The Big Picture
Your body has different levels of organization. It starts with tiny cells. Similar cells form tissues, different tissues form organs, and organs work together in organ systems.
These parts depend on one another. When the body’s organ systems work together to keep the internal environment stable, this steady state is called homeostasis.
Start Here If You’re Overwhelmed
Focus on these core ideas:
- A cell is the basic functional unit of body structure and function.
- A group of similar cells with a common function is called tissue.
- Structures made of different tissues and arranged to perform a specialized function are organs.
- Groups of organs that work together to perform special functions are organ systems.
- The body has four basic tissue types: epithelial, connective, muscle, and nerve tissue.
- A steady or stable internal environment maintained when the body’s organ systems work together is called homeostasis.
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Course Notes: Key Ideas You Need to Know
Cells Are the Basic Units of the Body
A cell is the basic functional unit of body structure and function. Cells share the same basic structure, but they can differ in function, size, and shape.
Cells are so small that you need a microscope to see them. They need nutrients, water, and oxygen to live and perform their functions.
Isotonic Solution
An isotonic solution is a solution of 0.09% NaCl, which is sodium chloride or salt, and water. It makes up the body’s cells and bodily fluids and is compatible with cells and the fluids around them.
Main Structures of a Cell
Cells contain several important structures.
Cell Membrane
The cell membrane forms the outer covering of the cell. It encloses the cell and helps the cell keep its shape.
The cell membrane also controls the movement of substances into and out of the cell.
Nucleus
The nucleus acts as the control centre of the cell. It directs the cell’s activities.
The nucleus also contains the chromosomes, including DNA, or deoxyribonucleic acid. It plays an important role in cell reproduction.
Cytoplasm
Cytoplasm is a gel-like fluid inside the cell. It supports the cell’s internal structures, including the nucleus.
Small structures called organelles are located within the cytoplasm. These organelles work together to carry out cell functions.
Chromosomes and Genes
Chromosomes are threadlike structures inside the nucleus. They contain instructions that direct cell processes.
Each cell has 46 chromosomes. These chromosomes contain genes.
Genes pass to new cells during reproduction. They determine physical and chemical traits that children inherit from their parents. Examples include height, eye colour, and skin colour.
Mitosis and Cell Reproduction
The nucleus helps control cell reproduction. Cells reproduce by dividing in half so that each half becomes a new cell.
This type of cell division is called mitosis. Mitosis is needed for the growth and repair of body tissues.
During mitosis, the chromosomes replicate, which means they duplicate. The chromosomes separate, and the cell divides. Each new cell is identical and has 46 chromosomes.
Tissues Are Groups of Similar Cells
Tissue is a group of cells that are similar in structure and function.
The body has four basic types of tissue:
- Epithelial tissue
- Connective tissue
- Muscle tissue
- Nerve tissue
Epithelial Tissue
Epithelial tissue covers internal and external body surfaces.
It is classified by:
- the shape of its cells
- the number of cell layers
Three epithelial cell shapes are described:
- Squamous: thin and flat
- Cuboidal: cube-shaped
- Columnar: tall and narrow
Epithelial tissue forms the thin outer layer of the skin. It also lines cavities that open to the outside, including areas of the nose, mouth, respiratory tract, stomach, urethra, vagina, and intestines.
Many glands also consist of epithelial tissue.
Functions of Epithelial Tissue
Epithelial tissue helps with:
- protection
- absorption
- secretion
- filtration
Secretion means the release of a substance into or out of the body following normal body functions.
Epithelial tissue does not have its own blood supply. It depends on the blood supply of the connective tissue beneath it for nourishment.
Because epithelial tissue consists almost entirely of cells and receives nourishment from the underlying connective tissue, it can repair or replace itself quickly.
Epithelial tissue also contains sensory cells. These cells allow it to perceive sensations such as pain and pressure.
Connective Tissue
Connective tissue anchors, connects, and supports other body tissues. It is found throughout the body.
There are two broad groups described here:
- connective tissue proper
- specialized connective tissue
Connective tissue proper can contain loose, irregular cells in a viscous, or thickened, fluid base. It can also contain dense irregular tissue, such as tissue found in joint capsules.
Specialized Connective Tissues
Several forms of specialized connective tissue have different functions.
Adipose Tissue
Adipose tissue is fat tissue. It:
- supplies energy to surrounding cells
- helps regulate body temperature
- cushions surrounding structures
- supports surrounding structures
Dense Fibrous Connective Tissue
Dense fibrous connective tissue includes tendons and ligaments.
- Tendons attach muscles to bones.
- Ligaments connect bones and support joints.
Cartilage
Cartilage is connective tissue that supports, protects, and cushions joints. It also provides framework.
Cartilage cushions joints so that the ends of bones do not rub together.
Bone Tissue
Bone tissue can protect organs, including the brain. It also supports the body’s weight for movement.
Blood
Blood transports:
- nutrients
- hormones
- respiratory gases such as oxygen and carbon dioxide
- waste materials
Lymph
Lymph drains interstitial fluids. It also takes part in the body’s immune response.
Muscle Tissue
Muscle tissue produces almost all body movements through contraction.
There are three types of muscle tissue.
Nerve Tissue
Nerve tissue is the major tissue of the nervous system.
It forms the main tissue of:
- the brain
- the spinal cord
- the nerves throughout the body
Nerve tissue relays information to and from the brain and throughout the body.
Organs Are Made of Different Tissues
Organs are structures made of different tissues arranged to perform a specialized function.
An organ performs one or more functions.
Examples of organs include:
- heart
- brain
- liver
- lungs
- kidneys
Skin is also considered an organ.
The Major Body Cavities
Organs are usually located inside body cavities.
The two major body cavities are:
- dorsal cavity
- ventral cavity
Dorsal Cavity
The dorsal cavity contains two cavities:
- cranial cavity
- spinal cavity
Ventral Cavity
The ventral cavity contains three cavities:
- thoracic cavity
- abdominal cavity
- pelvic cavity
Organ Systems
Organ systems are groups of organs that work together to perform special functions.
The major organ systems identified here are:
- integumentary
- musculoskeletal
- nervous, including the senses
- endocrine
- circulatory, containing the cardiovascular and lymphatic systems
- respiratory
- immune
- digestive
- urinary
- reproductive
Homeostasis
Homeostasis means maintaining a steady state or stable internal environment. This happens when the body’s organ systems work together.
One example involves body temperature. When the temperature outside drops, the body starts to shiver. Shivering creates warmth and helps keep the internal temperature at its normal level.
Fast Review
Use these points for a quick review before a quiz or exam:
- A cell is the basic functional unit of body structure and function.
- The cell membrane encloses the cell, helps maintain its shape, and controls movement of substances in and out.
- The nucleus controls cell activities and contains chromosomes.
- Each cell has 46 chromosomes containing genes.
- Mitosis produces identical new cells with 46 chromosomes and supports tissue growth and repair.
- The four basic tissue types are epithelial, connective, muscle, and nerve tissue.
- Epithelial tissue helps with protection, absorption, secretion, and filtration.
- Connective tissue anchors, connects, and supports other tissues.
- Organs contain different tissues arranged to perform specialized functions.
- The dorsal cavity contains the cranial and spinal cavities. The ventral cavity contains the thoracic, abdominal, and pelvic cavities.
- Organ systems contain organs that work together to perform special functions.
- Homeostasis is the stable internal environment maintained when the body’s organ systems work together.
Final Pep Talk
There are a lot of new names in this topic, so focus first on the pattern: cell, tissue, organ, then organ system. Once that structure feels familiar, review the cell parts, tissue types, and body cavities in smaller groups.