Organization of the Body

Organization of the Body

The human body is composed of distinct body parts, cavities, membranes, and organ systems that include various body systems. All of these are discussed in greater detail in the following sections.

Body Cavities and Membranes

The body is divided into two main cavities, the dorsal­ cavity and the ventral cavity. These two main cavities are divided into smaller subcavities. The dorsal cav-ity protects the organs of the nervous system. Its two subdivisions include the cranial cavity of the skull, which encases the brain, and the vertebral (spinal)cavity, located inside the vertebral column, whichencases the spinal cord. The vertebral­ cavity is also referred to as the vertebral canal. The cranial and spinal cavities are in continuation with each other. The ventral cavity contains­ most of the body’s organs. More anterior and larger than the dorsal cavity,­ it houses the viscera ­(visceral organs). The ventral cavity is divided into the thoracic cavity and the abdominopelvic cavity.

Thoracic Cavity

The thoracic cavity is surrounded by the chest mus-cles and ribs, and contains the lungs and heart; organs of the cardiovascular, respiratory, and lymphatic sys-tems; inferior esophagus; and the thymus. It is sub-divided into lateral pleural cavities, which surround each lung, and the medial mediastinum, which is a tissue mass that separates the cavities. Each pleural cavity is lined by a serous membrane, which is shiny and slippery, and functions to reduce friction as the lung expands and recoils during breathing. The pleura is the serous membrane lining a pleural cavity. The visceral pleuracovers the outer lung surfaces. The parietal pleuracovers the inner body wall and mediastinal surface.

The mediastinum is a mass of connective tissue surrounding and protecting the esophagus, trachea, thymus, and major blood vessels originating or end-ing at the heart. It also contains a small chamber sur-rounding the heart, which is called the pericardial cavity. The attached portion of the heart is calledthe base. The serous membrane of the heart is the pericardium, subdivided into the visceral pericar-dium(covering the heart) and its opposing surface, the parietal pericardium. As the heart changes size and shape while beating, the pericardial cavity also changes. Friction is prevented by the slipper pericardial­ lining, between the heart and thoracic cavity structures.

Abdominopelvic Cavity

The thoracic cavity and abdominopelvic cavity are separated internally by the diaphragm, which is a flat sheet of muscle. The major cavities of the body are shown in FIGURE 1-3. 

In the abdominopelvic cavity, which extends from the diaphragm to the pelvis, there are subdi-visions known as the superior abdominal cavity, and the inferior pelvic cavity. The abdominopelvic cavity contains the peritoneal cavity (FIGURE 1-4). This is a potential space that is lined by a serous membrane called the peritoneum. The peritoneal membranes include the parietal peritoneum lining the walls and the visceral peritoneum covering each organ. Move-ment of the digestive organs, while able to cause rumbling or gurgling sounds, does not cause damage because the peritoneum allows them to slide across each other.

The abdominal cavity extends from the infe-rior surface of the diaphragm to the level of the superior edges of the pelvis. It contains the liver, stomach, spleen, small intestine, and the major-ity of the large intestine. These organs are completely or ­partially enclosed by the peritoneal cavity. However, certain organs such as the kidney and pancreas lie in between the peritoneal lining and the muscular­ abdominal cavity wall. Organs lying behind the peritoneum­ are called retroperitoneal. Organs lying inside the peritoneum are called intraperitoneal. TABLE 1-2 lists intraperitoneal and retroperitoneal organs.

1. List the cavities of the head.

2. Which body cavity will be opened if an incision is made just inferior to the diaphragm?

3. Identify the subdivisions of the ventral body cavity.

Diagnostic Imaging

Diagnostic or “medical” imaging was developed in order to view the internal organs and body structures, in both normal and abnormal conditions. It began in the first decade of the 1900s when physicist Wilhelm Roentgen­ discovered X-rays. Until the 1950s, X-rays were the exclusive method of imaging available. In its early days, X-rays took much longer to produce and exposed the patient to significantly higher amounts of radiation. An example of an X-ray is shown in FIGURE 1-5.

Additional medical imaging developments are as follows:

■■ The development of fluorescent screens that were used with special glasses allowed real time viewing of X-ray images but also exposed physicians to radiation.

■■Contrast agents barium and iodine help to improve viewing of the esophagus, stomach, coronary arteries, and other structures. Examples of procedures that use contrast agents include intravenous pyelogram and angiogram (FIGURE 1-6).

■■ In 1955, X-ray image intensifiers allowed movingX-rays to be viewed by using television camerasand monitors.

■■ Radionuclide scanning, or nuclear medicine, wasdeveloped in the 1950s. This type of scan usesspecial gamma cameras and low level radioactivechemicals introduced into the body, allowing theevaluation of functional activity of organs. Resultsof nuclear medicine are recorded as a nuclearisotope scan (FIGURE 1-7).

■■ Ultrasound scanning appeared in the 1960s, using high frequency sound waves to penetrate thebody, bounce off the internal structures, and thenbe reconstructed into live pictures by a computer(FIGURE 1-8). Ultrasound is most useful for softtissues and body fluids and is commonly used toview the gallbladder, urinary bladder, and uterus.

■■ Digital imaging came along in the 1970s with the development of CT. All preexisting technologies were upgraded to digital forms. Digital X-ray detectors are replacing previous analog technologies,allowing better imaging and less healthrisks. CT acquires an image in less than a secondand instantly reconstructs it. It offers detailed cross-sectional images of body structures.­FIGURE 1-9 shows a CT machine and ­FIGURE 1-10shows a CT scan of the abdomen.

■■ MRI, first offered in 1984, allows detailed imaging without exposure to radiation (FIGURE 1-11).Images are produced by displacing protons inatomic nuclei with radiofrequency signals. However, it cannot be used on a patient who has anymetal implants because of its extremely powerful magnetization. Also, the person must remain completely still for a long period of time in asmall, confined space. MRI is often used for bone,joint, brain, and nerve imaging.

Organ Systems

In each organ system of the human body, the organs work together to maintain homeostasis. These organ systems include the integumentary, skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, digestive, respiratory, urinary, and reproductive systems (FIGURE 1-12).

Integumentary System

The integumentary system includes the skin, hair, nails, sebaceous (oil) glands, and sweat glands. The system protects the underlying tissues of the body, assists in the regulation of body temperature, contains various sensory receptors, and manufactures certain substances (such as vitamin D).

Skeletal System

The skeletal system supports and protects the soft tis-sues of the body and helps the body to move. It con-sists of bones, which are bound together by ligaments and cartilages. The skeletal system shields soft tissues and attaches to muscles. The bones also help in blood formation and provide storage of mineral salts.

Muscular System

The muscular system works with the skeletal system in helping the body to move. Body parts are moved by muscle contraction. Posture and body heat are main-tained by the muscular system. The muscular system also includes the tendons.

Nervous System

The nervous system, along with the endocrine system, controls and coordinates various organ functions, helping to maintain homeostasis. The nervous system consists of the brain, spinal cord, nerves, and sensory organs. Nerve impulses are electrochemical signals used by nerve cells to communicate with each other and with the glands and muscles of the body. Certain nerve cells (called sensory receptors) detect internal and external changes that affect the body. Other nerve cells interpret and respond to these stimuli. Additional nerve cells carry impulses from the brain or spinal cord to the glands and muscles. These nerves are able to stimulate the muscles to contract and cause the glands to secrete their products. The characteris-tics of the nervous system include short-term effects, rapid responses, and very specific responses, as well as a variety of other responses.

Endocrine System

The endocrine system consists of hormone-secreting glands. Hormones affect specific target cells, altering their metabolism. Hormones have a relatively long duration of action compared with nerve impulses and can last for several days or longer. The endocrine system also produces a slower response regarding body changes than the nervous system. The organs of the endocrine system include the hypothalamus (in the brain), pituitary gland, pineal gland, thyroid gland, parathyroid glands, adrenal glands, pancreas, and thymus. Other organs with endocrine function include the ovaries and testes, which are also the parts of the reproductive sys-tem. The endocrine system can produce effects involv-ing several organs or tissues at the same time.

Cardiovascular System

The cardiovascular system includes the heart, blood, arteries, veins, and capillaries. The heart muscle pumps blood through the arteries, transporting gases, hormones, nutrients, and wastes. Blood returns to the heart via the veins. Oxygen is carried from the lungs to the body, and nutrients are carried from the diges-tive system. The blood also transports biochemicals required for metabolism. Wastes are carried in the blood from body cells to the excretory organs.

Lymphatic System

The lymphatic system is composed of the lymphatic vessels, lymph nodes, thymus, spleen, and lymph fluid. It works with the cardiovascular system, transporting tissue fluid back into the bloodstream. It also carries specific fats from digestive organs into the bloodstream. Lymphatic cells (lymphocytes) defend the body against infection. The lymphatic vessels have two ducts in the chest, known as the thoracic duct and the right lymphatic duct.

Digestive System

The digestive system takes in food from outside the body, breaks it down and absorbs the nutrients. It then excretes wastes from its various processes. The digestive system also produces certain hormones and works in conjunction with the endocrine system. The structures of the digestive system include the mouth, teeth, salivary glands, tongue, esophagus, stomach, liver, gallbladder, pancreas, small intestine, large intestine, rectum, and anus. The pharynx is part of both the digestive and respiratory systems.

Respiratory System

The respiratory system takes in and expels air, exchanging oxygen and carbon dioxide via the lungs and bloodstream. The structures of the respiratory system include the nose, nasal cavity, larynx, trachea, bronchi, and lungs. Again, the pharynx is part of both the respiratory and digestive systems.

Urinary System

The urinary system functions to remove liquid wastes from the body. It consists of the kidneys, ureters, urinary bladder, and urethra; it is through the urethra that urine is expelled. The female urethra is located just above the vagina, while the male urethra runs through the penis. The kidneys filter wastes from the blood and maintain electrolyte concentrations. The urinary bladder stores the urine and the urethra car-ries it to outside the body.

Reproductive System

The reproductive system in females consists of the ova-ries, uterine tubes, uterus, vagina, clitoris, and vulva. The female sex cells are called oocytes or eggs. They are fertilized by male sex cells (sperm or spermatozoa). When a female is impregnated, the embryo develops within the uterus. The male reproductive system includes the scrotum, testes, epididymides, ductusdeferentia, seminal vesicles, prostate gland, bulbourethral glands, penis, and urethra. Reproduction is the process of producing offspring. As embryonic cells divide, they grow and produce new cells, and the pro-cess is continued.

1. Describe the general functions of the digestive system.

2. List the organs of the respiratory system.

Anatomic Planes

The body can be visually divided into specific areas, called planes. These planes “divide” the body at par-ticular angles and in particular directions (FIGURE 1 -13 and TABLE 1-3). They are also referred to as slices or Sections . There are three sectional planes, as follows:

■■ Transverse (horizontal) plane: It lies at right anglesto the long axis of the body and divides the bodyinto superior and inferior portions. A cut in thisplane is called a transverse (cross) section.

■■ Frontal (coronal) plane: It is parallel to the longaxis of the body and extends vertically, dividing the body into anterior and posterior portions.

■■ Sagittal plane: Also parallel to the long axis ofthe body, this plane, however, divides the body into left and right portions. A cut passing alongthe midline, dividing the body into equal leftand right halves, is called a midsagittal (median)section and a cut parallel to the midsagittal line iscalled a parasagittal section.

Directional Terms

Directional terms used in the study of anatomy include words that describe relative positions of body parts as well as imaginary anatomical divisions. The term anatomical position describes the body standing erect, facing forward, with the arms held to the sides of the body, palms of the hands facing forward. When the terms right and left are used, they refer to those specific sides of the body when it is in the anatomical position. Important directional terms used in anat-omy are listed in TABLE 1-4.

Abdominal Regions

Anatomists have divided the abdomen and pelvisinto nine imaginary regions that are helpful in identifying the location of particular abdominal organs.

They are also useful for describing the location of abdominal pain. FIGURE 1-14 shows the nine abdominal regions, identified from the left to the right and moving from top to bottom one row at a time. The abdomen may also be divided into four quadrants, as shown in FIGURE 1-15. TABLE 1-5 explains each abdom-inal region in greater detail.

1. Name the nine abdominopelvic regions.

2. Describe the anatomic planes of the body.

3. Describe the term “anatomical position.

Body Regions

The remainder of the body is classified into various regions that describe them clinically. For example, carpal tunnel syndrome refers to the carpal area the wrist where acute pain can occur from the development of this syndrome. The most common body regions are listed in TABLE 1-6.

TABLE 1-7 lists terms related to medicine andapplied sciences.