Respiratory System (Anatomy)

Overview of Respiratory System
     The respiratory system consists of all the tissue and organs designed to bring air to the gas exchange surface where oxygen is absorbed and carbon dioxide is released.
     The respiratory system can be divided into:
  upper respiratory system that includes the nose, nasal cavity, paranasal sinuses and pharynx; and
  lower respiratory system that includes the larynx, trachea, bronchi and lungs.
     The respiratory system is essentially designed to support the respiratory tract that consists of the passageways through which the air travels to reach the gas exchange surface. 
     The respiratory tract can be divided into:
  conducting portion from the nasal cavity to the terminal bronchioles through which no gas exchange occurs; and
  respiratory portion that includes the respiratory bronchioles and alveoli where gas exchange occurs.
       The primary functions of the respiratory system are to:
  1. Provide a gas exchange surface;
  2. Move air to and from exchange surface.
       These functions create problems for the body that are solved by features in its design. So the respiratory system also functions to:
  3. Protect respiratory surfaces from environment;
  4. Defend against invasion by pathogens.
       Auxiliary functions of the respiratory system include the:
  5. Production of sound;
   6. Involvement in regulation of blood volume and pressure, and control of body pH.
  Respiratory Epithelium
       Most of the mucous membrane lining the conducting portion of the respiratory tract is lined by pseudostratified, ciliated, columnar epithelium (PCCE), which is also know as respiratory epithelium.
       PCCE serves the protective function of the respiratory system. Goblet cells, and mucous glands found in the lamina propria, produce mucus that traps particles on the surface. The cilia move sheets of mucus with trapped debris and pathogens out of the tract and toward the esophagus.
  Upper Respiratory System  
  Nasal Cavity
     The nasal cavity is the chamber between external and internal nares. Coiled, shelf-like extensions of bones called conchae (a.k.a. turbinates) extend from the lateral walls of the nasal cavity toward the nasal septum. Narrow passageways between the conchae are called meatuses. The air that passes through the meatuses is filtered, warmed and humidified. 
     The pharynx is the passageway connecting the nasal cavity, oral cavity, esophagus and larynx. It is shared by the digestive and respiratory tracts.
  Lower Respiratory System
     Air enters the lower respiratory system through an opening called the glottis. The larynx is a cylindrical structure that surrounds, protects and controls the glottis. Vocal folds flank the glottis and contain elastic tissue that vibrate to produce sound. 
     The trachea is a tough, flexible tube that is supported by 15 to 20 C-shaped tracheal cartilages which are attached by annular ligaments. The trachea is lined by typical respiratory epithelium, PCCE. 
  Primary Bronchus
     The trachea divides into right and left primary bronchi which are similar to the trachea in design. The primary bronchi enter each lung at an indentation called the hilus.
  Superficial Anatomy of Lungs
     The right and left lungs are situated in the pleural cavities and are shaped like blunt cones with a blunt superior apex within the base of the neck and a concave base on the surface of the diaphragm.
  Lobes of lungs:
     Each lung is divided into distinct lobes. The right lung has three lobes, a superior, middle and inferior lobe. The left lung has only two lobes, a superior and inferior lobe.
  Internal Structure of the Lungs
     After the primary bronchi enter the lungs they immediately branch into smaller and smaller passageways giving rise to what is called the bronchial tree. The bronchi within the lungs are now referred to as intrapulmonary bronchi.
     The primary bronchi first divide into the secondary or lobar bronchi that supply the lobes of each lung. 
     Inside the lobes of the lungs, the lobar bronchi divide into tertiary or segmental bronchi that provide the bronchopulmonary segments within each lung. Each bronchopulmonary segment has its own blood supply and drainage in addition to its own segmental bronchi. Each lung has 10 bronchopulmonary segments.
     As the tertiary bronchi branch within a segment and become smaller, the cartilaginous plates that supported the wall disappear and the outer wall is dominated by smooth muscle. These passageways are now called bronchioles (“little bronchi”).
     The bronchioles continue to divide and give rise to smaller bronchioles until the bronchiole is supplying air to a pulmonary lobule. At this point the bronchiole is called a terminal bronchiole and this marks the end of the conducting system of the respiratory tract. The terminal bronchioles still have smooth muscle in their walls that control the flow of air into the lobule. Contraction of the smooth muscle is called bronchoconstriction which causes the opening to narrow. Relaxation is called bronchodilation which causes the opening to widen.
     Within the lobule the terminal bronchiole divides into bronchioles whose walls become thinner and develop thin pouches called alveoli. These bronchioles are now called respiratory bronchioles and the alveoli are line by simple squamous epithelium across which gas exchange occurs. The respiratory bronchioles are the beginning of the respiratory portion of the respiratory tract.
  Alveolar Ducts and Alveoli
     The respiratory bronchioles divide into multiple linear passageways lined by alveoli that end in sacs surrounded by alveoli. The linear passageways are alveolar ducts and the sacs are alveolar sacs. The alveolar walls are associated with capillaries and elastic fibers that enable the alveolar ducts and sacs to maintain their relative positions after expansion.
  Alveolus and Respiratory Membrane
     The alveolar epithelium is primarily simple squamous epithelium. The alveolar epithelium consists of:
  1. Squamous epithelial cells also called Type I cells or respiratory epitheliocytes;
  2. Large cells called septal cells, surfactant cells, Type II cells or large alveolar cells. This cell (of many names!) produces an oily secretion called surfactant. Surfactant consists of a mixture of phospholipids that reduce the surface tension of the fluid in alveoli;
  3. Alveolar macrophages that phagocytize any particulate matter or pathogens that managed to get through the defenses of the respiratory tract.
     The respiratory membrane consists of:
  1. alveolar epithelium;
  2. capillary endothelium;
  3. fused basal laminae of the alveolar and endothelial cells.
  The respiratory membrane is thin and permits rapid exchange of the lipid soluble respiratory gases.