The Human Body

The five senses—sight, hearing, smell, taste, and touch—help us gather information about the world. Each sense relies on specialized organs and nerve cells called receptors.

Reflexes are automatic responses that happen without conscious thought. When you touch something hot, your hand pulls away before your brain even registers the pain.

Ears detect sound waves that vibrate the eardrum. Tiny bones in the middle ear amplify vibrations, and the cochlea in the inner ear converts them into nerve signals.

Taste buds on the tongue detect five basic tastes: sweet, salty, sour, bitter, and umami. Smell greatly enhances the sense of taste, which is why food seems bland when you have a cold.

The skin is the body's largest organ. It protects against germs, helps regulate body temperature, and contains nerve endings that detect touch, pressure, pain, and temperature.

The brain is the control center of the nervous system. It processes information from the senses, controls movement, stores memories, and manages body functions like breathing.

Bones make up the skeleton, which supports the body, protects organs, and works with muscles to create movement. Adults have 206 bones, while babies are born with about 270.

Muscles work in pairs to move bones. When one muscle contracts (gets shorter), the opposite muscle relaxes (gets longer). The biceps and triceps are an example of this pairing.

The circulatory system includes the heart, blood vessels, and blood. Arteries carry oxygen-rich blood away from the heart, and veins return oxygen-poor blood to the heart.

The respiratory system brings oxygen into the body and removes carbon dioxide. When you breathe in, air enters the lungs where oxygen passes into the blood.

The digestive system breaks food into nutrients the body can use. Food travels from the mouth through the esophagus, stomach, and intestines, with nutrients absorbed along the way.

The immune system defends the body against disease-causing germs. White blood cells identify and destroy bacteria and viruses. Vaccines train the immune system to recognize specific threats.

Neurons are specialized cells that transmit electrical and chemical signals throughout the nervous system. Sensory neurons carry signals from receptors to the brain.

The heart pumps blood through blood vessels to deliver oxygen and nutrients to every cell in the body. It beats about 100,000 times per day.

Red blood cells carry oxygen using a protein called hemoglobin. White blood cells fight infection. Platelets help form blood clots to stop bleeding when you get a cut.

The excretory system removes waste products from the body. The kidneys filter blood to produce urine, the lungs remove carbon dioxide, and the skin releases some waste through sweat.

The skeletal system and muscular system work together as the musculoskeletal system. Tendons connect muscles to bones, and ligaments connect bones to other bones at joints.

The nervous system has two main parts: the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves throughout the body).

Eyes detect light using the retina, which contains rod cells (for dim light) and cone cells (for color). The lens focuses light onto the retina, creating an image the brain interprets.

Hormones are chemical messengers produced by glands that travel through the blood to target organs. They regulate growth, metabolism, mood, and many other body functions.

The five senses—sight, hearing, smell, taste, and touch—help us gather information about the world. Each sense relies on specialized organs and nerve cells called receptors.

Reflexes are automatic responses that happen without conscious thought. When you touch something hot, your hand pulls away before your brain even registers the pain.

Ears detect sound waves that vibrate the eardrum. Tiny bones in the middle ear amplify vibrations, and the cochlea in the inner ear converts them into nerve signals.

Taste buds on the tongue detect five basic tastes: sweet, salty, sour, bitter, and umami. Smell greatly enhances the sense of taste, which is why food seems bland when you have a cold.

The skin is the body's largest organ. It protects against germs, helps regulate body temperature, and contains nerve endings that detect touch, pressure, pain, and temperature.

The brain is the control center of the nervous system. It processes information from the senses, controls movement, stores memories, and manages body functions like breathing.

Bones make up the skeleton, which supports the body, protects organs, and works with muscles to create movement. Adults have 206 bones, while babies are born with about 270.

Muscles work in pairs to move bones. When one muscle contracts (gets shorter), the opposite muscle relaxes (gets longer). The biceps and triceps are an example of this pairing.

The circulatory system includes the heart, blood vessels, and blood. Arteries carry oxygen-rich blood away from the heart, and veins return oxygen-poor blood to the heart.

The respiratory system brings oxygen into the body and removes carbon dioxide. When you breathe in, air enters the lungs where oxygen passes into the blood.

The digestive system breaks food into nutrients the body can use. Food travels from the mouth through the esophagus, stomach, and intestines, with nutrients absorbed along the way.

The immune system defends the body against disease-causing germs. White blood cells identify and destroy bacteria and viruses. Vaccines train the immune system to recognize specific threats.

Neurons are specialized cells that transmit electrical and chemical signals throughout the nervous system. Sensory neurons carry signals from receptors to the brain.

The heart pumps blood through blood vessels to deliver oxygen and nutrients to every cell in the body. It beats about 100,000 times per day.

Red blood cells carry oxygen using a protein called hemoglobin. White blood cells fight infection. Platelets help form blood clots to stop bleeding when you get a cut.

The excretory system removes waste products from the body. The kidneys filter blood to produce urine, the lungs remove carbon dioxide, and the skin releases some waste through sweat.

The skeletal system and muscular system work together as the musculoskeletal system. Tendons connect muscles to bones, and ligaments connect bones to other bones at joints.

The nervous system has two main parts: the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves throughout the body).

Eyes detect light using the retina, which contains rod cells (for dim light) and cone cells (for color). The lens focuses light onto the retina, creating an image the brain interprets.

Hormones are chemical messengers produced by glands that travel through the blood to target organs. They regulate growth, metabolism, mood, and many other body functions.

The five senses—sight, hearing, smell, taste, and touch—help us gather information about the world. Each sense relies on specialized organs and nerve cells called receptors.

Reflexes are automatic responses that happen without conscious thought. When you touch something hot, your hand pulls away before your brain even registers the pain.

Ears detect sound waves that vibrate the eardrum. Tiny bones in the middle ear amplify vibrations, and the cochlea in the inner ear converts them into nerve signals.

Taste buds on the tongue detect five basic tastes: sweet, salty, sour, bitter, and umami. Smell greatly enhances the sense of taste, which is why food seems bland when you have a cold.

The skin is the body's largest organ. It protects against germs, helps regulate body temperature, and contains nerve endings that detect touch, pressure, pain, and temperature.

The brain is the control center of the nervous system. It processes information from the senses, controls movement, stores memories, and manages body functions like breathing.

Bones make up the skeleton, which supports the body, protects organs, and works with muscles to create movement. Adults have 206 bones, while babies are born with about 270.

Muscles work in pairs to move bones. When one muscle contracts (gets shorter), the opposite muscle relaxes (gets longer). The biceps and triceps are an example of this pairing.

The circulatory system includes the heart, blood vessels, and blood. Arteries carry oxygen-rich blood away from the heart, and veins return oxygen-poor blood to the heart.

The respiratory system brings oxygen into the body and removes carbon dioxide. When you breathe in, air enters the lungs where oxygen passes into the blood.

The digestive system breaks food into nutrients the body can use. Food travels from the mouth through the esophagus, stomach, and intestines, with nutrients absorbed along the way.

The immune system defends the body against disease-causing germs. White blood cells identify and destroy bacteria and viruses. Vaccines train the immune system to recognize specific threats.

Neurons are specialized cells that transmit electrical and chemical signals throughout the nervous system. Sensory neurons carry signals from receptors to the brain.

The heart pumps blood through blood vessels to deliver oxygen and nutrients to every cell in the body. It beats about 100,000 times per day.

Red blood cells carry oxygen using a protein called hemoglobin. White blood cells fight infection. Platelets help form blood clots to stop bleeding when you get a cut.

The excretory system removes waste products from the body. The kidneys filter blood to produce urine, the lungs remove carbon dioxide, and the skin releases some waste through sweat.

The skeletal system and muscular system work together as the musculoskeletal system. Tendons connect muscles to bones, and ligaments connect bones to other bones at joints.

The nervous system has two main parts: the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves throughout the body).

Eyes detect light using the retina, which contains rod cells (for dim light) and cone cells (for color). The lens focuses light onto the retina, creating an image the brain interprets.

Hormones are chemical messengers produced by glands that travel through the blood to target organs. They regulate growth, metabolism, mood, and many other body functions.

The five senses—sight, hearing, smell, taste, and touch—help us gather information about the world. Each sense relies on specialized organs and nerve cells called receptors.

Reflexes are automatic responses that happen without conscious thought. When you touch something hot, your hand pulls away before your brain even registers the pain.

Ears detect sound waves that vibrate the eardrum. Tiny bones in the middle ear amplify vibrations, and the cochlea in the inner ear converts them into nerve signals.

Taste buds on the tongue detect five basic tastes: sweet, salty, sour, bitter, and umami. Smell greatly enhances the sense of taste, which is why food seems bland when you have a cold.

The skin is the body's largest organ. It protects against germs, helps regulate body temperature, and contains nerve endings that detect touch, pressure, pain, and temperature.

The brain is the control center of the nervous system. It processes information from the senses, controls movement, stores memories, and manages body functions like breathing.

Bones make up the skeleton, which supports the body, protects organs, and works with muscles to create movement. Adults have 206 bones, while babies are born with about 270.

Muscles work in pairs to move bones. When one muscle contracts (gets shorter), the opposite muscle relaxes (gets longer). The biceps and triceps are an example of this pairing.

The circulatory system includes the heart, blood vessels, and blood. Arteries carry oxygen-rich blood away from the heart, and veins return oxygen-poor blood to the heart.

The respiratory system brings oxygen into the body and removes carbon dioxide. When you breathe in, air enters the lungs where oxygen passes into the blood.

The digestive system breaks food into nutrients the body can use. Food travels from the mouth through the esophagus, stomach, and intestines, with nutrients absorbed along the way.

The immune system defends the body against disease-causing germs. White blood cells identify and destroy bacteria and viruses. Vaccines train the immune system to recognize specific threats.

Neurons are specialized cells that transmit electrical and chemical signals throughout the nervous system. Sensory neurons carry signals from receptors to the brain.

The heart pumps blood through blood vessels to deliver oxygen and nutrients to every cell in the body. It beats about 100,000 times per day.

Red blood cells carry oxygen using a protein called hemoglobin. White blood cells fight infection. Platelets help form blood clots to stop bleeding when you get a cut.

The excretory system removes waste products from the body. The kidneys filter blood to produce urine, the lungs remove carbon dioxide, and the skin releases some waste through sweat.

The skeletal system and muscular system work together as the musculoskeletal system. Tendons connect muscles to bones, and ligaments connect bones to other bones at joints.

The nervous system has two main parts: the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves throughout the body).

Eyes detect light using the retina, which contains rod cells (for dim light) and cone cells (for color). The lens focuses light onto the retina, creating an image the brain interprets.

Hormones are chemical messengers produced by glands that travel through the blood to target organs. They regulate growth, metabolism, mood, and many other body functions.

The five senses—sight, hearing, smell, taste, and touch—help us gather information about the world. Each sense relies on specialized organs and nerve cells called receptors.

Reflexes are automatic responses that happen without conscious thought. When you touch something hot, your hand pulls away before your brain even registers the pain.

Ears detect sound waves that vibrate the eardrum. Tiny bones in the middle ear amplify vibrations, and the cochlea in the inner ear converts them into nerve signals.

Taste buds on the tongue detect five basic tastes: sweet, salty, sour, bitter, and umami. Smell greatly enhances the sense of taste, which is why food seems bland when you have a cold.