Our respiratory system is composed of the air passages provided by 1) nose, 2) pharynx, 3) larynx and 4) the respiratory surfaces of the lungs. The primary function of this system is ventilation and gas exchange. However, there are certain secondary roles played by this system viz. olfaction, phonation, and stabilization of the thorax during the mechanical exertion and certain other biochemical functions.
Anatomy and physiology of the larynx: Larynx (commonly known as Voice Box) is a part of lower respiratory tract and is the organ for phonation (generation of sounds). Basically, Larynx is an air passage and has a sphincter. It extends from the tongue to the trachea and projects ventrally between the vessels of the neck. It opens into laryngopharynx on the upper side and trachea on the lower side. Until puberty, the size of the larynx is similar in both males and females. However, it grows bigger in size in case of adult males as compared to adult females. The skeleton of the larynx is made up of cartilages that are interconnected by ligaments and fibrous membranes. The laryngeal cartilages may be single or paired. The single cartilages are cricoids, thyroid and epiglottic cartilages. The paired cartilages include arytenoids, cuneiforms and corniculates. Chart 1 presents the structural units of larynx.
Chart 1. Structural elements of the larynx.
Arytenoid cartilages are the most important of all as they influence the vocal cords. Epiglottis (epi: over and glottis: tongue) is a leaf shaped elastic cartilage covered with epithelium. During swallowing, the epiglottis is closed, and the food cannot enter the larynx and respiratory airways.
Epithelium lining of the upper respiratory tract: The upper respiratory tract is made up of nose, pharynx and associated structures. The nose can further be divided into external and internal nose. Internal nose consists of a cavity lined with mucous membrane. The nasal lining may be divided into two sections viz. the respiratory lining and the olfactory lining.
The mucous membrane contains capillaries and pseudostratified ciliated columnar or cuboidal epithelium with many goblet cells. Inhaled air is warmed by the blood present in the capillaries and the mucous secreted by the goblet cells moistens the air and traps the particles present in it. Since the epithelium is ciliated, the cilia move the particles trapped in the mucous toward the pharynx, which are subsequently removed from respiratory tract. Olfactory epithelium is the membrane containing the olfactory receptor cells. This sensory epithelium is thicker than respiratory epithelium.
Mechanism of inspiration and expiration: Inspiration (or inhalation) is the process of taking in the air during breathing and expiration (or exhalation) is the opposite process. The phenomena of inspiration and expiration are based on Boyle’s Law. Boyle’s law states that the pressure of a gas in a closed container is inversely proportional to the volume of the gas.
Before a normal inspiration, the air pressure inside the lungs is equal to the atmospheric air pressure i.e. 1 atm. During inspiration, the volume of the lungs is increased so that the air pressure inside the lungs decrease and air flow into the lungs (For an inspiration, the lungs must expand leading to increase in the lung volume and decrease in pressure). Similarly, during an expiration, the lung volume decreases (i.e. lungs contract), this increases the air pressure inside the lungs and forces the air out of the lungs.
The apparatus used to measure the volume of air exchanged during breathing is called spirometer or respirometer. The record showing the different aspects of breathing and lung volumes is called as a spirogram. A typical spirogram is a chart showing lung volumes and lung capacities and may serve as a diagnostic tool for respiratory diseases/disorders. The details of lung volumes and capacities are as follows:
- Tidal volume (TV): the quantity of air moving in/out of the lungs during a normal breath (0.5 L)
- Inspiratory Reserve Volume (IRV): the quantity of air that can be forcefully inhaled after a normal inspiration (3.0 L)
- Expiratory Reserve Volume (ERV): the quantity of air that can be forcefully exhaled after a normal expiration (1.2 L)
- Vital Capacity (VC): Maximum volume of air that can move in or out of the lungs (TV + IRV + ERV = 4.5 to 5.5 L)
- Functional Residual Capacity (FRC): the quantity of air that remains in the lungs after a normal expiration (exhalation) (ERV + RV = 2.4 L)
- Residual Volume (RV): the quantity of air present in the lungs after a forced expiration (1.0 L)
- Inspiratory Capacity (IC): The sum of tidal volume and inspiratory reserve volume (TV + IRV = 2.4 to 3.6)
- Total Lung Capacity (TLC): the maximum volume of air that lungs can accommodate (TV + IRV + ERV + RV 4.4 to 6.4 L)
Tortora, G. J., and Derrickson, B. Principles of Anatomy and Physiology (Eleventh edition). (2006): New Jersey, John Wiley & Sons Inc.
Williams, P. L. Gray’s Anatomy The Anatomical Basis of Medicine and Surgery (Thirty Eighth edition). (2000): Edinburgh, Harcourt Publishers Ltd.