Gaseous Exchange in Humans

In humans and other higher animals the exchange of gases is carried out by the respiratory system.

Division of Respiratory System

We can divide the Respiratory System in three parts:

  • The Air Passageway
  • The Lungs
  • The Mechanism of Breathing

The Air Passageway

The air Passageway consists of the parts through which the outside lungs and after the exchange of gases it goes out. This passageway of air consists of the following parts.

Nasal cavity & Nostrils:

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 mucus which filter the dust particles form the air. The mucus also moistens and warms the incoming air and keeps its temperature nearly equal to that of the body.

Fig:- The air passageway and the lungs

Internal nostrils:

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. It extends to the opening of the oesophagus and the larynx.


The air goes from the pharynx into the larynx. We known that glottis is a narrow opening at the floor of pharynx which leads into larynx.

The glottis is guarded by a flap of tissue called the epiglottis.


The larynx is a box, made of cartilages. It is present between pharynx and trachea. It is also called Voice Box. Two pairs of fibrous bands called vocal cords are stretched The vocal cords vibrate when the air passes through them. This vibration produces sounds.

The vibrations in vocal cords and the movements of lips, cheeks, tongue and jaws produce specific sounds which result in speech. Speech is an ability that only humans are gifted with and this is one of the characteristics which has put human beings superior to all.


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 cartilagenous rings in the wall of trachea. The cartilages keep the trachea from collapsing even when there is no air in it.



On entering the chest cavity, the trachea divides into two smaller tubes called bronchi (Singular: Bronchus). The bronchi also have cartilagenous plates in their walls. Each bronchus enters into the lung of its side and then divides into smaller branches.


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 fine tubules called the alveolar ducts. Each alveolar duct opens into a cluster of pouches called alveoli. The alveoli from 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.

The trachea and the bronchi are also lined with ciliated and glandular cells. The glandular cells secrete mucus which moistens the air and also traps any fine particles of dust or bacteria that have escaped from the nasal cavity. The cilia beat with an upward motion so that the foreign particles along the mucus are sent to the oral cavity from where it may be either swallowed or coughed out.

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.

Pulmonary Vein:

These then join together to from the venules which from pulmonary vein. The pulmonary vein carries the oxygenated blood back to the heart.

The Lungs

Intercostal muscles:

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 ribs 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.

Outer pleural membrane:

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 movements of the lungs.

Fig:- Lungs and Pleural-Membranes

The Mechanism of Breathing

The physical movements associated with the gaseous exchange are called breathing.


There are two phases of breathing:

  1. Inspiration or Inhalation
  2. Expiration or Exhalation

1. Inspiration or Inhalation:

Inspiration or Inhalation is the process of taking air into the lungs. During inspiration, the ribs 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 the 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 ribs muscles relax bringing the ribs back to the original position. The diaphragm muscles also also relax and it gets its raised dome shape. This reduces the space in the chest cavity and increase 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.

The breathing movements are involuntary to a large extent. However, we can control the rate of breathing but not for a long time.

When we do exercise or some hard job our muscle cells carry out cellular respiration at greater rate. It results in the production of more carbon dioxide which is released in the blood. This greater than normal concentration of carbon dioxide stimulates the respiratory centre of brain. The respiratory centre sends messages to the rib muscles and diaphragm to increase the rate of breathing so that the excess carbon dioxide present in blood can be removed out of body. During exercise of other hard physical works the breathing rate may increase up to 30 – 40 times per minute.

Table:- Comparison between the inspired and the expired air
Feature Inspired Air Expired Air
Amount of oxygen 21% 16%
Amount of carbon dioxide 0.04% 4%
Amount of nitrogen 79% 79%
Amount of water vapours Variable Saturated
Amount of dust particles Variable Almost none
Temperature Variable Almost equal to body temperature

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