First Human Body Encyclopedia

1. The Human Body: A Complex Machine Built from Cells and Organized into Systems.

The greatest machine you’ll ever own is your body.

Tiny building blocks. Your body is an incredibly complex machine, far more intricate than any computer, and it's built from microscopic units called cells. These cells are the fundamental building blocks, fitting together like bricks in a wall, and hundreds could fit on the head of a pin. Different types of cells perform specialized jobs, from carrying oxygen to sending electrical signals.

Cells form tissues and organs. Cells of the same type group together to form tissues, such as muscle, bone, or nerve tissue. These tissues then combine to create organs, which are body parts designed for specific tasks – your heart pumps blood, your kidneys clean it, and your brain is the control center. Organs work in teams called systems to carry out major functions.

Systems work together. These body systems collaborate seamlessly to keep you alive and functioning. Examples include:

• The blood system transports vital supplies.
• The muscle system enables movement.
• The nervous system carries signals for thought and reaction.
• The digestive system processes food.
• The skeletal system provides support.
  This intricate organization, from tiny cells to complex systems, makes your body truly amazing.

2. Structure and Movement: Skeleton and Muscles Work Together.

Your bones all join up to make a frame for your body called the skeleton.

The body's framework. The skeleton is the internal frame that supports your body, protects your vital organs, and allows you to move. An adult skeleton has 206 bones, though babies are born with over 300 softer bones that fuse together over time. Bones are living tissue, constantly growing and repairing themselves, made strong by calcium.

Muscles create movement. Muscles are tissues that work by contracting (shortening), which makes them pull. About 650 skeletal muscles are attached to your bones by tough bands called tendons. These muscles work in pairs, pulling in opposite directions to move different parts of your skeleton. For example, your biceps bends your arm by pulling, while your triceps straightens it by pulling.

Joints allow flexibility. Bones meet at joints, which come in different types to allow various movements.

• Hinge joints (like knees and elbows) allow bending in one direction.
• Ball and socket joints (like hips and shoulders) allow movement in many directions.
• Pivot joints (like in the neck and elbows) allow rotation.
  Ligaments, which are stretchy bands of tissue, hold bones together at joints while still allowing movement.

3. Control Center: The Brain, Nerves, and Senses.

The brain is the body’s control centre.

The command center. Your brain is a remarkable organ, acting like a brilliant, living computer that controls everything you do, think, and feel. It's divided into parts like the cerebrum (thinking, senses, movement), cerebellum (balance, muscle control), and brain stem (basic functions like breathing and heartbeat). The brain is protected by the skull and shock-absorbing fluid.

A vast communication network. The nervous system is a network of nerve cells that transport messages between the brain and the rest of the body. These messages travel as electrical signals, allowing for rapid communication. Some actions, like blinking or the knee-jerk reflex, are automatic reflex actions that happen without conscious thought.

Senses gather information. Your body has specialized sense organs that collect information from the world and send it to the brain via nerves.

• Eyes detect light for sight.
• Ears detect sound waves for hearing and help with balance.
• Skin contains receptors for touch, temperature, and pain.
• The nose detects scent particles for smell.
• The tongue detects chemicals for taste.
  The brain processes this sensory input to help you understand your surroundings and react accordingly.

4. Transport System: Heart, Blood, and Vessels.

Blood is the body’s transport system.

Delivering life. Blood is a vital liquid tissue that acts as the body's internal transport system. It carries essential supplies like oxygen from the lungs and nutrients from digested food to every cell. It also picks up waste products to be removed from the body. Blood is a warm, soupy mixture of liquid plasma and different types of cells.

The powerful pump. The heart is a strong muscle located in the center of your chest that acts as a double pump. One side pumps oxygen-poor blood to the lungs to pick up oxygen, while the other side pumps oxygen-rich blood to the rest of the body. Each heartbeat is the sound of valves shutting, ensuring blood flows in only one direction. Your heart beats faster when you are active to supply muscles with extra blood.

The network of tubes. Blood travels throughout the body in a vast network of tubes called blood vessels.

• Arteries carry oxygen-rich blood away from the heart (shown red).
• Veins carry oxygen-poor blood back to the heart (shown blue).
• Capillaries are tiny, hair-thin vessels connecting arteries and veins, where oxygen and nutrients are exchanged with cells.
  Your body contains enough blood vessels to circle the world twice.

5. Gas Exchange: Lungs and Breathing.

The air you breathe contains a life-giving gas called oxygen.

Essential for life. Breathing is an automatic process that happens constantly to supply your body with oxygen, a gas vital for cells to get energy from food. It also removes carbon dioxide, a waste gas produced by your body. Air enters through your nose or mouth, gets warmed and dampened, and travels down the windpipe to the lungs.

Lungs and air sacs. Your lungs are spongy organs filled with a maze of tunnels that end in millions of tiny air sacs called alveoli. This is where the crucial exchange of gases happens:

• Oxygen from the inhaled air passes through the thin walls of the alveoli into tiny blood capillaries.
• Carbon dioxide from the blood passes into the alveoli to be breathed out.
  Laid out flat, the inside surface of your lungs would be about a third the size of a tennis court.

How breathing works. The process of breathing is helped by your ribs and a large muscle under your lungs called the diaphragm.

• When you breathe in, the diaphragm moves down and the ribs move up and out, expanding the chest cavity and pulling air into the lungs.
• When you breathe out, the diaphragm moves up and the ribs move down, squeezing air out of the lungs.
  Tiny hairs and mucus in your airways trap dust and dirt, keeping the air you breathe clean.

6. Outer Protection: Skin, Hair, and Nails.

Skin covers your whole body.

The body's armor. Skin is the largest organ in your body, forming a tough, protective outer layer that covers everything. It acts as a barrier against germs, water, and harmful sunshine. Skin also helps regulate body temperature through sweating and contains nerve receptors that allow you to feel touch, heat, and cold. It has two main layers: the epidermis (outer, visible layer of dead cells) and the dermis (inner layer with nerves and blood vessels).

Hair for warmth and protection. Hair grows from tiny pockets called follicles all over your body, except on the palms of your hands, soles of your feet, and lips. It's mostly made of keratin, the same material as skin and nails. Hair on your head is thick and long to keep your brain warm, while finer body hairs can stand up (causing goose pimples) to trap a layer of warm air when you're cold. Hair color is determined by melanin.

Nails for grip and protection. Nails are hard plates made of keratin that grow from roots under your cuticles. They protect the sensitive tips of your fingers and toes and help you grip and pick up objects. Like hair, nails grow continuously throughout your life. The unique patterns of ridges on your fingertips are called fingerprints and help you grip things.

7. Internal Processing: Digestion and Waste Removal.

Food is made up of large, complicated chemicals that your body has to break into small chemicals that your blood can absorb.

Breaking down food. Digestion is the process of breaking down the food you eat into small chemicals that your body's blood can absorb and use for energy, growth, and repair. This happens in a long, tangled tube called the digestive system, which is about 9 meters (30 feet) long in an adult. The process involves both physical breakdown (chewing, churning) and chemical breakdown using enzymes.

The digestive journey. Food travels through several organs:

• Mouth: Chewing breaks food into chunks; saliva starts chemical breakdown (starch).
• Esophagus: Muscular tube pushes food down to the stomach.
• Stomach: Churns food into a liquid; acid and enzymes start protein digestion.
• Small Intestine: Long, narrow tube where most chemical digestion finishes and nutrients are absorbed into the blood through tiny finger-like villi.
• Large Intestine: Shorter, fatter tube where water and some vitamins are absorbed; undigested leftovers form poo.

Filtering waste. The urinary system is responsible for filtering waste chemicals and excess water from your blood to produce urine. This is primarily done by two organs called kidneys, which act like sieves. Urine travels from the kidneys down tubes called ureters to the bladder, a stretchy organ that stores urine until you go to the toilet. Urine then leaves the body through the urethra.

8. Defense System: Fighting Germs and Disease.

Your body has lots of clever ways of keeping them out.

Constant threat. Your body is constantly exposed to tiny living things called germs, such as viruses and bacteria, which can make you ill if they get inside. However, your body has multiple layers of defense to protect itself. The first line of defense is physical barriers and substances that trap or kill germs before they can enter.

First line defenses. These include:

• Skin: A tough, waterproof barrier.
• Tears: Wash away germs from eyes and contain germ-killing chemicals.
• Earwax: Traps dirt and germs in the ear canal.
• Saliva: Protects the mouth and teeth from bacteria.
• Mucus and Hairs: Trap germs in airways and sweep them out.
• Stomach Acid: Kills germs that are swallowed.

The immune system. If germs manage to get past the initial defenses and invade your tissues, your immune system fights back. This system involves specialized white blood cells that hunt and destroy germs. Some white blood cells called macrophages swallow germs, while others produce antibodies that mark germs for destruction. The immune system also "remembers" germs, providing immunity against future infections (like through vaccination).

9. Life Cycle: Reproduction, Growth, and Aging.

You need a mother and a father to make a baby.

The start of life. A new human life begins when a father's sperm cell joins with a mother's egg cell in a process called fertilization. The single fertilized cell then starts dividing rapidly, making exact copies of itself. This ball of cells travels to the mother's uterus (womb), where it implants and continues to grow and develop over about 40 weeks.

Growing in the womb. Inside the protective bag of liquid in the uterus, the developing baby, called a foetus after eight weeks, receives all its food and oxygen from the mother through a special umbilical cord. The foetus grows rapidly, developing organs, limbs, and senses. Twins can develop if the fertilized egg splits (identical) or if two eggs are fertilized by different sperm (non-identical).

Life stages. After birth, growth continues rapidly in the early years, with babies and children learning essential skills like walking, talking, and interacting with others. Adolescence brings a significant growth spurt and physical changes (puberty). Adults reach their peak physical condition in their twenties and thirties, after which the body gradually ages, with organs and tissues becoming less efficient over time.

10. Staying Healthy: Nutrition, Sleep, and Medical Care.

You need a variety of foods to keep your body in peak working condition.

Fueling the body. To function properly, your body needs energy and raw materials from food. A balanced diet is crucial, providing a variety of nutrients:

• Carbohydrates: Main source of energy (bread, pasta, cereals).
• Proteins: Needed for building and repairing cells (meat, fish, eggs, beans).
• Fats: Provide energy and help absorb vitamins (nuts, dairy); needed in small amounts.
• Vitamins and Minerals: Essential for various body processes (fruits, vegetables).
• Water: Makes up two-thirds of your body weight; needed for all functions.

Rest and repair. Sleep is vital for both your body and brain. While you sleep, your body rests and repairs itself, and your brain processes the events of the day and consolidates memories. The amount of sleep needed changes with age, with babies needing much more than adults. Lack of sleep can affect your physical and mental health.

Professional care. Doctors and dentists are healthcare professionals who help you stay healthy and treat you when you are ill. Regular check-ups allow them to monitor your health, catch problems early, and provide necessary treatments like medicine or fillings. They use tools like stethoscopes, otoscopes, and X-rays to examine your body and diagnose illnesses.

11. Expressing Yourself: Body Language and Communication.

You don’t just talk with words – you also use your hands, face, and body.

Beyond words. Communication is not just about the words you speak; your body language, facial expressions, and gestures also convey powerful messages about how you feel and what you mean. People often unconsciously mimic the body language of those they feel comfortable with. The way you stand or sit can signal confidence or nervousness.

Universal expressions. Certain facial expressions are recognized universally across different cultures to show basic emotions:

• Happy (smile)
• Surprised (raised eyebrows, wide eyes)
• Angry (lowered eyebrows, narrowed eyes)
• Sad (drooping mouth, inner eyebrows up)
• Afraid (raised eyelids, wide eyes, pale face)
• Disgusted (wrinkled nose, narrowed eyes)
  Babies use these expressions to communicate before they can talk.

Gestures and personal space. Hand gestures are often used while speaking, though their meaning can vary significantly between cultures. Pointing is one of the few gestures with a widely understood meaning. How close people stand to each other (personal space) also communicates the nature of their relationship, from intimate zones for close friends and family to social zones for strangers.

Last updated: May 15, 2025