Stomata are the microscopic pores in the epidermis of leaves. They are the passageways for gases and water vapours. Opening and closing of stomata controls the gaseous exchange.
In humans and other higher animals the exchange of gases is carried out by the respiratory system. We can divide the respiratory system in two parts i.e. the air passageway and the lungs.
All the alveoli on one side constitute a lung. There is a pair of lungs in the thoracic cavity. The chest wall is made up of 12 pairs of ribs and the rib muscles called intercoastal muscles. A thick muscular structure, called diaphragm, is present below the lungs.The left lung is slightly smaller and has two lobes and the right lung is bigger with three lobes
The physical movements associated with the gaseous exchange are called breathing. There are two phases of breathing i.e. inhalation and exhalation.
1. Inspiration or Inhalation
During inspiration, the rib muscles contract and ribs are raised. At the same time the dome-shaped diaphragm contracts and is lowered. These movements increase the area of the thoracic cavity, which reduces the pressure on lungs. As a result, the lungs expand and the air pressure within them also decreases. The air from outside rushes into the lungs to equalize the pressure on both sides.
2. Expiration or Exhalation
After the gaseous exchange in the lungs, the impure air is expelled out in exhalation. The rib muscles relax bringing the ribs back to the original position. The diaphragm muscles also relax and it gets its raised dome shape. This reduces the space in the chest cavity and increases the pressure on lungs. The lungs contract and the air is expelled out of them.
Humans breathe 16 -20 times per minute in normal circumstances i.e. at rest. The rate of breathing is controlled by the respiratory centre in the brain. The respiratory centre is sensitive to the concentration of carbon dioxide in the blood.
There are a number of respiratory disorders which affect people
3. Bronchitis is the inflammation of the bronchi or bronchioles
4. Emphysema is the destruction of the walls of the alveoli
5. Pneumonia is an infection of lungs. If this infection affects both lungs then, it is called double pneumonia
6. Asthma is a form of allergy, in which there is inflammation of the bronchi, more mucous production and narrowing of the airways
7. Lung cancer is a disease of uncontrolled cell divisions in the tissues of the lung. The cells continue to divide without any control and form tumours