Structure of the Human Body

Trunk, head, and neck

The trunk (truncus) comprises the chest, abdomen, and pelvis. The posterior region of the trunk is known as the back (dorsum). The upper region of the trunk, the chest (thorax), is formed from the bony rib cage, which protects the lungs.The chest cavity (cavitas thoracis) is separated from the abdominal cavity below by the diaphragm. The abdomen describes the area between the thorax and the pelvis, its bony structure being formed by the lumbar spine. The abdominal cavity (cavitas abdominalis) contains the abdominal viscera, which are bordered by the peritoneum. The lower part of the trunk, the pelvis, contains the pelvic organs and connects the trunk to the lower extremities.

The head (caput) includes the cavities that house the brain and the sensory organs. The head also includes the mouth and nose, responsible for food and air intake, respectively. The basic bony structure of the head is the skull (cranium), the tooth-bearing parts of which are formed by the skeletal tissue of the jaw.The crown (vertex) forms the highest point of the vault of the cranium. The front half of the head is known as the sinciput, the posterior area is known as the occiput, and the side regions are known as the temples (tempus). The front surface of the head is known as the face (facies), in which the eyes (oculi), nose (nasus), and mouth (os) are located. The ears (auris) are positioned on the sides of the head.

The neck (collum) connects the head to the chest. Its basic bony structure is the cervical spine, in front of which lie the windpipe and the gullet as well as the nerves and vessels supplying the head. The back of the neck is known as the nape (nucha).

Extremities

The four limbs (ie, arms and legs) are connected to the top of the trunk by the shoulder girdle and to the bottom of the trunk by the pelvic girdle.The arm is subdivided into the upper arm (brachium), lower arm or forearm (antebrachium), and hand (manus), which includes the fingers (digiti). The leg is composed of the thigh (femur), lower leg (crus), and foot (pes).

Complexity of the human body

There is a scale of increasing complexity within the structure of the human body, ranging from a single cell—the smallest unit of life—to a complete organism (see Fig 1-2c). Cells make up tissues, the next level of complexity, which then differentiate into organs with characteristic functions. Organ systems, which comprise combinations of several organs, form the next level of complexity, with the unit of highest complexity being the complete organism.

The cell is the smallest functional unit of all living creatures that is capable of independent life and reproduction. A cell is able to undertake various metabolic processes including growth, movement, and reaction to stimuli, and it can reproduce by cell division. Cells exist in a diversity of forms and, in multicellular organisms, become specialized to form specific types of tissue.

Tissues are structures composed of cells that have the same structure, function, and intercellular substances. Individual tissues are never independent, but as several tissues combine, they form more complex functional units such as organs and organ systems.Tissues can be classified according to their structure and function as protective, supporting, and connective tissue as well as muscle and nerve tissue.

The intercellular space is the gap between cells, and these spaces contain intercellular substances in which nutrients, active substances, and breakdown products are transported from the vessels to the cells. The intercellular substances determine the unique properties of different tissues (eg, the high tensile strength in tendons is due to collagenous fibers, and the strength of the hard tissue in teeth results from the deposition of calcium salts).

Organs are functional units composed of different tissues that are characterized by their specific function and their histologic microstructure. Examples include muscles, lungs, kidneys, etc. Autonomic (vegetative) organs are involved in nutrition, excretion, and procreation, while sensory organs are classified as somatic organs.

Organ systems are units of organs that work together to perform particular functions. Examples include the respiratory, digestive, excretory, and nervous systems. Interrelationships among organ systems are coordinated and regulated by nervous and hormonal control.

The organism is the whole system of organs in a living body. It is made up of various functional units (ie, the organs), which are responsible for the development, maintenance, and procreation of the organism.