Pregnancy, Growth, and Development Part 3: Postnatal Life and Aging

Overview and Introduction to the Postnatal Period

The postnatal period is defined as the stage of the human life cycle that lasts from birth until death. Immediately following birth, both the mother and the newborn undergo significant physiological and structural changes to adapt to the separation of their systems. The postnatal period is subdivided into the following chronological stages:

• The Neonatal Period

• Infancy

• Childhood

• Adolescence

• Adulthood

• Senescence

A critical conceptual takeaway is that dying is considered a natural and integral part of the life cycle.

The Neonatal Period ($\text{Birth to end of 4th week}$)

This period spans from the moment of birth until the end of the fourth week of life. During this time, the newborn must transition to independent biological functioning, including:

• Respiration: The newborn must begin carrying on respiration. The first breath must be exceptionally powerful to expand the lungs, which were collapsed in the womb. Lung expansion is facilitated by surfactant, which reduces surface tension within the lungs.

• Nutrition and Digestion: The newborn must obtain and digest its own nutrients. For the first few days, the infant relies on stored fats and colostrum for energy.

• Waste Excretion: The newborn must begin excreting its own metabolic wastes. For the first few days, the newborn secretes dilute urine, as water and electrolyte balance mechanisms may not be fully functional.

• Thermoregulation: The newborn must regulate its own body temperature, although these mechanisms may remain unstable for several days.

• Cardiovascular Adjustments: The circulatory system must undergo rapid structural changes to shift from fetal circulation to independent circulation.

Cardiovascular Adjustments in the Newborn

The transition from fetal to neonatal life involves specific structural transformations as outlined in the following table:

Infancy ($\text{End of 4th week to one year of age}$)

Infancy is characterized by rapid development and growth:

• Growth Rate: The growth rate is very high; an infant may triple its birth weight within the first year ($1\,\text{year}$).

• Physical Development: Primary teeth begin to erupt. The muscular and nervous systems mature, leading to coordinated movement.

• Motor Milestones: The infant begins to reach for and grasp objects, sit, crawl, and eventually stand.

• Communication: Social and communicative behaviors begin, including smiling, laughing, responding to sounds, and potentially speaking a few words.

• Nutritional Needs: Specific requirements for growth include proteins, calcium, vitamin $D$, iron, and vitamin $C$.

Childhood ($\text{One year of age to puberty}$)

Children continue to experience a high growth rate and refined biological control:

• Dentition: Primary teeth continue to erupt and are eventually replaced by permanent secondary teeth.

• Motor Control: High degrees of voluntary muscular control are achieved, enabling walking, running, and climbing.

• Physiological Control: Bladder and bowel controls are established.

• Cognitive and Emotional Maturity: Communication skills expand to include speaking, reading, writing, and complex thinking. Emotions continue to mature during this stage.

Adolescence ($\text{Puberty to adulthood}$)

Adolescence marks the transition to physical and reproductive maturity:

• Reproduction: The individual becomes reproductively functional. Secondary sex characteristics develop under hormonal control.

• Growth: Significant growth spurts occur in both the skeletal and muscular systems, often accompanied by an increased appetite.

• Gender Differences: Controlled by hormones, females typically mature and reach full growth at earlier ages than males.

• Maturity: Motor skills, intellectual abilities, and emotional maturity continue to develop and increase.

Adulthood ($\text{Adolescence to old age}$)

Also referred to as maturity, this stage involves the maintenance of the body followed by the onset of decline:

• Peak Performance: Muscle strength typically peaks in the $20$s for both genders.

• Efficiency Loss: By age $30$, the body becomes functionally less efficient every year.

• Degenerative Changes ($30$s and $40$s): After age $30$, senses become less acute and ligament elasticity decreases. During the $40$s, metabolic rates decrease (leading to weight gain), hair may gray or fall out, muscles lose strength, skin may wrinkle, and arteries/arterioles narrow.

• Aging in the $50$s: Close vision is often lost, nail growth slows, taste buds die, and skin loses elasticity. Women complete the cessation of menstrual cycles. Muscle mass and weight begin to decrease.

• Aging in the $60$s and $70$s: The $60$s may bring some memory loss, though intellect usually remains sharp. By age $70$, height has typically decreased by $1\,\text{inch}$, skin sags, and connective tissue is lost.

Senescence ($\text{Old age to death}$)

Senescence is the biological process of growing old through the accumulation of degenerative changes:

• Physical Decline: The body becomes less able to cope with physical demands. Cartilage wears away in joints, causing stiffness and pain.

• Cellular and Immune Decline: Cell division in certain types declines. Immune responses weaken, and sensory functions decline.

• Cognitive Impact: Some intellectual functions may be lost.

• Cause of Death: Death results from disturbances to body systems (mechanical disturbances in the cardiovascular system) or diseases affecting vital organs.

Statistics on Life Expectancy and Mortality

• Human Life Span: The theoretical maximum human life span is approximately $120\,\text{years}$. Most individuals die from injury or disease before reaching this age.

• Life Expectancy: This is a projection of how long a person will live based on epidemiology. In countries where life expectancy exceeds $80\,\text{years}$, people have survived past the typical ages for cancer and cardiovascular disease.

• $2014$ U.S. Statistics:     - Life expectancy for males: $76.4\,\text{years}$.     - Life expectancy for females: $81.2\,\text{years}$.

The Ten Leading Causes of Death in the United States ($2014$)

1. Heart disease: $23.4\,\%$ of total deaths

2. Cancer: $22.5\,\%$ of total deaths

3. Chronic lower respiratory diseases: $5.6\,\%$ of total deaths

4. Unintentional injuries: $5.2\,\%$ of total deaths

5. Stroke: $5.1\,\%$ of total deaths

6. Alzheimer disease: $3.6\,\%$ of total deaths

7. Diabetes mellitus: $2.9\,\%$ of total deaths

8. Influenza and pneumonia: $2.1\,\%$ of total deaths

9. Nephritis, nephrotic syndrome, and nephrosis: $1.8\,\%$ of total deaths

10. Suicide: $1.6\,\%$ of total deaths

The Process of Dying and End of Life

The end of life is a personal process influenced by domestic circumstances and beliefs. For the chronically ill, death often follows a specific two-stage sequence:

1. Preactive Dying

• Duration: May last up to $3\,\text{months}$.

• Behavioral Changes: Individuals may become aware of impending death and begin accepting mortality. This includes losing interest in news, friends, conversation, and food.

• Physical Signs: Increased sleep and the development of swallowing problems.

2. Active Dying

• Signs: Frequent sleep (though the person can still be awakened), confusion regarding time, place, and people.

• Physical Decline: Complete loss of appetite and the slow shutting down of organ systems.

Molecular and Cellular Aging

The medical field of gerontology examines biological aging. Aging is categorized into passive and active processes.

Passive Aging

• Description: A breakdown of structures and slowing of functions.

• Molecular Level: Degeneration of elastin and collagen proteins leads to sagging skin and loss of muscle firmness. DNA replication is disrupted, leading to cell death from damaged genetic instructions.

• Biochemical Level: Lipids break down; membranes and mitochondria leak and malfunction.

• Free Radicals: Cellular degradation is linked to free radicals (unpaired electrons that destabilize other molecules).

Active Aging

• Description: Involves the appearance of new substances or activities.

• Substances: Lipofuscin granules accumulate from lipid breakdown. Autoimmunity may develop, where the immune system attacks own cells.

• Apoptosis: Programmed cell death. This is a normal part of life that begins before birth (e.g., in the fetal brain, half of the neurons die so those with strong synaptic connections can remain). Mitosis and Apoptosis are opposite but complementary processes.

Centenarians and Longevity

Centenarians are individuals who live past $100\,\text{years}$ of age. While environment plays a major role in deaths occurring between ages $60$ and $85$, genetics become the dominant factor for survival to extreme old age.

• Centenarian Characteristics: High levels of HDL (high-density lipoprotein), which appears to add $20\,\text{years}$ of life; normal to low weight; non-smoking history; effective stress management; and absence of dementia.

• Genetic Factors: Centenarians inherit protective genes and normal variants of genes that are otherwise disease-causing when mutant. Key genes in aging influence:     - Glucose metabolism and insulin secretion.     - Immune system functioning and stress response.     - Control of the cell cycle.     - Cholesterol metabolism.     - Synthesis of antioxidant enzymes.