The Human Body Systems Guide

Nervous System

Your brain and spinal cord are the major parts of the central nervous system. Different parts of your body send messages to the brain through the nerves and spinal cord. Once your brain gets these messages, it responds by interpreting the messages and reacting. The brain can then send instructions out to the body.

• Divisions of the Nervous System: The nervous system works to send signals throughout the body.
• The Nervous System: Doctors estimate that every person has about 100 billion neurons in their brain, which are responsible for sending and receiving messages.
• About the Nervous System: The central nervous system includes the brain and the spinal cord.

Endocrine System

The endocrine system takes care of many different things. This system sends hormones out through the body, which are chemicals that tell cells what to do. Under the care of the endocrine system, lots of different activities occur. For instance, the body sleeps at night and wakes up in the morning, cells grow, and organs function in certain ways.

• Endocrine System: Hormones travel all over the body in the blood to take care of important functions thanks to the endocrine system.
• The Enteric Endocrine System: The endocrine system helps with digestion as the stomach and intestines secrete hormones.
• The Endocrine System: Think of hormones as tiny messengers that move around the body to take care of issues like organ function and appetite.
• Overview of the Endocrine System: Hormones affect growth and development as they send different messages about when to grow.

Respiratory System

Breathing in air and using the oxygen in it are the most important functions of the respiratory system. The body breathes in air through the nose and the mouth. The air moves through airways to the lungs. In the lungs, fresh oxygen goes into the blood for transport throughout the body. Carbon dioxide moves out of the body as you exhale.

• The Respiratory System: The respiratory system is in charge of getting rid of carbon dioxide from the body and keeping the blood full of oxygen.
• Information About the Respiratory System: The respiratory system includes the lungs and the airways that connect into and out of them.
• Anatomy of the Respiratory System: The nose, mouth, throat, windpipe, bronchi, and lungs are parts of the respiratory system.

Circulatory System

The circulatory system has the job of transporting substances throughout the body. With a system of veins and arteries, blood moves continuously all over the body. Blood carries chemicals to the places they need to go, and it also transports waste products to be eliminated from the body. The circulatory system is very important because it works together with every other system and organ in the body.

• Functions of the Circulatory System: The circulatory system has several important jobs in the body, including moving waste products and transporting hormones.
• Circulatory System Diseases: Someone suffering from a disease of the circulatory system might have trouble with blood pressure or an abnormal heart rate.
• Human Circulatory System: The circulatory system works with the lymphatic system to carry water and nutrients throughout the body.
• Effects of Exercise on the Cardiovascular System: Exercise is good for the circulatory system. When the heart beats faster, it moves oxygen through the body better.

Digestive System

The digestive system is in charge of taking the food you eat and turning it into the fuel your body needs to function. With every bite of food, saliva begins working immediately to get food ready to swallow. The tongue and teeth also help with this process. After swallowing food, it moves down the esophagus to the stomach for digestion. Food then moves through the intestines, where the digestive system breaks down and absorbs nutrients for the body to use.

• Why Do We Need a Digestive Tract? The digestive system is in charge of processing foods eaten to make them into energy.
• Overview of the Digestive System: Salivary glands help break down food to begin the digestive process.
• The Digestive System: Where Does Food Go? (PDF): It might surprise you to learn the length of the total human digestive tract. This system might be as long as nearly 30 feet if stretched out and measured.

Immune System

The body needs to defend itself against germs and infection. The immune system is the body’s shield against unfamiliar things like viruses and parasites. When the immune system detects something foreign, it will attack it to protect the body. For example, if bacteria get into your body through your mouth, the immune system would send white blood cells to kill the invading germs. Sometimes the white blood cells win and you don’t get sick. Other times, the white blood cells aren’t able to stop an illness and you will get sick. If this happens, the white blood cells will keep fighting until you are well again.

• The Immune System (video): The immune system is in charge of protecting the body from bacteria and viruses that could make it sick.
• Overview of the Immune System: A healthy immune system knows how to recognize harmful invaders.
• Rebooting the Immune System: When the body has an injury or illness, the immune system is in charge of healing to make the body healthy again.

Muscular System

The muscular system has three different kinds of muscles: cardiac, smooth, and voluntary. Cardiac muscles are in your heart, and these muscles have the job of pumping blood. The smooth muscles are the ones that work inside your body for functions like digesting food. Voluntary muscles are the ones that you control with your brain to move your body.

• The Muscular System: The muscular system might weigh as much as one-half of a person’s total body weight.
• Overview of the Muscular System (PDF): The heart is a muscle that contracts and relaxes to pump blood throughout the body.
• Muscular System: Muscle Tissue (PDF): The body has different types of muscles that are designed to perform different types of jobs, some voluntary and others involuntary.

Skeletal System

The bones, tendons, and ligaments in the body make up the skeletal system. This system is in charge of supporting the body and all of the systems in it. Teeth are also part of the skeletal system. Bones are strong so they can protect fragile organs inside the body. The ribs are in place in the chest to protect the heart and lungs.

• The Skeletal System: Bones have a tough outer covering and an inner spongy layer that contains marrow.
• The Skeletal System: Introduction: An adult human body contains 206 bones that are connected by ligaments, tendons, and cartilage.
• Bones and Skeletal Tissues (PDF): A baby has approximately 270 bones, some of which will fuse together as a child grows.
• The Human Skeletal System: Inside and Out: It wasn’t until 1895 that people first got a glimpse of the skeletal system with the first X-rays.

Excretory System

As the body performs its various functions, it creates waste products that have to be removed. The body has to have a way to remove both liquid and solid waste to stay healthy. The kidneys are in charge of keeping the right balance of fluids and salt in the body. After filtering, the kidneys move waste fluids out of the body as urine. The excretory system also removes waste when you breathe out and when you sweat. Defecation is the way the body removes solid waste.

• The Excretory System (PDF): The excretory system helps maintain the proper balance of water and salts in the body.
• Human Excretory System (PDF): The excretory system is responsible for removing wastes from the body using organs including the kidneys and the liver.
• Excretory (Urinary) System (PDF): Liquid waste leaves the body through the urinary system, which includes the kidneys, bladder, and urethra.