DIGESTIVE AND RENAL SYSTEMS:THE DIGESTIVE SYSTEM, Digesting Food

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Health Psychology PSY408

LESSON 09

DIGESTIVE AND RENAL SYSTEMS

THE DIGESTIVE SYSTEM

Whether we eat an apple, drink some milk, or swallow a pill, our bodies respond in the same general Way.

The digestive system breaks down what we have ingested, converts much of it to chemicals the body can

use, and excretes the rest. The chemicals the body uses are absorbed into the bloodstream, which transports

them to all of our body cells. Chemical nutrients in the foods we eat provide energy to fuel our activity,

body growth, and repair.

Food's Journey through Digestive Organs

Think of the digestive system as a long hose-- about 20 feet long--with stations along the way. The

journey of food through this hose begins in the mouth and ends at the rectum. These digestive organs and

the major organs in between are shown in the diagram.

Digesting Food

How does this system break down food? One way is mechanical: for example, we grind food up when we

chew it. Another way is chemical: by the action of enzymes, substances that act as catalysts in speeding up

chemical reactions in cells. How do enzymes work? You can see the effect of an enzyme by doing the

following experiment. Place a bit of liver in some hydrogen peroxide and watch what happens:

Health Psychology PSY408

An enzyme in liver called catalase causes the peroxide to decompose, frothing as oxygen is given off as a

gas. This is the same reaction you see when you use peroxide to disinfect a wound.

In most cases, the names for enzymes end in the letters -ase, and the remainder of each name reflects the

substance on which it acts. The following list gives some examples:

· Carbohydrase acts on carbohydrates.

· Lactase acts on lactose (milk).

· Phosphatase acts on phosphate compounds.

· Sucrase acts on sucrose (sugar).

As food is broken down into smaller and smaller units in the digestive tract, water molecules become

attached to these units. When food is in the mouth, there is more digestive action going on than just

chewing. Saliva moistens food and contains an enzyme that starts the process of breaking down starches.

The salivary glands release saliva in response to commands from the brainstem, which responds primarily to

sensory information from taste buds. Simply seeing, smelling, or even thinking about food can produce

neural impulses that cause the mouth to water.

The journey of food advances to the esophagus, a tube that is normally flattened when food is not passing

through it. The esophagus pushes the food down to the stomach by wavelike muscle contractions called

peristalsis. By the time food enters the esophagus, the stomach has already begun digestive activities by

releasing small amounts of gastric juice even before food reaches it. Tasting, smelling, seeing, or thinking

about food can initiate this process. Once food reaches the stomach, this organ amasses large amounts of

gastric juices, including hydrochloric acid and pepsin, an enzyme that breaks down proteins. The stomach

also produces a sticky mucus substance to protect its lining from the highly acidic gastric juices.

The muscular stomach walls produce a churning motion--that we are generally not aware of--which mixes

the food particles with the gastric juices. This mixing continues for 3 or 4 hours, producing a semi- liquid

mixture. Peristalsis in the stomach then moves this mixture on, a little at a time, to the initial section of the

small intestine called the duodenum.

Important digestive processes occur in the small intestine. First, the highly acidic food mixture becomes

chemically alkaline as a result of substances added from the pancreas, gallbladder, and wall of the small

intestine. This is important because the linings of the small intestine and remainder of the digestive tract are

not protected from high acidity, as the stomach is. Second, enzymes secreted by the pancreas into the

duodenum break down carbohydrates, proteins, and fats further. Third, absorption increases. Because the

stomach lining can absorb only a few substances, such as alcohol and aspirin, most materials we ingest are

absorbed into the bloodstream through the lining of the small intestine. If alcohol is consumed along with

fatty foods, very little alcohol is absorbed until it reaches the small intestine. By the time food is ready to be

absorbed through the intestine wall, nutrients have been broken down into molecules--carbohydrates are

broken down into simple sugars, fats into glycerol and fatty acids, and proteins into amino acids.

How does Absorption Occur?

The inside of the small intestine is made of a membrane that will allow molecules to pass through. To

increase the absorbing surface, the intestine wall has many folds that contain projections, as pictured in the

diagram.

Health Psychology PSY408

Each of the many thousands of projections contains a network of structures that will accept the molecules

and transport them away to other parts of the body. These structures include tiny blood vessels called

capillaries and a tube called a lacteal.

Capillaries absorb amino acids, simple sugars, and water; they also absorb some fatty acids, vitamins, and

minerals. Lacteals accept glycerol and the remaining fatty acids and vitamins.

The remaining food material continues its journey to the large intestine, most of which is called the colon.

Absorption, mainly of water, continues in the first half of the colon, and the remaining material is

transported on. Bacterial action converts the material into feces, which eventually reach the rectum, where

they are stored until defecation occurs.

Disorders of the Digestive System

Judging from the many media advertisements for stomach and "irregularity" remedies, it seems that people

have a good deal of trouble with their digestive processes. We will consider a few digestive problems. One

disorder of the digestive system is peptic ulcers.

Health Psychology PSY408

Peptic Ulcers are open sores in the lining of

the stomach or intestine, usually in the

duodenum. These sores appear to result from

excess gastric juices chronically eroding the

lining when there is little or no food in the

stomach, but bacterial infection can play a

role, too. Abdominal pain is the chief

symptom of the disorder. Although the

victims of ulcers are mostly adults, the

disorder also occurs in children, particularly

boys. People who experience high levels of

stress seem to be more susceptible to ulcers

than people who do not.

Hepatitis is a class of several viral diseases in

which the liver becomes inflamed and unable

to function well. The first symptoms often are

like those of flu. But the symptoms persist,

and jaundice, a yellowing of the skin, generally

follows. Hepatitis A appears to be transmitted

through contaminated food, water, and utensils. Hepatitis B and C infections occur through sexual contact,

transfusion of infected blood, and sharing of contaminated needles by drug addicts, but the modes of

transmission may be broader. Some forms of hepatitis can lead to permanent liver damage.

Cirrhosis is another disease of the liver. In this disease, liver cells die off and are replaced by nonfunctional

fibrous scar tissue. The scar tissue is permanent, and when it becomes extensive, the livers normal functions

are greatly impaired. As we will see later, the liver is not only important in the digestive process; it also

cleanses and regulates the composition of the blood. Cirrhosis can result from several causes, including

hepatitis infection and, particularly, alcohol abuse (AMA, 1989).

Cancer may occur in any part of the digestive tract, especially in the colon and rectum. People over 40 years

of age have a higher prevalence for cancers of the digestive tract than do younger individuals. Early

detection for many of these cancers is possible and greatly improves the person's chances of recovery.

THE RENAL SYSTEM

Overview

The renal system--consisting of the kidneys, ureters, urinary bladder, and urethra--is also critically

important in metabolism. The kidneys are chiefly responsible for the regulation of the bodily fluids; their

principal function is to produce urine. The ureters contain smooth muscle tissue that contracts, causing

peristaltic waves to move urine to the bladder, a muscular bag that acts as a reservoir for urine. The urethra

then conducts urine from the bladder out of the body.

Urine contains surplus water, surplus electrolytes; waste products from the metabolism of food, and surplus

acids or alkalis. By carrying these products out of the body, it main- tabs water balance, electrolyte balance,

and blood pH. Of the electrolytes, sodium and potassium are the most important because they are involved

in the normal chemical reactions of the body, muscular contractions, and the conduction of nerve impulses.

Thus, an important function of the kidneys is to maintain an adequate balance of sodium and potassium

ions. In the case of certain diseases, the urine will also contain abnormal amounts of some constituents;

therefore, urinalysis offers important diagnostic clues to many disorders. For example, an excess of glucose

Health Psychology PSY408

may indicate diabetes, an excess of red blood cells may indicate a kidney disorder, and so on. This is one of

the reasons why a medical checkup often includes a urinalysis.

As noted, one of the chief functions of the kidneys is to control the water balance in the body. For example,

on a hot day when a person has been active and has perspired profusely, relatively little urine will be

produced so that the body may retain more water. This is because much water has already been lost through

the skin. On the other hand, on a cold day when a person is relatively inactive and a good deal of liquid has

been consumed, urine output will be higher so as to prevent over-hydration.

To summarize, then, the urinary system regulates bodily fluids by removing surplus water, surplus

electrolytes, and the waste products generated by the metabolism of food.

Disorders of the Renal System

The renal system is vulnerable to a number of disorders. Among the most common are urinary tract

infections, to which women are especially vulnerable and which can result in considerable pain, especially

upon urination. If untreated, they can lead to more serious infection.

AGN (Acute Glomerular Nephritis) is a disease that results from an antigen-antibody reaction in which

the inner tissues of the kidneys become markedly inflamed. These inflammatory reactions can cause total or

partial blockage in the kidneys. In severe cases, total renal shutdown occurs. Acute glomerular nephritis is

usually caused by a kind of streptococcus infection.

Another common cause of acute renal shutdown is tubular necrosis, which involves destruction of the

epithelial cells in the tubules of the kidneys. Poisons that destroy the tubular epithelial cells or severe

circulatory shock are most common causes of tubular necrosis.

Kidney Failure is a severe disorder because the inability to produce an adequate amount of urine will cause

the waste products of metabolism as well as surplus inorganic salts and water to be retained in the body. An

artificial kidney, a kidney transplant, or kidney dialysis may be required in order to rid the body of its wastes.

Although these technologies can cleanse the blood to remove the excess salts, water, and metabolites, they

are highly stressful medical procedures. Kidney transplants carry many health risks, and kidney dialysis can

be extremely uncomfortable for patients. Consequently, health psychologists have been involved in

addressing the problems of the kidney patient.

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