The energy is required for various metabolic activities in an individual. This energy is obtained by the oxidation of end products of digestion at the cellular level. Hence oxygen is essential for the maintenance of life. Oxygen is involved in oxidation by releasing carbon dioxide along with energy.
According to William S. Hoar, the intake of oxygen and expulsion of carbon dioxide 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;
- External respiration
- Circulatory phase
- 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 the 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 other tissues is also brought to the respiratory organs through the blood. This transportation of gases through blood constitutes the Circulatory Phase.
Internal Respiration
In the third phase of respiration, the oxygen that enters the tissues involves in the oxidation of foodstuffs. As a result of oxidation, energy, carbon dioxide, water, and nitrogenous wastes are formed. The energy is utilized 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 the tissue level is called Cellular respiration or Internal respiration.
RESPIRATORY ORGANS IN RABBIT
The respiratory organs in rabbits are a pair of lungs. The respiratory system starts with a couple of external nostrils present at the anterior end of the snout. The nostrils open into a 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 nasal passage helps in (a) olfactory sensation (b) Filtering the air and (c) Warming up the inhaled air.
The pharynx in rabbits has two openings namely the gullet and the glottis. The gullet leads into the esophagus 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 the trachea by closing the glottis.
Larynx
The anterior part of the trachea consists of the larynx or voice box. It encloses a cavity called the 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. The 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 the dorsal side of the cricoid. There is also a pair of small nodules called the cartilage of Santorini present at the apex of the arytenoid.
Trachea
The larynx opens into the trachea or windpipe that runs along the length of the neck, ventral to the esophagus. The trachea enters the thoracic cavity and is 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 several alveolar ducts. The alveolar ducts terminate in Air sacs or Infundibuli formed of many alveoli. The alveoli are highly vascularised with blood capillaries. The smallest bronchioles are not supported by tracheal rings. Bronchioles are lined with mucous membranes. The wall of air sacs is made up of a thin layer of flattened cells supported by highly elastic connective tissue. It is also supplied with a large number of blood capillaries.
Lungs
The lungs in rabbits are hollow pinkish, spongy bed bags, lying in thoracic cavity or air-tight pleural cavities. They are surrounded dorsally by the vertebral column ventrally by the 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 the lungs is significantly increased and it is several times the surface area of the body. The left lung consists of two lobes namely the left anterior and left posterior lobes.
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.