The breathing system of a mammal consists of a pair of lungs which are thin-walled elastic sacs lying in the thoracic cavity. The thoracic cavity consists of vertebrae, sternum, ribs and intercostal muscles. The thoracic cavity is separated from the abdominal cavity by the diaphragm.


The lungs lie within the thoracic cavity. They are enclosed and protected by the ribs which are attached to the sternum and the thoracic vertebrae. There are twelve pairs of ribs, the last two pairs are called ‘floating ribs’ because they are only attached to the vertebral column. The ribs are attached to and covered by internal and external intercostals muscles.


The diaphragm at the floor of thoracic cavity consists of a muscLe sheet at the periphery and a central circular fibrous tissue. The muscles of the diaphragm are attached to the thorax wall. The lungs communicate with the outside atmosphere through the bronchi, trachea, mouth and nasal cavities. The trachea opens into the mouth cavity through the larynx.


A flap of muscles, the epiglottis, covers the opening into the trachea during swallowing. This prevents entry of food into the trachea. Nasal cavities are connected to the atmosphere through the external nares(or nostrils)which are lined with hairs and mucus that trap dust particles and bacteria, preventing them from entering into the lungs. Nasal cavities are lined with cilia.


The mucus traps dust particles, The cilia move the mucus up and out of the nasal cavities. The mucus moistens air as it enters the nostrils. Nasal cavities are winding and have many blood capillaries to increase surface area to ensure that the air is warmed as it passes along. Each lung is surrounded by a space called the pleural cavity. It allows for the changes in lung volume during breathing.

An internal pleural membrane covers the outside of each lung while an external pleural membrane lines the thoracic wall. The pleural membranes secrete pleural fluid into the pleural cavity. This fluid prevents friction between the lungs and the thoracic wall during breathing. The trachea divides into two bronchi, each of which enters into each lung.

Trachea and bronchi are lined with rings of cartilage that prevent them from collapsing when air pressure is low. Each bronchus divides into smaller tubes, the bronchioles. Each bronchiole subdivides repeatedly into smaller tubes ending with fine bronchioles. The fine bronchioles end in alveolar sacs, each of which gives rise to many alveoli. Epithelium lining the inside of the trachea, bronchi and bronchioles has cilia and secretes mucus.


Adaptations of Alveolus to Gaseous Exchange

Each alveolus is surrounded by very many blood capillaries for efficient transport of respiratory gases. There are very many alveoli that greatly increase the surface area for gaseous exchange. The alveolus is thin walled for faster diffusion of respiratory gases. The epithelium is moist for gases to dissolve.


Gaseous Exchange between the Alveoli and the Capillaries

The walls of the alveoli and the capillaries are very thin and very close to each other. Blood from the tissues has a high concentration of carbon (IV) oxide and very little oxygen compared to alveolar air. The concentration gradient favours diffusion of carbon (IV) oxide into the alveolus and oxygen into the capillaries. No gaseous exchange takes place in the trachea and bronchi.

These are referred to as dead space. Ventilation Exchange of air between the lungs and the outside is made possible by changes in the volumes of the thoracic cavity. This volume is altered by the movement of the intercostal muscles and the diaphragm.



The ribs are raised upwards and outwards by the contraction of the external intercostal muscles, accompanied by the relaxation of internal intercostal muscles. The diaphragm muscles contract and diaphragm moves downwards. The volume of thoracic cavity increases, thus reducing the pressure. Air rushes into the lungs from outside through the nostrils.



The internal intercostal muscles contract while external ones relax and the ribs move downwards and inwards. The diaphragm muscles relaxes and it is pushed upwards by the abdominal organs. It thus assumes a dome shape. The volume of the thoracic cavity decreases, thus increasing the pressure. Air is forced out of the lungs. As a result of gaseous exchange in the alveolus, expired air has different volumes of atmospheric gases as compared to inspired air.


Lung Capacity

The amount of air that human lungs can hold is known as lung capacity. The lungs of an adult human are capable of holding 5,000 cm3 of air when fully inflated. However, during normal breathing only about 500 cm3 of air is exchanged. This is known as the tidal volume.

A small amount of air always remains in the lungs even after a forced expiration. This is known as the residual volume. The volume of air inspired or expired during forced breathing is called vital capacity.


Control of Rate of Breathing

The rate of breathing is controlled by the respiratory centre in the medulla of the brain. This centre sends impulses to the diaphragm through the phrenic nerve. Impulses are also sent to the intercostal muscles. The respiratory centre responds to the amount of carbon (IV) oxide in the blood.

If the amount of carbon (IV) oxide rises, the respiratory centre sends impulses to the diaphragm and the intercostal muscles which respond by contracting in order to increase the ventilation rate. Carbon (IV) oxide is therefore removed at a faster rate.


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