The energy is required for various metabolic activities in an individual. This energy is obtained by oxidation of end products of digestion at cellular level. Hence oxygen is essential for the maintenance of life. The oxygen is involved in oxidation by releasing carbon dioxide along with energy.

According to William S.Hoar, the intake of oxygen and expulsion of carbondioxide is called respiration. However, Yapp defined the process of respiration as energy releasing oxidative process. %

The process of respiration in vertebrates occurs in three phases namely,

1. External respiration.

2. Circulatory phase.

3. Internal or Tissue respiration.

External respiration

In this phase of respiration the atmospheric oxygen enters into the blood supplied to respiratory organs and carbon dioxide from the blood is sent outside. The exchange of gases between the atmosphere and the respiratory organs is known as external respiration. It is also regarded as ventilation of respiratory organs.

Circulatory phase

In this phase the oxygen that diffuses into the blood is carried to the different tissues along with blood circulation. The carbon dioxide from different tissues is also brought to the respiratiory organs through the blood. This transportation of gases through blood constitutes Circulatory Phase.

Internal respiration

In the third phase of respiration the oxygen that enters into the tissues involves in oxidation of food stuffs. As a result of oxidation, energy, carbon dioxide,water and nitrogenous wastes are formed. , The energy is utilised for normal vital activities while the water and nitrogenous wastes are sent outside through excretory organs. Carbon dioxide is sent outside through respiratory organs. This ultimate energy releasing process at tissue level is called Cellular respiration or Internal respiration.



The respiratory organs in rabbit are a pair of lungs. The respiratory system starts with a pair of external nostrils present at the anterior end of the snout. The nostrils open into nasal passage that is situated above the buccal cavity.

The nasal passage is separated from the buccal cavity by a palate. The nasal passage opens posteriorly into the pharynx by internal nostrils. The nassal passage helps in (a) olfactory sensation (b) Filtering the air and (c) Warming up the inhaled air.

The pharynx in rabbit has two openings namely the gullet and the glottis. The gullet leads into oesophagus while the glottis leads into the trachea. The glottis is guarded by a cartilaginous flap like structure called Epiglottis. The epiglottis prevents the entry of food into trachea by closing the glottis.


The anterior part of trachea consists of larynx or voice box. It encloses a cavity called Laryngeal chamber. Two pairs of membranous folds called vocal cords are present inside the laryngeal chamber. One pair of vocal cords are true and the second pair are false. When air is sent outside the vocal cords vibrate to produce sound.


The wall of the larynx is supported by four cartilages namely a thyroid cartilage, a cricoid cartilage a pair of arytenoid cartilages. Thyroid cartilage is in the form of a broad ring, lying in the ventral and lateral walls of the pharynx. This cartilage is incomplete dorsally. The lower ring - like cartilage is cricoid which is broad dorsally and narrow ventrally. The arytenoids are present at the anterior end of dorsal side of cricoid. There is also a pair of small nodules called the cartilages of santorini present at the apex of arytenoid.


The larynx opens into trachea or wind pipe that runs along the length of neck, ventral to the oesophagus. The trachea enters into the thoracic cavity and divided into two branches called Bronchi.

The trachea and bronchi are supported by incomplete cartilaginous rings called tracheal rings. Each bronchus enters into the lung of its side. The bronchus is further divided into small branches called bronchioles within the lung. Each bronchiole divides into number of alveolar ducts. The alveolar ducts terminate in Air sacs or Infundibuli formed of many alveoli. The alveoli are highly vascularised with blood capillaries. Smallest bronchioles are not supported by tracheal rings. Bronchioles are lined with mucous membrane. The wall of air sacs is made up of thin layer of flattened cells supported by highly elastic connective tissue. It is also supplied with large number of blood capillaries.


The lungs in rabbit are hollow pinkish, spongyjobed bags, lying in thoracic cavity or air tight pleural cavities. They are surrounded dorsally by the vertebral column ventrally by sternum,posteriorly by the diaphragm, anteriorly by the neck and laterally by the ribs. The ribs are operated by two sets of intercostal muscles.


The internal surface of lungs is greatly increased and it is several times the surface area of the body. The left lung consists of two lobes namely left anterior and left posterior lobe.


The right lung consists of four lobes namely anterior azygos, right anterior, right posterior and posterior azygos. Inside each lung the bronchiole terminates in a cluster of air sacs or alveoli. Gaseous exchange occurs within the alveoli.

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