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INTRODUCTION TO THE HUMAN BODY

  2. SKIN

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 that region. Hence, this textbook is arranged regionally but, by way of introduction for someone studying anatomy for the first time, this initial chapter will briefly introduce 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):
FIGURE 1-1 Anatomical Position and the Terminology Used to Describe Various Body Regions

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):
FIGURE 1-2 Body Planes and Terms of Relationship
TABLE 1-1 Terms of Relationship
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Key terms of relationship used in anatomy and the clinic are summarized in Table 1-1. Sometimes these terms of relationship will be 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.
FIGURE 1-3 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:
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2. SKIN

The skin is the largest organ in the body, accounting for about 15% to 20% of the total body mass, and functions in:
The skin consists of two layers (Fig. 1-4):
FIGURE 1-4 Layers of the Skin
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Psoriasis

 

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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 of 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 of 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)
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Additional figures available online. See Table of Contents for Instructions for online access.

 

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Langer's Lines

3. SKELETAL SYSTEM

Descriptive Regions

The human skeleton is divided into two descriptive regions (Fig. 1-5):
FIGURE 1-5 The Axial and Appendicular Skeleton

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. Cartilage is attached to some bones, especially where flexibility is important, or covers the surfaces of some 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 five shapes (Fig. 1-6):
FIGURE 1-6 Bone Classification Based Upon Their Shape
The functions of the skeletal system include:
There are two types of bone:
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Long bones also are divided into the following descriptive regions (Fig. 1-7):
FIGURE 1-7 Growth and Ossification of Long Bones (Mid-Frontal Sections) (Contd.)
FIGURE 1-7 (Contd.) Growth and Ossification of Long Bones (Mid-Frontal Sections)
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:
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Bone Development

Bones develop in one of two ways:
  • Intramembranous formation: most flat bones develop in this way by direct calcium deposition into a mesenchymal (primitive mesoderm) precursor or model of the bone
  • Endochondral formation: most long and irregular shaped bones develop by calcium deposition into a cartilaginous model of the bone that provides a scaffold for the future bone
The sequence of events in endochondral bone formation include (see Fig. 1-7):

Types of Joints

Joints are the site of union or articulation of two or more bones or cartilages and are classified into one of three types (Fig. 1-8):
FIGURE 1-8 Types of Joints
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, while 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):
FIGURE 1-9 Types of Synovial Joints
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Fractures

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Degenerative Joint Disease

4. MUSCULAR SYSTEM

Muscle cells (fibers) produce contractions (shorten 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: non-striated 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 The 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:
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Muscle contraction shortens the muscle. Generally, skeletal muscle contracts in one of three ways:
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Muscle contraction that produces movements can act in several ways, depending on the conditions:

5. CARDIOVASCULAR SYSTEM

The cardiovascular system consists of the heart, which pumps blood into the pulmonary circulation for gas exchange and into the systemic circulation to supply the body tissues, and 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):
FIGURE 1-11 The Composition of Blood
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 comprise 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 comprises 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:
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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 the 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 (O2) by the various organ systems is 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 (Contd.)
FIGURE 1-13 (Contd.) 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 valves that aid in venous return to the heart. 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)
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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, with the danger that they 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.

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Heart

The heart is a hollow muscular (cardiac muscle) organ that is divided into four chambers (Fig. 1-14):
FIGURE 1-14 Chambers of the Heart
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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, also possess valves called the pulmonic and aortic (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:
  • Protect the body against infection by activating defense mechanisms that compose our immune system
  • Collect tissue fluids, solutes, hormones, and plasma proteins and return them to the circulatory system (bloodstream)
  • Absorb fat (chylomicrons) from the small intestine
Components of the lymphatic system include:
  • Lymph: a watery fluid that resembles plasma but contains fewer proteins and may contain fat, together with cells (mainly lymphocytes and a few RBCs)
  • Lymphocytes: the cellular components of lymph, which include T cells and B cells
  • Lymph vessels: an extensive network of vessels and capillaries in the peripheral tissues that transport lymph and lymphocytes
  • Lymphoid organs: collections of lymphoid tissue that include lymph nodes, aggregates of lymphoid tissue along the respiratory and GI passageways, tonsils, thymus, spleen, and bone marrow

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. Additionally, 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 lymphatic duct for delivery back to the venous system (joins the veins at the union of the left internal jugular and left subclavian veins) (Fig. 1-15). A much smaller right lymphatic duct drains the right upper quadrant of the body lymphatics to a similar site on the right side. Along the route of these lymphatic vessels, encapsulated lymph nodes are strategically placed to “filter” the lymph as it moves toward the venous system. Lymph nodes form an important site for phagocytosis of microorganisms and other particulate matter, and they initiate the body's immune responses.
FIGURE 1-15 Overview of the Lymphatic System

Immune Response

When a foreign microorganism, virus-infected cell, or cancer cell is detected within the body, the lymphatic system mounts what is called an immune response. The detected pathogens are distinguished from the body's own normal cells, and then a response is initiated to neutralize the pathogen. The human body has evolved three major responses to protect against foreign invaders:
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7. RESPIRATORY SYSTEM

The respiratory system provides oxygen to the body for its metabolic needs and eliminates carbon dioxide. Structurally, the respiratory system includes the following (Fig. 1-16):
FIGURE 1-16 The Respiratory System
Functionally, the respiratory system performs five basic functions:
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Asthma

Asthma can be intrinsic (no clearly defined environmental trigger) or extrinsic (has a defined trigger). Asthma usually results from a hypersensitivity reaction to an allergen (dust, pollen, mold), which leads to irritation of the respiratory passages and smooth muscle contraction (narrowing of the passages), swelling (edema) of the epithelium, and increased production of mucus. Presenting symptoms are often wheezing, shortness of breath, coughing, tachycardia, and feelings of chest tightness. Asthma is a pathologic inflammation of the airways and occurs in both children and adults.

Additional figures available online. See Table of Contents for Instructions for online access.

8. NERVOUS SYSTEM

General Organization

The nervous system integrates and regulates many body activities, sometimes at discrete locations (specific targets) and sometimes more globally. The nervous system usually acts quite rapidly and can also modulate effects of the endocrine and immune systems. The nervous system is separated into two structural divisions (Fig. 1-17):
FIGURE 1-17 General Organization of the Nervous System

Neurons

Nerve cells are called neurons, and their structure reflects the functional characteristics of an individual neuron (Fig. 1-18). Information comes to the neuron largely via processes called axons, which terminate on the neuron at specialized junctions called synapses. Synapses can occur on neuronal processes called dendrites or on the neuronal cell body, called a soma or perikaryon.
FIGURE 1-18 Cell Types Found in the CNS
Neurons convey efferent information via action potentials that course along a single axon arising from the soma that then synapses on a selective target, usually another neuron or target cell, for example, muscle cells. There are many different types of neurons. Some of the more common types include:
Although the human nervous system contains billions of neurons, they can be classified largely into one of three functional types:
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Neurons can vary considerably in size, ranging from several micrometers to over 100 micrometers in diameter. They may possess numerous branching dendrites, studded with dendritic spines that increase the receptive area of the neuron manyfold. The neuron's axon may be quite short or over 1 meter long. The axonal diameter may vary. Axons that are larger than 1 to 2 micrometers in diameter are insulated by myelin sheaths. In the CNS, axons are myelinated by a special glial cell called an oligodendrocyte, whereas axons in the peripheral nervous system (PNS) are surrounded by a glial cell called a Schwann cell. Schwann cells also myelinate many of the PNS axons they surround.
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Glia

Glia are the cells that support neurons, both within the CNS (the neuroglia) and within the PNS. Glial cells far outnumber the neurons in the nervous system and contribute to most of the postnatal growth seen in the CNS. Functionally, glia:
  • Provide structural isolation of neurons and their synapses
  • Sequester ions in the extracellular compartment
  • Provide trophic support to the neurons and their processes
  • Support growth and secrete growth factors
  • Support some of the signaling functions of neurons
  • Myelinate axons
  • Phagocytize debris and participate in inflammatory responses
  • Participate in the formation of the blood-brain barrier
The different types of glial cells include the following (see Fig. 1-18):

Peripheral Nerves

The peripheral nerves observed grossly in the human body are composed of bundles of thousands of nerve fibers that are enclosed within a connective tissue covering and are supplied by small blood vessels. The nerve “fibers” consist of axons (efferent and afferent) that are individually separated from each other either by the cytoplasmic processes of Schwann cells or are myelinated by a multilayered wrapping of continuous Schwann cell membrane (the myelin sheath).
The peripheral nerve resembles an electrical cable of axons that is further supported by three connective tissue sleeves or coverings (Fig. 1-19):
FIGURE 1-19 Features of a Typical Peripheral Nerve
Peripheral nerves include the 12 pairs of cranial nerves arising from the brain and the 31 pairs of spinal nerves arising from the spinal cord.

Meninges

The brain and spinal cord are surrounded by three membranous connective tissue layers called the meninges. These three layers include the following (Fig. 1-20):
FIGURE 1-20 The CNS Meninges
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Cranial Nerves

Twelve pairs of cranial nerves arise from the brain, and they are identified both by their names and by Roman numerals I to XII (Fig. 1-21). The cranial nerves are somewhat unique and can contain multiple functional components:
FIGURE 1-21 Overview of the Cranial Nerves
Hence, each cranial nerve may possess multiple functional components, such as:
In general, CN I and II arise from the forebrain and are really tracts of the brain for the special senses of smell and sight. CN III, IV, and VI move the extraocular skeletal muscles of the eyeball. CN V has three divisions: V1 and V2 are sensory, and V3 is both motor to skeletal muscle and sensory. CN VII, IX, and X are both motor and sensory. CN VIII is the special sense of hearing and balance. CN XI and XII are motor to skeletal muscle. CN III, VII, IX, and X also contain parasympathetic fibers of origin (visceral), although many of the autonomic fibers will “jump” onto the branches of CN V to reach their targets. Table 1-2 summarizes the types of fibers in each cranial nerve.
TABLE 1-2 Cranial Nerve Fibers

Spinal Nerves

The spinal cord gives rise to 31 pairs of spinal nerves (Figs. 1-22 and 1-23), which then form two major branches (rami):
FIGURE 1-22 Overview of the Spinal Cord and Spinal Nerves
FIGURE 1-23 Elements of the PNS. For clarity, this schematic shows the arrangement of the efferent and afferent somatic nerve components of a typical spinal nerve on the left side and the efferent components of the ANS of a typical spinal nerve on the right side.
Once nerve fibers (sensory or motor) are beyond, or peripheral to, the spinal cord proper, the fibers (axons) then reside in nerves of the peripheral nervous system (PNS). Components of the PNS include the following (see Fig. 1-23):
Features of the somatic nervous system include the following:
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The unilateral area of skin innervated by the somatic sensory fibers from a single spinal cord level is called a dermatome (see Chapter 2 for a complete description of a dermatome). Clinically, dermatome maps of the body can be helpful in localizing spinal cord or peripheral nerve lesions.
Features of the ANS division of the PNS include the following:

Sympathetic Division of the Autonomic Nervous System

The ANS is divided into the sympathetic and parasympathetic divisions. In contrast to the somatic division of the PNS, the ANS is a two-neuron system with a preganglionic neuron in the CNS that sends its axon into a peripheral nerve to synapse on a postganglionic neuron in a peripheral autonomic ganglion (Fig. 1-24). The postganglionic neuron then sends its axon to the target (smooth muscle, cardiac muscle, and glands). The ANS is a visceral system, since many of the body's organs are composed of smooth muscle walls or contain secretory glandular tissue.
FIGURE 1-24 Overview of the Sympathetic Division of the ANS
The sympathetic division also is known as the thoracolumbar division because:
Preganglionic axons exit the T1-L2 spinal cord in a ventral root, and then enter a spinal nerve via a white ramus communicans to enter the sympathetic chain. The sympathetic chain is a bilateral chain of ganglia just lateral to the vertebral bodies that runs from the base of the skull to the coccyx. Once in the sympathetic chain, the preganglionic axon may do one of three things:
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  • Synapse on a sympathetic chain postganglionic neuron at the T1-L2 level, or ascend or descend to synapse on a sympathetic chain neuron at any of the 31 spinal nerve levels
  • Pass through the sympathetic chain, enter a splanchnic (visceral) nerve, and synapse in a collateral ganglion in the abdominopelvic cavity
  • Pass through the sympathetic chain, enter a splanchnic nerve, pass through a collateral ganglion, and synapse on the cells of the adrenal medulla
Axons of the postganglionic sympathetic neurons may do one of four things:
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Preganglionic axons release acetylcholine (ACh) at their synapses, while norepinephrine (NE) is the transmitter released by postganglionic axons (except on sweat glands, where it is ACh). The cells of the adrenal medulla (modified postganglionic sympathetic neurons) release epinephrine and some NE into the blood, not as neurotransmitters but as hormones. The sympathetic system acts globally throughout the body to mobilize it in “fright-flight-fight” situations. The specific functions are summarized in Table 1-3.
TABLE 1-3 Effects of Sympathetic Stimulation on Various Structures

Parasympathetic Division of the Autonomic Nervous System

The parasympathetic division of the ANS also is a two-neuron system with its preganglionic neuron in the CNS and postganglionic neuron in a peripheral ganglion (Fig. 1-25). The parasympathetic division also is known as the craniosacral division because:
FIGURE 1-25 Overview of the Sympathetic Division of the ANS
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Preganglionic parasympathetic axons may do one of two things:
Axons of the postganglionic parasympathetic neurons may do one of two things:
CN X (vagus nerve) is unique. Its preganglionic axons exit the brainstem and synapse on terminal ganglia in or near the targets in the neck, thorax (heart, lungs, glands, smooth muscle), and abdominal cavity (proximal two-thirds of the GI tract and its accessory organs). Axons of the terminal ganglia neurons then synapse on their targets.
Parasympathetic axons do not pass into the limbs, as do sympathetic axons. Therefore, the vascular smooth muscle, arrector pili muscles of the skin (attached to hair follicles), and sweat glands are all innervated only by the sympathetic system. ACh is the neurotransmitter at all parasympathetic synapses.
The parasympathetic system is concerned with feeding and sexual arousal functions, and acts more slowly and focally than the sympathetic system. For example, CN X can slow the heart rate without affecting input to the stomach. In general, the sympathetic and parasympathetic systems maintain homeostasis, although as a protective measure, the body maintains a low level of “sympathetic tone” and can activate this division on a moment's notice. ANS function is regulated ultimately by the hypothalamus. The specific functions of the parasympathetic division of the ANS are summarized in Table 1-4.
TABLE 1-4 Effects of Parasympathetic Stimulation on Various Structures

Enteric Nervous System

The enteric nervous system was formally considered the third division of the ANS. The word enteric refers to the bowel. This component of the PNS consists of ganglia and nerve plexuses in the walls of the GI tract. These ganglia and their neural networks include the following (Fig. 1-26):
FIGURE 1-26 The Relationship of the Enteric Nervous System to the Sympathetic and Parasympathetic Divisions of the ANS
The enteric nervous system has important links to both divisions of the ANS, which are critical for optimal regulation of bowel secretion, absorption, and motility. More than 20 different transmitter substances have been identified in the intrinsic neurons of the enteric nervous system, pointing to the fine degree of regulation that occurs at the level of the bowel wall. Optimal GI functioning requires coordinated interactions of the ANS, the enteric nervous system, and the endocrine system.

9. ENDOCRINE SYSTEM

The endocrine system, along with the nervous and immune systems, facilitates communication, integration, and regulation of many of the body's functions (Fig. 1-27). Specifically, the endocrine system interacts with target sites (cells and tissues), many a great distance away, by releasing hormones into the bloodstream. Generally speaking, endocrine glands and hormones share several additional features:
FIGURE 1-27 Major Endocrine Organs
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Hormones can communicate via a variety of cell-to-cell interactions, including:
Major hormones and the tissues responsible for their release are summarized in Table 1-5.
TABLE 1-5 Major Hormones
Additionally, the placenta releases human chorionic gonadotropin (hCG), estrogens, progesterone, and human placental lactogen (hPL), whereas other cells release a variety of growth factors. Obviously, the endocrine system is widespread and critically important in regulating bodily functions.

10. GASTROINTESTINAL SYSTEM

The GI system includes the epithelial-lined tube that begins with the oral cavity and extends to the anal canal and also includes GI-associated glands, including the following:
The epithelial-lined tube that is the GI tract measures about 25 feet in length (from mouth to anal canal) and includes the following cavities and visceral structures (Fig. 1-28):
FIGURE 1-28 Overview of the Gastrointestinal System

11. URINARY SYSTEM

The urinary system includes the following components (Fig. 1-29):
FIGURE 1-29 The Urinary System
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The kidneys function to:
  • Filter plasma and begin the process of urine formation
  • Reabsorb important electrolytes, organic molecules, vitamins, and water from the filtrate
  • Excrete metabolic wastes, metabolites, and foreign chemicals (e.g., drugs)
  • Regulate fluid volume, composition, and pH
  • Secrete hormones that regulate blood pressure, erythropoiesis, and calcium metabolism
  • Convey urine to the ureters, which then pass the urine to the bladder
The kidneys filter about 180 liters of fluid each day. Grossly, each kidney measures about 12 cm long × 6 cm wide × 3 cm thick and weighs about 150 grams, although variability is common. Approximately 20% of the blood pumped by the heart passes to the kidney each minute for plasma filtration, although most of the fluid and important plasma constituents are returned to the blood as the filtrate courses down the tubules of the kidney's nephrons (the kidney's filtration units).
Each ureter is about 24 to 34 cm long, lies in a retroperitoneal position, and contains a thick smooth-muscle wall. The urinary bladder serves as a reservoir for the urine and is a muscular “bag” that expels the urine when appropriate. The urethra in the female is short (3 to 5 cm) and in the male is long (about 20 cm). The male urethra runs through the prostate gland, the external urethral sphincter, and the corpus spongiosum of the penis.

12. REPRODUCTIVE SYSTEM

Female Reproductive System

The female reproductive system is composed of the following structures (Fig. 1-30):
FIGURE 1-30 The Reproductive System

Male Reproductive System

The male reproductive system is composed of the following structures (see Fig. 1-30):

13. BODY CAVITIES

Organ systems and other visceral structures are often segregated into body cavities. These cavities can protect the viscera and also may allow for some expansion and contraction in size. Two major collections of body cavities are recognized (Fig. 1-31):
FIGURE 1-31 Major Body Cavities
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The CNS (brain and spinal cord) is surrounded by three membranes (see Fig. 1-20):
The thoracic cavity contains two pleural cavities (right and left) and a single midline space called the mediastinum (middle septum) that contains the heart and structures lying posterior to it, including the thoracic descending aorta and esophagus. The heart itself resides in the pericardial sac, which has a parietal and visceral layer.
The abdominopelvic cavity also is lined by a serous membrane, called the peritoneum, which has a parietal and visceral layer.
 

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Potential Spaces

Each of these spaces (pleural, pericardial, and peritoneal) is considered a “potential” space. This is because between the parietal and visceral layers, we usually find only a small amount of serous lubricating fluid (to keep organ surfaces moist and slick and hence reduce friction from movements such as respiration, heartbeat, or peristalsis); however, during inflammation or trauma (accumulation of pus or blood), fluids can collect in these spaces and restrict movement of the viscera. In this case, these “potential” spaces become real spaces and may necessitate removal of the offending fluid so as not to compromise organ function or exacerbate an ongoing infection.

14. OVERVIEW OF EARLY DEVELOPMENT

Week One of Development: Fertilization and Implantation

Fertilization normally occurs in the ampulla of the uterine tube (oviduct, fallopian tube) within 24 hours after ovulation (Fig. 1-32). The fertilized ovum (the union of sperm and egg nuclei, with a diploid number of chromosomes) is termed a zygote. Subsequent cell division (cleavage) occurs at the two-, four-, eight-, and 16-cell stages, and results in formation of a ball of cells that travels down the uterine tube toward the uterine cavity. When the cell mass reaches days three to four of development, it resembles a mulberry and is called a morula (16-cell stage). As the morula enters the uterine cavity at about day five, it develops a fluid-filled cyst in its interior and is then known as a blastocyst. At about days five to six, implantation occurs as the blastocyst literally erodes or burrows its way into the uterine wall (endometrium).
FIGURE 1-32 Schematic of Key Events of Week One of Human Development

Week Two of Development: Formation of the Bilaminar Embryonic Disc

As the blastocyst implants, it forms an inner cell mass (future embryo, embryoblast) and a larger fluid-filled cavity surrounded by an outer cell layer called the trophoblast (Fig. 1-33). The trophoblast undergoes differentiation and complex cellular interactions with maternal tissues to initiate formation of uteroplacental circulation. Simultaneously, the inner cell mass develops into two cell types (bilaminar disc):
FIGURE 1-33 Bilaminar Disc Formation During Week Two of Human Development
The epiblast forms a cavity on the dorsal side that gives rise to the amniotic cavity; the blastocyst cavity on the ventral side becomes the primitive yolk sac, which is lined by simple squamous epithelium derived from the hypoblast. About day 12, further hypoblast cell migration forms the true yolk sac, and the old blastocyst cavity becomes coated with extraembryonic mesoderm.
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Week Three of Development: Gastrulation

Gastrulation (development of the trilaminar embryonic disc) begins with appearance of the primitive streak on the dorsal surface of the epiblast (Fig. 1-34). This streak forms a groove demarcated at its cephalic end (head) by the primitive node. The node forms a midline cord of mesoderm that becomes the notochord. Migrating epiblast cells move toward the primitive streak, invaginate, and replace the underlying hypoblast cells to become the endoderm germ layer. Other invaginating epiblast cells develop between the endoderm and overlying epiblast and become the mesoderm. Finally, the surface epiblast cells form the ectoderm, the third germ layer. All body tissues are derived from one of these three embryonic germ layers.
FIGURE 1-34 Gastrulation Formation During Week Three of Human Development
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Overview of the Embryonic Germ Layer Derivatives

Figures 1-35 to 1-37 and the accompanying tables provide a general overview of the adult derivatives of the three embryonic germ layers that are formed during gastrulation. As you study each region of the body, refer to these summary pages to review the embryonic origins of the various tissues, as many clinical problems arise during the development in utero of these germ layer derivatives.
FIGURE 1-35 Ectodermal Derivatives (Contd.)
FIGURE 1-35 (Contd.) Ectodermal Derivatives
FIGURE 1-36 Mesodermal Derivatives (Contd.)
FIGURE 1-36 (Contd.) Mesodermal Derivatives
FIGURE 1-37 Endodermal Derivatives (Contd.)
FIGURE 1-37 (Contd.) Endodermal Derivatives
In general, ectodermal derivatives include the following (Fig 1-35):
In general, mesodermal derivatives include the following (Fig. 1-36):
In general, endodermal derivatives include the following (Fig. 1-37):
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15. IMAGING THE INTERNAL ANATOMY

General Introduction

In 1895, Wilhelm Roentgen (Wurzberg, Germany) used X-rays generated from a cathode ray tube to make the first radiographic image, for which he ultimately was awarded the first Nobel Prize in Physics in 1901. As the X-rays (a form of electromagnetic radiation) pass through the body, they lose energy to the tissues, and only the photons with sufficient energy to make it through then expose a sheet of photographic film. It is now largely collected as digital information rather than as a photographic image on film (Table 1-6).
TABLE 1-6 Attenuation of X-rays Passing through the Body*
*Greatest to least attenuation.

Plain (Conventional) Radiographs

A plain radiograph, also known as conventional or plain film radiography, provides an image in which the patient is positioned either anterior (anteroposterior, or AP) to or posterior (posteroanterior, or PA) to the X-ray source (Fig. 1-38A). The X-ray tube also may be placed in a lateral or oblique position in reference to the patient. Contrast media (radiopaque fluids such as barium sulfate or iodine compounds) can be administered to study tubular structures such as the bowel or vessels. A double contrast study uses barium and air to image the lumen of structures such as the distal colon (Fig. 1-38B). X-rays now are collected digitally in real time by producing a stream of X-rays. Techniques are now available that can even image moving structures in the body using angiography (contrast medium in the heart and larger vessels) and fluoroscopy.
FIGURE 1-38 Plain (Conventional) Radiographs. (Reprinted with permission from A. Wicke L: Atlas of Radiologic Anatomy, 7th Edition. Philadelphia, Saunders, 2004, and B. Major NM: A Practical Approach to Radiology. Philadelphia, Saunders, 2006.) (Contd.)
FIGURE 1-38 (Contd.) Plain (Conventional) Radiographs. (Reprinted with permission from A. Wicke L: Atlas of Radiologic Anatomy, 7th Edition. Philadelphia, Saunders, 2004, and B. Major NM: A Practical Approach to Radiology. Philadelphia, Saunders, 2006.)

Computed Tomography (CT)

CT was invented in the 1972 by Sir Godfrey Hounsfield (at EMI Labs, Hayes, England), who received the Nobel Prize in Medicine or Physiology in 1979 (shared with Allen McLeod Cormack of Tufts). A CT scanner uses X-rays generated by a tube that passes around the body and collects a series of images in the axial (transverse slices) plane. A sophisticated computer program then transforms the multiple images into a single slice (Fig. 1-39A). In the 1980s, multislice (multidetector) CT scanners were developed that capture many slices as the tube rotates in a helical pattern around the patient, who is moving through the scanner on a table. Three-dimensional images can be recreated by the computer from these slices. Bone is well imaged by CT, and contrast media may be employed to enhance the imaging of hollow viscera such as the GI tract. Additionally, CT angiography (CTA) can image larger blood vessels in 2-D and 3-D following the intravascular administration of contrast material (Fig. 1-39B).
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FIGURE 1-39 Computed Tomography (CT). (Reprinted with permission from Kelley LL, Petersen C: Sectional Anatomy for Imaging Professionals. Philadelphia, Mosby, 2007.)
Advantages of CT include lower costs (compared to MRI), availability, 3-D capabilities, the ability to image bony features, and speed (compared to MRI). Disadvantages include its high dose of X-rays (compared to plain films), artifacts (motion and scattering), and relatively poor tissue definition compared to MRI.

Positron Emission Tomography (PET)/CT

Glucose uptake in tissues (following 18-fluorodeoxy-D-glucose administration) can be imaged by PET/CT, an especially useful technique for detecting tissues or structures with a higher metabolic rate, such as malignant tumors and inflammatory lesions.

Magnetic Resonance Imaging (MRI)

Paul Lauterbur (Illinois) and Sir Peter Mansfield (Nottingham, England) were awarded the Nobel Prize in Medicine or Physiology in 2003 for their contributions to the development of MRI. Since the first MRI of a human subject in 1977, this process has become a versatile and safe diagnostic tool. Strong magnets align hydrogen's free protons (the hydrogen in molecules of water present in almost all biological tissues). Then a radio wave pulse passes through the patient and deflects the protons, which return to their aligned state but emit small radio pulses whose strength, frequency, and time produce distinct signals. Computers then analyze these signals and create axial, coronal, and sagittal images (Fig. 1-40).
FIGURE 1-40 Magnetic Resonance Imaging (MRI). (Reprinted with permission from Wicke L: Atlas of Radiologic Anatomy, 7th Edition. Philadelphia, Saunders, 2004.)
Advantages of MRI include the lack of ionizing radiation, the ability to image all planes, and the capability to image soft tissues at very high resolution (compared to CT). Disadvantages include high cost; inability to image patients with metallic implants or foreign bodies; inability to image bone well; longer procedure time compared to CT; potential for patients to become claustrophobic in the scanner; and tendency for artifacts (movement).
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 C L I N I C A L   F O C U S 

Online Figures

Myasthenia gravis

Additional figures available online. See Table of Contents for Instructions for online access.

Ultrasound

Ultrasound uses very-high-frequency longitudinal sound waves that are generated by a transducer. The waves produced by the transducer are reflected or refracted as they collide with the soft tissue interfaces. The proportion of sound reflected is measured as acoustic impedance and represents different densities of the soft tissues. A computer then interprets these signals and produces a real-time image (Fig. 1-41).
FIGURE 1-41 Ultrasound. (Reprinted with permission from Jackson S, Thomas R: Cross-Sectional Imaging Made Easy. Philadelphia, Churchill Livingstone, 2004.)