The muscular system is an organ system consisting of skeletal, smooth and cardiac muscles. It permits movement of the body, maintains posture and circulates blood throughout the body.
The Muscles in Your Body by 
The Muscular System by The primary organs of the respiratory system are lungs, which carry out this exchange of gases as we breathe. Red blood cells collect the oxygen from the lungs and carry it to the parts of the body where it is needed.
The Lungs in Your Body by 
The Respiratory System by The circulatory system is a vast network of organs and vessels that is responsible for the flow of blood, nutrients, hormones, oxygen and other gases to and from cells.
The Heart and Blood in Your Body by 
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The Circulatory System by The integumentary system is an organ system consisting of the skin, hair, nails, and exocrine glands. The skin is only a few millimeters thick yet is by far the largest organ in the body. The average person's skin weighs 10 pounds and has a surface area of almost 20 square feet.
The Skin on Your Body by The digestive system is a group of organs working together to convert food into energy and basic nutrients to feed the entire body. Food passes through a long tube inside the body known as the alimentary canal or the gastrointestinal tract (GI tract).
The Stomach and Intestines in Your Body by 
Taste and Digestion by 
The Digestive System by The excretory system is the system of an organism's body that performs the function of excretion, the bodily process of discharging wastes. The Excretory system is responsible for the elimination of wastes produced by homeostasis.
The reproductive system or genital system is asystem of sex organs within an organism which work together for the purpose of sexual reproduction. Many non-living substances such as fluids, hormones, and pheromones are also important accessories to the reproductive system.
Reproductive System by The skeletal system is the body system composed of bones, cartilages, ligaments and other tissues that perform essential functions for the human body. Bone tissue, or osseous tissue, is a hard, dense connective tissue that forms most of the adult skeleton, the internal support structure of the body.
The Bones in Your Body by 
The Skeletal System by The nervous system is a complex collection of nerves and specialized cells known as neurons that transmit signals between different parts of the body. It is essentially the body's electrical wiring. Structurally, the nervous system has two components: the central nervous system and the peripheral nervous system.
Understanding the Brain by 
The Brain in Your Body by 
The Nervous System by The immune system is the body's defense against infectious organisms and other invaders. ... The immune system is made up of a network of cells, tissues, and organs that work together to protect the body.
Fighting the Flu by 
Fighting Polio by 
Fighting Cancer by 
Viruses up Close by The endocrine system is the collection of glands that produce hormones that regulate metabolism, growth and development, tissue function, sexual function, reproduction, sleep, and mood, among other things.
Endocrine System by 
