National Geographic - Inside the living body

The Journey from Birth to Adulthood

Our lives can be divided into different stages: birth, infancy, childhood, puberty, adulthood, and old age.
Each stage of our lives comes with its own set of challenges and changes.
The story of our lives is a journey that starts from the moment we are born.
This journey takes place deep inside our bodies and involves the growth and development of our organs, cells, and systems.
Our bodies are made up of trillions of cells that work together in harmony to make us who we are.

"The Incredible Journey of life from birth through infancy, childhood, puberty, adulthood, and a slow maturity to old age. This is the story of Our Lives from a unique perspective, deep inside our bodies. This is the Journey of a human life from the outside in."

The Miracle of Cell Development

A fetus develops in the womb over a period of 40 weeks.
The body of a fetus is a miracle of microscopic design, with perfectly formed organs made up of billions of functioning cells.
Cells are the building blocks of our bodies and determine our physical characteristics and functions.
The DNA in each cell is unique to every person and acts as a blueprint for growth and development.

"A fetus develops in the womb. Its body is a miracle of microscopic design, with tiny, perfectly formed organs, each made up of billions of perfectly functioning cells. Cells are the building blocks of our bodies. They make us what we are, with a hundred thousand billion cells all working in harmony. Inside every cell is the same extraordinary engine, the machine that tells each cell how to grow and what functions to perform. DNA is unique to every person, a chemical blueprint of instructions that creates each new layer."

The First Breath of Life

At the moment of birth, everything changes for the baby.
One of the first challenges the newborn faces is the need to breathe or die.
The unborn baby's lungs are filled with amniotic fluid, which protected them during the nine months in the womb.
The body kicks into survival mode, and the adrenal glands send adrenaline surging through the body, shocking the lungs into life.
The muscles needed for breathing start to spasm, and the baby takes its first breath, which is the most important breath of its life.
From that moment on, the lungs will be pumping air every second for as long as the person lives.

"The journey starts with a challenge: breathe or die. His lungs have never breathed before, and they're still full of amniotic fluid that protected them for nine long months. The newborn is in danger of drowning. Then the body kicks into survival mode. The adrenal glands right above the kidneys send adrenaline surging around the body. It shocks the lungs into life, and muscles we need to breathe suddenly start to spasm. And we take our first breath. It's the most important breath of Our Lives, the first of 700 million our lungs will pump air every single second as long as we live."

The Constant Work of the Lungs

The air we breathe rushes down the windpipe and into thousands of branching tubes, ultimately reaching nearly 30 million tiny air sacs called alveoli.
The alveoli pull oxygen into our blood and remove carbon dioxide with every breath.
This process of oxygen exchange happens non-stop for 80 years.
The lungs work tirelessly, pumping air and ensuring our bodies receive the oxygen they need to function.

"Air rushes down the windpipe, down thousands of branching tubes, and into nearly 30 million tiny air sacs, the alveoli. These air sacs pull oxygen into our blood and pump out the carbon dioxide we exhale with every breath. And they do it non-stop for 80 years."

Adapting to Life Outside the Womb

At birth, the physical link between mother and baby is broken for the first time.
The baby's organs have to adapt rapidly to the new environment outside the womb.
The heart, which was the size of a walnut in the womb, begins pumping blood through thousands of miles of blood vessels.
Other systems, such as the digestive tract, also undergo changes to prepare for the outside world.

"The physical link between mother and baby is broken for the first time. The first hour brings rapid change. All the baby's organs have to adapt to life outside the womb, which is both challenging and risky. At this age, the heart is no bigger than a walnut. It's been pumping in the womb for eight months, but now it has flaws that could be fatal. Two holes, one in the aorta and one in the heart, which diverted blood away from our inactive lungs, now seal shut as we need the lungs. The heart is working normally, pumping blood through tens of thousands of miles of blood vessels. Other systems are also gearing up. The digestive tract is ready to clear itself out to make room for its first meal."

The Protection and Development of the Baby

The newborn baby faces various challenges, including maintaining body temperature in a colder environment.
Special tissues around the blood vessels and vital organs help generate heat to keep the baby warm.
The baby's immunity is still underdeveloped, and the mother's milk provides protection by transferring antibodies.
The baby's senses, such as smell and hearing, are also developing and becoming adjusted to the outside world.

"At four weeks since birth, the baby has drunk nearly 30 pints of milk and has put on two pounds, a quarter of its body weight. Time for its first trip outside. Even a visit to the grocery store can overload the senses. It's noisy, bright, and smelly. The nose is working overtime, with specialized nerves detecting chemicals in the air. The eardrum vibrates in response to sound waves, with tiny bones amplifying the vibrations. The baby's vision is still underdeveloped, but its eyesight is improving over time. The immune system is also developing, and the mother's milk provides antibodies to fight infections. It's time to take on the world."

Exploring the World Through Touch

At eight months, babies are beginning to explore the world and their sense of touch becomes dominant
Temperature sensors in the skin detect hot or cold objects, and nerve signals quickly travel up the arm, spinal cord, and into the brain at a speed of 200 miles per hour
The brain interprets these signals as pain and sends a signal back to the muscles to move the hand away

"The sense we use most is touch. Touch something too hot and temperature sensors in the skin and nerve signals racing up the arm, up the spinal cord, and into the brain, all at 200 miles per hour. The brain detects the signal, interprets it as pain, and fires another signal back to the muscles. Move the hand away"

Sensory Receptors in the Skin

Sensory nerve receptors are located all over our skin, but some areas, like the hands, face, and mouth, are more sensitive than others
The tongue alone has about 9,000 sensory receptors, which is why babies often use their mouths to explore objects

"We have sensory nerve receptors all over our skin, but some areas are more sensitive than others. The hands, face, and mouth have a higher concentration of sensory receptors. There are 9,000 sensory receptors on the tongue alone, which is why babies use their mouths to explore"

Teething and the Painful Milestone

At around eight months, babies start getting their first teeth, and the process can be painful
Milk teeth begin to form deep in the gums while the baby is still in the womb, and they gradually emerge, causing discomfort
The pain from teething motivates babies to chew and gnaw on objects

"Something painful is happening inside the baby's mouth. Her first teeth are coming through. Milk teeth form deep in our gums while we're still in the womb. Now, one by one, they burst through. It's painful, but it's progress"

The Journey of Digestion

Digestion begins in the mouth, where food is broken down by chewing
Special glands under the tongue produce saliva, which helps break down and lubricate the food for its journey through the digestive system
The food passes from the stomach to the small intestine, where most of the nutrients are absorbed
Contractions of muscles in the intestines, called peristalsis, move the food along

"Digestion starts in the mouth. Grind up the food, then special glands under the tongue pump out saliva to help break down and lubricate the food on its 12-hour, 13-foot journey through the gut. Waves of contracting muscle keep the food moving, a process called peristalsis"

The Power of the Stomach

The stomach is a muscular organ that churns, squeezes, and squashes food into a liquid form
Acids in the stomach break down the food, while a lining of mucus protects the stomach from being digested by its own acids
After about an hour, the stomach squeezes the broken-down food out through a hole called the pyloric sphincter

"The stomach is a bag of muscle that churns, squeezes, and squashes food into liquid. Acids in the stomach break down the food. The stomach walls protect themselves with a lining of mucus. About an hour later, the stomach squeezes the broken-down food out through a tiny hole called the pyloric sphincter"

Absorbing Nutrients in the Small Intestine

The small intestine is an 11-foot long tube where most of the nutrients from food are absorbed
The walls of the small intestine are lined with millions of microscopic projections called villi, which provide a large surface area for nutrient absorption
Pancreatic juices neutralize stomach acid, and bile from the liver breaks down fats into smaller droplets for easier absorption by the intestine
After about an hour and a half, the small intestine has absorbed most of the nutrients from the food

"The small intestine is an 11-foot coil of tube where we absorb most of the nutrients. The walls of the small intestine are lined with millions of microscopic projections called villi, which increase the surface area for nutrient absorption. First, the pancreas pumps out a juice that neutralizes stomach acid. Then, bile from the liver breaks down fats into tiny droplets. After an hour and a half, the small intestine has absorbed most of the nutrients from the food"

The Final Stages of Digestion

What remains after nutrient absorption enters the large intestine through the ileocecal sphincter
The large intestine's main job is to extract water from the waste material
Bacteria in the large intestine aid in breaking down complex carbohydrates that we can't digest ourselves
After about 12 hours, what's left of the first meal is expelled as waste

"What's left is a mix of waste food and dead cells from the walls of the gut. The large intestine's main job is to extract water from it. Lots of bacteria, which we actually need, live here. They produce enzymes that break down complex carbohydrates in our food. Finally, after about 12 hours, we expel what's left of our first meal"

Brain development in childhood (2005-2011)

In childhood, the brain is primed to learn and grows rapidly.
This rapid growth enables the brain to easily form new connections, a skill that diminishes with age.
Young brains have the ability to learn and adapt quickly due to the rapid growth and formation of connections.

"In childhood, our brains are primed to learn and they're growing fast."

Body's defenses against infection (2025-2092)

The immune system's job is to recognize and fight infections.
The body has multiple defenses against infection, including eyebrows, eyelashes, ear hairs, and nasal hairs that catch airborne bacteria.
Sweat, tears, and mucus help wash off bacteria.
The skin sheds its top layer of cells, taking bacteria with it.
The mouth has saliva that contains lysozyme, an enzyme that destroys bacteria.
Saliva glands produce a significant amount of saliva each day to aid in fighting infections.

"Our eyebrows and eyelashes, ear hairs, and nasal hairs catch airborne bacteria. Sweat, tears, and mucus wash them off. Our skin constantly sheds its top layer of cells, taking bacteria with it."

Immune response to pathogens (2126-2168)

When a pathogen breaks through the external defenses, the immune system reacts to prevent infection.
Viruses travel through the blood and probe skin cells for weakness.
Viruses hijack cells, produce thousands of copies, and spread them throughout the body.
The body's immune response to the virus causes symptoms like rashes and fever.
The immune system kicks into action, with white blood cells latching onto infected cells and killing both the cell and the viruses.
Crusty skin blisters are a sign of the battle between the immune system and viruses.

"Sometimes a pathogen breaks through these external defenses, and our immature immune system reacts to prevent infection."

Importance of childhood diseases for immune system development (2259-2299)

Getting diseases like chickenpox during childhood is vital for developing a strong immune system.
Our bodies create memory molecules and antibodies against the virus.
Antibodies from mother's milk used to be relied upon, but now our bodies can make their own antibodies.
If we catch the same disease when we're older, the antibodies allow our bodies to recognize the virus instantly and eliminate it before it can take hold.
Childhood is the ideal time for vaccination, as it strengthens the immune system by creating antibodies against specific diseases.

"It may be unpleasant, but getting diseases like this when we're young is vital for our developing immune systems."

Puberty and reproductive system changes (2341-2479)

Puberty marks the biggest change in the human body.
It begins between the ages of 10 and 13 and varies based on DNA, lifestyle, and diet.
Puberty starts in the brain's hypothalamus, triggering the release of hormones throughout the body.
In girls, puberty leads to the maturation of ovaries and ovulation.
The ovaries produce eggs and hormones such as estrogen.
Boys experience a growth spurt, changes in body shape, the production of sperm, and a surge in testosterone.
The larynx changes, causing the voice to deepen.
The brain undergoes rewiring, influencing mood and character.
Both sexes experience new emotions and a mutual sexual attraction.

"By age 11, puberty is already underway for most, but there's no schedule—it happens later for some and earlier for others."

Adulthood and body changes (2764-2830)

In our 20s, we reach adulthood and experience physical and emotional well-being.
Trillions of cells make up our organs and tissues, which continuously regenerate.
Our bodies replace entire organs over time.
We have the opportunity to prioritize our health by making choices like exercise, which positively influences overall body condition.
Our muscles undergo development and our appearance improves.
Our body is constantly regenerating, replenishing itself every 10 years.
Some tissues, like hair, regenerate even faster.

"In our 20s, we look and feel better than at any other time in our lives. We're in our prime both outside and in."

Muscles in the body

Muscles in the body work together and generate enough power to lift more than 11 tons, which is equivalent to the weight of four SUVs.

"Muscles in the body work together, they would generate enough power to lift more than 11 tons."

Muscle repair and growth

Muscles are made from bundles of fibers that can be ripped apart during a good workout.
Our cells repair the muscle fibers by adding extra material, which leads to muscle growth and strength.

"A good workout rips these muscles fibers apart, but our cells repair the damage by adding extra material. The muscle grows back bigger and stronger."

Choices and their impact on the body

Choices we make can have both beneficial and detrimental effects on our bodies.
Certain choices expose us to damage that even our youthful cells cannot repair.

"Through choices we make, some choices are less beneficial. Some expose us to damage that even our youthful cells can't repair."

Hazards to the body

Loud noise poses a threat to our healing, particularly in the delicate sensory hairs in the ear called stereocilia.
Loud noise can destroy these irreplaceable cells, especially those responding to high frequencies.
The effect of loud noise is small but contributes to the shrinking of our hearing frequency range.

"Our healing is under threat too from loud noise. Loud noise destroys these irreplaceable cells. The effect is too small to notice, but the frequency range of our hearing is already shrinking."

Alcohol's effects on the body

Alcohol affects both our organs and our state of mind when absorbed into our bloodstream.
It raises blood pressure, causes irregular heartbeat, relaxes us, and impairs coordination.
Alcohol's impact on the brain cells, especially in the cerebellum, affects coordination and balance.

"As we absorb alcohol into our bloodstream, it affects both our organs and our state of mind. When we drink, alcohol affects the cerebellum's brain cells."

Love and attraction

Love and attraction have both social, physical, and biological components.
Biology plays a key role in attraction, including the sense of smell and pheromones.
Pheromones carry information about genetic health and disease resistance and influence partner choice.

"We may think the attraction is social or physical, but a lot of it is biological. Pheromones carry information about our genetic health and our ability to resist disease."

The chemistry of love

Love involves both emotional and chemical components in the brain.
Chemistry of love includes the release of adrenaline and the flood of dopamine, the "feel-good hormone."
Dopamine can be as potent as cocaine, leaving us euphoric and craving more.

"Love is more than just an emotional; it's all about chemistry. The brain floods with dopamine, the feel-good hormone. It's as potent as cocaine."

Bonding and long-term relationships

Long-term bonding in relationships is also a chemical process.
Oxytocin, known as the bonding hormone, is released during sex and strengthens the bond between partners.
Oxytocin is a chemical that can endure the trials of parenthood and create a strong bond.

"It's a relationship we hope will last a lifetime, and the process of long-term bonding is chemical. Oxytocin pumps the blood full of a substance called oxytocin, sometimes called the bonding hormone."

The journey of sperm

Sperm's goal is to find and fertilize the egg.
They face challenges surviving the acidic environment of the vagina and making it to the cervix.
Only a few hundred sperm make it to the fallopian tube, where muscular contractions guide them towards the egg.
Finally, only one sperm out of millions succeeds in fertilizing the egg.

"Sperm have a tough journey ahead. Ejaculation releases 300 million sperm, but only thousands will make it as far as the cervix."

Pregnancy and fetal development

Pregnancy begins with a single cell developing into a fully formed baby over 40 weeks.
Morning sickness and nausea may be protective mechanisms or side effects of the immune system's changes.
The fetus relies on the placenta, where the mother's blood passes nutrients to support its growth.
The mother's organs rearrange internally to make space for the growing baby.

"Often, the first symptom of pregnancy is morning sickness. The fetus saps the mother's energy and draws what it needs from her body."

Changes in the body during pregnancy

The mother's organs become squeezed and may be pushed up into her chest.
To accommodate the growing baby, the spine curves, the stomach and intestines are compressed, and the bladder is rotated.
The organs work harder to support both the mother and the baby.

"In a pregnant woman, the organs get squeezed, and some are pushed up into her chest. The lungs and heart work harder than ever before."

The challenges of childbirth

Giving birth can be a challenging process, especially pushing out a baby weighing around seven pounds.
Softened tendons allow the pelvis to open up the birth canal.
Volumetric imaging shows the cramped route that the baby has to take, including tight twists to navigate.
If it's too tight for mother and baby, a cesarean section may be necessary.

"Pushing out a seven-pound baby can be quite a challenge. Softened tendons allow the pelvis to open up the birth canal."

Aging and changes in appearance

Aging starts from birth, and the body's repair systems compensate until the repair systems themselves start aging.
Changes in appearance become more noticeable in the 40s.

"Some experts believe that we all start aging from the moment we're born. The changes in our appearance are getting noticeable."

Aging Process

The aging process begins several decades before old age sets in. During old age, the body slows down and signs of aging become more prominent.
Aging affects every cell in our bodies and is a complex and mysterious process.

"The aging process begins several decades before old age sets in and affects every cell in our bodies."

Skin Aging

The sun's ultraviolet radiation triggers a molecular chain reaction that degrades collagen in the skin, leading to thin and broken fibers, loss of elasticity, and the appearance of wrinkles.
Our skin cells constantly replenish, with 30,000 new skin cells being produced every minute to replace dead cells. However, the problem lies in the degradation of collagen that binds the skin cells together.

"The sun's ultraviolet radiation degrades collagen, leading to thin and broken skin fibers and the formation of wrinkles."

Changes in Eyesight

Changes in eyesight with age are primarily due to the lens of the eye gradually stiffening, resulting in decreased ability to focus and dryness of the eyes.
The lens cells in our eyes are the same cells we had as babies, and as we age, the lens loses its flexibility.

"Changes in eyesight with age are primarily due to the lens of the eye gradually stiffening, resulting in decreased ability to focus."

Body Shape and Metabolism Changes

Middle age brings changes in body shape and metabolism. It becomes harder to maintain a fit body through exercise alone, as the metabolic rate slows down and it becomes easier to put on weight.
The decrease in several hormones such as estrogen, testosterone, and growth hormones, along with the loss of lean muscle, contributes to the decrease in metabolic rate.

"Middle age brings changes in body shape and metabolism, making it harder to maintain a fit body through exercise alone."

Fat Deposits and Health Risks

Aging leads to an accumulation of fat throughout the body, not just in the waistline, with fat deposits even found inside blood vessels. This poses health risks such as narrowing of blood vessels, increased workload on the heart, and the potential blockage of arteries.
Belly fat in men and fat around the hips in women have evolutionary reasons related to quick energy release and the provision of a steady energy supply for pregnancy, respectively.

"Aging leads to an accumulation of fat throughout the body, posing health risks such as narrowing of blood vessels and increased workload on the heart."

Effects of Stress

Middle-aged life is often stressful due to job responsibilities and raising teenage children. Stress not only affects our mental well-being but also speeds up the aging process and damages our cardiovascular network.
When stressed, our bodies release hormones such as adrenaline and cortisol, causing physiological responses like increased heart rate and blood pressure, putting a strain on our cardiovascular system.

"Stress speeds up the aging process and damages our cardiovascular network."

Menopause and Aging

Menopause, in combination with the natural aging process, has profound effects on a woman's body and accelerates the aging process. The hormonal changes during menopause destabilize regions of the brain that affect mood, sleep, and temperature control.
The cessation of estrogen and progesterone production during menopause further exacerbates the aging process, affecting bone and muscle tissues.

"Menopause has profound effects on a woman's body and accelerates the aging process."

Bone Aging and Osteoporosis

Aging dramatically affects our skeletons, and both men and women are at risk for osteoporosis. The balance between bone destruction and formation is disrupted, leading to weakened bones and a higher risk of fractures.
Women experience a faster rate of bone loss due to hormonal changes during menopause.

"Aging disrupts the balance between bone destruction and formation, leading to weakened bones and a higher risk of fractures."

Cell Cloning and Cell Replacement Process

Cells clone themselves every day to battle wear and tear on our organs.
Inside each cell, the copying system ensures that old cells die off and new ones take their place.
However, imperfections in our DNA are also duplicated during this process.
Over a lifetime, the accumulation of defects in the DNA can lead to negative effects on our cells and organs.
This copying system can be likened to using a photocopier, where copies made from copies tend to degrade in quality.

"The copying system isn't perfect any imperfections in our DNA are also duplicated."