PHYSIOLOGY OF NORMAL AGING
Many physiological changes have been found to occur as part of normal aging. There is a decrease in maximal aerobic capacity and maximal heart rate, and an increase in heart rate and blood pressure response to submaximal exercise. There is a decrease in tissue elasticity, muscle strength, muscle power, and endurance. Motor coordination and neural reaction time also decrease. Oxidative and glycolytic capacity decline with a reduction in mitochondrial volume. Gait parameters including step length, cadence, speed, and stability also decrease. Attention span, memory, cognitive processing speed, and accuracy decrease. Rapid eye movement (REM) and slow wave sleep duration decrease. Heat and cold intolerance increases, and temperature regulatory capacity decreases.
Most cardiovascular functions also decrease, including maximal cardiac output, stroke volume, endothelial reactivity, skeletal blood flow, and capillary density. Arterial distensibility and vascular sensitivity also decrease, as does glomerular filtration rate (GFR). Baroreflex function becomes more impaired, and postural hypotension can be exacerbated. Pulmonary function also decreases with aging, including a decline in vital capacity and peak flow rates.
There are also effects on the nutritional status of the body, including decreased metabolic rate and total energy expenditure. Total body water, potassium, calcium, and nitrogen also decrease, as do protein synthesis rates, amino acid uptake into skeletal muscle, nitrogen retention, and protein turnover. Appetite also decreases, and dehydration is more common as thirst decreases while the number of nephrons decreases, and the kidneys become less sensitive to the effects of antidiuretic hormone. Metabolic changes are also part of normal aging. Glycogen storage capacity, glycogen synthase, GLUT-4 transporter protein content, and translocation to the cell membrane all decrease. With aging, lipoprotein lipase activity decreases, while total cholesterol and low density lipoprotein (LDL)-C increase. High density lipoprotein (HDL)-C decreases, so does insulin-like growth factor (IGF)-1 and growth hormone.
Body composition also changes with aging. There is an increase in adipose mass and a decline in fat-free mass (FFM). Adipose tissue deposition patterns shift, increasing both visceral and truncal adiposity. There is a decrease in muscle mass (sarcopenia) and type 2 (fast twitch) muscle fiber types. There is an increase in intramuscular fat and connective tissue, with a decline in muscle quality. There is also decreased bone mass and density, with an increase in bony fragility.