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All vital organs begin to lose some function as you age during adulthood. Aging changes occur in all of the body's cells, tissues, and organs, and these changes affect the functioning of all body systems.

Living tissue is made up of cells. There are many different types of cells, but all have the same basic structure. Tissues are layers of similar cells that perform a specific function. The different kinds of tissues group together to form organs.

There are four basic types of tissue:

Connective tissue supports other tissues and binds them together. This includes bone, blood, and lymph tissues, as well as the tissues that give support and structure to the skin and internal organs.

Epithelial tissue provides a covering for superficial and deeper body layers. The skin and the linings of the passages inside the body, such as the gastrointestinal system, are made of epithelial tissue.

Muscle tissue includes three types of tissue:

• Striated muscles, such as those that move the skeleton (also called voluntary muscle)
• Smooth muscles (also called involuntary muscle), such as the muscles contained in the stomach and other internal organs
• Cardiac muscle, which makes up most of the heart wall (also an involuntary muscle)

Nerve tissue is made up of nerve cells (neurons) and is used to carry messages to and from various parts of the body. The brain, spinal cord, and peripheral nerves are made of nerve tissue.

AGING CHANGES

Cells are the basic building blocks of tissues. All cells experience changes with aging. They become larger and are less able to divide and multiply. Among other changes, there is an increase in pigments and fatty substances inside the cell (lipids). Many cells lose their ability to function, or they begin to function abnormally.

As aging continues, waste products build up in tissue. A fatty brown pigment called lipofuscin collects in many tissues, as do other fatty substances.

Connective tissue changes, becoming more stiff. This makes the organs, blood vessels, and airways more rigid. Cell membranes change, so many tissues have more trouble getting oxygen and nutrients, and removing carbon dioxide and other wastes.

Many tissues lose mass. This process is called atrophy. Some tissues become lumpy (nodular) or more rigid.

Because of cell and tissue changes, your organs also change as you age. Aging organs slowly lose function. Most people do not notice this loss immediately, because you rarely need to use your organs to their fullest ability.

Organs have a reserve ability to function beyond the usual needs. For example, the heart of a 20-year-old is capable of pumping about 10 times the amount of blood that is actually needed to keep the body alive. After age 30, an average of 1% of this reserve is lost each year.

The biggest changes in organ reserve occur in the heart, lungs, and kidneys. The amount of reserve lost varies between people and between different organs in a single person.

These changes appear slowly and over a long period. When an organ is worked harder than usual, it may not be able to increase function. Sudden heart failure or other problems can develop when the body is worked harder than usual. Things that produce an extra workload (body stressors) include the following:

• Illness
• Medicines
• Significant life changes
• Sudden increased physical demands on the body, such as a change in activity or exposure to a higher altitude

Loss of reserve also makes it harder to restore balance (equilibrium) in the body. Drugs are removed from the body by the kidneys and liver at a slower rate. Lower doses of medicines may be needed, and side effects become more common. Recovery from illnesses is seldom 100%, leading to more and more disability.

Side effects of medicine can mimic the symptoms of many diseases, so it is easy to mistake a drug reaction for an illness. Some medicines have entirely different side effects in the elderly than in younger people.

AGING THEORY

No one knows how and why people change as they get older. Some theories claim that aging is caused by injuries from ultraviolet light over time, wear and tear on the body, or byproducts of metabolism. Other theories view aging as a predetermined process controlled by genes.

No single process can explain all the changes of aging. Aging is a complex process that varies as to how it affects different people and even different organs. Most gerontologists (people who study aging) feel that aging is due to the interaction of many lifelong influences. These influences include heredity, environment, culture, diet, exercise and leisure, past illnesses, and many other factors.

Unlike the changes of adolescence, which are predictable to within a few years, each person ages at a unique rate. Some systems begin aging as early as age 30. Other aging processes are not common until much later in life.

Although some changes always occur with aging, they occur at different rates and to different extents. There is no way to predict exactly how you will age.

TERMS TO DESCRIBE TYPES OF CELL CHANGES

Atrophy:

• Cells shrink. If enough cells decrease in size, the entire organ atrophies. This is often a normal aging change and can occur in any tissue. It is most common in skeletal muscle, the heart, the brain, and the sex organs (such as the breasts and ovaries). Bones become thinner and more likely to break with minor trauma.
• The cause of atrophy is unknown, but may include reduced use, decreased workload, decreased blood supply or nutrition to the cells, and reduced stimulation by nerves or hormones.

Hypertrophy:

• Cells enlarge. This is caused by an increase of proteins in the cell membrane and cell structures, not an increase in the cell's fluid.
• When some cells atrophy, others may hypertrophy to make up for the loss of cell mass.

Hyperplasia:

• The number of cells increases. There is an increased rate of cell division.
• Hyperplasia usually occurs to compensate for a loss of cells. It allows some organs and tissues to regenerate, including the skin, lining of the intestines, liver, and bone marrow. The liver is especially good at regeneration. It can replace up to 70% of its structure within 2 weeks after an injury.
• Tissues that have limited ability to regenerate include bone, cartilage, and smooth muscle (such as the muscles around the intestines). Tissues that rarely or never regenerate include the nerves, skeletal muscle, heart muscle, and the lens of the eye. When injured, these tissues are replaced with scar tissue.

Dysplasia:

• The size, shape, or organization of mature cells becomes abnormal. This is also called atypical hyperplasia.
• Dysplasia is fairly common in the cells of the cervix and the lining of the respiratory tract.

Neoplasia:

• The formation of tumors, either cancerous (malignant) or noncancerous (benign).
• Neoplastic cells often reproduce quickly. They may have unusual shapes and abnormal function.

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As you grow older, you will have changes throughout your body, including changes in:

Baynes JW. Aging. In: Baynes JW, Dominiczak MH, eds. Medical Biochemistry. 5th ed. Philadelphia, PA: Elsevier; 2019:chap 29.

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Fillit HM, Rockwood K, Young J, eds. Brocklehurst's Textbook of Geriatric Medicine and Gerontology. 8th ed. Philadelphia, PA: Elsevier; 2017.

Walston JD. Common clinical sequelae of aging. In: Goldman L, Schafer Al, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 22.

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Updated by: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.