Ch. 3: Human Organization

and Introduction to Homeostasis

(text pp. 61-78)
Human Biology (BIOL100)

In chapter 3 we will explore the basic organization of the cells of the human body into tissues , organs, and organ systems :
1) atoms.
2) molecules.
3) cell organelles.
4) cells.
5) tissues.
6) organs.
7) organ systems.
A tissue is a mass of cells each of which performs a similar function. The four types of tissue are epithelial, connective, muscular, and nervous.
An organ is a structure composed of two or more tissues which combine to perform a specific function(s). Examples of organs are the stomach, heart, lungs, skin, etc.
An organ system is composed of two or more organs which combine to perform a specific function(s). Examples of organs systems are the cardiovascular system, muscular system, skeletal system, nervous system, etc.

We will take a brief look at the body cavities and membranes that contain and surround the various organs and organ systems of the human body.

Finally we will study homeostasis, the attempts of the tissues, organs, and organ systems of the human body to maintain a relatively constant internal environment regardless of the conditions in the external environment.

I. Types of Tissues

There are four types of tissues: epithelial, connective, muscular, and nervous.

a) Epithelial:
Epithelial tissue consists of cells that cover the internal and external surfaces of the body.
It functions to protect and allow for the secretion and absorbtion of substances out of and into the body.

The shapes of the cells that make-up epithelial tissue are:
1. Squamous.
2. Cuboidal.
3. Columnar.
These cells can be arranged as a single layer called simple or into multiple layers called stratified or pseudostratified. Epithelial cells may hold cilia or microvilli and can form glands that secrete their product into ducts or directly into the blood.

b) Connective:
Connective tissue consists of cells dispersed in a non-cellular matrix of several types.
It functions to bind other tissue types and the organs together, provides support and protection, produces blood cells, and stores fat.

The types of connective tissue are:
1. Loose fibrous .
2. Dense fibrous - proteins matrix forms tendons and ligaments.
3. Adipose - store fat.
4. Reticular
5. Cartilage
6. Bone - rigid, mineralized matrix.
7. Blood - plasma matrix containing erythrocytes, leukocytes, and thrombocytes.

c) Muscular:
Muscular tissue consists of cells called muscle fibers .
Muscle fibers contain filaments of the proteins actin and myosin which allow the cell to contract to produce movement.

The types of muscular tissue are:
1. Skeletal or voluntary muscle - multi-nucleate, striated cells. Contraction of skeletal muscle fibers is under voluntary control. It is attached to bones via tendons.
2. Smooth or involuntary muscle - single nucleus, cells are not striated. Contraction of smooth muscle fibers is involuntary. It is located in many internal organs and blood vessels.
3. Cardiac muscle - single nucleus, striated, branched cells. Contraction of cardiac muscle fibers is involuntary. It is located in the heart.

d) Nervous:
Nervous tissue consists of cells called neurons and neuroglia located in the brain, spinal cord, and nerves.
It functions to produce and conduct nervous impulses. These nervous impulses may be sensory (internal and external stimuli), integrative (within the brain and spinal cord), and motor (to muscles and glands).

II. Body Cavities and Membranes

The internal organs are located within specific body cavities:
1) Dorsal cavity contains the:
a) Cranial cavity - contains the brain.
b) Vertebral canal - contains the spinal cord.

2) Ventral cavity contains the:
a) Thoracic cavity - contains the heart (within pericardium) and lungs (within the pleura). Separated from the abdominopelvic cavity by the diaphragm.
b) Abdominopelvic cavity - contains the organs of the digestive, urinary, and reproductive system.

These cavities are lined by epithelial membranes:
1) Mucous membranes.
2) Serous membranes.
3) Synovial membranes.

III. Organ Systems

The organ systems of the human body work together to maintain homeostasis (see below):

1) Maintainance of the body - the digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing and tranporting functions that maintain the normal conditions of the body.

2) Integumentary system - the skin is sometimes called the integumentary system because it contains accessory organs such as hair, nails, sweat, and oil glands.

3) Support and Movement - the skeletal and muscular systems give the body support and allow it to move.

4) Integration and Control of the Body - the nervous system receives sensory input from internal and external sensory receptors and directs the musculoskeletal system and glands to respond to outside stimuli. The endocrine system produces hormones.

5) Continuance of the Species - The male and female reproductive systems contain the organs that produce gametes (sex cells = sperm and eggs) and the ducts that make gametes available so that fertilization can occur.

IV. Homeostasis

Homeostasis is the dynamic equilibrium of the internal environment of the human body.
The internal environment consists of tissue fluid , which bathes the cells and is derived from blood .
All organ systems contribute to maintaining the relatively constant composition and condition of the blood and tissue fluids in order that the cells may function normally.
Coordination of the activity organ systems in order to maintain homeostasis is achieved through nervous and endocrine control.
A negative feedback mechanism is utilized in both systems.