Biology of Human Aging

Khalil Khollesi

Introduction to human aging

• Aging is a complicated process involving all of the many subtle changes that occur in our bodies with the passage of time.

• These changes tend to decrease our ability to withstand stress, and death is the inevitable end result.

Focus of the book:• Considers changes that occur in body structure — at both the microscopic and the

macroscopic levels — as we get older and the functional restrictions and impairments that are often associated with increasing age.

• Although age-related dys-functions is an important part of overview of aging, aging is a normal process and does not, by itself, necessarily cause nay dysfunctions.

• Factors such as genetics, lifestyle, and environment are thought more likely to cause dysfunctions than is age alone.

Methods used to study aging • Longitudinal study

– Baltimore Longitudinal Study of Aging (BLSA): >1000 men, since 1978 women– Humans have long life spans (difficult to study generations)

• Cross-sectional studies– Study parameters are measured and compared simultaneously for persons in

various groups (20-30 years old or 50-60 years old)– Allows to obtain average value– Can be conducted rapidly in large groups – Difficult to distinguish between age-related changes from environmental effects

Rationale for using lower animals– Humans are among the most hybridized species– Lower animals are genetically uniform– Easier to control their environment– Extrapolation of aging processes from one phylum to another is risky

General effects of aging

Biological aging:

• doesn’t happen only to old people

• It is a continuing, normal process that begins at maturity and eventually ends with death

The manner in which a person ages is unique to that person and is influenced by interactions between genetics and environmental factors

• Chronological age: is measured in years

• Physiological age: measure of functional capacity

“These two usually do not coincide”

A person may look younger or older than his/her age and may have greater or lesser functional capacities than expected for someone of that age

• Aging is a normal process and by itself does not cause a disease

• A well-established relationship between aging and disease exists

• Death rates for all major diseases increase with age

– Largely due to a decline in the ability of the elderly to respond to stress

“General effects that are fundamental to the aging process”

Lowered functional capacity

– Due to tendency of the structural elements in the body to deteriorate

– Older persons are incapable of responding to external and/or internal stimuli

as effectively as younger persons

– Difficult to maintain stable homeostasis (chemical and physical states)

– Absence of efficient homeostasis, organ dysfunction is more prevalent, thus,

aging increases the likelihood of death

Common terms related to aging

• Aging: the process of growing old, regardless of chronological age

• Senescence: the group of deleterious effects that lead to a decrease in the efficient

functioning of an organism with increasing age increased probability of death

• Aging and senescence describe the same process, though, senescence is more

restrictive; often refers to the state of old age characteristics of the later years of

life (implies pathological effects)

• Senility: physical and mental deterioration often associated with old age

WHO: elderly: 60-75 years old old: 76-90 years old very old: >90 years old

• Gerontology: scientific study of aging and associated problems

(biological, sociological, historical)

• Geriatrics: health care delivery for the elderly. Branch of medicine that treats the

conditions and diseases associated with aging

• Longevity: duration of an individual’s life (life span)

a. Mean longevity: average longevity of a population

added ages of all population at death/number of individuals in the population

This is life expectancy; is continuously increasing due to better sanitation and medical care (1776: 35, 1975: 72, unlikely to exceed 85)

b. Maximum longevity: the age at death of the longest-lived member of the population

• Demography: statistical study of human populations collectively

Includes: geographic changes & trends in birth, marriages, diseases, deaths

Rates of aging

• All organs in an individual do not age at the same rate.

• Any specific organ does not necessarily age at the same rate in different individuals

• Thus, different individuals undergo aging at different rates.

• Since aging occurs at different rates in different individuals, the goal of most studies on aging is not so much to significantly interfere with the process of senescence as it is to attempt to understand the aging process.

This way diseases may be recognized and treated earlier.

General Aging Changes in the Body

• Though changes in our body, as we get older, alert us to the fact that AGING

is occurring, in most cases we are unaware of the many cellular changes

that are directly responsible for these structural and functional alterations

Rather, the signs of aging most obvious to the individual occur in the structure

and composition of the body and in the functionality of body systems

• Body Structural Changes

• Body Compositional Changes

• Body Functional Changes

Body Structural Changes

• Cross-sectional studies (measure various parameters in various people in the same age group) show loss of height with age (around 30), with slow progress afterwards

• Longitudinal studies (obtained from the same individual over a number of years): small increase in height beyond 30 years gradual decline start at 40

• Loss of height: equally divided between trunk length and lower limb length

1. The length of bones in lower limbs shows minimal age-related changes, loss in lower limb length is due to changes in the joints and in the arches of the feet

2. Loss in trunk length due to increase in normal anterior-posterior curvatures of the spinal column and compression of the discs between the spinal vertebrate

Body Structure Changes (continued)

1. Decrease in shoulder width (gradual muscle deterioration w/ age), increase in hip diameter with age (increased fat deposition)

2. Loss of lung elasticity w/ age breathing needs effort; circumference of the thorax increases w/ age

3. Different patterns of changes in body weight in men and women:

– Men: increase in weight till mid. 50s gradual loss– Women: continue to gain weight till 60s start to lose (slower rate than in men)

Body Compositional Changes

Non-invasive methods to measure body composition change without sampling body tissues:

1. measurement of total body water

• Administration of indicator compound (enters body water) analyze the indicator dilution: provides estimation of amount of fat in body

• Fat contains little water and affects dilution of indicator

2. Measurement of the specific gravity (SG) of the body

Weighing an individual in air and then in water

SG measures the buoyancy of the body (indicative of the amount of fat)

3. Measuring the skinfold thickness in specific areas

Indication of subcutaneous (just below the skin) fat present

Variations in these parameters:1. Variations in Total Body Water (TBW)

Young man: TBW that is 60% of body weight is average age: 54%Young woman: TBW of 52% of body weight is average TBW w/ age: 46%

This reduction is not primarily due to dehydration, rather changes in the ration of lean to fat, thus reducing the space available to accommodate body water

2. Changes in skinfold thickness

Depends on where the measurement is take: The skinfold thickness increases in some areas until later in life (abdominal wall)

Other areas: gradual but steady decrease in skinfold thickness with age

Skinfold measurements in women (most areas) remains constant till age 64

The results obtained by skinfold measurement suggest that most of the extra adipose tissue that accumulates in the elderly is laid internally, rather than just beneath the skin in or around organs or in the mesenteries that attach the intestine to the body wall

•Decline in the diameter of calf and arm with age, while the amount fat remains relatively constant in these structure suggestive of loss of lean tissue with age

Explains characteristics of older persons: thinner extremities and thicker trunk

Major body water storage comportments

• Intracellular water (within individual cells)• Extra-cellular water (spaces between the cells)

• They both decrease with age, but not equally• Greater amount of reduction in fluid occurring w/ age is loss

of water from the cellsthe cells than the extra-cellular space– Certain amount of dehydration w/ age– Loss in the total number of body cells

• Most organs show age-related mass reduction

(most of the loss of lean body mass occurs in muscles)

Body functional changes

Though numerous changes exist in the functioning of the body systems (discussed in future lectures), here we focus on changes that a person experiences senescence are most

likely noticeable

Movement: becomes more difficult as joints stiffen and muscle strength lessens

Breathing: Oxygen and carbon dioxide exchange in lungs become less efficientmakes it difficult to breath in older persons

Elevated blood pressure: changes in the strength of hearth muscle and artery walls Digestive system functional changes require alterations in the dietary habits Urinary system: increased frequency of urination is one of the most common functional

changes

Body functional changes responsible for nursing home care for 1.6 M persons >65 in 1990, they don’t occur to the same degree in all aging persons

Functional changes are largely dependent on: genetic make up, lifestyle history, and environmental factors

Importance of Regular Health Examination