Human Anatomy

What Is the Digestive System?

Food is our fuel, and its nutrients give our bodies' cells the energy and substances they need to work. But before food can do that, it must be digested into small pieces the body can absorb and use.

The first step in the digestive process happens before we even taste food. Just by smelling that homemade apple pie or thinking about how delicious that ripe tomato is going to be, you start salivating — and the digestive process begins in preparation for that first bite.

Almost all animals have a tube-type digestive system in which food:

• enters the mouth
• passes through a long tube
• exits the body as feces (poop) through the anus

Along the way, food is broken down into tiny molecules so that the body can absorb nutrients it needs:

• Protein must be broken down into amino acids.
• Starches break down into simple sugars.
• Fats break down into into fatty acids and glycerol.

The waste parts of food that the body can't use are what leave the body as feces.

How Does Digestion Work?

The digestive system is made up of the alimentary canal (also called the digestive tract) and other organs, such as the liver and pancreas. The alimentary canal is the long tube of organs — including the esophagus, stomach, and intestines — that runs from the mouth to the anus. An adult's digestive tract is about 30 feet (about 9 meters) long.

Digestion begins in the mouth, well before food reaches the stomach. When we see, smell, taste, or even imagine a tasty meal, our salivary glands in front of the ear, under the tongue, and near the lower jaw begin making saliva (spit).

As the teeth tear and chop the food, spit moistens it for easy swallowing. A digestive enzyme in saliva called amylase (pronounced: AH-meh-lace) starts to break down some of the carbohydrates (starches and sugars) in the food even before it leaves the mouth.

Swallowing, done by muscle movements in the tongue and mouth, moves the food into the throat, or pharynx (pronounced: FAIR-inks). The pharynx is a passageway for food and air. A soft flap of tissue called the epiglottis (pronounced: ep-ih-GLAH-tus) closes over the windpipe when we swallow to prevent choking.

From the throat, food travels down a muscular tube in the chest called the esophagus (pronounced: ih-SAH-fuh-gus). Waves of muscle contractions called peristalsis (pronounced: per-uh-STALL-sus) force food down through the esophagus to the stomach. A person normally isn't aware of the movements of the esophagus, stomach, and intestine that take place as food passes through the digestive tract.

At the end of the esophagus, a muscular ring or valve called a sphincter (pronounced: SFINK-ter) allows food to enter the stomach and then squeezes shut to keep food or fluid from flowing back up into the esophagus. The stomach muscles churn and mix the food with digestive juices that have acids and enzymes, breaking it into much smaller, digestible pieces. An acidic environment is needed for the digestion that takes place in the stomach.

By the time food is ready to leave the stomach, it has been processed into a thick liquid called chyme (pronounced: kime). A walnut-sized muscular valve at the outlet of the stomach called the pylorus (pronounced: pie-LOR-us) keeps chyme in the stomach until it reaches the right consistency to pass into the small intestine. Chyme is then squirted down into the small intestine, where digestion of food continues so the body can absorb the nutrients into the bloodstream.

The small intestine is made up of three parts:

1. the duodenum (pronounced: due-uh-DEE-num), the C-shaped first part
2. the jejunum (pronounced: jih-JU-num), the coiled midsection
3. the ileum (pronounced: IH-lee-um), the final section that leads into the large intestine

The inner wall of the small intestine is covered with millions of microscopic, finger-like projections called villi (pronounced: VIH-lie). The villi are the vehicles through which nutrients can be absorbed into the blood. The blood then brings these nutrients to the rest of the body.

The liver (under the ribcage in the right upper part of the abdomen), the gallbladder (hidden just below the liver), and the pancreas (beneath the stomach) are not part of the alimentary canal, but these organs are essential to digestion.

The liver makes bile, which helps the body absorb fat. Bile is stored in the gallbladder until it is needed. The pancreas makes enzymes that help digest proteins, fats, and carbs. It also makes a substance that neutralizes stomach acid. These enzymes and bile travel through special pathways (called ducts) into the small intestine, where they help to break down food. The liver also helps process nutrients in the bloodstream.

From the small intestine, undigested food (and some water) travels to the large intestine through a muscular ring or valve that prevents food from returning to the small intestine. By the time food reaches the large intestine, the work of absorbing nutrients is nearly finished.

The large intestine's main job is to remove water from the undigested matter and form solid waste (poop) to be excreted.

The large intestine has three parts:

1. The cecum (pronounced: SEE-kum) is the beginning of the large intestine. The appendix, a small, hollow, finger-like pouch, hangs at the end of the cecum. Scientists believe the appendix is left over from a previous time in human evolution. It no longer appears to be useful to the digestive process.
2. The colon extends from the cecum up the right side of the abdomen, across the upper abdomen, and then down the left side of the abdomen, finally connecting to the rectum.
   
   The colon has three parts: the ascending colon and the transverse colon, which absorb fluids and salts; and the descending colon, which holds the resulting waste. Bacteria in the colon help to digest the remaining food products.
3. The rectum is where feces are stored until they leave the digestive system through the anus as a bowel movement.

Functional Organ Systems

The heart has four valves, namely the mitral, aortic, tricuspid, and pulmonary, that regulate the flow of blood through the heart's four chambers. Each valve consists of a flap, or leaflet, that regulates the blood flow to adjacent chambers, then snaps shut to prevent blood from flowing backwards. As in an automobile engine, valves can experience leakage, a situation in which valves do not close completely, allowing blood to flow in reverse. A second valve disorder is stenosis, in which the malfunctioning valve limits the volume of blood flow.

Both conditions can significantly reduce the heart's ability to pump blood. In many cases, heart disease progresses slowly, as the heart compensates for irregularities in blood flow, so symptoms may not seem severe. One may appear symptom-free, yet have serious heart valve disease, requiring immediate hospitalization. In general, irregular valve activity creates abnormal heart sounds, such as murmurs and clicks, that can be heard with a stethoscope. Finally, an echocardiogram may be called for in order to confirm the diagnosis. Further diagnostics can be performed, such as CT-angiography and cardiac MRI.

Anatomy of the Lungs

The lungs have miles of tiny passages, easily clogged by pollutants such as smoke, and other microscopic irritants. Asthma is a chronic lung disease that narrows the airways. In the US, more than 25 million people are known to have asthma, and new research indicates that a chemical compound found in many air fresheners, bathroom cleaners, and deodorizing products, may be harmful to the lungs.

Air first enters your body through your nose or mouth, which wets and warms the air. Conversely, cold, dry air can irritate your lungs. The air then travels through your voice box and down your windpipe, which splits into two bronchial tubes entering the lungs. A thin flap of tissue called the epiglottis covers your windpipe when you swallow, preventing food and drink from entering the air passage.

Except for the mouth and some parts of the nose, all of the airways are covered by cilia, which contain a sticky, mucus coating. The cilia trap germs and other foreign particles that enter your airways when you breathe in. Fine hairs then sweep the particles up to the nose or mouth. From there, they're swallowed, coughed, or sneezed out of the body.

Your lungs and associated blood vessels deliver oxygen to your body and remove carbon dioxide. Interestingly, the left lung is slightly smaller than the right lung, allowing additional room for your heart. Within the lungs, individual bronchi branch into thousands of thinner tubes called bronchioles. These tubes end in bunches of tiny round air sacs, the alveoli. Each air sac is covered by a mesh of tiny capillaries. The pulmonary artery delivers blood rich in carbon dioxide (lacking in oxygen) to the capillaries that surround the air sacs. Inside, carbon dioxide migrates from the blood back into the air. At the same time, oxygen is absorbed. The oxygen-rich blood then travels to the heart through the pulmonary vein, completing respiration.

Disorders of the Ear

The ear depends on coordinated events that transform sound waves into electrical impulses. The auditory nerve transmits these signals to the brain. Initially, sound waves enter the outer ear and traverse the outer ear canal, leading to the eardrum. The eardrum vibrates from the incoming sound waves and sends vibrations to three tiny bones in the middle ear.

These bones couple the sound waves from the air to fluid vibrations in the cochlea of the inner ear. Hair-like sensory cells perched on top of the basilar membrane ride the ripple of fluid thus created. As the hair cells move up and down, microscopic stereocilia sitting on top of the hair cells bump against an overlying structure and bend, which causes pore-like channels at the tips of the stereocilia to open. When that happens, chemicals rush into the cell, sparking an electrical signal. The auditory nerve then carries this signal to the brain, which translates it into a sound that we can recognize and understand.

When exposed to loud noises over an extended period, hearing losses may occur. Over time, sounds become distorted, and it may be difficult to understand other people when they talk. Sometimes exposure to continuous noise causes a temporary hearing loss, but there also may be residual long-term damage. Loud noise exposure also may be responsible for tinnitus, which is perceived as a ringing in the ears or cranium.

Auditory Neuropathy
Auditory Processing Disorder
Ear Infections in Children
Enlarged Vestibular Aqueducts (EVA)
Hearing Loss/Deafness, Sudden
Hearing Loss and Older Adults
Hearing Loss, Noise Induced
Hearing Loss, Ten Ways to Recognize
Ménière's Disease
Otosclerosis
Pendred Syndrome
Presbycusis
Tinnitus
Usher Syndrome
Vestibular Schwannoma and Neurofibromatosis


Common noise levels (in decibels):

Humming of a refrigerator = 45 decibels
Normal conversation = 60 decibels
Heavy city traffic = 85 decibels
Motorcyle engine = 95 decibels
An MP3 player at maximum volume = 105 decibels
Police siren = 120 decibels
Firecrackers = 150 decibels

Advances in ear replacement surgery:

Biological scientists used 3-D printing of cartilage cells and nano-sized materials to create functional ears that receive radio signals. The experiments demonstrated that it may be possible to create bionic tissues and organs. Scientists used 3-D printing to merge living tissue with an antenna that is able to receive radio signals. In tissue engineering, cells and other biological materials are used to augment or replace deteriorating muscle matter, bone and cartilage. Currently, it’s difficult to create 3-D structures for use in the body, especially organs with complex geometry such as the ear.

    Source: National Institutes of Health, www.nih.gov.

Medical Jobs - Listings

Healthcare jobs such as Registered Nurses, LPN's, LVN's and related Medical Technicians provide over 15 million jobs, and ten of the 20 fastest growing occupations are healthcare-related. Most healthcare workers have jobs that require less than 4 years of college education, such as health technologists and technicians, medical records, billing and coding, health information technicians, diagnostic medical sonographers, radiologic technologists and technicians, and dental hygienists. As people age they have more medical problems, and hospitals will require more staff. Wages vary by the employer and area of the county. Aside from their salary, most medical jobs include excellent benefits, as well as retirement plans.

Each link below lists current openings:	Starting Salary (up to)	10 Year Salary (up to)
Audiologists	$61,110	$89,160
Diagnostic Medical Sonographers	$36,090	$68,520
Dietitians	$34,450	$63,250
Emt, Paramedic Jobs	$29,390	$65,280
Family Medicine	$78,850	$108,320
Fitness Trainers	$31,710	$56,750
Home Health Aides	$30,100	$57,030
Licensed Vocational Nurses (LVN)	$54,480	$84,780
Massage Therapist Jobs	$33,000	$62,670
Medical Assistants	$26,980	$37,140
Medical Laboratory Technicians	$30,550	$59,260
Mental Health Counselors	$26,550	$46,370
Nursing	$37,760	$74,130
Occupational Therapist Assistants	$42,110	$58,270
Occupational Therapists	$66,010	$87,330
Physical Therapist Assistants	$41,410	$56,220
Physical Therapists	$58,050	$94,810
Physician Assistants	$41,270	$62,230
Psychiatry	$86,990	$129,990
Psychologists	$50,360	$77,840
Public Health	$92,250	$92,250
Radiation Therapists	$47,580	$62,110
Radiologic Technicians	$52,110	$77,160
Registered Nurses (RN)	$49,730	$83,440
Rehabilitation	$49,350	$72,940
Respiratory Therapists	$68,610	$94,190
Respiratory Therapy Technicians	$39,860	$56,220
Skin Care Specialists	$25,300	$48,510
Surgical Technologists	$39,680	$73,630

Source: Nursing Jobs Outlook, Bureau of Labor Statistics.