40.1. Animals Are Organized (p. 718)
A. Levels of Organization
1. Animal levels of organization are cells, tissues, organs, and organ systems. (Fig. 40.1)
2. Structure and function of each level is dependent on the structure and function of the organ, tissue, and cell type.
B. Three Embryonic Germ Layers
1. In the embryological development of animals, three fundamental germ layers give rise to all tissues:
2. Ectoderm is outer germ layer; gives rise to epidermis, epithelium of mouth and rectum, and nervous system.
3. Mesoderm is the middle germ layer; gives rise to skeleton, dermis of skin, skeletal system, muscular system, excretory system, reproductive system, and outer layers of respiratory and digestive systems.
4. Endoderm is inner germ layer; gives rise to epithelium of digestive and respiratory tract, associated glands of digestive and respiratory systems, and lining of urinary bladder.
C. Tissues Have Structure and Functions
1. Epithelial tissue covers body surfaces and lines body cavities.
2. Connective tissue binds and supports body parts.
3. Muscular tissue causes body parts to move.
4. Nervous tissue responds to stimuli and transmits impulses.
D. Epithelial Tissue Covers
1. Epithelial tissue forms a continuous layer over most body surfaces including inner cavities.
2. There are three types of epithelial tissue. (Fig. 40.2) [transp. 213]
a. Squamous epithelium is composed of flat cells.
b. Cuboidal epithelium has cube-shaped cells.
c. Columnar epithelium has elongated cells that resemble pillars or columns.
3. Epithelium varies in number of cell layers. (Fig. 40.2) [transp. 213]
a. Simple epithelium has one cell layer; all cells contact basement membrane.
b. Pseudostratified epithelium appears layered; actually, all cells contact basement membrane.
c. Stratified epithelium is composed of more than one layer of cells.
4. Epithelial cells can have cilia. (Fig. 40.2) [transp. 213]
a. Ciliated epithelium cells are covered with cilia (e.g., the lining of the human respiratory tract).
b. Cilia can bend and move material over the surface of the epithelium.
5. Secretory epithelia can be unicellular or multicellular glands.
a. Glands are a single cell or a group of cells that secrete products into the lumen of or onto the lining of a tube or cavity, into the blood, or to the outside of the body; they are classified in two types:
1) Exocrine glands secrete their products into ducts or directly into a tube or cavity.
2) Endocrine glands secrete their product directly into the bloodstream.
6. Epithelium forms outer layer of skin of most animals; it may produce an outer, nonliving protective cuticle.
7. Epithelial tissue cells are packed tightly and joined to one another in one of three ways:
a. Tight junctions have plasma proteins extending between neighboring cells and binding the cells tightly.
b. Adhesion junctions have cytoskeletal elements joining internal plaques present in neighboring cells.
c. Gap junctions form when two identical plasma membrane channels of neighboring cells join allowing ions and small molecules to pass between the cells.
E. Connective Tissue Connects
1. Connective tissue binds structures together, fills spaces, stores fat, and forms blood cells.
2. Connective tissue provides source cells for muscle and skeletal cells in animals that can regenerate lost parts.
3. Connective tissue cells are separated widely by a matrix, a noncellular material found between cells.
4. Loose connective tissue and fibrous connective tissue are flexible.
a. Cells of loose and fibrous connective tissues are fibroblasts.
b. Fibroblasts are spaced apart, separated by jelly matrix of white collagen fibers and yellow elastic fibers.
c. Collagen fibers provide flexibility and strength; elastic fibers provide elasticity.
d. Loose fibrous connective tissue is located under skin and supports epithelium and provides support, flexibility, and protective covering encasing internal organs. (Fig. 40.3a) [micro. slide 82]
e. Dense fibrous connective tissue contains closely packed collagenous fibers; found in tendons, which attach muscles to bone, and ligaments, which bind bones to other bones at joints.
f. Adipose tissue is a type of loose connective tissue that serves to insulate, provide protective padding, and store fat; in mammals, it is found beneath skin, around kidneys, and on surface of heart. (Fig. 40.3b) [micro. slide 83]
g. Reticular connective tissue is present in the lymph nodes, the spleen, and the bone marrow; reticular fibers, associated with reticular cells, support free blood cells.
5. Cartilage and bone are solid.
a. Cartilage and bone are rigid connective tissue in which structural proteins (cartilage) or calcium salts (bone) are deposited in the intercellular matrix.
b. In cartilage, cells lie in small chambers or lacunae embedded in a strong, flexible matrix.
(Fig. 40.3c) [micro. slide 84]
1) In some animals, such as sharks and rays, the entire skeleton is cartilage.
2) The human fetal skeleton is entirely cartilage but is gradually replaced by bone.
3) Various types of cartilage are classified according to type of collagen and elastic fiber found in matrix.
4) Cartilage is at end of long bones, the human nose, framework of the human ear, in walls of respiratory ducts, and within intervertebral discs.
c. In bone, a matrix of calcium salts is deposited around protein fibers. [micro. slide 85]
1) The calcium salts give bone rigidity, and the protein fibers provide elasticity and strength.
2) In compact bone, bone cells (osteocytes) lie within lacunae arranged in concentric circles within osteons (Haversian systems) around tiny tubes called central canals.
3) The canals contain nerve fibers and blood vessels.
4) Nutrients brought by the blood reach all of the cells via minute canals (canaculi) containing thin processes of the osteocytes that connect them with one another and with the central canals.
5) Spongy bone at end of long bones is designed for strength, has numerous long bony bars and plates.
G. Blood Is a Liquid
1. Blood is a connective tissue in which the cells are separated by liquid plasma.
2. In vertebrates, the blood cells are mainly of two types. (Fig. 40.4)
a. Red blood cells (erythrocytes) carry oxygen.
b. White blood cells (leukocytes) aid in fighting infection.
3. Platelets, also present in plasma, are fragments of giant cells found in bone marrow; play a key role in initiating blood clotting.
4. Unlike other types of connective tissue, the intercellular matrix of blood (i.e., plasma) is not made by cells; instead, plasma is a mixture of molecules that enter the blood at various locations. (Table 40.1)
H. Muscular Tissue Contracts
1. Muscular (contractile) tissue is composed of cells called muscle fibers.
2. Muscle fibers contain actin and myosin filaments; interactions result in animal movement. (Fig. 40.5)
3. There are three types of vertebrate muscle tissue: skeletal, cardiac, and smooth.
4. Skeletal muscle is attached via tendons to bones of skeleton. (Fig. 40.5a) [micro. slide 86])
a. Skeletal muscle moves body parts, is under voluntary control, and contracts faster than other muscle types.
b. Skeletal muscle fibers are very long, cylindrical, multinucleate cells that arose from fusion of several cells.
c. Skeletal fibers are striated with light and dark bands due to overlapping actin and myosin filaments.
5. Cardiac muscle is found only in the wall of the heart and powers the heartbeat that pumps blood.
a. Cardiac muscle combines features of both smooth muscle and skeletal muscle. (Fig. 40.5b) [micro. slide 87]
b. Unlike skeletal muscles with many nuclei, cardiac muscles have one centrally placed nucleus.
c. Although a heart appears to be one mass of muscle fibers, cardiac muscle fibers are individual cells.
d. Cardiac muscle cells are bound end to end at intercalated disks where folded membranes between two fibers contain desmosomes and gap junctions; impulses move from cell to cell so heartbeat is coordinated.
6. Smooth (visceral) muscle is not striated.
a. Spindle-shaped fibers form layers with the thick middle portion of one fiber opposite the thin ends of adjacent fibers. (Fig. 40.5c) [micro. slide 88]
b. Smooth muscle is not under voluntary control and is therefore involuntary.
c. Smooth muscle is found in walls of viscera (intestine, stomach, and other organs) and blood vessels.
d. Smooth muscles drive intestinal contractions and blood vessel constrictions.
I. Nervous Tissue Conducts
1. Nervous tissue is composed of neurons in the brain and spinal cord.
2. A neuron has three parts. (Fig. 40.6) [transp. 214]
a. Dendrites receive a stimulus and conduct signals to the cell body.
b. Cell body contains most of the cytoplasm and the nucleus of the neuron.
c. Axon conducts nerve impulses away from the cell body.
3. Long axons and dendrites form neuron fibers; they are bound together by connective tissue to form nerves.
4. Neurons detect stimuli and conduct signals to brain or spinal cord; nerves lead to muscles or glands.
J. Neuroglial Cells Are Versatile
1. There are several types of neuroglial cells in the CNS. (Fig. 40.6)
2. Neuroglial cells outnumber neurons nine to one; previously considered to only support or nourish neurons.
3. Oligodendrocytes form myelin, and microglial cells also phagocytize bacterial and cellular debris.
4. In addition to providing nutrients, neuroglial cells produce a growth factor known as glial-derived growth factor (GDGF) that might be useful to cure diseases of neural degeneration.
5. Although they lack long processes, they do communicate among themselves and with neurons.
40.2. Organs Have Structure and Functions (p. 724)
A. Organs are combinations of two or more different tissues performing common functions.
1. Organ systems are a group of two or more different organs performing common functions.
2. The skin is sometimes considered the integumentary system since it cannot be placed in another system; it is a system composed of the skin and accessory organs (i.e., nails, hair, glands, and sensory receptors).
B. Human Skin is an Organ
1. Human skin has many functions.
a. It serves to protect underlying tissues from trauma, desiccation, radiation damage, and microbial invasion;
b. Skin produces a precursor molecule that is converted to vitamin D after it is exposed to UV light.
c. Skin also helps regulate body temperature.
d. Laden with sensory receptors, the skin collects information about the external environment.
2. Skin has an outer epidermal layer (epidermis) and a deeper layer (dermis). (Fig. 40.7) [transp. 215]
C. Skin Has Layers
1. The epidermis is the outer, thinner layer of the skin.
a. The epidermis is composed of stratified squamous epithelium.
b. Cells are derived from a basal layer of stem cells that undergo continuous cell division.
c. Newly formed cells push to the surface, away from their blood supply; they gradually flatten and harden as they accumulate keratin, a hard, waterproof protein.
d. Eventually, keratinized cells die and are sloughed off.
e. Melanocytes are cells located in the basal layer of the skin, that produce the pigment melanin, which absorbs UV light, thereby protecting surrounding cells from radiation damage.
f. Nails grow from special epidermal cells at the base of the nail in a region called the nail root.
1) The visible portion of the nail is the nail body.
2) The cells become keratinized as they grow out over the nail bed.
3) Vascular dermal tissue under nail provides pink color; white half-moon area is a thicker germinal area.
2. The dermis is the deeper and thicker layer of the skin.
a. The dermis contains elastic fibers and collagen fibers; these run parallel with the skin surface.
b. The hair follicle contains the nonliving hair shaft and the living hair root that produced it.
1) A hair shaft is formed of dead, keratinized epidermal cells, which protects the surface of the skin.
2) Arrector pili muscle is smooth muscle attached to hair follicle; it contracts to cause hair to erect.
3) Follicles have sebaceous glands producing sebum, an oil secreted to lubricate hair and skin.
c. Sweat (sudoriferous) glands are coiled tubules present in all regions of the skin that secrete a fluid (sweat) onto the surface of the skin that contributes to evaporative cooling.
d. Many small receptors are present in the dermis.
1) There are separate receptors for pressure, touch, temperature, and pain.
2) Pressure receptors have onionlike sense organs buried deep in the dermis and around joints.
3) In cats, Pacinian corpuscles are concentrated in paws, leg joints, and the abdomen.
4) Closely related sensors in the tongue of woodpeckers help them find insects in tree bark.
5) Touch receptors are flat and oval shaped: concentrated in fingertips, palms, lips, tongue, nipples, penis, and clitoris.
6) Heat and cold sense organs are encapsulated inside sheaths of connective tissue.
7) Nerve fibers branch out through the skin; free nerve endings are pain receptors.
e. The dermis contains blood vessels that constrict (turn pale) and dilate (blush).
3. The subcutaneous layer lies below the dermis.
a. It is composed of loose connective tissue, including adipose tissue.
b. The adipose tissue helps to insulate the body by minimizing both heat gain and heat loss.
c. This layer gives a rounded appearance to the body.
d. Excessive development of adipose tissue occurs with obesity.
4. Skin Cancer Can Develop (p. 725)
a. There has been a great increase in persons with skin cancer due to sunbathing and use of tanning machines.
b. Excessive exposure to UV radiation can convert cells in the basal layer of epidermis to cancer cells.
D. Organs Form Systems
1. Individual organs function as part of an organ system.
2. Organ systems carry out life processes.
E. Bodies Have Cavities
1. Human body has two main cavities: dorsal cavity with brain and spinal cord, and larger ventral cavity. (Fig. 40.8) [transp. 216]
2. The ventral cavity located on front side of body develops from coelom and is divided by the muscular diaphragm in humans and other mammals.
3. The thoracic (chest) cavity is located in the upper part of the ventral cavity, above the muscular diaphragm, and contains the heart and the lungs.
4. The abdominal cavity is located in the lower part of the ventral cavity, below the muscular diaphragm, and contains major portions of the digestive system, the excretory system, and much of the reproductive system.
40.3. Homeostasis Is Necessary (p. 728)
A. The cells of the body live in an internal environment, the tissue fluid that bathes the cells of an animal's body.
1. This concept was first proposed by Claude Bernard, a famous French physiologist in 1859.
2. The quality of the internal environment (e.g., composition and temperature) must stay within normal range.
3. This relative internal stability allows animals to tolerate considerable external variation.
B. Homeostasis is maintenance of internal conditions in a cell or an organism by means of a self-regulating mechanism that curtails fluctuations above and below a normal range.
1. Most organ systems of the human body contribute to homeostasis.
a. The respiratory system adds oxygen and removes carbon dioxide; amounts can be increased to meet needs.
b. The liver removes and stores glucose as glycogen, then replaces blood glucose levels when low.
c. The hormone insulin is secreted by the pancreas to regulate glucose levels.
d. Kidneys are under hormonal control to excrete wastes and salts and to maintain blood pH.
2. Although homeostasis is controlled by hormones, it is ultimately controlled by the nervous system.
3. The brain contains centers that regulate temperature and blood pressure.
4. Regulation requires a receptor that detects unacceptable levels and signals a regulator center that can direct an adaptive response; once normalcy is obtained, the receptor is no longer stimulated.
5. A negative feedback mechanism involves a response in which the output is counter to and cancels the input, thereby decreasing the likelihood of a response. (Fig. 40.9) [transp. 217]
a. A house thermostat is an analogy.
b. Negative feedback initiates the heater or air conditioner to maintain temperature within narrow limits.
6. A positive feedback mechanism involves output that intensifies and increases the input, thereby increasing the likelihood of a response.
a. Once childbirth begins, each event makes the process continue until completion.
b. The sequences in blood clotting likewise progress to form a blood clot.
C. How Body Temperature Is Regulated (Fig. 40.10) [transp. 218]
1. The regulator center for body temperature is located in the hypothalamus, a part of the brain.
2. When body temperature falls below normal, a regulator center directs smooth muscles of the blood vessels in skin to constrict, which reduces blood flow to peripheral tissues, and thereby reduces loss of heat to the external environment.
3. In hairy animals, the arrector pili muscles pull hairs erect forming a thicker insulation.
4. If temperature falls even lower, regulator center sends nerve impulses to skeletal muscles, initiating shivering to generate heat.
5. If body temperature is too warm, regulator center directs skin blood vessels to dilate, which increases blood flow to peripheral tissues, and thereby increases loss of heat.
6. Regulator center also activates sweat glands, increasing sweat production, which increases evaporative cooling.