HUMAN PHYSIOLOGY

14.1 RESPIRATORY ORGANS

Respiration is the vital process by which organisms exchange gases, primarily oxygen (O₂) and carbon dioxide (CO₂), with their environment. The respiratory organs and mechanisms vary widely among animals, adapted to their habitat and complexity. Lower invertebrates such as sponges, coelenterates, and flatworms rely on simple diffusion of gases across their entire body surface due to their thin body walls and aquatic environment. Earthworms utilize their moist cuticle for gas exchange, while insects possess a specialized tracheal system consisting of a network of tubes that directly transport atmospheric air to body cells. Aquatic arthropods and molluscs use gills, which are vascularized structures adapted for extracting oxygen from water, a process known as branchial respiration. Terrestrial animals, including amphibians, reptiles, birds, and mammals, have evolved lungs—vascularized sacs that facilitate pulmonary respiration. Amphibians like frogs also supplement lung respiration with cutaneous respiration, exchanging gases through their moist skin. In humans, the respiratory system is a complex network of organs designed to efficiently bring atmospheric air to the lungs and facilitate gas exchange. The external nostrils open above the upper lip and lead into the nasal chamber through the nasal passage. The nasal chamber connects to the pharynx, a shared pathway for food and air. The pharynx opens into the larynx, a cartilaginous structure known as the sound box due to its role in voice production. The epiglottis, a thin elastic flap, covers the glottis during swallowing to prevent food from entering the respiratory tract. From the larynx, air passes into the trachea, a straight tube supported by incomplete cartilaginous rings that prevent collapse. The trachea bifurcates at the level of the fifth thoracic vertebra into right and left primary bronchi, which further branch into secondary and tertiary bronchi and then into bronchioles. The terminal bronchioles end in alveoli, tiny vascularized sacs where gas exchange occurs. The lungs, comprising this branching network, are enclosed within the thoracic cavity and covered by a double-layered pleura with pleural fluid between the layers to reduce friction during breathing movements. The respiratory system is divided into two parts: the conducting part, which includes the nostrils up to the terminal bronchioles and is responsible for transporting, filtering, humidifying, and warming the air; and the respiratory or exchange part, consisting of alveoli and their ducts where actual gas diffusion occurs. The lungs are housed in the thoracic chamber, an airtight space bounded by the vertebral column, sternum, ribs, and diaphragm. Changes in the volume of the thoracic cavity directly affect lung volume, facilitating breathing. Respiration involves several steps: (i) breathing or pulmonary ventilation, which moves air in and out of the lungs; (ii) diffusion of gases across the alveolar membrane; (iii) transport of gases by the blood; (iv) diffusion of gases between blood and tissues; and (v) cellular respiration where oxygen is utilized and carbon dioxide produced.

• Respiratory mechanisms vary among animals based on habitat and complexity.
• Lower invertebrates use simple diffusion; insects use tracheal tubes; aquatic animals use gills; terrestrial animals use lungs.
• Human respiratory system includes nostrils, nasal chamber, pharynx, larynx, trachea, bronchi, bronchioles, and alveoli.
• Lungs are covered by pleura and housed in the thoracic cavity formed by ribs, sternum, vertebral column, and diaphragm.
• Conducting part transports and conditions air; respiratory part (alveoli) is the site of gas exchange.
• Respiration steps include ventilation, diffusion, transport, tissue exchange, and cellular respiration.
• 📌 Respiration: Exchange of oxygen and carbon dioxide between an organism and its environment.
• 📌 Branchial respiration: Respiration using gills in aquatic animals.
• 📌 Pulmonary respiration: Respiration using lungs in terrestrial animals.

14.2 MECHANISM OF BREATHING

Breathing, or pulmonary ventilation, consists of two phases: inspiration (inhalation) and expiration (exhalation). These processes involve the movement of air into and out of the lungs by creating pressure gradients between the atmosphere and the lung alveoli. Inspiration occurs when the intra-pulmonary pressure (pressure inside the lungs) falls below atmospheric pressure, causing air to flow inward. Expiration occurs when the intra-pulmonary pressure rises above atmospheric pressure, pushing air out. The diaphragm and intercostal muscles play crucial roles in generating these pressure changes. The diaphragm is a dome-shaped muscle forming the floor of the thoracic cavity. During inspiration, the diaphragm contracts and flattens, increasing the volume of the thoracic cavity in the antero-posterior axis. Simultaneously, the external intercostal muscles contract, elevating the ribs and sternum, which increases the thoracic volume in the dorso-ventral axis. The combined effect is an overall increase in thoracic volume, which is transmitted to the lungs, increasing pulmonary volume. This increase lowers intra-pulmonary pressure below atmospheric pressure, causing air to rush into the lungs (Figure 14.2a). During expiration, the diaphragm and external intercostal muscles relax, returning the thoracic cavity to its original size. This decreases lung volume, raising intra-pulmonary pressure above atmospheric pressure, and air is expelled from the lungs (Figure 14.2b). Expiration is usually passive during normal breathing but can be active during forceful breathing with the help of internal intercostal and abdominal muscles. The volume of air moved during breathing can be measured using a spirometer, which is useful for clinical assessment of lung function. The average breathing rate in a healthy adult is 12-16 breaths per minute. Additional muscles can be recruited to increase the strength and depth of breathing during physical exertion. The respiratory volumes and capacities are important parameters describing the amount of air involved in various phases of breathing. Tidal Volume (TV) is the volume of air inspired or expired during normal respiration, approximately 500 mL. Inspiratory Reserve Volume (IRV) is the additional volume that can be forcibly inspired after a normal inspiration, about 2500-3000 mL. Expiratory Reserve Volume (ERV) is the additional volume that can be forcibly expired after a normal expiration, about 1000-1100 mL. Residual Volume (RV) is the volume of air remaining in the lungs after a forcible expiration, about 1100-1200 mL. From these volumes, various capacities are derived: Inspiratory Capacity (IC) = TV + IRV; Expiratory Capacity (EC) = TV + ERV; Functional Residual Capacity (FRC) = ERV + RV; Vital Capacity (VC) = IRV + TV + ERV; Total Lung Capacity (TLC) = VC + RV.

• Breathing involves inspiration and expiration driven by pressure gradients.
• Diaphragm contraction increases thoracic volume causing inspiration.
• External intercostal muscles elevate ribs and sternum during inspiration.
• Relaxation of these muscles decreases thoracic volume causing expiration.
• Spirometer measures respiratory volumes and capacities.
• Respiratory volumes include TV, IRV, ERV, and RV; capacities are combinations of these volumes.
• 📌 Inspiration: Process of drawing air into the lungs.
• 📌 Expiration: Process of expelling air from the lungs.
• 📌 Tidal Volume (TV): Volume of air inhaled or exhaled during normal breathing.