Physiology of Aging
Aging is a process that begins at conception and continues for as long as we live. At any given time throughout our lifespan, the body reflects:
In other words, our bodies reflect our genetic capacity to adapt and repair, as well as the cumulative damage from disease processes. Aging highlights our strengths and our weaknesses.
In our society we currently think of the "young old" as being around 65 to 74 years of age, the "middle old" 75 to 84 and the "old old" 85 years +. With advancing age, all of the body systems eventually demonstrate reduced efficiency, slowed building & replacement and actual loss of tissue. While an individual' s aging experience is unique, there are generalizations which can be observed for each of the body systems.
The primary function of the skin is to protect the organism from the environment. It accomplishes this by providing a barrier that regulates temperature, retains fluid and absorbs shock and ultraviolet radiation, among other things. As we age, the dermis decreases in thickness by about 20%. As it thins it loses vascularity, cellularity and sensitivity. Its ability to exchange or retain internal heat is diminished. The skin becomes thin, fragile and slow to heal. Sweat and sebaceous glands are reduced both in number and effectiveness. Sensory neurons are decreased by 30% from the age of 10 years to 90 years old.
Subcutaneous fat deposition is altered in the elderly. Muscle, blood vessels and bone become more visible beneath the skin due to thinning of subcutaneous fat on the extremities. Fat deposition occurs mainly on the abdomen and thighs.
Muscle mass is a primary source of metabolic heat. When muscles contract, heat is generated. The heat generated by muscle contraction maintains body temperature in the range required for normal function of its various chemical processes.
As early as the third decade of life there is a general reduction in the size, elasticity and strength of all muscle tissue. The loss of muscle mass continues throughout the elder years. Muscle fibers continue to become smaller in diameter due to a decrease in reserves of ATP, glycogen, myoglobin and the number of myofibrils. As a result, as the body ages, muscular activity becomes less efficient and requires more effort to accomplish a given task. The elderly are less efficient at creating the heat necessary to drive the important biochemical reactions necessary for life.
Lung function diminishes with age. The major contributing factors are the progressive loss of elastic recoil within lung tissue, the chest wall becomes stiff, and there is a decrease in alveolar surface area. These changes diminish the efficiency of gas exchange and make it more difficult to exercise.
Despite cardiovascular disease, often combined with a slowdown in the autonomic nervous response, the cardiovascular function of a resting healthy elder is usually adequate to meet the body's needs. Cardiac output of healthy exercising elders can usually be maintained, allowing moderate continued physical activity throughout their lives.
Endocrine and metabolism
Old age is accompanied by a generalized reduction in hormone production and activity. This reduction affects most metabolic functions of the body. Water,mineral, electrolyte, carbohydrate, protein, lipid and vitamin disorders are all more common in the elderly. Nutrition and the ability to use food for energy is seriously affected in the elder population.
Diabetes is common in the elderly. There are many causes but a primary mechanism involves the inability of skeletal muscle to absorb glucose. Over time skeletal muscle becomes less responsive to insulin.
Recent research indicates that the elderly are at risk for nutritional deficiencies due to anorexia. Age related anorexia has been linked to a lower satiety threshold. Elders feel "full" sooner which may be due to changes in hormone receptor or trigger mechanisms.
Like other systems, the nervous system changes with age. There is loss of neurons in both the brain and spinal cord. There is loss of neuronal dendrites which reduces the amount of synaptic transmission. The sense of smell, taste, sight, touch and hearing are all diminished over time. Depression can be the result of impaired synaptic activity. Research indicates that as many as 25% of nursing home residents are clinically depressed. Depression is one of the most common reversible causes of weight loss.
Clearly elders are at a disadvantage when it comes to generating metabolic heat. They have less muscle mass and therefore less generating apparatus. They have less alveolar surface, therefore less oxidative reserve. Their skin provides less protection from heat loss. They have impaired neurotransmission, therefore less ability and/or desire to initiate activity. All of these factors put the elderly at risk for hypothermia if their environmental circumstances expose them to heat loss greater then their resting heat generating capacity.