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Humans—and other complex, multicellular organisms—have systems of organs that work together, carrying out the processes that keep us alive.

The body has levels of organization that depend on each other. Cells form tissues, tissues form organs, and organs form organ systems.

The function of the organ system depends on the integrated activity of its organs. For example, the organs of the digestive system cooperate to process food.

The survival of the organism depends on the integrated activity of all organ systems, which is often coordinated by the endocrine system and the nervous system.

A group of two or more tissues that work together to perform a specific function

If you are a single-celled organism and live in a place rich in nutrients, surviving is very simple. For example, if you are an amoeba living in a pond, you can absorb nutrients directly from your environment. The oxygen you need for metabolism can diffuse across the cell membrane, and carbon dioxide and other waste products can diffuse. When it’s time to breed, you can just split yourself in two!

However, you’re probably not an amoeba – since you’re currently using Khan Academy – and things aren’t quite that simple for large, multicellular organisms like humans. Your complex body contains more than 30 trillion cells, and most of these cells are not in direct contact with the external environment.

A group of two or more tissues that work together to perform a specific function

Start with a high line 1, end high A cell deep inside your body – in one of your bones, for example, or in your liver – can’t get the nutrients or oxygen you need directly from the environment.

So how does the body nourish its cells and keep itself functioning? Let’s take a closer look at how organizing your amazing body makes this possible.

Multicellular organisms need specialized systems

Most cells in large, multicellular organisms do not exchange substances such as nutrients and waste directly with the external environment, instead, they are surrounded by an internal environment of extracellular fluid – literally, extracellular fluid. 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 constant and able to provide for the needs of cells.

A group of two or more tissues that work together to perform a specific function

Different body systems perform different functions. For example, the digestive system is responsible for eating and processing food, while the respiratory system – which works with the circulatory system – is responsible for absorbing oxygen and getting rid of carbon dioxide. The muscular system and the skeletal system are essential to movement. The reproductive system handles reproduction; The excretory system gets rid of metabolic waste.

Because of their specialization, these different systems depend on each other. The cells that make up the digestive, muscular, skeletal, reproductive, and excretory systems all need oxygen from the respiratory system to function, and the cells of the respiratory system—as well as all other organs—need nutrients and must be eliminated as metabolic waste. All body systems work together to keep the organism functioning.

A group of two or more tissues that work together to perform a specific function

An overview of the organization of the body

All living things consist of one or more cells. Unicellular organisms, such as amoebas, consist of only one cell. Multicellular organisms, like humans, are made up of many cells. Cells are the basic units of life.

Cells in complex multicellular organisms such as humans are organized into tissues, which are groups of similar cells that work together on a specific task. Organs are structures consisting of two or more tissues organized to perform a particular function, and groups of organs with related functions form the various organ systems.

A group of two or more tissues that work together to perform a specific function

At every level of organization—cells, tissues, organs, and organ systems—structure is closely related to function. For example, the cells of the small intestine that absorb nutrients look very different from the muscle cells that are needed for the body’s movement. The structure of the heart reflects its function of pumping blood throughout the body, while the structure of the lungs increases the efficiency with which it can absorb oxygen and release carbon dioxide.

tissue types

As we saw above, each 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 types of tissue: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.

epithelial tissue

Epithelial tissue is made up of tightly packed sheets of cells that cover surfaces — including the outside of the body — and line body cavities. For example, the outer layer of your skin is epithelial tissue, as is the lining of your small intestine.

A group of two or more tissues that work together to perform a specific function

Epithelial cells are polarized, which means they have an upper and a lower side. The apical, upper, side of the epithelial cell faces the inner part of the cavity or the outer part of the structure and is usually exposed to fluid or air. basal

control and coordination

Many of the body’s functions are controlled by the nervous system and the endocrine system. These two regulatory systems use chemical messengers to influence the function of other organ systems and to coordinate activity at different sites in the body.

How are the endocrine system and the nervous system different?

In the endocrine system, chemical messengers are hormones that are released into the blood.

A group of two or more tissues that work together to perform a specific function

In the nervous system, chemokines are neurotransmitters that are sent directly from one cell to another through a small gap.

Because hormones must travel through the bloodstream to their targets, the endocrine system usually coordinates processes on a slower time scale than the nervous system in which messages are delivered directly to the target cell. In some situations, such as the fight-or-flight response to an acute threat, the nervous and endocrine systems work together to produce a response.

Humans—and other complex, multicellular organisms—have systems of organs that work together, carrying out the processes that keep us alive.

The body has levels of organization that depend on each other. Cells form tissues, tissues form organs, and organs form organ systems.

The function of the organ system depends on the integrated activity of its organs. For example, the organs of the digestive system cooperate to process food.

The survival of the organism depends on the integrated activity of all organ systems, which is often coordinated by the endocrine system and the nervous system.

A group of two or more tissues that work together to perform a specific function

If you are a single-celled organism and live in a place rich in nutrients, surviving is very simple. For example, if you are an amoeba living in a pond, you can absorb nutrients directly from your environment. The oxygen you need for metabolism can diffuse across the cell membrane, and carbon dioxide and other waste products can diffuse. When it’s time to breed, you can just split yourself in two!

However, you’re probably not an amoeba – since you’re currently using Khan Academy – and things aren’t quite that simple for large, multicellular organisms like humans. Your complex body contains more than 30 trillion cells, and most of these cells are not in direct contact with the external environment.

A group of two or more tissues that work together to perform a specific function

Start with a high line 1, end high A cell deep inside your body – in one of your bones, for example, or in your liver – can’t get the nutrients or oxygen you need directly from the environment.

So how does the body nourish its cells and keep itself functioning? Let’s take a closer look at how organizing your amazing body makes this possible.

Multicellular organisms need specialized systems

Most cells in large, multicellular organisms do not exchange substances such as nutrients and waste directly with the external environment, instead, they are surrounded by an internal environment of extracellular fluid – literally, extracellular fluid. 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 constant and able to provide for the needs of cells.

A group of two or more tissues that work together to perform a specific function

Different body systems perform different functions. For example, the digestive system is responsible for eating and processing food, while the respiratory system – which works with the circulatory system – is responsible for absorbing oxygen and getting rid of carbon dioxide. The muscular system and the skeletal system are essential to movement. The reproductive system handles reproduction; The excretory system gets rid of metabolic waste.

Because of their specialization, these different systems depend on each other. The cells that make up the digestive, muscular, skeletal, reproductive, and excretory systems all need oxygen from the respiratory system to function, and the cells of the respiratory system—as well as all other organs—need nutrients and must be eliminated as metabolic waste. All body systems work together to keep the organism functioning.

A group of two or more tissues that work together to perform a specific function

An overview of the organization of the body

All living things consist of one or more cells. Unicellular organisms, such as amoebas, consist of only one cell. Multicellular organisms, like humans, are made up of many cells. Cells are the basic units of life.

Cells in complex multicellular organisms such as humans are organized into tissues, which are groups of similar cells that work together on a specific task. Organs are structures consisting of two or more tissues organized to perform a particular function, and groups of organs with related functions form the various organ systems.

A group of two or more tissues that work together to perform a specific function

At every level of organization—cells, tissues, organs, and organ systems—structure is closely related to function. For example, the cells of the small intestine that absorb nutrients look very different from the muscle cells that are needed for the body’s movement. The structure of the heart reflects its function of pumping blood throughout the body, while the structure of the lungs increases the efficiency with which it can absorb oxygen and release carbon dioxide.

tissue types

As we saw above, each 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 types of tissue: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.

epithelial tissue

Epithelial tissue is made up of tightly packed sheets of cells that cover surfaces — including the outside of the body — and line body cavities. For example, the outer layer of your skin is epithelial tissue, as is the lining of your small intestine.

A group of two or more tissues that work together to perform a specific function

Epithelial cells are polarized, which means they have an upper and a lower side. The apical, upper, side of the epithelial cell faces the inner part of the cavity or the outer part of the structure and is usually exposed to fluid or air. basal

control and coordination

Many of the body’s functions are controlled by the nervous system and the endocrine system. These two regulatory systems use chemical messengers to influence the function of other organ systems and to coordinate activity at different sites in the body.

How are the endocrine system and the nervous system different?

In the endocrine system, chemical messengers are hormones that are released into the blood.

A group of two or more tissues that work together to perform a specific function

In the nervous system, chemokines are neurotransmitters that are sent directly from one cell to another through a small gap.

Because hormones must travel through the bloodstream to their targets, the endocrine system usually coordinates processes on a slower time scale than the nervous system in which messages are delivered directly to the target cell. In some situations, such as the fight-or-flight response to an acute threat, the nervous and endocrine systems work together to produce a response.

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