Although humans are aging from the time they are born, aging is generally used to describe adults past their physical peak. In biological terms, aging adults undergo senescence, which is the degeneration of organ systems that takes place between the age of peak physical form and death. Senescence involves a decreased ability to repair damage to cells and tissues, heal from disease, and recover from physical stress, as well as an increased susceptibility to disease.Senescence occurs at different rates across all body systems. Aging adults lose bone mass or bone density in a condition called osteopenia, or the more severe osteoporosis. Females lose more bone mass than males. In females estrogen production is directly related to protecting bones, and after menopause, the reduction in estrogen production results in bone loss. These skeletal changes increase the risk of fractures and decrease the ability to heal from them. Osteoarthritis, which is degeneration of cartilage and bone in joints, and degenerative joint diseases become more common. In the integumentary system, hair grays and thins, sebaceous glands atrophy, skin loses elasticity and sensory function, and blood vessels become fragile. Senescence of the muscular system leads to weakness and muscular atrophy. In the nervous system, adults lose brain tissue and synapses. Although the aging adult nervous system continues to produce new synapses, the process is slower than in a child's brain, and loss in aging adults exceeds production, thus reducing the number of synapses. Motor coordination, short-term memory, and intellectual function diminish. Several sensory organs also lose function; hearing, vision, and the ability to taste and smell all begin to decline. Glaucoma and cataracts are degenerative eye conditions that reduce vision. Glaucoma increases pressure within the eye, which results in loss of vision. Cataracts are a clouding of the eye lens, which is caused by the breakdown of natural proteins in the eye. This normally begins to occur at 40 years of age, but by age 60, clouding is more severe and may require corrective surgery to remove.
Bone Mass Declines in Aging Adults
Senescence of the respiratory system occurs because of the thoracic cage's decreased flexibility and a decline in lung alveoli. This change causes reduced pulmonary function, which decreases oxygen levels to the brain and muscles. Because organ function is directly related to oxygen flow through the blood, reduction of oxygen levels shows negative effects on brain and muscle function. The urinary system exhibits marked senescence, as kidneys atrophy significantly over time. Older adults are thus susceptible to water imbalances and problems clearing toxins and medications. Many elderly males experience an enlarged prostate and increased urine retention. The enlarged prostate reduces urine flow through the ureter, which results in an inability to efficiently empty urine from the bladder. Aging females are prone to increased urinary incontinence. In the aging digestive system, the stomach atrophies and liver function declines. Older adults experience decreased salivation and have poorer dental health. They have more difficulty in swallowing and have increased issues with gastroesophageal reflux, constipation, and loss of appetite.
Diseases Associated with Aging
As adults age and undergo senescence, they are more likely to develop or contract diseases and become less able to resist or recover from them. The incidence of type 2 diabetes, which is an endocrine disease in which blood glucose levels are too high, climbs as target cells have a reduced number of insulin receptors because of aging of the brain. A reduction in insulin receptors makes it more difficult for the body to maintain normal metabolism, resulting in increased thirst, irritability, fatigue, and/or confusion. In addition, increased body fat makes cells less sensitive to insulin, so overweight adults at any age are more likely to develop type 2 diabetes. Aging adults experience muscle atrophy, which increases their percentage of body fat and raises their likelihood of developing type 2 diabetes.
Senescence in the cardiovascular system contributes significantly to the leading causes of human death. Increased incidence and severity of atherosclerosis causes heart muscles to weaken and increases the likelihood of heart failure, myocardial infarction (heart attack), stroke, aneurysm, and organ atrophy. Risks of these events are increased by the rise in hypertension common in aging adults. Heart failure, or congestive heart failure, is the condition in which the heart pumps blood inadequately. It can result from several conditions, including coronary artery disease, high blood pressure, and type 2 diabetes—all ailments that are more common in older adults. Heart failure causes fluid buildup in lungs and other body parts, which makes the heart work even harder. Myocardial infarctions are triggered by blocked coronary arteries. They cause tissue death in the heart. Strokes typically occur when an artery to the brain is blocked or ruptures, and strokes result in brain tissue death. Severe myocardial infarctions or strokes are likely to cause death or long-term disability. An aneurysm is a bulge in the wall of the aorta, typically as it passes through the thorax (chest) or the abdomen. A large aneurysm can rupture and will cause death if not repaired.
Respiratory diseases also become more common with age. Reduced pulmonary function drastically increases the risk of pneumonia, especially in very elderly individuals. Pneumonia and chronic obstructive pulmonary diseases (COPDs), such as emphysema and chronic bronchitis, are significant causes of death. COPDs become serious during senescence because their degenerative effects build over long periods of time.
Dying and Death
In biological terms, death is when a body completely ceases respiratory or circulatory functions or when the brain ceases electrical activity. Whether an individual's dying process takes place rapidly or over a long period of time, death results from failures of multiple body systems. Failure of one organ can trigger cascading effects in other organs. For instance, a prolonged disease affecting the kidneys can lead to a dangerous buildup of wastes in the blood. High toxicity levels can, in turn, diminish brain function and respiration and eventually cause cardiac arrest when the heart stops beating. A rapid death such as that from a myocardial infarction (heart attack) affects multiple systems quickly. Cardiac arrest shuts down cardiopulmonary functions, depriving all other body systems of needed blood and oxygen. In turn, a brain deprived of oxygen halts processes critical to sustaining life.
For many elderly people, organ senescence, vulnerability to infection, and inability to fight disease eventually bring on life-threatening conditions that lead to death. Older adults may experience long periods in which they deal with health complications before they encounter one—or a combination of many—that proves fatal.
The actual instance of biological death is hard to define, particularly as some organs continue to function for a brief period after heart activity ends. Legally, death was previously considered to occur at the moment the heart stopped beating and respiration ceased. Now, however, cardiopulmonary function can be maintained by artificial means in a hospital. The medical community currently uses brain death as the necessary condition to declare an individual legally dead. Brain death is defined as the permanent loss of brain activity—a condition that shuts down respiration and all other vital functions. Doctors use specific criteria to diagnose and confirm brain death.In the United States, the leading cause of death is heart disease, followed closely by cancer. Unintentional injuries, respiratory diseases, and strokes make up the next most common causes of U.S. deaths. However, these occur at much lower rates than heart disease and cancer. The remainder of the top ten causes of U.S. deaths are, in descending order, Alzheimer's disease, type 2 diabetes, influenza and pneumonia, kidney disease, and suicide.