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Nov 3, 2023 · The human body is a biological machine made of body systems; groups of organs that work together to produce and sustain life. Sometimes we get lost while studying about cells and molecules and can’t see the forest for the trees. It can be helpful to step back and look at the bigger anatomical picture.
An organ is an anatomically distinct structure of the body composed of two or more tissue types. Each organ performs one or more specific physiological functions. An organ system is a group of organs that work together to perform major functions or meet physiological needs of the body.
- Lindsay M. Biga, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Mat...
- 2019
- Overview
- Key points
- Introduction
- Multicellular organisms need specialized systems
- Overview of body organization
- Types of tissues
- Epithelial tissue
- Connective tissue
- Muscle tissue
- Nervous tissue
Learn about the main tissue types and organ systems of the body and how they work together.
•Humans—and other complex multicellular organisms—have systems of organs that work together, carrying out processes that keep us alive.
•The body has levels of organization that build on each other. Cells make up tissues, tissues make up organs, and organs make up organ systems.
•The function of an organ system depends on the integrated activity of its organs. For instance, digestive system organs cooperate to process food.
•Humans—and other complex multicellular organisms—have systems of organs that work together, carrying out processes that keep us alive.
•The body has levels of organization that build on each other. Cells make up tissues, tissues make up organs, and organs make up organ systems.
•The function of an organ system depends on the integrated activity of its organs. For instance, digestive system organs cooperate to process food.
•The survival of the organism depends on the integrated activity of all the organ systems, often coordinated by the endocrine and nervous systems.
If you were a single-celled organism and you lived in a nutrient-rich place, staying alive would be pretty straightforward. For instance, if you were an amoeba living in a pond, you could absorb nutrients straight from your environment. The oxygen you would need for metabolism could diffuse in across your cell membrane, and carbon dioxide and other wastes could diffuse out. When the time came to reproduce, you could just divide yourself in two!
However, odds are you are not an amoeba—given that you're using Khan Academy right now—and things aren’t quite so simple for big, many-celled organisms like human beings. Your complex body has over 30 trillion cells, and most of those cells aren’t in direct contact with the external environment.1 A cell deep inside your body—in one of your bones, say, or in your liver—can’t get the nutrients or oxygen it needs directly from the environment.
Most cells in large multicellular organisms don't directly exchange substances like nutrients and wastes with the external environment, instead, they are surrounded by an internal environment of extracellular fluid—literally, fluid outside of cells. The cells get oxygen and nutrients from this extracellular fluid and release waste products into it. Humans and other complex organisms have specialized systems that maintain the internal environment, keeping it steady and able to provide for the needs of the cells.
Different systems of the body carry out different functions. For example, your digestive system is responsible for taking in and processing food, while your respiratory system—working with your circulatory system—is responsible for taking up oxygen and getting rid of carbon dioxide. The muscular and skeletal systems are crucial for movement; the reproductive system handles reproduction; and the excretory system gets rid of metabolic waste.
All living organisms are made up of one or more cells. Unicellular organisms, like amoebas, consist of only a single cell. Multicellular organisms, like people, are made up of many cells. Cells are considered the fundamental units of life.
The cells in complex multicellular organisms like people are organized into tissues, groups of similar cells that work together on a specific task. Organs are structures made up of two or more tissues organized to carry out a particular function, and groups of organs with related functions make up the different organ systems.
As we saw above, every organ is made up of two or more tissues, groups of similar cells that work together to perform a specific task. Humans—and other large multicellular animals—are made up of four basic tissue types: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.
Epithelial tissue consists of tightly packed sheets of cells that cover surfaces—including the outside of the body—and line body cavities. For instance, the outer layer of your skin is an epithelial tissue, and so is the lining of your small intestine.
Epithelial cells are polarized, meaning that they have a top and a bottom side. The apical, top, side of an epithelial cell faces the inside of a cavity or the outside of a structure and is usually exposed to fluid or air. The basal, bottom, side faces the underlying cells. For instance, the apical sides of intestinal cells have finger-like structures that increase surface area for absorbing nutrients.
Connective tissue consists of cells suspended in an extracellular matrix. In most cases, the matrix is made up of protein fibers like collagen and fibrin in a solid, liquid, or jellylike ground substance. Connective tissue supports and, as the name suggests, connects other tissues.
Loose connective tissue, show below, is the most common type of connective tissue. It's found throughout your body, and it supports organs and blood vessels and links epithelial tissues to the muscles underneath. Dense, or fibrous, connective tissue is found in tendons and ligaments, which connect muscles to bones and bones to each other, respectively.
Muscle tissue is essential for keeping the body upright, allowing it to move, and even pumping blood and pushing food through the digestive tract.
Muscle cells, often called muscle fibers, contain the proteins actin and myosin, which allow them to contract. There are three main types of muscle: skeletal muscle, cardiac muscle, and smooth muscle.
Skeletal muscle, which is also called striated—striped—muscle, is what we refer to as muscle in everyday life. Skeletal muscle is attached to bones by tendons, and it allows you to consciously control your movements. For instance, the quads in your legs or biceps in your arms are skeletal muscle.
Cardiac muscle is found only in the walls of the heart. Like skeletal muscle, cardiac muscle is striated, or striped. But it's not under voluntary control, so—thankfully!—you don’t need to think about making your heart beat. The individual fibers are connected by structures called intercalated disks, which allow them to contract in sync.
Nervous tissue is involved in sensing stimuli—external or internal cues—and processing and transmitting information. It consists of two main types of cells: neurons, or nerve cells, and glia.
The neurons are the basic functional unit of the nervous system. They generate electrical signals called conducted nerve impulses or action potentials that allow the neurons to convey information very rapidly across long distances. The glia mainly act to support neuronal function.
5 days ago · The human body is the physical substance of the human organism. Characteristic of the vertebrate form, the human body has an internal skeleton with a backbone, and, as with the mammalian form, it has hair and mammary glands.
- The Editors of Encyclopaedia Britannica
- Chemically, the human body consists mainly of water and organic compounds—i.e., lipids, proteins, carbohydrates, and nucleic acids. The human body...
- The four main types of tissue in the human body are epithelial, muscle, nerve, and connective.
- The nine major organ systems in the human body are the integumentary system, the musculoskeletal system, the respiratory system, the circulatory sy...
An organ is an anatomically distinct structure of the body composed of two or more tissue types. Each organ performs one or more specific physiological functions. An organ system is a group of organs that work together to perform major functions or meet physiological needs of the body.
The main organs that perform these functions are the brain, spinal cord, and peripheral nerves. The endocrine system is responsible for secreting hormones to regulate bodily processes. Some major organs of the endocrine system include the pituitary gland, thyroid gland, pancreas, adrenal glands, testes, and ovaries.
Like anatomists, physiologists typically specialize in a particular branch of physiology. For example, neurophysiology is the study of the brain, spinal cord, and nerves and how these work together to perform functions as complex and diverse as vision, movement, and thinking.
