Study Notes on the Respiratory System from Hole's Essentials of Human Anatomy & Physiology
16.1: Introduction to the Respiratory System
The respiratory system is vital for cellular oxygen needs and carbon dioxide removal. It acquires oxygen, expels , filters, warms, and moistens air, regulates blood pH, and aids in vocalization and smell.
16.2: Organs of the Respiratory System
The Upper Respiratory Tract includes the nose, nasal cavity, sinuses, and pharynx. The Lower Respiratory Tract consists of the larynx, trachea, bronchial tree, and lungs.
Nose and Nasal Cavity
The nose allows air entry, supported by bone and cartilage. The nasal cavity, divided by the nasal septum, contains nasal conchae that increase surface area for warming, moisturizing, and filtering air. Mucus traps particles carried to the pharynx by cilia.
Paranasal Sinuses and Pharynx
Air-filled sinuses in cranial bones reduce skull weight and act as resonant chambers. The pharynx serves as a common passageway for air and food, aiding speech, and divides into the nasopharynx, oropharynx, and laryngopharynx.
Larynx
Superior to the trachea, the larynx transports air, prevents particle entry into the trachea, and houses vocal cords. It is composed of cartilage (e.g., thyroid, cricoid, epiglottic) and muscles.
Vocal Folds of the Larynx
Two pairs of vocal folds exist: false (upper, no sound) and true (lower, sound production). The glottis is the opening between true vocal cords. Sound is produced when air vibrates these cords, with pitch controlled by tension and loudness by air force.
Trachea (Windpipe)
This cylindrical tube connects the larynx to the bronchial tree. It is lined with ciliated mucous membrane for particle trapping and supported by 20 incomplete C-shaped cartilaginous rings to keep the airway open.
Bronchial Tree
This branched system of airways extends from the trachea to the alveoli. It includes primary, secondary (lobar), and tertiary (segmental) bronchi, leading to smaller bronchioles.
Bronchioles and Alveoli
Bronchioles branch into terminal (air conduction) and respiratory (gas exchange) bronchioles, which lead to alveolar ducts and sacs where clusters of alveoli, rich in capillaries, are the primary sites of gas exchange.
Characteristics of the Bronchial Tree
Cartilage decreases as airways narrow, replaced by increased smooth muscle in bronchioles, enabling bronchodilation or bronchoconstriction. Alveoli are lined with simple squamous epithelium for efficient gas exchange.
Lungs
Soft, spongy, cone-shaped organs occupying the thoracic cavity, separated by the mediastinum. They contain air passages, alveoli, blood vessels, and nerves.
Pleura and Lobes of the Lungs
The lungs are encased by a double-layered serous membrane (pleura), with visceral (lung surface) and parietal (thoracic cavity) layers, separated by lubricating serous fluid. The right lung has three lobes, and the left lung has two.
Mechanics of Breathing
Ventilation is the movement of air, comprising inspiration (inhalation) and expiration (exhalation). Air moves from high to low pressure.
Inspiration Mechanics
Inspiration is an active process where the diaphragm contracts and moves downward, and external intercostal muscles contract, expanding the thoracic cavity. This decreases intrapulmonary pressure, drawing air in. Surfactant prevents alveolar collapse.
Expiration
Normal expiration is a passive process resulting from the elastic recoil of the lungs and relaxation of the diaphragm and intercostal muscles, reducing thoracic volume and expelling air.
Respiratory Volumes and Capacities
Spirometry measures lung volumes:
Tidal Volume (TV): ~$500\,mL (normal breath).
Inspiratory Reserve Volume (IRV): ~$3,000\,mL (additional inhale).
Expiratory Reserve Volume (ERV): ~$1,100\,mL (additional exhale).
Residual Volume (RV): ~$1,200\,mL (air remaining).
Respiratory Capacities
Vital Capacity (VC): TV + IRV + ERV ( ~$4,600\,mL).
Inspiratory Capacity (IC): IRV + TV ( ~$3,500\,mL).
Functional Residual Capacity (FRC): ERV + RV ( ~$2,300\,mL).
Total Lung Capacity (TLC): VC + RV ( ~$5,800\,mL).
Alveolar Gas Exchange
Alveoli are sites of gas exchange across the respiratory membrane (alveolar epithelium and capillary endothelium). Gases diffuse down their partial pressure gradients: from alveoli to blood, from blood to alveoli. Factors favoring diffusion include increased surface area, shorter distance, greater gas solubility, and steeper gradients.
Gas Transport
Oxygen is primarily transported as oxyhemoglobin (98%) in red blood cells, with some dissolved in plasma. Carbon dioxide is mainly transported as bicarbonate ions (70%), with smaller amounts bound to hemoglobin or dissolved in plasma.