Chapter 33 Tissues, Organ Systems, and Homeostasis

I. Animal Structure and Function: An Overview
A. Animals are structurally and physiologically adapted to be able to:
1 Maintain internal operating conditions within some tolerable range.
2 Locate and acquire nutrients and dispose of wastes.
3 Protect themselves against injury or attack from viruses, bacteria, and other agents. 4 Reproduce and often provide for the offspring during early development.
B. Complex animals exhibit levels of organization.
1 A tissue is an aggregation of cells and intercellular substances that function in one or more
specialized activities (division of labor).
2 Various types of tissues can combine to form organs, such as the heart.
3 Organs may interact to form organ systems, such as the digestive system.

II. Animal Tissues
A. Tissue Formation
1 Germ cells in the parental gonads produce either sperm or eggs by meiosis; all other cells of
the body are called somatic cells.
2 Fusion of gametes forms a zygote, which undergoes mitosis to form an embryo. 3 Cells in the embryo become arranged into three primary tissues:
a. Ectoderm gives rise to skin and nervous system.
b. Mesoderm gives rise to muscle, skeleton, and the organs of circulation, reproduction, and
excretion.
c. Endoderm gives rise to the lining of the gut and its associated organs.
B. Epithelial Tissue (protection, secretion, absorption)
1 General Features
a. In epithelial tissues, cells are linked tightly together; there may be one or more layers.
b. One surface is free (may have cilia or microvilli), and the other adheres to a noncellular basement
membrane.
c. Simple epithelium consists of a single layer of cells (squamous, cuboidal, columnar); example:
wall of capillaries.
d. Stratified epithelium consists of one or more layers of cells; example: skin.
2 Glandular Epithelium
a. Glands are single cells or multicelled secretory structures.
b. Exocrine glands often secrete through ducts to free surfaces; they secrete mucus, saliva, wax, milk,
and so on.
c. Endocrine glands secrete hormones directly into intercellular fluid for distribution by the blood.
3 Cell-to-Cell Contacts in Epithelium
a. Epithelial cells adhere tightly to one another by means of special attachment sites; this prevents
leakage (example: stomach acid).
b. The types of junctions include: tight junctions, desmosomes (“spot welds”), and gap junctions
(protein channels).
C. Connective Tissue
1 Most connective tissue contains cells and fibers (collagen and/or elastin) scattered in a ground
substance.
2 Connective Tissue Proper
a. Loose connective tissue supports epithelia and organs and surrounds blood vessels and nerves; it
contains collagen and elastin fibers plus fibroblasts and macrophages.
b. Dense, irregular connective tissue has thicker fibers and more of them, but fewer cells.

3 Specialized Connective Tissue
a. Dense, regular connective tissue has its fibers in parallel; this is the arrangement found in tendons
(muscle to bone) and ligaments (bone to bone).
b. Cartilage contains a dense array of fibers in a jellylike ground substance.
1) It cushions and maintains the shape of body parts; it resists compression and is resilient.
2) Locations include the ends of bones, parts of the nose, external ear, and disks between
vertebrae.
c. Bone stores mineral salts, produces blood cells, and provides spaces for its own living osteocytes.
1) Organized as flat plates and cylinders, bones support and protect body tissues and organs.
2) Bones work with muscles to perform movement.
d. Adipose tissue cells are specialized for the storage of fat, which can be used as an energy reserve
and as cushions to pad organs.
e. Blood transports oxygen, wastes, hormones, and enzymes; it also contains clotting factors to protect
against bleeding and components to protect against disease-causing agents.
D. Muscle Tissue
1 Muscle tissue contracts in response to stimulation, then passively lengthens.
2 There are three varieties of muscle:
a. Skeletal muscle tissue attaches to bones for voluntary movement; it contains striated, multinucleated, long cells.
b. Smooth muscle tissue contains spindle-shaped cells; it lines the gut, blood vessels, and glands; its
operation is involuntary.
c. Cardiac (heart) muscle is composed of short, striated cells that can function in units.
E. Nervous Tissue
1 Neurons are organized as lines of communication throughout the body.
2 Some types detect stimuli; others coordinate the body’s responses; still others relay signals to muscles
and glands for response.

III. Major Organ Systems
A. Eleven organ systems in vertebrates contribute to the survival of the living cells of the body. B. They are: integumentary, muscular, skeletal, nervous, endocrine, circulatory, lymphatic, respiratory,
digestive, urinary, and reproductive.

IV. Homeostasis and Systems Control
A. The Internal Environment
1 The trillions of cells in our bodies must draw nutrients from, and dump wastes into, the same fluid. 2 The extracellular fluid consists of interstitial fluid (between the cells and tissues) and plasma (blood fluid). 3 The component parts of an animal work together to maintain the stable fluid environment required by its
living cells.
B. Mechanisms of Homeostasis
1 Homeostasis refers to stable operating conditions in the internal environment, maintained through
homeostatic controls.
2 A common homeostatic mechanism is negative feedback.
a. It works by detecting a change in the internal environment that brings about a response that tends to
return conditions to the original state.
b. It is similar to the functioning of a thermostat in a heating/cooling system.
3 Positive feedback mechanisms may intensify the original signal; sexual arousal is an example. 4 Homeostatic control mechanisms require three components:
a. Sensory receptors detect specific changes in the environment.
b. Integrators (brain and spinal cord) act to direct impulses to the place where a response can be made. c. Effectors (muscles and glands) perform the appropriate response.