Adrenal glands   A pair of endocrine glands that produce hormones in response to stress, including adrenaline, which makes the heart beat faster, preparing the body for action.

Allergy   An abnormal reaction of the immune system to a harmless substance. During an allergic reaction, the body attacks a harmless substance as through it were a pathogen.

Alveoli   Tiny air sacs, clustered at the end of the  bronchioles and surrounded by capillaries. Oxygen passes easily through the thin walls of the alveoli into the blood in the capillaries.

Amino acid   One of the building blocks from which proteins are made by the body’s cells. During digestion, protein in food is broken down into amino acids.

Amniotic fluid   The fluid that surrounds and protects a baby when it is inside the uterus.

Antibody   A substance carried in the blood that helps destroy germs.

Anus   An opening at the end of the digestive tract through which faeces leave the body. It is surrounded by a ring of muscle which controls its opening.

Aorta   The largest artery in the body. It carries oxygen-rich blood away from the heart and towards the tissues.

Appendix   A small tube attached to the first section of the large intestine. Once thought to have no function, the appendix is now believed to play a part in the body’s defence system.

Aqueous humour   A watery fluid in the eye between the cornea and the lens.

Arteriole   A very small artery. Arterioles branch into even smaller blood vessels called capillaries.

Artery   A blood vessel that carries blood away from the heart. Most arteries carry oxygen-rich blood to the body’s organs and tissues, but the pulmonary artery, which runs from the heart to the lungs, carries oxygen-poor blood.

Atria   The two upper chambers of the heart. The left atrium receives oxygen-rich blood from the lungs. The right atrium receives oxygen-poor blood from the body.

Automatic nervous system (ANS)   Part of the nervous system that automatically controls processes such as heartbeat, breathing and digestion.

Axon   The long, thin projection that carries nerve impulses away from a neuron’s cell body.

Backbone   The column of irregular bones (vertebrae) that run the length of the back, supporting the body and protecting the spinal cord. The backbone is also known as the spine or vertebral column.

Bacterium (plural bacteria)   A tiny organism made of just one cell. Some bacteria invade living things and cause diseases, but most types are harmless.

Ball-and-socket joint   A type of joint that allows bones to move in any direction. The round end of the bone sits in the joint like a ball in a cup. The hips and shoulders are both examples of ball-and-socket joints.

Bases   Chemicals that make up the “rungs” on a double helix of DNA. There are four different types: adenine (A), thymine (T), guanine (G) and cytosine (C). Each “rung” is a combination of two bases. A always links to T, and G always links up to C. The order in which the bases appear forms the coded instructions in genes.

Bile   A thick, greenish fluid that helps break down fat in food. Bile is produced by the liver and is stored in the gall bladder.

Bladder   The organ that collects and stores urine after it leaves the kidneys.

Blood   A fluid that carries oxygen and nutrients to the body’s tissue cells and removes their wastes. It contains several different types of cells, each with a different job to perform, that are suspended in liquid plasma. Pumped by the heart, blood travels through a network of blood vessels to the tissues and then back to the heart. Together, the heart, blood and blood vessels make up the circulatory system.

Bone marrow   The jelly-like substance in the middle of bones. Red bone marrow makes new blood cells. Yellow bone marrow stores fat and minerals. Most of the bones in a baby’s body contain red marrow. With age, this changes mostly into yellow.

Bones   Hard, strong structures that fit together to make the body’s basic framework—the skeleton. Bone is made of living cells, surrounded by hard minerals, such as calcium phosphate, and tough, flexible collagen fibres. Bones support the body’s weight and give it shape, protect its internal organs, and work together with the muscles to make the body move.

Brain   The large organ inside the skull that controls almost every aspect of body activities, including feeling, thinking, moving and breathing. A column of nervous tissue, the spinal cord, extends downwards from, and relays messages to and from, the brain. The brain and spinal cord are linked to the rest of the body by cable-like nerves, which carry messages in both directions. The brain, spinal cord and nerves make up the nervous system. This works alongside the body’s second control system, the endocrine system, to ensure body activities are co-ordinated.

Brain stem   The part at the base of the brain that links it to the spinal cord and helps control the automatic nervous system (ANS).

Bronchioles   Branches of the narrowest bronchi. They end in tiny respiratory bronchioles, which end in alveoli.

Bronchus   The airway connecting each lung to the trachea. Like the trachea, it is supported by cartilage. Inside the lungs, bronchi divide into smaller bronchi. 

Canines   The four pointed teeth near the front of the mouth, used to grip food.

Capillary   The smallest type of blood vessel. The walls of capillaries are so thin that oxygen and other nutrients can seep through them into the body’s tissues.

Carbohydrate   A group of nutrients that includes starch, found in bread, pasta, potatoes and rice, and energy-rich sugars such as glucose.

Carbon dioxide   A waste gas produced by cells as they release energy from glucose. It is carried in the bloodstream to the lungs, where it is breathed out.

Cardiac muscle   A specialized muscle found only in the wall of the heart. It contracts automatically and without tiring for a whole lifetime, to pump blood around the body.

Cartilage   A strong, flexible tissue that covers and cushions the ends of bones in synovial joints. Some body parts, such as the outer ear, are shaped by cartilage.

Cartilaginous joint   A slightly flexible joint connected solely by cartilage, for example, the joints between ribs and the breastbone.

Cell   The basic “building block” of the body. There are over 200 kinds, such as blood cells, nerve cells and muscle cells. They vary in size and shape, depending on their function, but nearly all are far too small to see without a powerful microscope. A typical cell is a bag of jelly, or cytoplasm, containing tiny organelles. At the heart of the cell is the nucleus, which controls the rest of the cell. Coiled up inside the nucleus are strands of a substance called DNA, which contains genes, a kind of “instruction manual” for the body.

Cell division   The process by which a cell divides to make new cells. There are two types of division. Mitosis, which enables growth to take place and replaces worn-out cells, makes two new cells which are each identical to the original cell. Meiosis, a process that makes sex cells (sperm and eggs) in the testes and ovaries, makes four new cells that have half the number of chromosomes found in the original cell.

Cell membrane  The thin outer boundary of the cell. It allows only certain substances to pass in and out of the cell.

Cerebellum   The region at the back of the brain that deals with muscle control to make movements coordinated.

Cerebral cortex   The thin outer layer of the cerebrum that consists of grey matter. It is divided into different areas, each with a particular function.

Cerebral hemispheres   The two halves of the cerebrum. The right hemisphere controls the left side of the body and deals with artistic skills and imagination. The left hemisphere controls the right side of the body and deals with language, numbers and logic.

Cerebrum   The largest part of the brain. It deals with sensing, movement, thought, reasoning, memory, problem solving and other conscious actions.

Chambers   The four sections in the heart.

Chest cavity   The space enclosed by the ribs and diaphragm, containing the lungs and the heart.

Choroid   A layer of the eye between the retina and sclera, containing blood vessels.

Chromosome   A long, thread-like structure inside the nucleus of a cell. A chromosome is made of tightly wound coils of DNA. Most cells in the human body contain two sets of 23 chromosomes.

Cilia   Tiny “hairs” that line the airways. They bend from side to side, sweeping dust, dirt and germs trapped in mucus to the throat so it can be swallowed.

Ciliary muscle   The muscle that controls the shape of the eye's lens so that it can focus light from objects at varying distances on to the retina.

Circulation   The movement of blood through the heart and around the body.

Cochlea   A tiny, coiled tube in the inner ear. It contains a fluid that ripples as sound vibrations pass through it. "Hairs" detect the movement and convert it to electrical signals, which are passed to the brain.

Collagen   The tough protein found inside bone. It makes bones slightly flexible.

Colon   The main part of the large intestine from which water is absorbed into the bloodstream from watery, undigested food. The remaining semi-solid waste (faeces) is pushed towards the rectum.

Compact bone   Hard, strong bone tissue that forms the outer shell of most bones. It is the strongest type of bone in the body.

Cones   One of two types of receptor cells in the retina that send nerve signals to the brain when hit by light. Cones detect colour and detail but do not work well in low light levels.

Conjunctiva   The membrane that covers the sclera at the front of the eye. It produces mucus, which lubricates the eye.

Cornea   The transparent dome at the very front of the eye. The cornea does most of the work of focusing light on the retina.

Cranial nerves   Nerves that extend directly from the brain.

Crown   The part of a tooth that is visible above the gum.

Cytoplasm  The jelly-like liquid inside a cell in which organelles float and move.

Dendrite   One of several thin projections that extends from a neuron’s cell body and through which nerve impulses are received from other neurons.

Dentine   A hard, yellowish, bone-like material inside teeth.

Dermis   The lower layer of skin, containing blood vessels, nerve endings and hair roots.

Diaphragm   The sheet of muscle beneath the lungs that helps breathing. To inhale, the diaphragm pulls down, increasing the volume of the chest cavity and causing air to rush into the lungs. To exhale, it relaxes, moving up and pushing air out of the lungs.

Digestion   The process of taking in food and breaking it down into tiny particles, or nutrients, that can be absorbed into the blood and used by the body. Food supplies all of the body’s energy requirements, the raw materials for growth and repair, and the nutrients such as vitamins that are essential for good health. The body organs—including the mouth, stomach and small intestine—that take in and break down food, make up the digestive system.

Disease   A disorder that affects how the body functions. A disease may affect a certain body part and may have symptoms, such as a cough or high temperature.

DNA (Deoxyribonucleic acid)   A chemical in the nucleus of each cell that can copy itself, and carries the instructions (genes) needed to build and run a cell. DNA’s structure resembles a twisted ladder, with two strands linked by “rungs” made of bases. The sequence of bases in DNA forms the “letters” of the coded instructions within genes.

Double helix   The name given to the twisted-ladder shape of a DNA molecule.

Ear bones   Three tiny bones in the middle ear, also known as the ossicles. They are: the malleus (hammer), attached to the eardrum, the stapes (stirrup), attached to the inner ear and the incus (anvil), which connects the two. They transmit and amplify vibrations from the eardrum.

Ear canal   A tube that runs from the pinna to the eardrum, and carries sound waves into the ear.

Ear wax   A yellow, waxy substance produced in the ear. It protects the ear by trapping dirt and fighting off infections.

Eardrum   A thin piece of skin stretched tight between the middle and outer ear. When sounds hit the eardrum it vibrates, transmitting sound waves to the ear bones.

Ears   Organs that detect sound, positioned on either side of the head. Ears collect sound vibrations from the air, and change them into nerve signals, which are sent to the brain. They also help the body to balance.

Embryo   An unborn baby in the first eight weeks of gestation. During this period, all of the main body parts and organs, such as the brain, heart, eyes, ears and toes form.

Enamel   The hard white coating on the outside of teeth. Enamel is the hardest substance in the body.

Endocrine gland   A type of gland that produces and releases hormones.

Endoplasmic reticulum   A network of membranes inside a cell’s cytoplasm. The membranes make and transport proteins and other substances, including fats.

Enzyme   A substance that speeds up chemical reactions. Enzymes in the digestive system speed up the break down of food.

Epidermis   The protective upper layer of the skin. It constantly renews itself as dead cells are worn away.

Epiglottis   A flap of cartilage attached to the top of the larynx. It stops food from entering the trachea during swallowing.

Exhale   To breathe out, expelling air from the lungs through the mouth or nose.

External muscles   Six strap-like muscles that control the movement of each eyeball.

Eyes   Two round organs at the front of the head. They detect light rays and convert them to nerve signals which are sent to the brain. Around three-quarters of all information processed in the brain comes from the eyes.

Faeces   Solid food waste that cannot be digested. This is mostly vegetable and fruit fibre and harmless bacteria.

Fallopian tubes   Two tubes, also known as oviducts, that connect each ovary to the uterus. Eggs made in the ovaries pass down the fallopian tubes into the uterus. 

Fats   A group of nutrients found in meat, fish, dairy products, nuts and some fruit and vegetables. Fats, also known as lipids, are needed to build cell membranes and maintain nerves.

Fertilization   The coming together of a sperm and an egg. A fertilized egg is ready to grow into a baby.

Fibre   The part of fruit and vegetables that the body cannot digest. Fibre adds bulk to food. This improves the pushing power of the intestines as they squeeze food along.

Fibril   A fine bundle of long, chain-like muscle proteins called actin and myosin. Bundles of fibrils make up fibres, and bundles of fibres form a muscle. When a muscle contracts, actin and myosin slide past each. These tiny movements inside every fibril make the muscle shorter.

Fibrous joint   A fixed, immovable joint connected by fibrous tissue.

Flat bone   A broad, flat bone, consisting of spongy bone between compact bone. Flat bones either protect internal organs, as in the pelvis, or form a surface for muscles to attach to, as in the shoulder blades.

Foetus   An unborn baby after the eighth week of gestation until it is born, during which time it shows considerable growth and development. 

Fovea   A small region of the retina, densely packed with cones. It is responsible for sharp, central vision, as used when reading.

Gall bladder   A small, sac-like organ underneath the liver where bile is stored.

Gamete   A sex cell (egg or sperm). Sex cells contain 23 chromosomes rather than the 46 found in other body cells. During fertilization, a sperm and egg come together to make one cell with a complete set of 46 chromosomes.

Gastric juice   A liquid mixture of acid and enzymes, made by glands in the lining of the stomach. It helps to break down proteins.

Genes   A set of instructions that tell the body how to grow and maintain itself. Because genes control the way a body’s cells are built, they help determine a person’s characteristics and appearance. Genes are contained within DNA.

Genetics  The branch of science that deals with heredity—how features are passed on, or inherited, from parent to child via genes.

Genotype  The combination of all a person’s genes, inherited from both parents.

Gestation   The period during which an unborn baby develops inside the uterus, also called pregnancy. The gestation period for humans is about 38 weeks.

Gland   An organ that produces a substance needed by the body, such as tears or saliva.

Glucose   The main source of energy for the body. It is released from carbohydrates in food during digestion and stored in the liver in the form of glycogen.

Glycogen   A store of glucose found inside liver cells. Glucose is released from glycogen into the bloodstream, or removed from the bloodstream to make glycogen, according to the body’s demands.

Grey matter   Darker tissue in the central nervous system (CNS) that is packed with neurons and is where information-processing happens.

Gum   The pink tissue that forms a tight seal around the teeth.

Haemoglobin   The red protein found in red blood cells. Haemoglobin combines with oxygen to carry it to the body’s tissues.

Hair   A thread-like strand growing from the skin. Hair consists mainly of dead cells packed with tough, waterproof keratin, the substance also found in nails. The visible part of the hair is called the shaft. Living cells at the base of a hair follicle divide to make the hair grow.

Hair erector muscle   A muscle attached to each hair follicle. In cold conditions, it contracts to pull the hair upright, producing goosebumps.

Hair follicle   A tiny pit in the skin, from which a hair grows. The shape of hair follicles determines whether hair is curly, wavy or straight.

Heart   A large muscular organ in the chest that pumps blood around the body.

Hepatic portal vein   The vein that carries blood rich in nutrients from the small intestine to the liver.

Hepatic vein   The vein that empties blood from the liver into the inferior vena cava.

Hinge joints   A type of joint that allows bones to move in one plane, like an opening and closing door. The elbow is a type of hinge joint.

Hormones   Chemical messengers carried in the bloodstream. They alter the activities of certain cells and tissues, controlling body activities such as growth and reproduction.

Hypothalamus   A small but important brain area that monitors and controls thirst, hunger, body temperature and many other activities. It sends signals to the rest of the body via the automatic nervous system (ANS) and hormones produced in the pituitary gland.

Immune system   The body system that protects it from invasion by pathogens. It consists mainly of the defensive white blood cells that are found in the circulatory and lymphatic systems, and in body tissues.

Incisors   The thin, sharp teeth at the front of the mouth. During chewing, they come together like scissors to cut up food.

Infectious disease   A disease caused by pathogens entering and multiplying inside the body. Some infectious diseases, including cold, flu and measles, can be passed on from person to person.

Inhale   To breathe in, drawing air through the mouth or nose and into the lungs.

Inner ear   The innermost part of the ear, containing the cochlea, which deals with hearing, and the vestibular system, which deals with balance.

Insulin   A hormone produced in the pancreas that removes excess glucose in the blood.

Intercostal muscles   The muscles between the ribs. During breathing in, they contract (shorten) to lift the rib cage, increasing the volume of the chest cavity so that air rushes into the lungs. During breathing out they relax, so the rib cage moves down, decreasing the volume of the cavity.

Intestines   The continuous tube linking the stomach to the anus. It is divided into the small intestine and the large intestine.

Iris   The coloured part of the eye, a ring of muscle that controls the amount of light entering the eye by changing the size of the pupil.

Irregular bone   A bone that cannot be grouped as flat, long or short but is shaped for the particular task it serves. Irregular bones include vertebrae and facial bones.

Jaws   The part of the skull enclosing the mouth. The upper jaw cannot move. The lower jaw is moved by muscles in the head.

Joint   The place where two or more bones meet. Joints make the skeleton flexible and allow it to move.

Kidneys   A pair of organs that each contain around 1 million microscopic nephrons. These filter wastes and excess water and salts from the blood to make urine.

Larynx   The small organ, also known as the voice box, that enables us to make sounds. It sits at the top of the trachea, where it contains the vocal cords.

Lens   The clear, elastic part of the eye behind the iris. Ciliary muscles pull the lens into shape to adjust the sharpness of the image projected onto the retina.

Ligament   A strong, flexible strap that holds two bones together at a joint. Ligaments are made of collagen fibres.

Lips   The two fleshy, flexible folds of skin that surround the mouth. The lips close during chewing to keep food in the mouth. They also help to form words.

Liver   A large organ that processes nutrient-rich blood from the small intestine, storing some nutrients and dispatching others. The liver also removes poisonous substances from the blood and makes bile.

Long bone   A long, thin bone with a knob at each end. The shaft is reinforced by compact bone, while spongy bone is at the ends. Long bones are found in the arms and legs.

Lungs   The two spongy, sac-like organs in the chest cavity, through which oxygen enters the bloodstream. 

Lymph   Fluid that is drained from the body's tissues by lymph vessels, which return it to the bloodstream. Fluid in the bloodstream passes through capillary walls to carry nutrients to tissue cells and pick up wastes. Most of it passes back into the blood but some remains in the tissue until removed, as lymph, by lymph vessels. Along the way, it passes through lymph nodes, where white blood cells kill pathogens in the lymph.

Lymphatic system   The network of vessels that carries lymph around the body.

Lysosome   Found in cells, an organelle filled with enzymes that break down and recycle worn-out organelles.

Medicines   Also called drugs, these are substances used to prevent or treat diseases. They may be obtained from natural sources such as plants or made in a laboratory.

Melanin   The dark pigment that gives skin its colour. It also filters harmful rays, which damage skin cells, from sunlight. Melanin is made by cells in the epidermis called melanocytes. When the skin is exposed to the sun for longer periods, melanocytes make more melanin for extra protection, resulting in a tan.

Menstrual cycle   A monthly cycle, controlled by hormones, in which the female body prepares itself for possible pregnancy by increasing the thickness of, and the blood supply to, the lining of the uterus. If an egg is fertilized, it will bury itself in the lining to develop into a baby. If not, the lining breaks down and leaves the body through the vagina. This is called menstruation.

Mental illness   An illness that affects a person’s emotions, behaviour or learning.

Middle ear   The part of the ear between the eardrum and cochlea. This is where sounds are converted to vibrations.

Minerals   Chemicals that are needed by the body in order to function properly. For example, calcium, needed for healthy teeth and bones, is found in milk, cheese and vegetables. Meat, bread and nuts contain iron, used to make the haemoglobin inside red blood cells.

Mitochondrion   An organelle that takes in glucose and other “fuels” and breaks them down using oxygen to release energy for use inside the cell.

Molars   The wide teeth at the back of the mouth, used to crush and chew food.

Motor neuron   A neuron that carries nerve impulses from the brain and spinal cord to the body’s muscles and some glands.

Mucus   A sticky liquid that protects the insides of hollow organs such as the stomach. In the respiratory system, it lines the nose, throat, trachea and lungs and traps dirt and germs.

Muscles   Tissues made up of long cells called fibres that are able to contract (shorten) in order to create movement. Skeletal muscles are attached to bones and work in opposing teams to move body parts, such as the arms or legs, in different directions. The walls of body organs also contain muscle tissue. For example, in the intestines, layers of muscle push food along; muscle in the heart wall enables it to pump blood.

Musculoskeletal system   The skeletal system and muscular system combined. It consists of: bones, muscles and joints. Bones and muscles work together to make the body move.

Nail   A hard, flat plate, made of keratin, that protects the ends of toes and fingers. Nails grow from roots tucked into folds in the skin. The visible part of the nail is dead.

Nail cuticle   A skin fold at the base of a nail.

Nasal cavity   The large space behind the nose, divided into two passageways, each connecting one nostril to the throat. The lower, respiratory region of the nasal cavity warms and moistens air before it enters the lungs. The upper, olfactory region is lined with receptors that detect smells.

Nerve   A bundle of neurons that relays nerve impulses between the central nervous system and the body.

Nerve impulse   A tiny electrical signal that travels along a neuron at high speed. It can only travel in one direction along a neuron.

Nervous system   The network made up of the brain, spinal cord and nerves. The brain and spinal cord form the central nervous system (CNS). The nerves comprise the peripheral nervous system (PNS).

Neuron   An individual nerve cell. It has a cell body from which shorter dendrites and a single, long axon project.

Non-infectious disease   A disease that is not caused by a pathogen. Non-infectious diseases may be inherited via genes, caused by environmental factors such as pollution or brought on by an unhealthy lifestyle. Unlike infectious diseases, they cannot be passed on through contact.

Nostrils   The two openings in the nose, separated by a thin wall of cartilage called the septum.

Nucleolus   A small structure inside a nucleus, where RNA is produced.

Nucleus   A large structure inside a cell. It contains the gene-carrying chromosomes and therefore controls all cell activities.

Nutrients   Substances in food, including carbohydrates, proteins, fats, vitamins and minerals, that humans and other living things need in order to build and run their bodies and maintain good health.

Oesophagus   The tube through which food passes from the mouth to the stomach.

Oestrogen   The main hormone in the female reproductive system. It is made in the ovaries and controls many of the changes in a girl’s body during puberty.

Optic nerve   The nerve at the back of the eye that carries messages to the brain.

Organ   A structure made of two or more different types of tissues that does a particular job in the body. The brain, stomach, thyroid gland and skin are all examples of organs.

Organelle   A tiny structure inside a cell, such as a mitochondrion. Each type of organelle has its own specific function. Organelle means “little organ”.

Ossification   The process of bone growth. At birth, babies have about 300 bones, made partly from cartilage. Over time, this is replaced by bone, reducing the total number of bones to 206 in adults. Bones stop growing at around the age of 20.

Osteoblasts   Cells that grow new bone and repair damaged bone.

Osteoclasts   Cells that break down bone tissue by dissolving calcium phosphate and other minerals in the bone.

Outer ear   The fleshy external part of the ear, including the pinna and ear canal. It funnels sounds into the middle ear.

Ovaries   Two female reproductive organs that produce eggs, or ova. Every 28 days an egg is released from one ovary and travels down the fallopian tube into the uterus. This is called ovulation.

Ovum   A female sex cell, or egg.

Oxygen   A gas that makes up about one-fifth of the air around us. Oxygen is needed for a process called cell respiration that happens inside cells and releases energy from food.

Palate   The roof of the mouth. The front hard palate, is a surface that the tongue can push food against when chewing. The back part, the soft palate, prevents food going upwards into the nasal cavity during swallowing.

Pancreas   An organ that produces enzymes and releases them into the small intestine. The pancreas also releases two hormones that control glucose levels in the blood.

Papillae   Tiny bumps on the top of the tongue. They help grip food, and some contain the tongue’s taste buds.

Pathogen   A bacterium, virus or other micro-organism that causes disease. Pathogens multiply inside the body.

Pelvis   A bowl-shaped ring of bones consisting of two hip bones joined to the sacrum of the backbone. It protects the bladder, intestines and reproductive organs.

Penis   The male external sex organ. During sexual intercourse, the penis is inserted into the female’s vagina, where it releases sperm. The penis is also used to release urine from the body.

Periosteum   A membrane that covers the outside of bones. Periosteum contains blood vessels that nourish the bones.

Peristalsis   The squeezing action in the oesophagus, stomach and intestines, which pushes food along.

Pharynx   Also called the throat, this extends from the mouth and nasal cavity to the openings of the larynx and oesophagus.

Phenotype   A person’s appearance as a result of both genotype and environmental factors, such as diet.

Pinna   The visible part of the ear, made of skin and cartilage. It acts as a funnel to direct sound waves into the ear.

Pituitary gland   A tiny endocrine organ at the base of the brain. It releases several hormones, some of which control other endocrine glands. It is itself controlled by the brain's hypothalamus.

Placenta   A plate-shaped organ that develops in the uterus during pregnancy. It provides the developing foetus with nourishment and oxygen via the umbilical cord.

Plasma   The watery part of blood that carries blood cells and contains dissolved nutrients, hormones and other substances.

Platelets   Tiny fragments of cells found in the blood. When a blood vessel is damaged, they collect at the wound, stick together and help form a clot. This prevents blood loss or infection by invading bacteria.

Pleural membranes   Two membranes that cover the surface of the lungs and line the inside of the chest cavity. In the narrow gap between the membranes there is a small amount of liquid. This helps the layers to slide against each other, reducing friction as the lungs inflate and deflate.

Pore   An opening in the skin, through which cooling sweat seep out on to the skin.

Premolars   The flat, broad teeth between the canines and the larger molars. Premolars are used for grinding food.

Progesterone   A female hormone that helps to control the menstrual cycle and prepare the body for pregnancy.

Proteins   A group of substances obtained from food and used by the body for many purposes, including building cells and making enzymes. Foods rich in protein include meat, poultry, fish, beans and nuts.

Puberty   The period when a child’s body develops so that it is able to reproduce. This usually takes place between the ages of 11–14 in girls and 13–16 in boys. Girls develop a more rounded body shape and grow breasts. Boys grow facial hair and their voices deepen.

Pulp   The soft tissue at the centre of teeth, containing nerves and blood vessels.

Pulse   The rhythmic beat felt in the arteries as blood is pushed through them. Each beat is caused by the heart squeezing and relaxing once.

Pulse rate   The number of pulses felt per minute. During exercise, the heart pumps faster and the pulse rate increases. At rest, the pulse rate is much lower. You can feel your pulse by placing your fingers on the inside of your wrist, where an artery passes just under the skin.

Pupil   An opening in the iris, through which light enters the eye. The pupil varies in size to control the amount of light reaching the retina, shrinking in bright light and growing larger in dim light.

Rectum   The last part of the large intestine, in which faeces are stored before they are passed out through the anus.

Red blood cells   The doughnut-shaped cells that pick up oxygen in the lungs and release it in the body’s tissues where it is needed.

Reflex   An automatic, unconscious response made by the body, often without involving the brain. If you touch something sharp, pain sensors in the skin send nerve signals along the arm to the spinal cord. Signals go straight back to the arm to make it move.

Respiration   Breathing, the process that takes oxygen into the body and gets rid of waste carbon dioxide. The set of organs involved in breathing make up the respiratory system. They include the nose, trachea, diaphragm and lungs. Breathing in draws air into the lungs, where oxygen passes into the bloodstream and is carried to every cell in the body. Meanwhile, the bloodstream transports carbon dioxide in the opposite direction, from the cells to the lungs, where it is breathed out. Breathing out also enables us to speak.

Retina   The layer that contains light-sensitive rods and cones. The retina lines the inside of the back of the eye.

Rib cage   The rib cage consists of 12 pairs of ribs; the sternum at the front of the chest, to which most ribs are attached; and the 12 thoracic vertebrae in the backbone, to which all ribs are attached.

Ribosome   An organelle often attached to the endoplasmic reticulum. Ribosomes are the site of protein production.

RNA (Ribonucleic acid)   A chemical related to, but smaller than, DNA. It copies sections of DNA (genes) in the nucleus and carries the instructions for making cell-building proteins to ribosomes in the cytoplasm, where proteins are made.

Rods   The most numerous of the two types of receptor cells in the retina that send nerve signals to the brain when hit by light. Rods work best under low light levels but cannot detect colour or detail.

Root   The part of a tooth under the gum that fixes the tooth firmly to the jaw bone. It is attached to the bone by a hard substance called cementum.

Saliva   A slippery liquid that makes food moist and easier to swallow. It contains an enzyme that starts starch digestion and moistens food, enabling the taste buds to detect dissolved tastes. Saliva is produced by six glands located around the mouth.

Sclera   The tough, white outer layer of the eyeball that helps to protect it.

Sebaceous glands   Glands attached to hair follicles that produce sebum.

Sebum   The natural oil that keeps the skin's surface moisturized and waterproof.

Semicircular canals   Three tiny tubes in the inner ear that detect head movements in all directions. They are filled with liquid that moves as the head moves. Microscopic “hairs” sense this movement and send signals to the brain, which makes adjustments to posture so the body can remain balanced.

Senses   The means by which the body can detect what is happening in the outside world. There a five main senses: sight, hearing, taste, smell and touch.

Sensory neuron   A type of neuron that carries nerve impulses to the brain and spinal cord from sense organs—the eyes, ears, tongue, nose and skin—and other body organs.

Sesamoid bone   A small, flat bone, such as the patella (knee cap), that is embedded in a tendon.

Sex chromosomes   Two chromosomes that determine sex. Females have two X chromosomes and males have one X and one Y. Sex cells, sperm and eggs, have only one sex chromosome each. When a sperm and egg fuse during fertilization, the sex of the future baby depends on whether the sperm carries an X chromosome or a Y.

Short bone   A bone that is as wide as it is long. Short bones include the tarsals (ankle and heel bones) in the foot, and the carpals (wrist bones) in the hand, which provide support and stability but allow little movement.

Sinuses   Small air pockets in the skull bones around the nasal cavity. They make the skull lighter and help moisten inhaled air.

Skeletal muscle   Muscle connected to two bones across a joint, and responsible for moving part of the skeleton. Skeletal muscle is also known as voluntary muscle because it can be moved at will, or striped muscle because its long fibres have a striped appearance under a microscope.

Skin   The outer covering of the human body and its largest organ. Tough, waterproof and germ-proof, skin provides a barrier between the body’s insides and the harsh outside environment. It also filters the sun’s harmful rays and enables the brain to detect touch and pain.

Skull   A strong, bony structure that supports and protects the brain, and shapes the head and face. The skull is made up of 22 bones, all fused together at suture joints, except for the lower jaw.

Small intestine   A long tube linking the stomach to the large intestine. In the first section, the duodenum, bile and pancreatic juices continue the digestive process. Then, in the jejunum and ileum, digestion is completed and nutrients are absorbed into the bloodstream.

Smooth muscle   A type of muscle found in the walls of hollow internal organs, such as the bladder, blood vessels and stomach. It is also known as involuntary muscle because it works automatically.

Socket   A hollow in one body structure into which another body part fits. For example, the socket in a hip bone into which the ball-shaped head of the femur (thighbone) fits; or an eye socket in the skull which surrounds and protects most of the eye.

Sperm   Tiny, tadpole-like male sex cells which have a compact "head" and a long, beating flagellum ("tail"). During sexual intercourse, sperm pass out of the penis, into the female’s vagina, and swim towards the egg.

Spinal cord   A column of nervous tissue, no wider than a pencil at its thickest, that runs from the brain down the back, protected by the backbone.

Spongy bone   Also called cancellous bone. A tissue that has a honeycomb structure of struts and spaces that makes it both strong and light. It is found in the interior of bones and is covered by compact bone.

Stomach   The organ where digestion begins in earnest. While an enzyme in gastric juice digests proteins in food, its muscular walls churn food into a creamy paste.

Subcutaneous fat   A layer of fat just beneath the skin. It helps to keep the body warm and cushions it from knocks.

Suture   A type of fibrous joint only found in the skull.

Sweat   Watery fluid produced by sweat glands and released through pores. Sweat helps the body to cool down. When it evaporates from the skin’s surface (turns from a liquid into a gas), it takes some of the body’s heat with it.

Sweat glands   Glands in the dermis that produce sweat.

Synapse   The junction between two neurons, in which they are separated by a tiny gap. Nerve impulses cannot cross a synapse in their electrical form, so one neuron releases a chemical (neurotransmitter) that passes across the gap and triggers an electrical impulse in the next neuron.

Synovial joint   A mobile joint, such as the elbow joint, where bone ends are separated by synovial fluid produced by a capsule that surrounds the joint. This oily fluid lubricates the joint and enables bones to move against each other without rubbing.

Taste buds   Clusters of taste-detecting cells housed in certain papillae on top of the tongue. Taste buds detect tastes that have dissolved in saliva as food is being chewed. Five tastes—sweet, sour, salty, bitter and umami (meaty)—can be detected.

Tears   A salty liquid that keeps the eyes moist and clean. Tears are made in the lacrimal glands above the eyes and drain from the eye into the nose.

Tendon   A strong, tough cord that attaches a muscle to a bone. Bones are moved when muscles pull on tendons. Tendons contain fibres of tough collagen.

Testes   The main organs of the male reproductive system, where sperm are made. Testes hang outside the body in a sac of skin called the scrotum.

Testosterone   A hormone made by the testes that produces many of the changes in a boy’s body during puberty.

Thalamus   Part of the brain that relays incoming information to the cerebrum.

Thymus   A gland located in the upper chest that helps to develop the body’s immune (defence) system during early childhood.

Thyroid   An endocrine gland in the neck which releases hormones that control the speed at which body cells work and how much energy they consume.

Tissue   Groups of cells that are similar in structure and function.

Tongue   A muscular organ attached in part to skull bones. The tongue tastes food, and helps to move it around the mouth. It also helps us to form words during speech.

Tonsils   Patches of tissue containing immune (defence) system cells that surround the back of the mouth at the entrance to the throat. The tonsils help to trap and kill pathogens.

Trachea   The airway connecting the lungs to the larynx, through which air flows to and from the mouth and nose. It is supported by rings of cartilage that keep it open during breathing.

Trait  A characteristic that is inherited, for example eye colour or hair type.

Umbilical cord   A cord containing blood vessels that connects a foetus to the placenta during pregnancy, and through which the foetus receives food and oxygen.

Ureter   The tube through which urine passes from the kidneys to the bladder.

Urethra   The tube through which urine passes from the bladder out of the body.

Urinary system   The organs that produce, store and remove urine, including the kidneys, ureters, bladder and urethra.

Urine   A mixture of water, salts and waste substances, produced in the kidneys and stored in the bladder before being expelled from the body. 

Uterus   Also known as the womb. An organ in a woman’s lower body in which a baby develops during pregnancy. The wall of the uterus contains powerful muscles. When birth starts, these contract to squeeze the baby out of the mother’s body through her vagina.

Vaccination   A treatment that prevents disease by injecting the body with a harmless version of a pathogen, which the immune system learns to recognize. If the body later encounters the actual pathogen, it will destroy it rapidly before it can cause harm.

Vagina   The tube-shaped part of the female reproductive system, through which a baby leaves the mother’s body. During sexual intercourse, the male’s penis is inserted into the female’s vagina.

Valves   Flaps in the heart and in larger veins that open to allow blood through them and shut to prevent backflow. Valves ensure blood only flows in one direction.

Vein   A blood vessel that carries blood towards the heart. Most veins carry blood low in oxygen but the pulmonary veins, which run from the lungs to the heart, carry blood that has just picked up oxygen.

Vena cava   A large vein that carries blood low in oxygen from the body to the heart. The inferior vena cava carries blood from the lower body and the superior vena cava carries blood from the upper body.

Ventricles   The two lower chambers of the heart. The left ventricle pumps blood to the body. The right ventricle pumps blood to the lungs.

Venule   A very small vein. Venules branch into even smaller vessels called capillaries.

Vertebrae   The 26 separate bones—including the sacrum and coccyx—that fit together as a column to make the backbone. Discs of cartilage between most vertebrae cushion them so that they can twist and tilt against each other, enabling the back to move.

Vestibular system   The part of the inner ear that controls balance. It includes the semicircular canals and two tiny sacs, the saccule and utricle. These contain tiny grains of chalk that shift as the head moves, passing over nerve endings, which send messages to the brain.

Villi   Tiny projections lining the inside of the small intestine. They absorb nutrients from the intestine into the bloodstream.

Virus   A microscopic infectious agent that can reproduce itself only inside the cell of a living organism.

Vitamins   Essential nutrients that are needed in small amounts to make the body work properly and stay healthy. For example, vitamin A, found in leafy green vegetables and carrots, is important to eyesight and skin. Vitamin C, which helps to heal wounds, is found in citrus fruits. Eggs and fish are rich in vitamin D, which is needed for strong bones.

Vitreous humour   A clear, jelly-like substance that fills the space in the back of the eyeball between the lens and retina.

Vocal cords   Two folds of elastic tissue that stretch across the top of the larynx. To speak, muscles pull the cords together so there is only a narrow slit between them. Air rushing through the slit makes the cords vibrate, which produces sounds.

White blood cells   Cells carried in the blood that protect the body from disease. There are three main kinds. Lymphocytes identify disease-causing germs and either kill them with chemicals or release antibodies to disable them. Neutrophils and monocytes track down germs, surround them and eat them.

Wisdom teeth   Four molar teeth at the very back of the mouth that usually appear between the ages of 18 and 21. Some people’s wisdom teeth never come through.

X-ray   An imaging technique that uses radiation projected through the body to produce an image of the inside of that body region. X-rays can, for example, reveal a broken bone, or show if the lungs are infected.

Consultant: Richard Walker