Gaseous Exchange in Humans
In humans and other higher animals the exchange of gases is carried out by the respiratory system.
Parts of Respiratory System
We can divide the respiratory system in two parts i.e. the air passageway and the lungs. Figure 10.3.
1. The air passageway
The air passageway consists of the parts through which the outside air comes in the lungs and after the exchange of gases it goes out. This passage of air consists of the following parts.
(i) Nasal cavity
The nose encloses the nasal cavity. It opens to the outside through the openings called the nostrils. The nasal cavity is divided into two portions by a wall. Each portion is lined by fine hairs and mucous.
Function of Nasal Cavity
It filters the dust particles from the air. The mucous also moistens and warms the incoming air and keeps its temperature nearly equal to that of the body.
ii) Pharynx
The nasal cavity opens into the pharynx by means of two small openings called internal nostrils.
Pharynx is a muscular passage and is common to both food and air. Itextends to the opening of the oesophagus and the larynx. Glottis is a narrow opening at the floor of pharynx which leads into larynx.
(iii) Larynx
The larynx is a box, made of cartilage. It is present between pharynx and trachea. It is called the vice box. Two pairs of fibrous bands called vocal cords are stretched across the larynx. The vocal cords vibrate when the air passes through them. This vibration produces sounds.
(iv) Trachea
Larynx continues to the trachea, which is also called the windpipe. It is about 12 cm long tube which lies in front of the oesophagus. There are C-shaped cartilaginous rings in the wall of trachea. The cartilages keep the trachea from collapsing even when there is no air in it.
(v) Bronchi
On entering the chest cavity, the trachea divides into two smaller tubes called bronchi (singular: bronchus). The bronchi also have cartilaginous plates in their walls. Each bronchus enters into the lung of its side and then divides into smaller branches.
(vi) Bronchioles
The bronchi continue dividing in the lungs until they make several fine tubes Called, bronchioles. The bronchioles progressively lose the cartilages as they become narrower. The bronchioles end as find tubules called the alveolar ducts.
(vli) Alveoli
Each alveolar duct opens into a cluster of pouches called alveoli. The alveoli form the respiratory surface in human body. Each alveolus is a sac-like structure lined by a single layer of epithelial cells. It is bound on the outside by a network of capillaries.
2. 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 12 pairs of ribs and the rib muscles called intercostal 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. They are spongy and elastic organs.The lungs also have blood vessels that are the branches of the pulmonary arteries and veins. Each lung is enclosed by two membranes called the outer pleural membrane and the inner pleural membrane.The membranes enclose a fluid which provides lubrication for the free expanding and contracting of the lungs.
3.Blood Circulation in Lungs
The pulmonary artery from the heart containing deoxygenated blood enters the lungs and branches into arterioles and then into capillaries which surround the alveoli. These then join together to form the venules which form pulmonary vein. The pulmonary vein carries the oxygenated bold back to the heart.