Respiratory System

Chapter 4

Respiratory System  

Introduction 

• Our body tissues utilize inhaled oxygen for metabolic purposes and produce carbon dioxide in the process. The main aim of the respiratory system is to extract oxygen from the atmosphere and supply it to body tissues and take out CO2 from the tissues and expel it into the atmosphere.

• The lungs expand to take air from the atmosphere which is rich in oxygen. The oxygen enters the lungs and is transferred to the blood in the pulmonary capillaries. This blood, which is now rich in oxygen, delivers it to the tissues. The tissues utilize oxygen and produce CO2 which passes into the blood. Blood rich in CO2 is called venous blood. The venous blood brings the CO2 to the lungs where the CO2 is diffused into the lungs and the lungs discharge it into the atmosphere.

• Respiration is thus a process, which involves exchange of gases between the atmosphere and blood and cells. 

Pulmonary Ventilation 

Pulmonary ventilation is the inspiration and expiration of air between atmosphere and lungs.

• External Respiration: Exchange of gases between the atmosphere and lungs; that is, the blood gains O2 and loses CO2.

• Internal Respiration: Internal respiration involves exchange of gases between blood and tissue cells. The blood gives O2 and receives CO2 from the tissues.

The respiratory system consists of the nose, pharynx (throat), larynx (voice box) trachea (windpipe), bronchi, and the lungs. But structurally, the respiratory system consists of two parts:

1. The upper respiratory tract which refers to the nose, pharynx and associated structures. 

2. The lower respiratory tract refers to the larynx, trachea, bronchi and lungs.

Functionally, there is the conducting part which consists of nose, pharynx, larynx, trachea, bronchi, bronchioles and terminals of bronchioles. All these conduct air into the lungs. The respiratory part, on the other hand, consists of respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli. This part serves as a site for the exchange of gases.

Nose

• The nose consists of an external part and an internal part in the side of skull. The nasal bone forms the bridge of the nose which holds it in a fixed position. It has a framework of pliable hyaline cartilage. The hyaline cartilage is covered with muscle and skin and is lined with mucous membrane. There are two nares or nostrils present on the surface of the external nose. The nose performs three main functions.

1. It receives olfactory stimuli.
2. It receives olfactory stimuli.
3. Its large hollow resonating chambers modify speech sounds

• Internally, it communicates with the pharynx through two openings called the internal nares. Various sinuses from frontal, sphenoid, maxillary, ethmoidal and lachrymal ducts open into the internal nose; the ethmoidal, maxillae, lachrymal, palatine form walls of the internal nose. The roof is formed by the ethmoidal bone whereas floor of the nose is formed by palatine bones. The nasal cavity is divided into two parts by the nasal septum. Anteriorly, the septum consists of hyaline cartilage and remaining part is formed by vomer which is a perpendicular plate of the ethmoidal bone. 

• The oropharynx is the intermediate of the pharynx. It lies posterior to the oral cavity and extends from the soft palate and inferior to the level of the hyoid bone. The portion of the pharynx used for digestive as well as respiratory purposes, is called throat or faucets. The palatine and lingual tonsils are present in the oropharynx.

• The laryngopharynx starts from hyoid bone and connects to the oesophagus with the larynx. Both, the oropharynx and laryngopharynx are lined by non-keratinised stratified squamous epithelium.

 

The Larynx or Voice box

• This has a short passage which connects the laryngopharynx with trachea. It lies between 4th-6th cervical vertebrae. The larynx is composed of nine pieces of cartilage. The three single cartilages are thyroid cartilage, epiglottis or epiglottis cartilage and circoids cartilage. The three paired cartilages are arytenoids, cuneiform and articulate cartilages. Arytenoids cartilage influences the position and tension of the vocal cords. 
• The thyroid cartilage consists of two fused plates of hyaline cartilage which forms the anterior wall of the larynx.

The Epiglottis

• It is a large leaf shaped elastic cartilage piece. The stem is attached to anterior rim of thyroid cartilage whereas the leaf portion remains unattached. This leaf portion is free to move up and down like a trap door. The opening of the glottis is closed by the epiglottis during swallowing.

• The glottis consists of a pair of mucous membrane folds (i.e., vocal cord) in the larynx. When larynx is closed off, the liquid and food enter into the esophagus. The inferior wall of the larynx is formed by circloids cartilage. This is attached to the first ring of cartilage of trachea.

The Bronchioles

• The bronchi divide into smaller and smaller branches called bronchioles. These bronchioles do not have cartilage in their walls. Initially, they are composed of muscle tissue, fibrous tissue and elastic tissues with an inner lining of cubical epithelium. As the tubes become smaller, fibrous and muscular tissues disappear and the cubical epithelium is converted into a single layer of flattened epithelium. The minute branches of bronchioles are known as terminals of bronchioles. These further bifurcate to form respiratory bronchioles. The respiratory bronchiole is expanded at its to form an alveolar duct. The alveolar duct leads to minute sac like structures called alveoli, where the exchange of gases occurs between air in the alveoli and blood in the capillaries.

The Lungs 

• There are two lungs. They are cone shaped organs that lie in the thoracic cavity. The lungs are separated by the heart and the great blood vessels. The space between the two lungs is called mediastinum. Each lung has an apex and a base. The lungs are convex on the outer surface and concave on the inner surface. The right lung is divided into three lobes, i.e., superior lobe, middle lobe and inferior lobe. The left lung is divided into two lobes, i.e., superior lobe and inferior lobe. 

• The convex surface of the lung which is called the costal surface is smooth and follows the shape of the chest wall. The concave surface is called the medial surface. The lung is covered by a serous membrane known as pleura, which is composed of epithelial cells. The pleura are divided into two layers:

1. Parietal pleura 

2. Visceral pleura.

• The parietal pleura line the ribs, sternum, costal cartilage, and the intercostal muscle fibres and also covers the superior surface of the diaphragm. The visceral pleura is completely attached to the lungs covering the lung surface. It also enters into fissures, assists for dividing the lungs into respective lobes. At the base of the lung, it is reflected backward to form parietal pleura. The flattened epithelial cells secrete a serous fluid which occupies the space between the two layers, i.e., the pleural cavity. This fluid reduces friction between the two membranes and allows them to slide easily over one on another during respiration. The internal structure of the lung shows bronchi, bronchioles, alveolar ducts, alveoli, pulmonary artery, and bronchial artery, branches of vagus nerve, pulmonary veins, bronchial veins and lymphatic vessels. These structures occup the lobules of the lungs. The lobules are separated by interlobular tissue 

Mechanism of Breathing

When, the lungs expand air from the atmosphere it enters the lungs and upon contraction of the lungs the air is expelled out. The pressure gradient assists in breathing. When pressure inside the lungs is less than the atmospheric pressure air enters the lungs while when pressure inside the lungs is greater than the atmospheric pressure, the air moves out of the lungs.

This mechanism occurs in three phases.

• Aspiratory phase: The respiration cycle occurs about 16 times/minute. As a result of muscular activity, the chest expands during inspiration. This activity may be partly voluntary and partly involuntary.

• The intercostal muscle: The space between each of the twelve pairs of ribs is called the intercostal space. This space is occupied by muscle fibres. These muscle fibres are arranged in two layers.

• External intercostal muscle fiber: It is present between two ribs and extends in downward and forward direction.

• The internal intercostal muscle fiber: It is extended in downward and backward direction between two ribs. In this way, these two muscle fibres form a network or cross pattern occupying intercostal spaces. There is no movement of the first rib; it is immovably attached to the thoracic vertebra and the manubrium. When intercostal muscles contract, they pull all the ribs towards the first rib. Due to curvy nature of the ribs, when they move upward, they also move outwards. This helps to enlarge the thoracic cavity. These muscle fibres are stimulated by intercostal nerves.
• The diaphragm: Due to the relaxation of the diaphragm the thoracic cavity is separated from the abdominal cavity. But when the diaphragm contracts, it is pulled downwards to its original position helps in enlarging the thoracic cavity in length. During contraction, the diaphragm exerts pressure on the abdominal organs and the abdominal wall which moves under this pressure.