All cells of the body require oxygen for the breakdown of food particles to get energy. The burning of food is an oxidation-reduction process. The food is broken down to CO₂ and water with simultaneous release of energy. This process is called Cellular Respiration.

Carbohydrates (glucose) produce large amount of energy when they are broken down in the presence of O₂ and water.

The complete breakdown of glucose occurs in two steps:

In this step:

• Glucose is broken into pyruvic acid.
• This process occurs in cytoplasm.
• It doesn’t require oxygen.

In this step:

• Pyruvic acid completely broken into CO₂ and H₂O and energy.
• This process occurs in mitochondria.
• It requires oxygen.
• ATP (Adenosine triphosphate) are formed by the addition of high energy phosphate bond to ADP. Whenever energy is needed to do work, the high energy phosphate bond is broken and the energy is used to do work.

In some animals, the presence of break down of food particles to get energy occurs in the absence of oxygen. This type of respiration is called Anaerobic Respiration.

During heavy exercise, the muscles of the body also respire anaerobically. In the absence of oxygen Glucose is broken down into Lactic acid with small amount of energy.

In unicellular organisms gaseous exchange takes place by diffusion. Oxygen is carried in from surrounding water and carbon dioxide diffused out.

The process of respiration in Hydra and Planaria occurs by means of diffusion.

Exchange of gases takes place by diffusion through skin but transportation of these gases occurs by blood vascular system. Oxygen after reaching the blood capillaries of skin, combines with hemoglobin and is carried to the all cells of the body. Carbon dioxide is transported similarly from the cells to the skin from where it diffused out into the environment.

Blood vascular system is of open type in grasshopper, therefore they have a very efficient respiratory system with which they transport air directly to all cells of the body. The system consists of Spiracles and Trachea.

There are ten pairs of spiracles. Two of them are present in thorax while eight paris are present in abdomen. These are the openings which lead into an special type of system, the tracheal system.

These are tubular structures form a network in the body of animal. It divides into small branches, tracheoles. The tracheoles directly reach the cells of the body. In thorax and abdomen trachea shows several dilations called Air sacs. The tracheoles contain fluid in which oxygen dissolves so that it can diffuse into the cells. The contraction and relaxation of air sacs maintain the movement of gases to all parts of the body.

The muscles of abdomen help in the process of breathing. Due to the relaxation of muscles, body expands air from outside is drawn into the trachea through spiracles and transported to all cells of the body. Carbon dioxide diffuses into the tracheoles and when the body contracts, it is transported by the same path and is expelled through the spiracles.

In human there is a highly developed respiratory system. Gaseous exchange occurs in lungs from where transportation of gases to all cells of the body takes place by means of blood.

There is a passage for air to reach the lungs which starts from external nostrils.

External nares lead into a small chamber where the purification of air occurs by small hairs.

The air leave the nasal chamber through small holes called internal nares.

It is the common chamber for respiratory and digestive system.

Pharynx leads anteriorly into a cartilagenous structure called Larynx or sound box. It has vocal cords which when vibrate due to the passage of sound produce sound.

It is also called windpipe, made up of C shaped cartilage. It divides into right and left Bronchi before entering the lungs.

Lungs are the main respiratory organs where gaseous exchange takes place. These are paired structures, located in the thoracic cavity.

Internally in the lungs, the bronchi is divided into small bronchioles. These branches divide further into sac called Alveoli. The alveoli are richly supplied with blood capillaries. The air from the atmosphere diffuses in these capillaries. When several millions alveoli of the lungs are fitted with air, the lungs swell to fill the thoracic cavity.

The body cavity is divided into thorax and abdomen by a muscular Diaphragm. This diaphragm forms the lower boundary of thoracic cavity. The thorax is supported by a body cage which is made up of sternum infront, backbone (vertebral column) behind and twelve pairs of ribs on the side. The ribs are supported by intercostal muscles.

During breathing the inspiration and expiration helps in the entry and exit of air.

It is an active process. Due to the contraction of muscles of diaphragm, the longitudinal diameter (length) of the thoracic activity increases. Similarly, the contraction of intercostal muscles, move the ribs upward and outward and increases the transverse diametre (width) of the thoracic cavity. All these cavity and air rushes in through the nares, into the trachea and finally into the alveoli of the lungs. Here exchange of gases occurs.

Expiration is caused by the relaxation of muscles of diaphragm and intercostal muscles. The volume of thoracic cavity decreases and the air is forced out of the lungs.

Oxygen from the alveoli diffuses into the blood capillaries where it combine with haemoglobin and forms oxyhaemoglobin and carried to all cells of the body. It detaches from the haemoglobin on reaching the cell and diffuses into the cells. Carbon dioxide transported similarly from cells to the alveoli.