Learning Objective:
Understand the physiologic changes associated with normal aging across major organ systems and recognize how these changes increase susceptibility to disease without necessarily causing pathology.
Normal Aging
Normal aging is a time-dependent, progressive decline in organ function that leads to increased vulnerability to disease and reduced physiologic reserve. These changes result from cumulative genetic (eg, telomere shortening), epigenetic (eg, DNA methylation), and metabolic (eg, mitochondrial dysfunction) alterations rather than a single disease process.
Cardiovascular
- ↓ Arterial compliance → ↑ arterial stiffness
- ↑ Aortic diameter
- ↓ Left ventricular cavity size with a sigmoid-shaped interventricular septum due to myocardial hypertrophy
- ↑ Left atrial size
- Aortic and mitral valve calcification
- ↓ Maximum heart rate
Gastrointestinal
- ↓ Lower esophageal sphincter tone
- ↓ Gastric mucosal protection
- ↓ Colonic motility
Hematopoietic
- ↓ Bone marrow cellularity
- ↑ Bone marrow fat
- Reduced ability to respond to stressors (eg, hemorrhage, infection)
Immune (Immunosenescence)
- Predominant decline in adaptive immunity
- ↓ Naïve B and T cells
- Preserved memory B and T cells
- Impaired response to new antigens, including pathogens and vaccines
Musculoskeletal
- ↓ Skeletal muscle mass (sarcopenia)
- ↓ Bone mass (osteopenia)
- Thinning of joint cartilage
Nervous System
- ↓ Brain volume due to neuronal loss
- ↓ Cerebral blood flow
- Overall neurologic function largely preserved despite mild cognitive slowing
Special Senses
- Impaired accommodation (presbyopia)
- ↓ Hearing (presbycusis)
- ↓ Smell and taste
Skin
- Epidermal and dermal atrophy with flattening of the dermal–epidermal junction
- ↓ Dermal collagen and elastin → wrinkles, senile purpura
- ↓ Sweat glands → increased risk of heat stroke
- ↓ Sebaceous glands → xerosis cutis
Types of Skin Aging
- Intrinsic aging: Chronologic aging with ↓ biosynthetic activity of dermal fibroblasts
- Extrinsic aging (photoaging): UVA-induced degradation of collagen and elastin with accumulation of abnormal elastic fibers (solar elastosis)
Renal
- ↓ Glomerular filtration rate due to ↓ nephron number
- ↓ Renal blood flow
- ↓ Hormonal function
- Increased voiding dysfunction (eg, urinary incontinence)
Reproductive
- Males: Testicular atrophy with ↓ spermatogenesis, prostate enlargement, slower erection and ejaculation, prolonged refractory period; libido relatively preserved
- Females: Vulvovaginal atrophy, vaginal shortening and dryness, ↑ vaginal pH due to ↓ estrogen from ovarian follicle depletion (menopause)
Respiratory
- ↑ Lung compliance due to ↓ elastic recoil
- ↓ Chest wall compliance (↑ stiffness)
- ↓ Respiratory muscle strength
- ↓ FEV₁ and FVC, ↑ residual volume (total lung capacity unchanged)
- ↑ A–a gradient and ventilation–perfusion mismatch
- Blunted ventilatory response to hypoxia and hypercapnia
- Less effective cough and slower mucociliary clearance
