Human anatomy

Definition

Human anatomy is the study of the structure of the human body and the relationship between its parts.

Description

Anatomic position

Discussions on the subject of human anatomy assume that the person is standing erect, feet slightly apart, arms by the side with palms facing forward, thumbs pointing away from the body. This is referred to as the anatomic position and acts as a common reference point for anatomists (an expert or student of anatomy). If the body is lying face up, it is referred to as supine, if it is lying face down, the body is in a prone position.

Various structures can be described in relation to any number of imaginary flat surfaces or planes that bisect the body. The median sagittal plane is a vertical plane that passes through the center of the body, dividing it into right and left sides. The coronal plane, on the other hand, is a vertical plane that is perpendicular to the sagittal plane and divides the body into front and back halves. A paramedian plane passes vertically through the body at any point parallel to the median sagittal plane, while a horizontal or transverse plane is perpendicular to both the sagittal and coronal planes. A plane on an angle to the transverse plane is called an oblique plane.

A number of terms can be used to describe the location of body structures in relation to the above planes. One structure that is closer to the sagittal plane than another is said to be medial, while one that is farther from the sagittal plane is lateral (e.g. the heart is medial to the left lung). Structures found on the same side of the median sagittal plane are referred to as ipsilateral, while those on opposite sides of the body are called contralateral. A structure is anterior or ventral if it is before or in front of

another structure, and posterior or dorsal if it found after or behind (e.g. the trachea or windpipe is anterior to the esophagus). Superior or cranial structures are found closer to the top or crown of the head than inferior or caudal structures (e.g. the heart is superior to the stomach ). It is important to remember that the terms left and right are used from the perspective of the person being viewed, not the observer; for instance, the right lung would be on the left hand side of a body as viewed by an observer.

The head, neck, and trunk (the main axis) make up the axial portion of the body while the appendages or limbs compose the appendicular portion. To describe the limbs, the term proximal refers to a structure that is closer to the limb origin; a distal structure is farther from the limb origin (e.g. the wrist is distal to the elbow). The hand has a palmar surface (the anterior side) and a dorsal surface (the posterior side). The upper surface of the foot is called the dorsal surface, while the bottom side is called the plantar surface. Superficial structures are located nearer to the surface of the body; those found farther from the surface are referred to as deep structures (e.g. the skin is superficial to the skeletal muscles ). Internal structures are found inside of an organ or cavity, in contrast to external surfaces which are found outside (e.g. alveoli are internal to the lungs ).

Terms of movement

A joint is a site where two or more bones come together. Some joints allow no movement between bones(e.g. the bones of the skull ); others allow only slight movement (e.g. the joints between bones of the vertebral column ). Still other joints allow free movement (e.g. the hip joint). There are subsequently numerous terms to describe the various movements that joints can make.

Flexion is the bending of a joint, typically in the anterior direction; an example is the bending of the arm at the elbow. In contrast, the term extension describes the straightening of a joint; an example is the straightening of the leg at the knee. Abduction is the movement of a joint away from the midline along the coronal plane, while adduction is the movement toward the midline along the same plane; the arm, for example, abducts at the shoulder if it moves away from the torso in the same plane. Rotation occurs when a body part moves around its axis; medial rotation results in the anterior surface facing medially and lateral rotation results in the anterior surface facing laterally. The term pronation describes movement of the hand so that it faces posteriorly; supination causes the hand to face anteriorly. Circumduction is a movement that combines flexion, extension, abduction, and adduction; circling the arm around the shoulder joint is an example.

The term protraction describes a motion forward and retraction a motion backward; these describe the movement of the mandible (jaw) at the temporomandibular joints. Inversion occurs when the sole of the foot is turned to face the median (this would happen if you tried to stand on the outer side of your foot); eversion is movement that causes the sole of the foot to face laterally.

Cavities of the body

Within the axial portion of the body lie two major cavities: the dorsal and ventral body cavities. The dorsal body cavity lies posterior to the ventral body cavity and protects the organs of the central nervous system . It is composed of the cranial cavity (enclosing the brain ) and the vertebral cavity (containing the spinal cord ).

The ventral body cavity is the larger of the two and encloses the viscera or visceral organs (internal organs). It also has two major subdivisions: the thoracic cavity and the abdominopelvic cavity. The thoracic cavity is surrounded anteriorly by the ribs and chest muscles. It encloses two pleural cavities, each encasing a lung, and the mediastinum. The pericardial cavity is located inside of the mediastinum and encloses the heart. The mediastinum also surrounds the esophagus, trachea, and other thoracic organs. The abdominopelvic cavity is separated from the thoracic cavity by the diaphragm, a thin muscle below the lungs and heart that is important for breathing. The abdominopelvic cavity contains the abdominal cavity (enclosing the stomach, liver , spleen, intestines, and other digestive organs) and the pelvic cavity (containing the bladder, reproductive organs, and rectum).

There are also smaller body cavities that exist throughout the axial portion of the body. The oral cavity (mouth) contains the teeth and tongue. The digestive cavity includes the oral cavity and extends down to the anus, including all digestive organs. The nasal cavity is found posterior to the nose and is part of the respiratory system .

The middle ear cavity is medial to the external ear and contains three small bones (the malleus, the incus, and the stapes) that are essential to normal hearing . The orbital cavities are found in the skull and contain the eyes, as well as skeletal muscles and nerves. The freely movable joints of bones are found in synovial cavities, where synovial fluid is secreted that helps lubricate joints and reduce friction between bones.

VENTRAL BODY CAVITY MEMBRANES. The ventral body cavity is lined with a thin membrane called the parietal serosa; internal organs are covered with a similar membrane called the visceral serosa. These membranes secrete a small amount of fluid called serous fluid that separates and lubricates them. Different parietal and visceral membranes have different names for the cavities and organs that they protect. The parietal pericardium lines the cavity that contains the heart and the visceral pericardium covers the surface of the heart. Likewise, the parietal pleura lines the thoracic cavity and the visceral pleura covers the surface of the lungs.

Regions and quadrants

The abdominopelvic region is often further divided into regions or quadrants for reference in study or clinical examination. The abdomen may be partitioned into nine regions, with two transverse planes and two parasagittal planes positioned in a grid. The central grid surrounds the umbilicus (navel) and is subsequently called the umbilical region; the left and right lumbar regions are found lateral to the umbilical region. The epigastric region is located superior to the umbilical region and includes the area of the stomach; the left and right hypochondriac regions are found to each side. Inferior to the umbilical region is the hypogastric or pubic region in which the urinary bladder is found; to each side are the left and right iliac or inguinal regions.

The quadrant system is often used by health care workers during examinations to localize pain , tumors, or abdominal structures. The median sagittal plane marks the vertical division while a transverse plane across the umbilicus marks the horizontal division. The subsequent four divisions are named the left upper quadrant (LUQ), right upper quadrant (RUQ), left lower quadrant (LLQ), and right lower quadrant (RLQ).

Different regional terms exist that help to specifically identify different regions or parts of the body. The following are some of the more common regional terms: nasal (nose), oral (mouth), otic (ear), cervical (neck), sternal (sternum or breastbone), axillary (armpit), thoracic (chest), mammary (breast), brachial (arm), abdominal (abdomen), olecranal (elbow), carpal (wrist), digital (fingers and toes), manus (hand), pubic (genitals), patellar (kneecap), crural (leg), femoral (thigh), tarsal (ankle), pedal (foot), vertebral (spinal column), dorsal (back), and gluteal (buttock). If an individual broke a bone in or near the elbow, for example, he or she would experience pain in the olecranal region.

Organization of the body

The complex structures of the human body are organized into numerous hierarchies. The chemical level is the most basic level and is the foundation on which life is based. Many different chemicals are essential to sustain life; these include carbon (C), oxygen (O), nitrogen(N), potassium (K), sodium (Na), and calcium (Ca). Atoms (single particles of an element) combine to form molecules that in turn combine to form various structures that are the building blocks of cells. The cell is the basic functional unit of life; on the cellular level, however, it is evident that it is an extremely complex structure. Different types of cells found in the human body include muscle cells, nerve cells, blood cells, and epithelial cells.

Similar cells may unite to perform a specific function; these groups of cells are called tissues. These are organized on the tissue level into four major groups: epithelial tissue that covers the internal and external surfaces of the body, nervous tissue that transmits electrical signals, muscle tissue that is specialized for contraction, and connective tissue that provides a structural matrix for other tissues. At the organ level, different tissues (at least two types) combine to form an organ, a structure that is capable of performing specialized tasks. Examples of organs are the stomach, lungs, kidneys , and liver.

A system consists of groups of organs that have a common function. This next level of organization includes systems such as the respiratory system, reproductive system, and endocrine system . The organs of the respiratory system, for example, work together to accomplish the intake of oxygen and the output of carbon dioxide. The organ systems together make up the organism, the individual human being. The organismic level is the highest level in the structural hierarchy; it represents the unification of all body structures and their complex interaction.

ORGAN SYSTEMS. Numerous organ systems are found in the human body. Each corresponds to a group of specialized organs that perform related activities. The following list represents the major organ systems of the human body and their key functions:

• Integumentary system : This includes the skin, nails, hair, and sweat and sebaceous glands. The integumentary system provides an external protective covering for the body; helps to regulate body temperature; protects internal organs from injury; excretes sebum, a oily substance with antimicrobial activity; synthesizes vitamin D ; and has sensory receptors that are sensitive to pain, pressure, temperature, and touch.
• Skeletal system : Bones, cartilage, and ligaments make up the skeletal system. It provides a point of attachment for muscles and a framework that supports them. Blood cells are produced in bone marrow (the spongy material found in the center of most large bones); bones also store essential minerals such as calcium.
• Muscular system : Muscles are the organs of the muscular system. They provide the force necessary for body movement, support organs or body parts, help to maintain posture, provide the main source of body heat, and help in breathing.
• Nervous system: This includes the brain, spinal cord, nerves, and sense organs. Nervous impulses are the means by which organs and tissues communicate with the brain; nerve tissues carry impulses from various structures to the brain and vice versa. The nervous system can quickly respond to changes in the internal or external environment.
• Endocrine system: The hormone-secreting glands (pituitary gland , pineal gland, thyroid gland , thymus, adrenal gland, pancreas , testis, ovary, and parathyroid gland) make up the endocrine system. They are important in the regulation of different processes such as growth, metabolism , reproduction, and milk production in nursing women.
• Cardiovascular system : This is composed of the heart, blood vessels (arteries and veins), and the blood. The cardiovascular system is the means by which gases, nutrients, and wastes are transported throughout the body; it is also responsible for disseminating hormones, maintaining the acid-base balance in blood, and preventing extensive blood loss by the formation of clots.
• Lymphatic system : The lymphatic vessels, thymus, spleen, lymph nodes, and red bone marrow are all components of the lymphatic system. It is important in activating the immune response against foreign substances, returning tissue fluid to the blood, and supporting the maturation and proliferation of white blood cells.
• Digestive system : This includes the oral cavity (mouth, tongue, and teeth), salivary glands, pharynx, esophagus, stomach, liver, gallbladder , pancreas, small and large intestines, rectum, and anus. The purpose of the digestive system is to break down food and absorb necessary nutrients. It is also important in the process of detoxification.
• Respiratory system: The nasal cavity, pharynx, larynx, trachea, bronchi, and lungs together make up the respiratory system. It is responsible for the intake of oxygen and the output of carbon dioxide, the exchange of gases through the walls of alveoli (air sacs), and the vocalization of sounds.
• Urinary system : The kidneys, ureters, urinary bladder, and urethra are the major components of the urinary system. It is important for removing wastes from the body; maintaining a balance of water and electrolytes in the blood; and producing, storing, and transporting urine (a fluid made up of water, electrolytes, and nitrogenous wastes such as urea, uric acid, and creatinine).
• Reproductive system: In males, the reproductive system is made up of the testes, scrotum, penis, epididymes, vas deferens, seminal vesicles, prostate gland, and urethra. Its purpose is to produce the male sex cell (sperm) and transfer sperm to the female reproductive tract. The reproductive system in females is composed of the ovaries, uterus, fallopian tubes, vagina, vulva, and mammary glands. Its purpose is to produce the female sex cell (egg), provide an environment for sperm to fertilize an egg, support a developing fetus, and produce milk to nourish a newborn baby.

KEY TERMS

Abduction —Movement away from the midline along the coronal plane.

Adduction —Movement toward the midline along the coronal plane.

Anatomic position —Standing erect with feet slightly apart, arms at the side with palms facing forward, thumbs pointing away from the body.

Anterior —Situated before or in front of.

Circumduction —Movement that combines flexion, extension, abduction, and adduction.

Deep —Situated farther from the surface of the body.

Distal —Situated farther from the origin of a limb.

Eversion —Movement that turns the sole of the foot laterally.

Extension —Movement that straightens a joint.

Flexion —Movement that bends a joint.

Inferior —Situated farther from the crown of the head.

Inversion —Movement that turns the sole of the foot medially.

Lateral —Situated farther from the midline.

Medial —Situated closer to the midline.

Organ —A structure composed of different tissues that is capable of performing specialized tasks.

Plane —An imaginary line or surface that passes through the body.

Posterior —Situated after or behind.

Pronation —Movement that turns the hand so that it faces posteriorly.

Prone —The body lying face down.

Proximal —Situated closer to the origin of a limb.

Superficial —Situated closer to the surface of the body.

Superior —Situated closer to the crown of the head.

Supine —The body lying face up.

Tissue —Groups of similar cells that unite to perform a specific function.

Resources

BOOKS

Marieb, Elaine N. Essentials of Human Anatomy and Physiology. Boston: Benjamin Cummings, 2001.

Shier, David, Jackie Butler, and Ricki Lewis. Hole's Human Anatomy and Physiology. New York: McGraw-Hill, 1999.

Snell, Richard S. Clinical Anatomy for Medical Students. Baltimore: Lippincott Williams & Wilkins, 2000.

Thibodeau, Gary A., and Kevin T. Patton. Structure and Function of the Body. St. Louis, MO: Mosby-Year Book, Inc., 1997.

Tortora, Gerard J. Introduction to the Human Body: The Essentials of Anatomy and Physiology. New York: Biological Sciences Textbooks, Inc., 1997.

ORGANIZATIONS

American Association of Anatomists. 9650 Rockville Pike, Bethesda, MD 20814-3998. (301) 571-8314. <http://www.anatomy.org/anatomy>.

OTHER

The Visible Human Project. U.S. National Library of Medicine, 2001. <http://www.nlm.nih.gov/research/visible/visible_human.html>.

Stéphanie Islane Dionne