• Cell is the smallest structural and functional unit of an organism.
• Each cell performs certain functions such as nutrition, transportation, excretion and reproduction.
• To perform all these functions, the cell needs energy.
• The food that we eat is digested in our stomach and small intestine. This food has energy which is released during respiration.
• Therefore, all livings organisms respire to get energy from the food.
• In the respiratory system, exchange of gases takes place. The important organs under the respiratory system are:
• Nasal Passage
• Pharynx
• Larynx
• Trachea
• Bronchi
• Lungs

What is ATP?

ATP is the smallest unit of energy required for cellular processes. The energy released during the respiration is used to make a molecule of ATP molecule from ADP and inorganic phosphate.

• Animals have evolved different organs for the uptake of oxygen from the atmosphere and for getting rid of the carbon dioxide produced.
• Terrestrial animals can breathe the oxygen present in the atmosphere.
• Whereas the aquatic animals can only breath the oxygen present in the water. Therefore, the terrestrial animals have an advantage over the aquatic animals with regard to obtaining oxygen for the process of respiration.
• The oxygen is absorbed by different organs in the terrestrial organisms.
• In humans, the oxygen is absorbed by the lungs.

• In human beings, lungs perform the function of breathing in oxygen and exhaling out the carbon dioxide.
• Lungs have the structure that increases the surface area which is in contact with the oxygen-rich atmosphere.
• Since the exchange of oxygen and carbon dioxide has to take place across this surface, this surface is very fine and delicate.

• The taking in of air rich in oxygen into the body is called as inhalation.
• Whereas, the giving out of air rich in carbon dioxide from the body is called exhalation.

• The number of times a person breathes in a minute is called as breathing rate.
• A breath means one inhalation and one exhalation. While breathing, inhalation and exhalation takes place alternatively.

• Nasal Passage: The air passes through the nostrils and it is filtered by the fine hairs in the passage. The passage is lined with mucus which helps in this process. It prevents the particles of dust, bacteria or small organisms from entering into the body.
• Pharynx: It is a common passage for both respiratory and digestive system.
• Larynx: Vocal cords are inside the larynx which help in producing sound.
• Trachea: It is a tube from the base of larynx through the neck to the middle of the thorax. The rings of cartilage are present which ensure that the air passage do not collapse.
• Bronchi: The trachea divides into two branches called bronchi. Each lung has bronchus on both side.
• Bronchioles: Each bronchus is divided further into smaller and smaller fine tubes called bronchioles.
• Alveoli: Bronchioles terminate into balloon-like structures called alveoli. It is the site of gaseous exchange. The walls of alveoli contain an extensive network of blood-vessels.
• Lungs: Lungs are pink and red in colour. Each lung is surrounded by a membrane which is called pleural membrane. It consists of extensive network of capillaries where transportation of oxygen and carbon dioxide takes place.
• Diaphragm: It is a large muscular sheet which forms the floor of the chest cavity. When we breath in the air, our ribs lift and the diaphragm moves down. During exhalation, the ribs move down and the diaphragm moves up.

• The process of bringing the carbon dioxide from the body to the lungs and passing the oxygen to different organs of the body is called transportation of gases.
• The blood brings carbon dioxide from the rest of the body for release into alveoli, and the oxygen in the alveolar air is taken up by the blood in alveolar blood vessels to be transported to all the cells of the body.
• In small animals or plants, transportation of gases can take place through the process of diffusion.

• In big animals, diffusion alone cannot deliver oxygen to all the cells of the body.
• Therefore, it has respiratory pigments that take up oxygen from the air in the lungs and carry it to tissues which are deficient in oxygen before releasing it.
• In humans, respiratory pigment is haemoglobin which has very high affinity for the oxygen.
• Haemoglobin is present in the RBCs of the blood.

• Transportation of carbon dioxide from the cells to lung takes place by haemoglobin to the extent of 10-20%.
• It is more soluble in water than oxygen and transported in the dissolved form in the blood.