The Liver & Anatomy/Physiology of Digestion

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" Misty L. Trepke " <mistytrepke

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Subject:[s-A] The Liver & Anatomy/Physiology of Digestion

 

 

 

 

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The Role of the Liver in Normal Digestion

 

Article 6/21/2004

 

Through the process of digestion, carbohydrates, fats, and proteins

are reduced into substances that can be used by the body for energy,

physiologic regulation and tissue maintenance.

 

This article reviews those processes, with special attention paid to

the role of the liver in the digestive cycle.

 

Normal Carbohydrate Digestion

 

Carbohydrates are molecules that are fundamentally composed of

simple sugars.

 

Carbohydrate metabolism begins in the mouth, where enzymes in saliva

begin breaking down complex carbohydrates into disaccharides (2-

sugar molecules) and trisaccharides (3-sugar molecules).

 

Stomach acids inactivate these enzymes, but carbohydrate processing

resumes in the small intestine through the action of enzymes

secreted by the pancreas and intestinal mucosa.

 

Eventually, digestive enzymes convert all digestible carbohydrates

into simple sugars: glucose, galactose, fructose, and maltose.

 

These simple sugars are absorbed through the intestinal wall, and

are transported to the liver by the portal vein. Galactose,

fructose and maltose are converted to glucose by the liver.

 

The liver uses some glucose immediately to power its many metabolic

processes.

 

The liver also buffers and regulates the fluctuating absorption of

glucose (and other sugars) from the intestinal mucosa, keeping the

level of serum glucose (blood sugar) within range of 90 to 150

mg/dL.

 

When surplus glucose is available in serum, the pancreas is

stimulated to release insulin.

 

Insulin is a hormone that decreases the level of glucose in the

blood by increasing the uptake of glucose by the cells of the body,

and causing the liver to convert glucose to glycogen.

 

Glycogen is then stored in the liver and skeletal muscle.

 

When the quantity of glycogen stored in the liver exceeds roughly 5%

of the liver's weight, the liver converts surplus glucose to

triglyceride, a lipid (fatty) substance that binds with proteins in

the blood to form lipoprotein.

 

Although small quantities of lipids are stored in the liver, most of

the lipoproteins are circulated in the blood and then stored in

fatty

tissues of the body.

 

When serum glucose levels decline below 80 mg/dL, the pancreas is

stimulated to release glucagon.

 

This hormone causes the liver to break down stored glycogen into

glucose, increasing levels of blood glucose and ensuring a

relatively steady

level of serum glucose for the body's metabolic needs.

 

If glycogen reserves become depleted and serum glucose levels

decrease to roughly 70 mg/dL, the liver can synthesize glucose from

smaller carbon fragments.

 

Normal Protein Metabolism

 

Proteins form components of many body structures, such as muscle and

connective tissue. Proteins also are components of hemoglobin,

enzymes, hormones, neurotransmitters and antibodies.

 

Proteins contribute to the regulation and distribution of fluid in

the body, and act as " carrier " molecules for other substances,

assisting in the transportation of these substances throughout the

body.

 

Proteins can be used to generate energy, but not as easily as

carbohydrates or fats. In total, there are roughly 100,000

different types of proteins in the body.

 

Proteins are tightly coiled molecules composed of amino acids held

together by peptide bonds, and must be reduced to amino acids by the

digestive process before they can be used by the body.

 

The digestion of protein begins when powerful acids in the stomach

denature (uncoil) proteins, allowing digestive enzymes to break the

protein's peptide bonds.

 

Digestion continues in the small intestine, where enzymes reduce

protein into amino acids. Eventually, amino acids are transported

through the intestinal wall and are carried to the liver in the

blood by the portal vein.

 

Once amino acids reach the liver, they can be:

 

Reassembled to form new proteins

 

Converted into other, nonessential amino acids

 

Converted to glucose for energy

 

Converted to fat for energy storage

 

Released into circulation for use by other cells. Normal Fat

Metabolism

 

Fats belong to a class of substances known as lipids. Fats differ

from carbohydrates and proteins in that they are not water-soluble.

 

Although there are various types of fats, fundamentally they are all

composed of one to three fatty acid chains attached to a " backbone "

molecule of glycerol.

 

Monoglycerides are composed of a single fatty acid and a glycerol

molecule

 

Diglycerides are composed of two fatty acids and a glycerol molecule

 

Triglycerides are composed of three fatty acids and a glycerol

molecule. Triglycerides, in the form of dairy products, meats, oils,

butter, and margarine, are the most common type of dietary fat and

represent a major source of energy.

 

Fats have a number of important roles in the body. Fats are the

most concentrated form of stored energy, containing 9 calories per

gram. In contrast, proteins and carbohydrates yield 4 calories per

gram each.

 

Fats carry linoleic acid, which is essential to the maintenance of

cellular membranes and normal growth.

 

Fats provide a medium for the fat-soluble vitamins A, D, E and K,

and fat-based components of cell membranes regulate the movement of

substances into and out of cells.

 

Fat provides a layer of insulation to maintain body temperature and

also provides a cushion layer for the kidneys, heart and pancreas.

As you may have noticed, many Americans have more insulation than

they need.

 

While not technically a fat, cholesterol is a fat-like substance

that is critical to a number of physical processes, including the

maintenance of cell membranes, the production of the sex hormones

testosterone and estrogen, and steroid hormones.

 

Steroid hormones influence the formation of glucose, control serum

electrolytes (sodium and potassium), and promote wound healing.

 

Interestingly, there is no dietary requirement for cholesterol. The

liver is capable of building as much cholesterol as the body needs

from other fatty acids.

 

Fat digestion begins in the mouth as salivary enzymes begin to break

apart fats into fatty acids. The process continues in the stomach,

but most of the digestion of fat occurs in the small intestine.

 

Once in the small intestine, fats are exposed to bile, which

emulsifies the fat. Bile is produced in the liver, stored and

concentrated in the gallbladder, and released into the small

intestine in response to eating.

 

Bile emulsification increases fat's exposure to other digestive

enzymes, facilitating the reduction of fat into fatty acids of

varying lengths.

 

The other products of fat digestion in the small intestine are

glycerol, phospholipids, monoglycerides and cholesterol.

 

Long-chain fatty acids, monoglycerides, phospholipids and

cholesterol are absorbed by the lining of the small intestine and

combined to form chylomicrons, which are water-soluble molecules.

 

Chylomicrons are absorbed by the lymphatic system and then routed to

the blood, where they circulate to adipose (fat) tissue, liver and

muscle cells.

 

During circulation, chylomicrons are broken apart into

monoglycerides and fatty acids, which are absorbed by muscle and fat

tissues. The residual cholesterol circulates and is eventually

taken up by the liver.

 

Short and medium length fatty acids are absorbed by the intestinal

lining and combined with a protein to form lipoprotein, and are then

transported to the liver by the portal vein.

 

Glycerol is water-soluble, and so is absorbed directly into the

blood from the intestine and is transported to the liver.

 

Once fatty acids, glycerol and cholesterol arrive at the liver, they

can be processed in several different ways.

 

They can be stored in the liver in the form of fat droplets

 

They can be used to produce cholesterol and other fat-like

substances, which are secreted back into blood and delivered to

other cells in the body

 

Triglycerides can be assembled from fatty acids and glycerol, and

used as an energy source.

 

Summary

 

The liver is central to the process of digestion, the regulation of

physical processes and appropriate utilization of nutrients.

 

Clinical symptoms can develop as liver disease decreases the organs

functional ability.

 

Source

 

Brown E. The Science of Human Nutrition. Harcort Brace Jovanovich,

New York, 1990.

 

McCance K, Huether S. Pathophysiology: The Biologic Basis for

Diseases in Adults and Children, 2nd ed. Mosby, St. Louis, 1994.

 

Jacob S, Francone C. Structure and Function in Man, 3rd ed. W. B.

Saunders, Philadelphia, 1974.

 

Martini F. Fundamentals of Anatomy and Physiology, 2nd ed. Prentice

Hall, New Jersey, 1992.