Human life span, or longevity, is divided into two parts: mean longevity (also known as life expectancy) and maximal longevity. The average age of death of all individuals in a population is referred to as mean longevity. Human life expectancy has risen throughout history.
Biology of Aging
In the
late eighteenth century, for example, life expectancy in the United States was
thirty-five years. It has risen to 72 years by the final quarter of the
twentieth century. Maximum longevity, the second component of life span, is the
age at which the most long-lived individuals in a group die.
In humans,
this is difficult to calculate, however it is widely regarded to be between 110
and 120 years. The trend
of increasing life expectancy has been linked to advancements in diet,
sanitation, and medical treatment.
In
reality, maximum longevity appears to be independent of these environmental
influences and is an absolute limit, most likely set by gene activity.
The genes
that govern maximal lifespan are thought to be in charge of fixing faults in
genetic information, mending errors in protein synthesis, and determining the
moment of death.
Aging Changes that Occur in Humans
The skin
and its derivatives exhibit some of the most visible age-related changes in
humans. These include pigment loss in the hair, skin wrinkling, pigment
increase in the skin, and nail thickening.
Other visible changes include a decrease in size due to muscle and bone mass loss, a decrease in muscle strength, a decrease in joint mobility, and a variety of neurological changes such as decreased sensory function (vision, hearing, smell, and taste), increased response time, and a decreased capacity for learning and memory.
The latter
have been linked to a decrease of brain mass, which has been linked to a loss
of brain cells.
Less
obvious alterations include a drop in metabolic rate, impaired kidney, lung,
and pancreatic function, cardiovascular disease, impaired immunological
function, increased susceptibility to cancer, and a decline (in males) or
cessation (in females) of reproductive function. All of these alterations have
been linked to cellular events and processes explained by various aging
hypotheses.
Theories of Aging
It is well
acknowledged that the aging process cannot be attributed to a single factor. A
variety of ideas have been presented to explain the changes that occur as
people age.
The
changes provided by the theory must match the following requirements in order
to be a good contender for an explanation of the aging process:
(1)
they will occur in all or most individuals
(2) as
a person ages, these changes will become more prominent
(3)
the alterations will lead to cellular or organ malfunction, resulting in organ
or system failure. The following are the most widely accepted reasons for the
aging process.
Free Radicals.
Free
radicals are highly reactive chemical particles that contain an unpaired
electron. Aerobic metabolism, as well as radiation and other environmental
factors, create them. Their consequences are far-reaching.
They change or degrade the structure of many other molecules in the cell, impairing their activities. Proteins having enzymatic, structural, and regulatory roles react with free radicals. They create breaks in deoxyribonucleic acid (DNA), altering the information required for protein synthesis.
They
induce lipids to clump together, causing cell membranes to degrade.
Their effects on carbs have received little
attention. Free radicals are most prevalent in mitochondria, which are cellular
organelles where oxidative processes occur.
Mitochondrial
damage, particularly mitochondrial DNA damage, has been hypothesized as a role
in the aging process. Certain enzymes (superoxide dismutase and catalase) that
disrupt the cycle of processes that generate free radical damage reduce their
effects. Vitamins C and E, for example, defend against free radical damage by
quenching the reactions.
Crosslinking of Proteins
Proteins
can be transformed by the spontaneous and uncontrolled combining of protein
molecules by glucose, in addition to the impacts of free radicals. The result
of all of this glycosylation is that the proteins cling together.
The fibrous extracellular protein collagen, present in connective tissue, for example, stiffens as a result of this process, contributing to skin wrinkling and joint mobility loss.
Events
that have an impact on genetic material. Mutations, or alterations in DNA, are
widespread and can cause changes in protein structure and function.
There are a variety of processes that can repair these mutations, but their efficiency may reduce with age since they are carried out by enzyme proteins, which are degraded by the aging process. Another theory is that certain genes are responsible for the demise of individual cells.
It is also
known that cells in tissue culture will only divide a fixed amount of times.
This limit is roughly fifty cell divisions in human cells.
The
increasing shortening of the telomere, the portion of each DNA molecule
essential for starting DNA replication, has been proposed as a preliminary
explanation for the so-called Hayflick limit (named for the scientist who
originally described it).
As the telomere shortens, an increasing number of errors develop in the duplicated DNA.
Hormonal Effects
Normally, these chemical messengers have well-controlled effects on physiological tissues. Abnormally high levels of some hormones might alter tissue sensitivity to the hormones and promote the release of other hormones, the uncontrolled consequences of which could be harmful. Candidates for this process include insulin, growth hormone, glucocorticoid hormones, and reproductive hormones.
Immune System Alterations
This body's principal defensive system may undergo two
types of changes, either of which may contribute to the aging process.
First, the
immune system may progressively lose its capacity to differentiate between
bodily cells and foreign cells, culminating in an immunological attack on the
body itself.
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