and genes are linked together in long strings called chromosomes. Genes perform two functions. Dur- ing the normal life of the cell, they act as blueprints for making proteins. During cell reproduction, they act as the material of heredity, forming copies of themselves so that each new cell formed has a com- plete and identical set of blueprints. Defects in genes are responsible for hereditary diseases such as hemophilia and sickle cell disease. All cells in the body begin with a nucleus, but red blood cells push out theirs at maturity.
Mitochondria
Mitochondria are the cell’s power plants, burning fuels such as sugar and fat with oxygen to supply energy for the cell in the form of adenosine triphos- phate (ATP). Neurons and muscle cells contain very high numbers of mitochondria.
Cytoplasm
Cytoplasm refers to all the cellular material except the plasma membrane and the nucleus. Cytosol is the fluid portion of the cytoplasm.
Plasma Membrane
The plasma membrane encloses the cell and tightly regulates the flow of materials in and out of it.
Membranes are flexible, allowing cells to change shape if necessary. For instance, the red blood cell must squeeze through tiny capillaries that are thin- ner than its normal diameter, a feat made possible by the flexibility of the membrane.
Tissues
Cells of similar structure and function combine to form tissues. The human body is composed of the following four basic types of tissues:
1. Epithelial 2. Muscle 3. Nerve 4. Connective Epithelial Tissue
Epithelial tissue forms flat sheets and is most often found on surfaces where exchange with the envi- ronment takes place, such as the lining of the gut (Figure 6-2), or where rapid regeneration must
Lysosome Cytoplasm Free ribosome Microtubule
Mitochondrion
Centrioles
Nucleus Nuclear membrane
Nucleolus
Golgi apparatus
Cilia
Cell membrane
Rough endoplasmic reticulum (covered with ribosomes) Smooth endoplasmic reticulum
FIGURE 6-1 Cell structures include the plasma membrane, nucleus, mitochondria, and other organelles respon- sible for maintaining cellular homeostasis. (From Herlihy B: The human body in health and illness, ed. 5, St. Louis, 2015, Saunders.)
occur to protect internal structures, such as the skin or the surface of the eye. Epithelium may contain glands, which produce and secrete substances such as saliva, sweat, or insulin.
Muscle Tissue
Muscle tissue is contractile, meaning that it can shorten its length. Muscle cells contain long fibers of the proteins actin and myosin, whose movements produce muscle contraction. Muscles receive the stimulus to contract when the axon terminals of motor neurons (neurons that stimulate muscle) make contact with the muscle. The neuron releases a chemical (called a neurotransmitter) onto the muscle cell surface at the neuromuscular junction, causing the chemical changes within the muscle that lead to contraction.
Muscle tissue occurs in three forms, which differ in both structure and function (Figure 6-3):
1. Skeletal muscle, or striated voluntary muscle, is the most widespread type, constituting all the muscles that move the skeleton. Under the microscope, striated muscle has a striped appearance.
2. Cardiac muscle, or striated involuntary mus- cle, is found in the heart. It looks similar to skel- etal muscle but has features that are unique to it.
Cardiac muscle cells do not need stimulation by the nervous systems to start a contraction.
However, electrical stimulation is required to maintain the coordinated rhythm of the cells.
This function is performed by the heart’s physi- ologic pacemaker, found in the right atrium.
3. Smooth muscle, or nonstriated involuntary muscle, lines blood and lymph vessels within the
body just below the epithelial tissue, such as around the gut, lungs, bladder, and circulatory and reproductive systems. Smooth muscle con- tractions regulate the passage of materials through the vessel. For instance, contraction of intestinal smooth muscle propels food along the digestive system. Contraction of smooth muscle surrounding veins constricts the vessel, decreas- ing flow through the vein. Smooth muscle also is found in the skin, where it is responsible for hair erection.
Nerve Tissue
Nerve tissue is specialized for intercellular commu- nication by the conduction of electrical impulses and release of chemical messages. Nerve tissue is composed of neurons and neuroglial cells. Neurons are excitable cells, meaning that they can be stimu- lated to undergo electrical and chemical changes.
Neurons are found in the brain and spinal cord and throughout the body. Neuroglial cells nourish and support neurons in the brain and spinal cord.
The three major portions of the neuron are the dendrite, cell body, and axon (Figure 6-4). Axons can be extremely long; each of the motor neurons controlling the toes, for instance, has an axon that stretches from the spinal cord, down the leg, through the ankle, and to the muscles of the foot. Axons are insulated by a fatty sheath of myelin.
One neuron conveys information to another by releasing a chemical, called a neurotransmitter, at the small gap where they meet, called the synaptic cleft, or synapse. The neurotransmitter leaves the axon of the first neuron, crosses the synaptic cleft, and lands on a receptor on the dendrite of the second neuron, beginning a cascade of chemical changes down the length of the neuron. This may ultimately cause the second neuron to release its own neurotransmitter, thereby conveying informa- tion farther along the neural chain.
Connective Tissue
The general function of connective tissue is to bind and support the other three types of tissue. Connec- tive tissue is characterized by a relative scarcity of cells and a relative abundance of extracellular ground substance secreted by the cells. Bone is a
Stratified squamous
FIGURE 6-2 An example of epithelial tissue. (From Herlihy B: The
human body in health and illness, ed. 5, St. Louis, 2015, Saunders.) FLASH FORWARD
As you will learn in Chapter 7, primary hemostasis (bleeding control) involves smooth muscle constriction.
connective tissue—its ground substance is collagen, impregnated with mineral crystals. Blood is also a connective tissue, with plasma as its ground substance.
Organs
An organ is a distinct structural unit in the body, specialized for some complex function. Organs such as the heart, lungs, and kidneys incorporate all four tissue types. In the lungs, for instance, epithelial tis- sue lines the airways, cleaning and moistening them
and providing a barrier to infection. Smooth muscle constricts or relaxes to regulate the size of the air- way. Neurons in the bronchi detect the presence of excess mucus or other irritants and provoke a cough.
Cartilage—a type of connective tissue—supports the larger airways to prevent collapse.
Body Systems
Body systems are groups of organs functioning together for a common purpose. The respiratory system, for instance, involves not only the lungs but
Nucleus
Striations
A
B
C
Nucleus Striations
Junction between cells
Nucleus
FIGURE 6-3 The three forms of muscle tissue. A, Skeletal. B, Cardiac. C, Smooth.
also the upper airway, including the nasal passages and throat. It also includes the muscles that inflate the lungs, such as the diaphragm, the rib muscles, and the muscles of the neck.
After a brief discussion of anatomic terminology, we review the structure and function of the body systems, discuss how they may be involved in dis- ease, and indicate what tests can be ordered to monitor their function.