1
The human body

C Meurier

Learning objectives

• Use appropriate anatomical terminologies to describe the location of the different organs of the body.
• Discuss the relationship between cells, tissues, organs and systems in relation to the whole organism.
• List the components of each body system and explain how they contribute to the function of the system.
• Discuss how the different parts and systems of the body work together to maintain homeostasis.

Section 1: Introduction

Whether a health assessment is being performed to identify a health problem or to evaluate risk factors for health education purposes, a good understanding of biological knowledge is important (Carroll, 2004). Familiarisation with the common terminologies used for the different structures of the body, for example, enables effective communication of assessment findings to colleagues and other health professionals. Disease states and their impact can only be fully appreciated against the background of normal body structure and functions. In this chapter, the body will be looked at in a systematic way, starting from an examination of its basic organisation and the maintenance of internal stability to looking at individual organ systems.

Anatomical terms

To begin with, it is useful to provide an orientation of the body - looking at body regions, directional terms to describe one body part relative to another, and spaces and cavities that contain the different internal organs. This will facilitate precise and concise reporting of the assessment of the body. By using the exact anatomical term to describe the area of complaint of a particular symptom, attention can be focused more quickly to that specific area (Thibodeau & Patton, 2004).

Body regions

The body is conventionally divided into two major regions:

• Axial. This consists of the head, face, neck and trunk or torso.
• Appendicular. This consists of the shoulder girdles, the upper limbs, pelvic girdles and lower limbs.

Directional terms

Directional terms are used to locate body structures. They are usually grouped in opposite pairs, e.g. superior/inferior, anterior/ posterior. Directional terms only make sense when they are used to describe one structure relative to another. We refer, for example, to the elbow being superior to the wrist although they are both located in the superior aspect of the body. Directional terms are shown in Table 1.1.

Body cavities

The internal organs are located within spaces in the body called cavities. There are two main cavities: dorsal and ventral. The dorsal cavity, situated near the dorsal surface of the body, contains the brain in the cranial cavity and the spinal cord in the vertebral canal. The ventral cavity, located near the anterior part of the body, can be further divided into three cavities, namely thoracic, abdominal and pelvic. The diaphragm separates the thoracic cavity from the abdominal cavity. The largest organs in the thoracic cavity are the lungs. The heart is embedded in the mediastinum, i.e. the space between the two lungs. There is no physical separation between the abdominal and pelvic cavities and they are often referred to as the abdominopelvic cavity. The cavities contain internal organs that are collectively called viscera. To enable the precise location of organs, the abdominopelvic cavity is divided into nine smaller compartments as shown in Figure 1.1.

Table 1.1 Directional terms.

• Superior

• Towards the top. Upper part of a structure. Towards the head (cephalic or cranial)

• The head is superior to the lower limbs and the knee is superior to the ankle

• Inferior

• Towards the bottom. The lower part of a structure. Away from the head

• The diaphragm lies inferiorly to the lungs

• Anterior or ventral

• Towards the front

• The sterrnum is anterior to the heart

• Posterior or dorsal

• Towards the back

• The thoracic vertebrae are posterior to the heart

• Medial

• Towards the midline

• The heart is medial to the lungs

• Lateral

• Further away from the midline. Towards the sides

• The lungs are lateral to the heart

• Proximal

• Refers to a structure that is closer to any point of reference

• The proximal part of the nerve running along the arm is the part closest to the spinal cord

• Distal

• Further away from a point of reference

• In the hand, the phalanges are distal to the carpals

• Superficial

• Towards or on the surface of the skin

• The skin is superficial to the muscles

• Deep

• Away from the surface of the body

• The intestines are deep to the surface of the skin of the abdomen

Figure 1.1 Abdomino-pelvic regions

Levels of organisation of the body

The human body and its many parts are categorised into six levels of organisation, which influence body structure and functions. Ranging from the smallest to largest, these differing levels are as follows.

Chemical level

The body is made up of atoms and molecules. Atoms (e.g. carbon, oxygen, hydrogen) are the smallest units of matter. When two or more atoms joined together, they become molecules (e.g. water). Molecules in turn combine with other atoms and molecules to form macromolecules in the cytoplasm of the cells, which enables normal cellular functions. If this is not maintained, disease or death may result.

Cellular level

Cells are the basic structural and functional units of an organism. They consist of atoms and molecules. Each human organism begins life as a single cell, when the sperm fertilises the ovum. The fertilised ovum, now called a zygote, then divides into two cells, four cells, eight cells and so on. During development, these cells undergo differentiation, i.e. the transformation of unspecialised cells into specialised cells.

Tissue level

Tissues are aggregates of cells that work together to perform a particular function. The cells of the body are organised into four primary tissues: epithelial, connective, muscle and nervous. Epithelial tissue covers body surfaces and, lines cavities, hollow organs and ducts. Connective tissue is mainly a support tissue, connecting, anchoring and supporting the structures of the body. Connective tissue is characterised by large amount of extracellular material, called matrix, in the spaces between the connective tissue cells. Muscle tissue is responsible for movement. Nervous tissue consists of neurons and neuroglia. Neurons generate and transmit messages whereas neuroglia provide neurons with anatomical and functional support.

Organ level

Two or more different types of tissues join together to form an organ. For example, the heart is an organ and is composed of three different types of tissues: epithelial (endothelial), muscle and connective. Organs have specific functions. The specific function of the heart is to pump blood.

System level

A system is a group of organs that work together to perform a common function. The heart and blood vessels work together to transport blood around the body and is referred to as the cardiovascular system. The other systems of the body are integumentary, skeletal, muscular, nervous, respiratory, lymphatic and immune, gastrointestinal, urinary, endocrine and reproductive.

Organism level

All structures and systems in the body combine to make the human organism.

Hierarchy of functions

The levels of organisation show something of a hierarchy but each level in the hierarchy is as important as the other. Disturbance at one level may affect the activity of the other levels. This can go up or down the hierarchy as shown in Figure 1.2. For example, the chemicals within the cells influence their function, which in turn affects the activity at the next level and so on. Events at the level of the organism (and ultimately in the environment) can also affect activities of the lower levels. For instance, the availability or choice of diet can affect the functions of the cells.

The basic structure of cells

The human body is composed of billions of cells and the functions of these cells ultimately determine the function of the human...