BONES

ORGANIZATION IN HUMAN BODY

Levels of Organization

The living world can be organized into different levels. For example, many individual organisms can be organized into the following levels:

• Cell: Basic unit of structure and function of all living things.
• Tissue: Group of cells of the same kind. 
• Organ: Structure composed of one or more types of tissues. The tissues of an organ work together to perfume a specific function.Human organs include the brain, stomach, kidney, and liver. Plant organs include roots, stems, and leaves. 
• System: Group of organs that work together to perform a certain function. Examples of organ systems in a human include the skeletal, nervous, and reproductive systems.
• Organism: Individual living thing that may be made up of one or more organ systems.

TISSUES IN HUMAN BODY

We will focused on the different types of tissues, and of what type of cells are made up of:

Epithelial tissue

Functions of the Epithelium

In its role as an interface tissue, epithelium accomplishes many functions, including:

1. Protection for the underlying tissues from radiation, desiccation, toxins, and physical trauma.
2. Absorption of substances in the digestive tract lining with distinct modifications.
3. Regulation and excretion of chemicals between the underlying tissues and the body cavity.
4. The secretion of hormones into the blood vascular system. The secretion of sweat, mucus, enzymes, and other products that are delivered by ducts come from the glandular epithelium.
5. The detection of sensation.

Characteristics of Epithelial Layers

Epithelial tissue is composed of cells laid out in sheets with strong cell-to-cell attachments. These protein connections hold the cells together to form a tightly connected layer that is avascular but innervated in nature.

The epithelial cells are nourished by substances diffusing from blood vessels in the underlying connective tissue. One side of the epithelial cell is oriented towards the surface of the tissue, body cavity, or external environment and the other surface is joined to a basement membrane. The basement layer is non-cellular in nature and helps to cement the epithelial tissue to the underlying structures.

Types of Epithelial Tissue

Epithelial tissue is classified based on the number of cells, the shape of those cells, and the types of those cells.

*Simple squamous epithelium     

Locations	Function
Air sacs of the lungs and  the lining of the heart, blood vessels and lymphatic vessels	Allows materials to pass through by diffusion and filtration, and secretes lubricating substances

*Simple cuboidal epithelium

In ducts and secretory portions of small glands and in kidney tubules

Secretes and absorbs

*Simple columnar epithelium

Ciliated tissues including the bronchi, uterine tubes, and uterus; smooth (nonciliated tissues) are in the digestive tract bladder

Absorbs; it also secretes mucous and enzymes.

*Pseudostratified columnar epithelium

Ciliated tissue lines the trachea and much of the upper respiratory tract

Secrete mucous; ciliated tissue moves mucous

*Stratified squamous epithelium

Lines the esophagus, mouth, and vagina

Protects against abrasion

*Stratified cuboidal epithelium

Sweat glands, salivary glands, and mammary glands

Protective tissue

*Stratified columnar epithelium

The male urethra and the ducts of some glands.

Secretes and protects

*Transitional epithelium

Lines the bladder, urethra and ureters

Allows the urinary organs to expand and stretch

Connective tissue

Connective tissue fibers provide support. Three types of fibers are found in connective tissue:

1. Collagen
2. Elastic fibers
3. Reticular fibers

Collagen fibers are the strongest and most abundant of all the connective tissue fibers. Collagen fibers are fibrous proteins and are secreted into the extracellular space and they provide high tensile strength to the matrix.

Elastic fibers are long, thin fibers that form branching network in the extracellular matrix. They help the connective tissue to stretch and recoil.

Reticular fibers are short, fine collagenous fibers that can branch extensively to form a delicate network.

Functions of Connective Tissue

1. Binding and supporting.
2. Protecting.
3. Insulating.
4. Storing reserve fuel.
5. Transporting substances within the body.

Loose Connective Tissue

In vertebrates, the most common type of connective tissue is loose connective tissue. It holds organs in place and attaches epithelial tissue to other underlying tissues. The three main types of loose connective fibers include collagenous, elastic, and reticular fibers.

• Collagenous fibers are made of collagen and consist of bundles of fibrils that are coils of collagen molecules. These fibers help to strengthen connective tissue.

• Elastic fibers are made of the protein elastin and are stretchable. They help to give connective tissue elasticity.
• Reticular fibers join connective tissues to other tissues.

Loose connective tissues provide support, flexibility, and strength required to support internal organs and structures such as blood vessels, lymph vessels, and nerves.

Dense Connective Tissue

Another type of connective tissue is dense or fibrous connective tissue, which can be found in tendons and ligaments. These structures help attach muscles to bones and link bones together at joints. Dense connective tissue is composed of large amounts of closely packed collagenous fibers. In comparison to loose connective tissue, dense tissue has a higher proportion of collagenous fibers to ground substance.

Dense connective tissue can be categorized into dense regular, dense irregular, and elastic connective tissues.

• Dense regular: Tendons and ligaments are examples of dense regular connective tissue.
• Dense irregular: Much of the dermis layer of the skin is composed of dense irregular connective tissue. The membrane capsule surrounding several organs is also dense irregular tissue.
• Elastic: These tissues enable stretching in structures such as arteries, vocal cords, the trachea, and bronchial tubes in the lungs.

Specialized forms of connective tissue include:

Adipose

Adipose tissue is a form of loose connective tissue that stores fat. Adipose lines organs and body cavities to protect organs and insulate the body against heat loss. Adipose tissue also produces endocrine hormones.

Cartilage

Cartilage is a form of fibrous connective tissue that is composed of closely packed collagenous fibers in a rubbery gelatinous substance called chondrin. The skeletons of sharks and human embryos are composed of cartilage. Cartilage also provides flexible support for certain structures in adult humans including the nose, trachea, and ears.

Bone

Bone is a type of mineralized connective tissue that contains collagen and calcium phosphate, a mineral crystal. Calcium phosphate gives bone its firmness.

Blood

Interestingly enough, blood is considered to be a type of connective tissue. Even though it has a different function in comparison to other connective tissues, it does have an extracellular matrix. The matrix consists of the plasma with red blood cells, white blood cells, and platelets suspended in the plasma.

Lymph

Lymph is another type of fluid connective tissue. This clear fluid originates from blood plasma that exits blood vessels at capillary beds. A component of the lymphatic system, lymph contains immune system cells that protect the body against pathogens

Muscle tissue

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.

Smooth muscle cells do not have striations, while skeletal muscle cells do.

Cardiac muscle cells have striations, but, unlike the multinucleate skeletal cells, they have only one nucleus. Cardiac muscle tissue also has intercalated discs, specialized regions running along the plasma membrane that join adjacent cardiac muscle cells and assist in passing an electrical impulse from cell to cell.

Skeletal muscle, which is also called striated 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.

Smooth muscle is found in the walls of blood vessels, as well as in the walls of the digestive tract, the uterus, the urinary bladder, and various other internal structures. Smooth muscle is not striped, striated, and it's involuntary, not under conscious control. 

Nervous tissue

Nervous tissue is involved in sensing stimuli and processing and transmitting information. It consists of two main types of cells: neurons 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.