Q.6 Describe the structure and functions of human respiratory system
Answer:
Introduction
In humans and other higher animals, the exchange of gases is carried out by the respiratory system. We can divide respiratory system in two parts i.e.
(1)The air passageway
(2)The lungs
1. Air passageway
Definition
It consists of the parts through which the outside air comes in the lungs and after the exchange of gases it goes out.
Parts of air passageway
This passage of air consists of the following parts:
a) Nasal cavity and nostrils
The nose encloses the nasal cavity. It opens to the outside through the openings called the nostrils.
Structure
The nasal cavity is divided into two portions by a wall. Each portion is lined by fine hairs and mucous.
Function
Fine hairs and mucous filter 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.
b) Pharynx
The nasal cavity opens into the pharynx by means of two small openings called internal nostrils. It is a muscular passage and is common to both food and air. It extends to the opening of the oesophagus and the larynx.
c) Glottis and epiglottis
It is a narrow opening at the floor of pharynx which leads into larynx. The air goes from the pharynx into the larynx. The glottis is guarded by a flap of tissue called the epiglottis.
d) Larynx or voice box
The larynx is a box made of cartilage. It is also called the voice box.
Location
It is present between pharynx and trachea.
Vocal cords and sound production
Two pairs of fibrous bands called vocal cords are stretched across the larynx. When the air passes through them, the vocal cords vibrate. This vibration produces sounds. Movements of lips, cheeks, tongue and jaws produce specific sounds which results in speech: Speech is an ability of only humans.
e) Trachea
Larynx continues to the trachea, which is also called the windpipe.
Length and location
It is about 12 cm long tube which lies in front of the oesophagus.
Structure
There are C-shaped cartilagenous rings in the wall of trachea.
Function of cartilageย The cartilages keep the trachea from collapsing even when there is no air in it.
f) Bronchi
On entering the chest cavity, the trachea divides into two smaller tubes called bronchi (singular: bronchus).
Structure
The bronchi also have cartilagenous plates in their walls.
Division of Bronchi
Each bronchus enters into the lung of its side and then divides into smaller branches.
Other functions of trachea and bronchi
Both trachea and the bronchi are also lined with ciliated and glandular cells.
Function of glandular cells
These cells secrete mucous which moistens the air and also traps any fine particles of dust or bacteria that have escaped from the nasal cavity.
Function of cilia
The cilia beat with an upward motion so that the foreign particles along the mucous are sent to the oral cavity from where it may be either swallowed or coughed out.
g) 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.
h) Alveolar Ducts
The bronchioles end as fine tubules called the alveolar ducts.
1) Alveoli
Each alveolar duct opens into a cluster of pouches called alveoli.
Structure
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.
j) Pulmonary artery
The pulmonary artery from the heart containing deoxygenated blood enters the lungs and branches into arterioles and then into capillaries which surround the alveoli.
k) Pulmonary vein
Capillaries join together to form the venules which form pulmonary vein. The pulmonary vein carries the oxygenated blood back to the heart.
2) Lungs :
All the alveoli on one side constitute a lung. There is a pair of lungs in the thoracic cavity.
Chest wall formation
It is made up of 12 pairs of ribs and the rib muscles called intercoastal muscles.
Diaphragm
A thick muscular structure present below the lungs is called diaphragm.
Lobes of 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.
Blood vessels of lungs
The lungs also have blood vessels that are the branches of the pulmonary arteries and veins.
Pleural membranes
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.
