Introduction to Human Body

The human body is a well organized unit which has different parts namely, head, neck, torso and two arms and two legs which work in a coordinated manner to help in its survival and existence.

In technical terms, the human body is divided into various regions or parts. These are (Fig. 1 and 2):

1. Head: Head consists of skull and face. Head houses the brain while the face is specially designed to accommodate the eyes, mouth (opening of digestive tract) and nose (opening respiratory tract). Head is covered by a soft tissue known as scalp with the characteristic hairs.
2. Neck: Neck is the narrow part of the body which connects head to thorax.
3. Thorax: It is the part of the body which lies between neck and abdomen. It is also known as chest. It houses the two vital organs of human body which are the heart and lungs.
4. Abdomen and pelvis: This region lies below thorax. It contains organs of digestive system, urinary and genital system. The area present between upper parts of two thighs is known as perineum.
5. Upper limbs: A pair of upper limbs is attached at the upper end of trunk of body, one on each side. Each upper limb consists of shoulder region, scapular region, arm, forearm and hand. Upper limb is meant to manipulate external environment e.g., grasping, moving, throwing, catching, screwing etc. In common language upper limbs are called arms.
6. Lower limbs: A pair of lower limbs is attached at the lower end of trunk of body, one on each side. Each lower limb consists of hip and buttocks (gluteal region), thigh, leg, ankle, heel, foot and toes. Lower limb is meant for locomotion. In common language lower limbs are called legs.

The chest and abdomen form the central trunk of the body also known as the torso.

Fig. 1. Different parts of body (seen from front)

Fig. 2. Different parts of body (seen from behind)

Physical composition of human body

The substance of human body is made up of living cells and extracellular materials and is organized into tissues, organs, and systems.

Cell is the primary structural and functional unit of a human body is the cell. A number of cells with similar embryonic origin and function form a tissue, e.g., muscle, bone, nerve etc. A number of tissues are organised to form an organ that performs a specific function, e.g., stomach, heart etc.

The various organs and tissues of body are arranged in a systematic manner in order to perform different functions of the body, e.g., gastrointestinal tract, nervous system, etc. These systems perform specific function for survival and maintenance of human body. Each system has an independent function but is interdependent on other systems for its proper functioning.

Biochemical composition of body
In average adults, 60% of total body weight is water, 18% is protein and related substances, 15% is fat and 7% is minerals.

The body water is divided into two parts:

1. Intracellular fluid (ICF): It is the fluid contained within the cells. It forms 2/3rd of total body water and 40% total body weight.
2. Extracellular fluid (ECF):It forms 1/3rd of total body water and 20% of total body weight. ECF is further divided into two components:
   1. Circulating plasma:It is 25% of ECF and forms 5% of total body weight. Plasma along with various cellular elements forms blood which circulates in the vascular system.
   2. Extra-cellular or Interstitial fluid:It is 75% of ECF and forms 15% of total body weight. This fluid lies outside the blood vessels and bathes the cells.

The normal cell function depends on the composition of interstitial fluid. The internal environment of the body is kept constant by multiple regulatory factors. This is called homeostasis. This is a dynamic process that regulates volume, composition, pH, temperature and contents of the various body fluids within physiological limits.

The primary aim of any living organism is its survival and existence. In human body this aim is achieved by the various basic life processes performed by it namely:

• Metabolism: This includes all chemical processes occurring in the body which facilitate survival and existence.
• Homeostasis: This is the ability of an organism to respond to the external environment and regulate its own internal environment.
• Movement: It includes movement of cells within the body and movement of the organism as a whole.
• Growth: It involves increase in size and number of cells, replacement of cells and removal of older cells of the body.
• Differentiation: This process is fundamental in formation of various organs of an organism. It is the process wherein an unspecialized cell gets converted into a specialized cell during growth and development and organise swith other cells to form a special unit named organ.
• Reproduction: It includes new cell formation for growth and repair of the body and production of a new individual.

The various systems of a human body are:

• Skeletal system
• Muscular system
• Nervous System
• Cardiovascular system
• Respiratory system
• Digestive system
• Endocrine system
• Urinary system
• Male reproductive system
• Female reproductive system
• Skin
• Organ of sight – Eye
• Organ of hearing – Ear
• Organ of smell – Nose
• Organ of taste – Tongue