Introduction to the Human Body

1 Terminology

Anatomical Position

The study of anatomy requires a clinical vocabulary that defines position, movements, relationships, and planes of reference, as well as the systems of the human body. The study of anatomy can be by body region or by body organ systems. Generally, courses of anatomy in the United States approach anatomical study by regions, integrating all applicable body systems into the study of a particular region. This textbook therefore is arranged regionally, and for those studying anatomy for the first time, this initial chapter introduces you to the major body systems that you will encounter in your study of anatomy. You will find it extremely helpful to refer back to this introduction as you encounter various body systems in your study of regional anatomy.

By convention, anatomical descriptions of the human body are based on a person in the anatomical position (Fig. 1-1), as follows:

FIGURE 1-1 Anatomical Position and Terminology for Body Regions.

• Standing erect and facing forward

• Arms hanging at the sides with palms facing forward

• Legs placed together with feet facing forward

Terms of Relationship and Body Planes

Anatomical descriptions often are referenced to one or more of three distinct body planes (Fig. 1-2 and Table 1-1), as follows:

TABLE 1-1

General Terms of Anatomical Relationship

TERM	DEFINITION
Anterior (ventral)	Near the front
Posterior (dorsal)	Near the back
Superior (cranial)	Upward, or near the head
Inferior (caudal)	Downward, or near the feet
Medial	Toward the midline or median plane
Lateral	Farther from the midline or median plane
Proximal	Near a reference point
Distal	Away from a reference point
Superficial	Closer to the surface
Deep	Farther from the surface
Median plane	Divides body into equal right and left parts
Midsagittal plane	Median plane
Sagittal plane	Divides body into unequal right and left parts
Frontal (coronal) plane	Divides body into equal or unequal anterior and posterior parts
Transverse plane	Divides body into equal or unequal superior and inferior parts (cross sections)

• Sagittal plane: vertical plane that divides the body into equal right and left halves (median or midsagittal plane) or a plane parallel to the median sagittal plane (parasagittal) that divides the body into unequal right and left portions.

• Frontal (coronal) plane: vertical plane that divides the body into anterior and posterior portions (equal or unequal); this plane is at right angles to the median sagittal plane.

• Transverse (axial) plane: horizontal plane that divides the body into superior and inferior portions (equal or unequal) and is at right angles to both the median sagittal and the frontal planes (sometimes called cross sections).

Key terms of relationship used in anatomy and the clinic are summarized in Table 1-1. A structure or feature closer to the front of the body is considered anterior (ventral), and one closer to the back is termed posterior (dorsal). The terms medial and lateral are used to distinguish a structure or feature in relationship to the midline; the nose is medial to the ear, and in anatomical position, the nose also is anterior to the ear. Sometimes these terms of relationship are used in combination (e.g., superomedial, meaning closer to the head and nearer the median sagittal plane).

Movements

Body movements usually occur at the joints where two or more bones or cartilages articulate with one another. Muscles act on joints to accomplish these movements and may be described as follows: “The biceps muscle flexes the forearm at the elbow.” Figure 1-3 summarizes the terms of movement.

Anatomical Variability

The human body is remarkably complex and remarkably consistent anatomically, but normal variations do exist, often related to size, gender, age, number, shape, and attachment. Variations are particularly common in the following structures:

• Bones: fine features of bones (processes, spines, articular surfaces) may be variable depending on the forces working on a bone.

• Muscles: vary with size and fine details of their attachments (it is better to learn their actions and general attachments rather than focus on detailed exceptions).

• Organs: the size and shape of some organs will vary depending on their normal physiology or pathophysiologic changes that have occurred previously.

• Arteries: surprisingly consistent, although some variation is seen in the branching patterns, especially in the lower neck (subclavian branches) and in the pelvis (internal iliac branches).

• Veins: consistent, although variations, especially in size and number of veins, can occur and often can be traced to their complex embryologic development; veins generally are more numerous than arteries, larger, and more variable.

2 Skin

The skin is the largest organ in the body, accounting for about 15% to 20% of the total body mass, and has the following functions:

• Protection: against mechanical abrasion and in immune responses, as well as prevention of dehydration

• Temperature regulation: largely through vasodilation, vasoconstriction, fat storage, or activation of sweat glands

• Sensations: to touch by specialized mechanoreceptors such as pacinian and Meissner’s corpuscles; to pain by nociceptors; and to temperature by thermoreceptors

• Endocrine regulation: by secretion of hormones, cytokines, and growth factors, and by synthesis and storage of vitamin D

• Exocrine secretions: by secretion of sweat and oily sebum from sebaceous glands

The skin consists of two layers (Fig. 1-4):

• Epidermis: outer protective layer consisting of a keratinized stratified squamous epithelium derived from the embryonic ectoderm.

• Dermis: dense connective tissue layer that gives skin most of its thickness and support, and is derived from the embryonic mesoderm.

Fascia is a connective tissue sheet that may contain variable amounts of fat. It can interconnect structures, provide a conduit for vessels and nerves (termed neurovascular bundles), and provide a sheath around structures (e.g., muscles) that permits them to slide over one another easily. Superficial fascia is attached to and lies just beneath the dermis of the skin and can vary in thickness and density; it acts as a cushion, contains variable amounts of fat, and allows the skin to glide over its surface. Deep fascia usually consists of a dense connective tissue, is attached to the deep surface of the superficial fascia, and often ensheathes muscles and divides them into functional groupings. Extensions of the deep fascia encasing muscles also may course inward and attach to the skeleton, dividing groups of muscles with intermuscular septa.

Clinical Focus 1-1 Psoriasis

Psoriasis is a chronic inflammatory skin disorder that affects approximately 1% to 3% of the population (women and men equally). It is characterized by defined red plaques capped with a surface scale of desquamated epidermis. Although the pathogenesis is unknown, psoriasis seems to involve a genetic predisposition.

Clinical Focus 1-2 Burns

Burns to the skin are classified into three degrees of severity based on the depth of the burn:

• First-degree: burn damage is limited to the superficial layers of the epidermis; termed a superficial burn, clinically it causes erythema.

• Second-degree: burn damage includes all the epidermis and extends into the superficial dermis; termed a partial-thickness burn, it causes blisters but spares the hair follicles and sweat glands.

• Third-degree: burn damage includes all the epidermis and dermis and may even involve the subcutaneous tissue and underlying deep fascia and muscle; termed a full-thickness burn, it causes charring.

Clinical Focus 1-3 Langer’s Lines

Collagen in the skin creates tension lines called Langer’s lines. Often, surgeons may use these lines to make skin incisions. The resulting incision wounds tend to gape less when the incision is parallel to Langer’s lines, and this usually leaves a smaller scar after healing of the incision.

3 Skeletal System

Descriptive Regions

The human skeleton is divided into two descriptive regions (Fig. 1-5):

FIGURE 1-5 Axial and Appendicular Regions of Skeleton.

• Axial skeleton: bones of the skull, vertebral column (spine), ribs, and sternum, which form the “axis” or central line of the body (80 bones).

• Appendicular skeleton: bones of the limbs, including the pectoral and pelvic girdles, which attach the limbs to the body’s axis (134 bones).

Shapes and Function of Bones

The skeleton is composed of a living, dynamic, rigid connective tissue that forms the bones and cartilages. Generally, humans have about 214 bones, although this number varies, particularly in the number of small sesamoid bones that may be present. (Many resources claim we have only 206 bones but have not counted the eight sesamoid bones of the hands and feet.) Cartilage is attached to some bones, especially where flexibility is important, or covers the surfaces of bones at points of articulation. About 99% of the body’s calcium is stored in bone, and many bones possess a central cavity that contains bone marrow—a collection of hemopoietic (blood-forming) cells. Most of the bones can be classified into one of the following five shapes (Fig. 1-6):

FIGURE 1-6 Bone Classification Based on Shape.

The functions of the skeletal system include:

• Support

• Protection of vital organs

• A mechanism, along with muscles, for movement

• Storage of calcium and other salts

• A source of blood cells

There are two types of bone:

• Compact: a relatively solid mass of bone, commonly seen as a superficial layer of bone, that provides strength.

• Spongy (trabecular or cancellous): a less dense trabeculated network of bone spicules making up the substance of most bones and surrounding an inner marrow cavity.

Long bones also are divided into the following descriptive regions (Fig. 1-7):

FIGURE 1-7 Growth and Ossification of Long Bones (Midfrontal Sections).

• Epiphysis: the ends of long bones, which develop from secondary ossification centers.

• Epiphysial plate: site of growth in length; contains cartilage in actively growing bones.

• Metaphysis: site where the bone’s shaft joins the epiphysis and epiphysial plate.

• Diaphysis: the shaft of a long bone, which represents the primary ossification center and the site where growth in width occurs.

As a living, dynamic tissue, bone receives a rich blood supply from:

• Nutrient arteries: usually one or several larger arteries that pass through the diaphysis and supply the compact and spongy bone, as well as the bone marrow.

• Metaphysial and epiphysial arteries: usually from articular branches supplying the joint.

• Periosteal arteries: numerous small arteries from adjacent vessels that supply the compact bone.

Clinical Focus 1-4 Fractures

Fractures are classified as either closed (the skin is intact) or open (the skin is perforated; often referred to as a compound fracture). Additionally, the fracture may be classified with respect to its anatomical appearance (e.g., transverse, spiral).

Clinical Focus 1-5 Degenerative Joint Disease

Degenerative joint disease is a catch-all term for osteoarthritis, degenerative arthritis, osteoarthrosis, or hypertrophic arthritis; it is characterized by progressive loss of articular cartilage and failure of repair. Osteoarthritis can affect any synovial joint but most often involves the foot, knee, hip, spine, and hand. As the articular cartilage is lost, the joint space (the space between the two articulating bones) becomes narrowed, and the exposed bony surfaces rub against each other, causing significant pain.

4 Muscular System

Muscle cells (fibers) produce contractions (shortenings in length) that result in movement, maintenance of posture, changes in shape, or the propulsion of fluids through hollow tissues or organs. There are three different types of muscle:

• Skeletal: striated muscle fibers that are attached to bone and are responsible for movements of the skeleton (sometimes simplistically referred to as voluntary muscle)

• Cardiac: striated muscle fibers that make up the walls of the heart and proximal portions of the great vessels

• Smooth: nonstriated muscle fibers that line various organs, attach to hair follicles, and line the walls of most blood vessels (sometimes simplistically referred to as involuntary muscle)

Skeletal muscle is divided into fascicles (bundles), which are composed of muscle fibers (muscle cells) (Fig. 1-10). The muscle fiber cells contain longitudinally oriented myofibrils that run the full length of the cell. Each myofibril is composed of many myofilaments, which are composed of individual myosin (thick filaments) and actin (thin filaments) that slide over one another during muscle contraction.

FIGURE 1-10 Structure of Skeletal Muscle.

Skeletal muscle moves bones at their joints and possesses an origin (the muscle’s fixed or proximal attachment) and an insertion (the muscle’s movable or distal attachment). At the gross level, anatomists classify muscle on the basis of its shape:

• Flat: has parallel fibers, usually in a broad flat sheet with a broad tendon of attachment called an aponeurosis.

• Quadrate: has a four-sided appearance.

• Circular: forms sphincters that close off tubes or openings.

• Fusiform: has a wide center and tapered ends.

• Pennate: has a feathered appearance (uni-, bi-, or multipennate forms).

Muscle contraction shortens the muscle. Generally, skeletal muscle contracts in one of three ways:

• Reflexive: involuntary or automatic contraction; seen in the diaphragm during respiration or in the reflex contraction elicited by tapping a muscle’s tendon with a reflex hammer.

• Tonic: maintains “muscle tone,” a slight contraction that may not cause movement but allows the muscle to maintain firmness necessary for stability of a joint and important in maintaining posture.

• Phasic: two types of contraction; isometric contraction, where no movement occurs but the muscle maintains tension to hold a position (stronger than tonic contraction), and isotonic contraction, where the muscle shortens to produce movement.

Muscle contraction that produces movements can act in several ways, depending on the conditions:

• Agonist: the main muscle responsible for a specific movement (the “prime mover”).

• Antagonist: the muscle that opposes the action of the agonist; as an agonist muscle contracts, the antagonistic muscle relaxes.

• Fixator: one or more muscles that steady the proximal part of a limb when a more distal part is being moved.

• Synergist: complements (works synergistically with) the contraction of the agonist, either by assisting with the movement generated by the agonist or by reducing unnecessary movements that would occur as the agonist contracts.

5 Cardiovascular System

The cardiovascular system consists of (1) the heart, which pumps blood into the pulmonary circulation for gas exchange and into the systemic circulation to supply the body tissues; and (2) the vessels that carry the blood, including the arteries, arterioles, capillaries, venules, and veins. The blood passing through the cardiovascular system consists of the following formed elements (Fig. 1-11):

• Platelets

• White blood cells (WBCs)

• Red blood cells (RBCs)

• Plasma

Blood is a fluid connective tissue that circulates through the arteries to reach the body’s tissues and then returns to the heart through the veins. When blood is “spun down” in a centrifuge tube, the RBCs precipitate to the bottom of the tube, where they account for about 45% of the blood volume. This is called the hematocrit and normally ranges from 40% to 50% in males and 35% to 45% in females. The next layer is a “buffy coat,” which makes up slightly less than 1% of the blood volume and includes WBCs (leukocytes) and platelets. The remaining 55% of the blood volume is the plasma (serum is plasma with the clotting factors removed and includes water, plasma proteins, and various solutes). The functions of blood include:

• Transport of dissolved gases, nutrients, metabolic waste products, and hormones to and from tissues

• Prevention of fluid loss via clotting mechanisms

• Immune defense

• Regulation of pH and electrolyte balance

• Thermoregulation through blood vessel constriction and dilation

Blood Vessels

Blood circulates through the blood vessels (Fig. 1-12). Arteries carry blood away from the heart, and veins carry blood back to the heart. Arteries generally have more smooth muscle in their walls than veins and are responsible for most of the vascular resistance, especially the small muscular arteries and arterioles. Alternatively, at any point in time, most of the blood resides in the veins (about 64%) and is returned to the right side of the heart; thus veins are the capacitance vessels, capable of holding most of the blood, and are more variable and numerous than their corresponding arteries.

FIGURE 1-12 General Organization of Cardiovascular System.
The amount of blood flow per minute (), as a percent of the cardiac output, and the relative percent of oxygen used per minute () by the various organ systems are noted.

The major arteries are illustrated in Figure 1-13. At certain points along the pathway of the systemic arterial circulation, large and medium-sized arteries lie near the body’s surface and can be used to take a pulse by compressing the artery against a hard underlying structure (usually a bone). The most distal pulse from the heart is usually taken over the dorsalis pedis artery on the dorsum of the foot.

FIGURE 1-13 Major Arteries, Pulse Points, and Veins.

The major veins are also illustrated in Figure 1-13. Veins are capacitance vessels because they are distensible and numerous and can serve as reservoirs for the blood. Because veins carry blood at low pressure and often against gravity, larger veins of the limbs and lower neck region have numerous valves that aid in venous return to the heart (several other veins throughout the body may also contain valves). Both the presence of valves and the contractions of adjacent skeletal muscles help to “pump” the venous blood against gravity and toward the heart. In most of the body, the veins occur as a superficial set of veins in the subcutaneous tissue that connects with a deeper set of veins that parallel the arteries. Types of veins include:

• Venules: very small veins that collect blood from the capillary beds

• Veins: small, medium, and large veins that contain some smooth muscle in their walls, but not as much as their corresponding arteries

• Portal venous systems: veins that transport blood between two capillary beds (e.g., the hepatic portal system)

Clinical Focus 1-6 Atherogenesis

Thickening and narrowing of the arterial wall and eventual deposition of lipid into the wall can lead to one form of atherosclerosis. The narrowed artery may not be able to meet the metabolic needs of the adjacent tissues, which may become ischemic. Multiple factors, including focal inflammation of the arterial wall, may result in this condition. When development of a plaque is such that it is likely to rupture and lead to thrombosis and arterial occlusion, the atherogenic process is termed unstable plaque formation.