to imagine by considering the jobs the liver performs. Blood sugar falls, and without a proper level, brain damage can result. Failure of protein production, including manufacture of those which cause clotting in the blood, make the patient bleed easily; it also leads, for various technical reasons, to complications such as accumulation of fluid in the abdomen, called as cites.
Nutrients enter the liver in the portal vein, and oxygenated blood, to ‘fuel’ the cells, in the hepatic artery Processed blood collects in the hepatic vein to return to the heart . Waste drains via the common bile duct for storage in the gall bladder.
The liver’s lobes are divided into lobules encircled by veins and arteries.
Within the lobes branches of the portal vein and hepatic artery deliver nutrients and oxygenated blood to the sinusoids – reservoirs surrounding the rows of liver cells (hepatocytes). These absorb and process nutrients and waste. Process substances are returned to the sinusoids and leave via the interlobular vein. Waste leaves in the bile canaliculated.