Introduction to the Human Body



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:





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:





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).




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:



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




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.







3 Skeletal System


Descriptive Regions


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





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):




The functions of the skeletal system include:



There are two types of bone:



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




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




Markings on the Bones


Various surface features of bones (ridges, grooves, and bumps) result from the tension placed on them by the attachment of tendons, ligaments, and fascia, as well as by vessels or other structures that pass along the bone. Descriptively, these features include the following:



• Condyle: rounded articular surface covered with articular (hyaline) cartilage


• Crest: a ridge (narrow or wide) of bone


• Epicondyle: prominent ridge or eminence superior to a condyle


• Facet: flat, smooth articular surface, usually covered with articular (hyaline) cartilage


• Fissure: very narrow “slitlike” opening in a bone


• Foramen: round or oval “hole” in the bone for passage of another structure (nerve or vessel)


• Fossa: a “cuplike” depression in the bone, usually for articulation with another bone


• Groove: a furrow in the bone


• Line: fine linear ridge of bone, but less prominent than a crest


• Malleolus: a rounded eminence


• Meatus: a passageway or canal in a bone


• Process: bony prominence that may be sharp or blunt


• Protuberance: protruding eminence on an otherwise smooth surface


• Ramus: thin part of a bone that joins a thicker process of the same bone


• Spine: sharp process projecting from a bone


• Trochanter: large, blunt process for muscle tendon or ligament attachment


• Tubercle: small, elevated process


• Tuberosity: large, rounded eminence that may be coarse or rough



Bone Development


Bones develop in one of the following two ways:



The following sequence of events defines endochondral bone formation (Fig. 1-7, A-F):




Types of Joints


Joints are the sites of union or articulation of two or more bones or cartilages and are classified into one of the following three types (Fig. 1-8):




Fibrous joints include sutures (flat bones of the skull), syndesmoses (two bones connected by a fibrous membrane), and gomphoses (teeth fitting into fibrous tissue-lined sockets).


Cartilaginous joints include primary (synchondrosis) joints between surfaces lined by hyaline cartilage (epiphysial plate connecting the diaphysis with the epiphysis), and secondary (symphysis) joints between hyaline-lined articular surfaces and an intervening fibrocartilaginous disc. Primary joints allow for growth and some bending, whereas secondary joints allow for strength and some flexibility.


Synovial joints generally allow for considerable movement and are classified according to their shape and the type of movement that they permit (uni-, bi-, or multiaxial movement) (Fig. 1-9), as follows:








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 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.



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:



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



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




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):




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:




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.



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.



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:





Heart


The heart is a hollow muscular (cardiac muscle) organ that is divided into four chambers (Fig. 1-14):




The atria and ventricles are separated by atrioventricular valves (tricuspid on the right side and mitral on the left side) that prevent the blood from refluxing into the atria when the ventricles contract. Likewise, the two major outflow vessels, the pulmonary trunk from the right ventricle and the ascending aorta from the left ventricle, possess the pulmonic valve and the aortic valve (semilunar valves), respectively.



6 Lymphatic System


General Organization


The lymphatic system is intimately associated with the cardiovascular system, both in the development of its lymphatic vessels and in its immune function. The lymphatic system functions to:



Components of the lymphatic system include the following:




Lymphatic Drainage


The body is about 60% fluid by weight, with 40% located in the intracellular fluid (ICF) compartment (inside the cells) and the remaining 20% in the extracellular fluid (ECF) compartment. The lymphatics are essential for returning ECF, solutes, and protein (lost via the capillaries into the ECF compartment) back to the bloodstream, thus helping to maintain a normal blood volume. On average, the lymphatics return about 3.5 to 4.0 liters of fluid per day back to the bloodstream. The lymphatics also distribute various hormones, nutrients (fats from the bowel and proteins from the interstitium), and waste products from the ECF to the bloodstream.


Lymphatic vessels transport lymph from everywhere in the body (except the central nervous system) to major lymphatic channels. The majority of lymph ultimately collects in the thoracic duct

Jun 16, 2016 | Posted by in ANATOMY | Comments Off on Introduction to the Human Body
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