Cells Teaming Up
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Tissues and organs work together in related groups called systems. Different organs perform specific jobs within a system. All those jobs add up to a more general service the system performs for the organism.
Scientists classify the systems of the human body in various ways. Eleven organ systems of the human body are pictured below. You can blend different systems together. Add each system by moving its slider to the right.
Function: Protects the body from the outside world and keeps moisture inside. Helps regulate body temperature. Contains sense receptors for temperature, pain, and touch.
Parts: Skin, sweat glands, hair, and fingernails.
Function: Inhales oxygen and allows it to be dissolved into the bloodstream. Removes carbon dioxide from the bloodstream and exhales it.
Parts: Nose, larynx, trachea, bronchi, lungs, and diaphragm (the muscle that works the lungs).
Function: Provides shape, support, and protection to the body, while allowing it to move. Blood cells are also produced in the marrow of bones.
Parts: Bones, cartilage, and joints (ligaments and tendons). Fibroblasts are the main cells in dense connective tissue that makes up ligaments and tendons. Fibroblasts secrete collagen and elastic fibers.
Function: Controls and coordinates body functions. Receives signals, processes information, and transmits responses to organs.
Parts: Brain, spinal chord, and nerves.
Function: The digestive system breaks down food, absorbs its useful substances, and gets rid of the waste. The useful substances—or nutrients—are used by cells throughout the body for energy and building materials.
Parts: The digestive system is made up of the mouth, esophagus, stomach, liver, small intestines, large intestines, rectum, and anus.
Function: Moves the body, moves materials within the body, and generates heat.
Parts: Skeletal muscles, which are attached to bones and move the body around. (Smooth muscle tissue and cardiac muscle tissue are included in other systems).
Function: The cardiovascular system delivers oxygen and nutrients to cells throughout the body. It also carries waste materials away from all the cells. It is also the main distribution system for all sorts of chemical signals, and for white blood cells that travel around the body fighting infectious diseases. The cardiovascular system also helps regulate body temperature, by controlling how much blood flows near the body’s surface at different times.
Parts: The cardiovascular system is made up of the heart, blood vessels, and blood. (Weird fact: blood is considered a connective tissue, even though most tissues are solid.)
Function: The lymphatic system takes fluid that has leaked out of blood vessels and returns it to the cardiovascular system. As this lymph fluid filters through the system, white blood cells also check it for signs of infection, and manage the body’s immune response to disease. (Sometimes scientists identify the immune system as a separate system.)
Parts: lymph vessels, lymph nodes, tonsils, thymus, and spleen.
Function: Removes excess fluid and many dissolved waste products from the body.
Parts: Kidneys, ureters, bladder, and urethra.
Function: Regulates and controls growth, development, and various body functions by releasing chemical signals called hormones into the bloodstream.
Parts: Various glands throughout the body, including the pituitary, adrenal, and thyroid glands; and also ovaries in women and testes in men.
Function: The reproductive system produces children through sexual reproduction.
Parts: Vagina, uterus, ovaries, penis, and testes.
All the systems working together make a whole multicellular organism—in this case a human being. Other multicellular organisms (from mice to mushrooms to maple trees) have different cells, tissues, organs, and systems. Some are more different than others. Mice have hearts and lungs roughly similar to ours. But maple trees have very different kinds of cells, and their organs include things like roots and leaves.
An organism has all the parts it needs to live. A single human cell, or a piece of tissue, or an organ, or an organ system, cannot normally live by itself for long. Those parts depend on each other. Alone, they would die. But the whole multicellular organism, with all of its parts working together, can perform all the basic biological functions it needs to survive.
So can a single-celled organism, like a yeast cell or an amoeba. A yeast cell in a blob of bread dough does not depend on its neighboring yeast cells the way the cells of a human body depend on each other. True, a yeast cell can’t do all the interesting things a human can do. But the yeast cell’s needs are relatively simple, and it is able to cope on its own.
Single-celled living and multicellular living are two strategies for solving the basic problems of how to stay alive. Both strategies accomplish the same four basic necessities:
All the cells in a multicellular organism need to be able to meet these four basic needs at a cellular level. Their cooperative organization makes it easier for each cell to meet those necessities. The systems of the organism work together to create an internal environment where nutrients are delivered to each cell for building materials and energy, and where waste is carried away so it doesn’t build up in and around cells.
The digestive system breaks down food and absorbs nutrients (and disposes of the unusable portion of food as feces). The respiratory system brings in oxygen for the chemical reactions that get energy out of nutrients in the cells, and disposes of waste carbon dioxide. The cardiovascular system picks up oxygen from the respiratory system and nutrients from the digestive system and delivers them to cells throughout the body. And the cardiovascular system also picks up waste carbon dioxide from cells and delivers it to the respiratory system for exhalation, and picks up waste products from processed nutrients and delivers them to the urinary system for elimination from the body. All of this activity and more is supported by the other systems in the body.
A single yeast cell and a whole human being are both considered organisms. But a single human cell is not considered an organism. Why is a yeast cell an organism while a human cell is not?