What is an “organ” in the human body?

An organ is a group of several different tissues working together as a unit to perform a specific function or functions in the body. Each organ performs functions that none of the component tissues can perform alone. This cooperative interaction of different tissues is a basic feature of animals, including humans. For example, the heart is an organ that consists of cardiac muscle wrapped in connective tissue, with nerve tissues controlling the rhythmic contractions of the heart’s muscles.

How many organs are in the human body?

About 78 organs are in the human body, each one with a different size, shape, and function (or sometimes the same, as with two kidneys). The largest organ, with respect to its size and weight, is the skin. Not everyone has organs in the same place. For example, sometimes the kidney may be located closer to the pelvis, or only one kidney may be present. These differences can be due to genetics, differences in the organ’s cell growth, or even disease.

Several body systems are negatively affected by diabetes, which can damage everything from the heart and kidneys to causing blindness or damaging peripheral nerves, which could even necessitate amputations.

What body systems are most affected by diabetes, especially when the disease is left untreated?

If a person’s diabetes is left untreated, then many-often life-threatening-complications will arise in other parts of the body. In particular, major problems can develop with the heart, eyes, kidneys, nerves, and circulatory system. Often just as much of a problem, the muscular, reproductive, immune, and integumentary (skin) systems can be affected by untreated diabetes. (For more information about how diabetes affects these systems, see their respective chapters.)

What are the major organ systems in the human body?

An organ system is a group of organs working together to perform a vital body function. There are 12 major organ systems in the human body. The following lists those organ systems, along with their components and functions:

Major Organ Systems

Organ System




Heart, blood, and blood vessels

Transports blood throughout the body, supplying nutrients and carrying oxygen to the lungs and wastes to the kidneys


Mouth, esophagus, stomach,

Allows the ingestion of food and breaks intestines, liver, and pancreas it down into smaller (chemical) units


Pituitary, adrenal, thyroid, and

Coordinates and regulates the activities other ductless glands of the body, including metabolism


Kidneys, bladder, and urethra

Helps to remove wastes from the bloodstream


Lymphocytes, macrophages,

Removes foreign substances from the and antibodies bloodstream


Skin, hair, nails, and sweat

Protects the body; the skin holds in the glands organs


Lymph nodes, lymphatic

Captures body fluids and returns them capillaries, lymphatic vessels, to the cardiovascular system spleen, and thymus


Skeletal muscle, cardiac

Allows the body to move muscle, and smooth muscle


Nerves, sense organs, brain,

Receives external stimuli, processes and spinal cord information, and directs activities


Testes, ovaries, and related

Carries out reproduction organs


Lungs, trachea, and other air

Exchanges gases (captures oxygen [O2] passageways and disposes of carbon dioxide [CO2])


Bones, cartilage, and ligaments

Protects and provides support for the body’s motions and movements


What is diabetic neuropathy, and how does it affect a person who has diabetes?

In general, diabetic neuropathy is the loss of sensation-including touch-because of diabetes. This occurs because over time, diabetes can cause nerve damage as high blood glucose levels damage the body’s nerve fibers. This damage often causes several symptoms, including loss of feeling, especially on the skin and in the extremities. (For more about nerve damage and diabetes, see the chapter “How Diabetes Affects the Nervous System.”)

Can diabetes cause hearing loss?

No one truly knows whether diabetes can cause hearing loss, but it is known that many people with diabetes have some hearing loss. Some researchers believe that, as with nerve damage in the eyes, high blood glucose levels in a person with diabetes may damage the small vessels and/or nerves in the inner ear, causing hearing loss. Other reasons can add to the hearing loss, including exposure to toxins, injury (loud noises such as heavily amplified music through headphones), or genetic disorders.

What is tinnitus?

Tinnitus is the perception of sound in the ears or head where no external source is present. In almost all cases, tinnitus is a subjective noise, meaning that only the person who has tinnitus can hear it. It is often referred to as “ringing in the ears.” Persistent tinnitus usually indicates the presence of hearing loss. The exact cause of tinnitus is not known, but there are several likely sources, all of which are known to trigger or worsen the condition. They include noise-induced hearing loss, wax buildup in the ear canal, medicines that are toxic to the ear, ear or sinus infections, head or neck trauma, jaw misalignment (often associated with the temporal mandibular joint), and Meniere’s disease.

How are tinnitus and diabetes connected?

According to many studies, there may be a connection between tinnitus and diabetes. This is because those who experience tinnitus often have elevated blood glucose levels, which can lead to diabetes, or they already have diabetes with uncontrolled blood glucose levels. Because the inner ears depend on oxygen and glucose from the bloodstream for their energy (as in the brain, there are no fat reserves to use for energy in the ears), when a person’s glucose is interrupted, the parts of the ear called the cochlea and vestibular (inner ear) system cannot function well, resulting in tinnitus. In fact, one study indicated that close to 90 percent of the participants who had inner ear problems (called peripheral vestibular disorder) had higher glucose levels and problems metabolizing insulin. This is why many health care professionals suggest that people with diabetes who have “ringing in the ears” keep good control over their blood glucose levels to lessen the effects of-or even help stop the development of-tinnitus.

How does a person taste and smell?

A person’s ability to taste comes from certain cells located in clusters (taste buds) on the tongue, roof of the mouth (palate), and within the lining of the throat. A person tastes thanks to the thousands of nerve endings associated with the taste buds (everyone is born with about 10,000, but lose many with age). In general, each taste bud strongly responds to a certain taste according to its position on the tongue. Sweet receptors are concentrated at the tip of the tongue, while sour receptors are more common at the sides of the tongue. Salt receptors occur most frequently at the tip and front edges of the tongue, and bitter receptors are most numerous at the back of the tongue. These nerve endings help a person to distinguish among spicy, sweet, and sour foods. The nerves also help a person to distinguish smells. Because the aroma of a particular food also contributes to its taste, most people can tell the difference between foods, such as an orange from an apple. This is also why, if a person has diabetes, the sense of smell and taste can also be affected, as the nerve endings can be damaged by high blood glucose levels (for more about neuropathy, see the chapter “How Diabetes Affects the Nervous System”).

Do many people have impaired taste and smell?

According to the National Institute on Deafness and Other Communication Disorders (NIDCD), around 15 percent of adults may have a problem with their ability to taste and/or smell. As with many other conditions, it is estimated that only around 200,000 seek medical help for either condition. In fact, the NIDCD estimates that almost 75 percent of all adults ages 57 to 85 have some type of taste impairment, which is why it is often difficult to persuade older people to eat healthful foods to stay well.

Does type 1 or type 2 diabetes affect a person’s ability to taste and smell?

For many people who have type 1 or type 2 diabetes, the disease will not affect taste or smell. But there are some indications that taste in some people with diabetes can change. For example, one study (although the number of participants was small) indicated that of the people who had type 1 diabetes, around 70 percent had impaired taste, compared with 16 percent who did not have diabetes. In yet another study, participants who were newly diagnosed with type 2 diabetes seemed to have more trouble tasting sweet foods. In addition, some medications can affect a person’s taste (and often smell), including metformin, one of the most common medications to treat type 2 diabetes.

About 70 percent of people with type 1 diabetes have an impaired ability to taste foods.


What are the parts of the eye and their functions?

The major parts of the eye and their functions are summarized in the following chart:

Parts of the Eye




Maintains shape of eye; protects eyeball; site of eye muscle attachment; it is also referred to as “the white of the eye”


Refracts incoming light; focuses light on the retina


Admits light


Regulates amount of incoming light


Refracts and focuses light rays

Aqueous humor

Helps maintain shape of eye; maintains intraocular pressure; nourishes and cushions cornea and lens

Ciliary body

Holds lens in place; changes shape of lens

Vitreous humor

Maintains intraocular pressure; transmits light to retina; keeps retina firmly pressed against choroids


Absorbs light; stores vitamin A; forms impulses which are transmitted to brain; the macula of the retina is responsible for clear, sharp central vision

Optic nerve

Transmits impulses to the brain


Absorbs stray light; nourishes retina

The extra (accessory) structures of the eye include the eyebrows, eyelids, eyelashes, conjunctiva (mucus membrane that covers the front of the eye and inside the eyelids), and tear duct areas (called the lachrymal apparatus). These structures have several functions, including protecting the front portion of the eye, preventing the entry of foreign particles, and keeping the eyeball moist.

Can there be a connection between eye diseases and diabetes?

There is often a connection between diabetes and eye diseases. According to the American College of Family Physicians, almost all people who have had diabetes for 15 years or more-they estimate 98 percent-have some sort of eye disease that is connected with their diabetes. Some studies also suggest that male and female hormonal changes during puberty, along with diabetes, can increase a child’s odds of developing eye problems. One study notes that children who develop type 1 diabetes before age ten have a higher risk of developing diabetic retinopathy as they get older (for more about diabetic retinopathy, see below).

What are cataracts?

According to the American Academy of Ophthalmology, a cataract is a cloudy or opaque lens in a person’s eye. Because the incoming light is not properly reflected onto the light-sensitive tissue lining the back of the eye (the retina), the person’s vision becomes blurred, distorted, or cloudy.

Why do people develop cataracts?

Although cataracts are often associated with people who have diabetes (see below), there are other reasons for them. Cataracts mainly occur in older people and are more common in smokers. Development of cataracts also runs in families. They can also occur in people who take high doses of steroids and/or steroids for a long time; if they take certain other medications; if they have had certain surgeries that contribute to cataract development; or if they have other types of eye diseases and/or eye injuries that lead to the clouding of the lens. In addition, it is thought that eye exposure to the sun also can contribute to cataract development, which is why so many health care professionals recommend wearing sunglasses outside in the sunshine (or on snow or water) and/or having special ultraviolet coating added to eyeglasses.

When the lens of an eye becomes cloudy or opaque, a cataract has formed, compromising one’s vision. Diabetes increases the chance for cataracts to develop.

Do people with diabetes develop more cataracts than those without diabetes?

Yes, according to the American Diabetes Association. In fact, it is estimated that people with diabetes are 60 percent more likely to develop cataracts. It is also estimated that around 20 percent of people who have cataract operations have diabetes. People with diabetes also seem to develop cataracts earlier in life and more quickly than people who do not have the disease.

Why does diabetes often lead to the development of cataracts?

The main reason for the development of cataracts in people with diabetes has to do with blood glucose levels. The eye’s lens-the object that becomes cloudy when a person has cataracts-gets its nutrients from the aqueous humor (the fluid in the front of the eye). When a person’s blood glucose levels are not in control, the sugar levels rise in the aqueous humor and lens. When this occurs, the lens swells, causing the person’s vision to blur. This is why many people with diabetes often mention blurred vision, an increased problem with glare (especially during nighttime driving), and yellowish or faded views around them. As with most problems encountered by people with diabetes, one of the best ways to keep cataracts at bay is to maintain healthy blood glucose levels.

What is diabetic retinopathy?

Diabetic retinopathy is a phrase commonly used to describe many different eye disorders caused mainly by diabetes. It is also the major cause of blindness in the United States among adults ages 20 to 65, and is most associated with people who have diabetes-both types 1 and 2. According to Harvard Medical School, diabetic retinopathy is considered a degenerative eye disorder; it occurs when chronic high blood sugar causes the tiny blood vessels in the retina (capillaries) to break down and leak fluid into the surrounding tissue. Not only can the leakage leave what are called hard exudates, or deposits of fat and protein, but if it is near the macula, it will impair the person’s sight-a condition called macular edema.

Why does diabetes often cause blindness?

According to Harvard Medical School, diabetes is the leading cause of new cases of blindness in people ages 20 to 74. The high blood-sugar level in diabetes weakens blood vessel walls in the retina and choroids of the eye. This weakening, in turn, increases the eye’s susceptibility to hemorrhaging, scarring, and retinal detachment, all of which can contribute to blindness.

These vessels also nourish the eyes, and if the capillaries are damaged, it is more difficult for them to supply nutrients and eliminate wastes from the surrounding tissues. The vessels can also eventually become blocked, and although new vessels form to compensate in the later stages of retinopathy, the newer vessels are more fragile. This fragility can often lead to bleeding into the vitreous humor (the gel-like substance in the inner eye); the bleeding, in turn, temporarily blocks light and causes a sudden change in vision. Although the blood is often reabsorbed, scar tissue forms. This tissue can pull at the retina, often causing retinal detachment. If this condition is not caught in time, it can lead to permanent vision loss or blindness.

What are the major types of diabetic retinopathy?

According to the American Diabetes Association, there are commonly two major types of diabetic retinopathy-proliferative and nonproliferative. In proliferative retinopathy, new blood vessels form as others become blocked. The growth of these new vessels often leads to scar tissue, severe bleeding, and even retinal detachment. The more common type, nonproliferative retinopathy, forms as capillaries in the back of the eye form pockets. This type of retinopathy is also caused when blood vessels become blocked but usually does not require treatment.

How do eye specialists detect diabetic retinopathy?

According to the National Eye Institute, there are certain ways in which an ophthalmologist (eye specialist) can detect diabetic retinopathy in a person with diabetes. During a dilated-eye exam, in which the doctor dilates, or widens, a person’s pupil to better see the retina and optic nerve, the tests include a visual acuity test (using an eye chart to measure a person’s ability to see at various distances), a tonometry test (to measure the pressure inside the eye), and an optical coherence tomography or OCT (similar to an ultrasound but using light waves, not sound waves, to produce an image of the tissues inside the eye).

Normal vision compared to pre-proliferative retinopathy, non-proliferative retinopathy, and proliferative retinopathy.

How does an ophthalmologist treat diabetic retinopathy?

In reality, it is the person with diabetes who can “treat” diabetic retinopathy by keeping his or her blood glucose levels under control. If an ophthalmologist suspects that the patient has diabetic retinopathy, then the doctor may suggest a test called a fluorescein angiogram, in which a fluorescent dye is injected (most often into an arm vein). As the dye reaches the eyes, images of the retinal blood vessels are taken and examined for possible retinopathy. If diabetic retinopathy is confirmed, there are usually two major ways of helping the person’s eyesight. According to the American Academy of Ophthalmology, one procedure uses lasers and is usually performed in a doctor’s office. The laser treatment causes abnormal new vessels to shrink and often prevents them from growing. Another procedure is vitrectomy, which is usually performed in a hospital or surgery center operating room (usually on an outpatient basis). The procedure removes blood and scar tissue that often accompany abnormal blood vessels (that cause the bleeding) in the eye. When such hemorrhaging is eliminated, light rays are able to focus on the retina again.

What is diabetic macular edema?

Diabetic macular edema (DME) is most often associated with diabetic retinopathy. With retinopathy, chronic high blood sugar can cause the blood vessels in the retina to break down and leak fluid. If the leakage is near the center of the macula, it can cause the macula to swell. This condition is called diabetic macular edema. Because the macula is the part of the eye responsible for sharp, straight-ahead vision, the person’s vision will blur. DME is the most common cause of diminished vision in people with diabetes.

Are there ways to prevent macular degeneration?

According to Harvard Medical School, there are several ways to prevent macular degeneration in many people. These include the following: don’t smoke; wear sunglasses (especially in reflective places, such as around the ocean or snow); eat dark, leafy greens (kale and spinach are two of the best sources); exercise (choose a type that fits your lifestyle); if not allergic, eat nuts and oily fish (they both have omega-3, which some research indicates helps prevent macular degeneration); and especially avoid developing diabetes, mainly by eating healthfully and keeping weight in a normal range for your age and height.

Macular degeneration occurs when the center of the retina deteriorates. It affects about 10 million Americans.

What is macular degeneration?

Macular degeneration occurs when a small central portion of the retina called the macula deteriorates. The most common symptoms are slightly blurred vision, difficulty recognizing faces, and the need for brighter light with which to see. According to the American Diabetes Association, age-related macular degeneration is the most common cause of vision loss in people over age 55 (in the United States). Macular degeneration cases are expected to grow as the population ages.

Can a person with diabetes develop eye aneurysms?

Yes, if people with diabetes have damaged blood vessels in the retina from chronic high blood sugar, they can eventually develop what are called microaneurysms, or small sac-like pouches that affect the smaller blood vessels, or capillaries. (The pouches have been associated with diabetic retinopathy since 1879; the microaneurysms of the eye from diabetes were first described in 1943.) The recognition of eye microaneurysms is often used to predict the development of diabetic retinopathy. In some studies, the microaneurysms have been found to be directly correlated with HbA1c values. For example, in one study, the researchers found that every 10 percent increase in HbA1c value (for instance, from 8.1 percent to 9.0 percent) was associated with an increase of 0.7 microaneurysms. This is not a huge amount, but it does indicate that a person’s overall blood glucose level can have an effect on the formation of eye microaneurysms.