Respiratory System: Organization and Functions

• Chapter 23, part 1 of 2, will cover the respiratory system.

General Organization of the Respiratory System

• The respiratory system is structurally organized into upper and lower respiratory tracts.
• Functionally, it's organized into conducting and respiratory zones.
• The upper respiratory tract includes the nose, nasal cavity, pharynx, and larynx.
• The lower respiratory tract includes the trachea, bronchus, bronchioles (including terminal bronchioles), respiratory bronchiole, alveolar duct, and alveoli.
• The conducting zone is for air transport.
• The respiratory zone is where gas exchange occurs.

Functions of the Respiratory System

• The respiratory system serves as an air passageway between the atmosphere and the alveoli for breathing.
• It is the site for oxygen and carbon dioxide exchange.
• Oxygen diffuses from alveoli into blood and carbon dioxide diffuses from blood into alveoli.
• This system is involved in the detection of odors via olfactory receptors in the superior nasal cavity.
• It also produces sound as air moves across the vocal cords of the larynx, known as the voice box.

Respiratory Mucosa

• The mucosa, or mucous membrane, produces mucous secretions.
• Secretions are produced from goblet cells of the epithelial lining, as well as mucous and serous glands.
• Mucin protein increases mucus viscosity to trap dust, dirt, pollen, and other particles.
• The mucosa contains defenses against microbes like lysozyme, defensins, and Immunoglobulin A.
• Sputum is a term for saliva and trapped substances when coughed up.

Upper Respiratory Tract and External Nose

• Nasolacrimal ducts drain lacrimal secretions from eye surfaces to the nasal cavity.
• The nasal cavity conditions the air by warming, cleansing, and humidifying it.
• Air is warmed by extensive blood vessels.
• Mucus traps dust, microbes, and foreign material.
• Cilia sweep mucus toward the pharynx to be swallowed.
• A moist environment humidifies the air.
• Air turbulence created by conchae enhances all three processes.

Paranasal Sinuses

• These are cavities inside skull bones.
• They are named for the bones in which they are found.
• The sinuses are lined by pseudostratified ciliated columnar epithelium.
• Mucus caught in the sinuses is swept into the pharynx and swallowed.

Pharynx

• Is divided into three sections: the nasopharynx, oropharynx, and laryngopharynx.
• It ends at the larynx where the vocal cords are.

Lower Respiratory Tract

• Includes conducting pathways from the larynx to terminal bronchioles.
• Includes structures involved in gas exchange such as respiratory bronchioles, alveolar ducts, and alveoli.
• The larynx consists of 9 different cartilage pieces and extrinsic and intrinsic ligaments.

Vocal Folds

• Also known as true vocal cords, vibrate to create sound.
• Males generally have longer and thicker vocal folds than females, resulting in a deeper voice.

Trachea

• Also known as the windpipe.
• Extends from the larynx to the bronchi.
• It's supported by C-shaped rings called tracheal cartilages, which are rigid structures that ensure the trachea stays open.
• The trachea bifurcates into a left and right main bronchus.

Bronchial Tree

• It's a highly branched system of air conducting passages.
• It originates at the main bronchus and progressively branches into narrower tubes.
• The tubes diverge throughout the lungs before ending in the alveoli.

Bronchial Wall and Clinical Application

• Bronchi contain irregular cartilage rings for support.
• Bronchioles lack cartilage but have a large proportion of smooth muscle to allow for bronchoconstriction and bronchodilation.

Clinical View: Asthma

• Is a chronic condition.
• It is characterized by episodes of bronchoconstriction, wheezing, coughing, shortness of breath, and excess mucus.
• Attacks can occur due to sensitivity to an airborne agent or can be exercise-induced, and are worse in cold weather.
• Chronic inflammation of bronchioles due to asthma causes bronchiole walls to become permanently thickened.

Bronchioles and Alveoli

• The respiratory zone consists of respiratory bronchioles, alveolar ducts, and alveoli.
• Pulmonary capillaries form a vascular network (with a web/hair net appearance) around each alveolus.
• Gas exchange (oxygen and carbon dioxide) occurs at the alveoli.

Respiratory Zone

• Consists of Respiratory Bronchioles, Alveolar Ducts, and Alveoli.

The Lungs

• Located within the thoracic cavity on either side of the mediastinum.
• They are enclosed and protected by the thoracic cage.
• Each lung has a base (rests on diaphragm) and an apex (upper peak of the lung).
• There are three surfaces of the lungs described in relation to the structures that they sit up against, which are the Costal Surface (against ribs), Diaphragmatic Surface (against diaphragm), and Mediastinal Surface (against mediastinum).

Lungs

• They are composed of lobes separated by fissures.
• The hilum of the lung is the area where the pulmonary vessels and bronchi extend into/out of the lung.
• The left lung has 2 lobes.
• The right lung contains 3 lobes.

Pulmonary Circulation of the Lungs

Innervation of the Lungs

• Innervated by the autonomic nervous system.
• The autonomic nervous system innervates smooth muscles and glands of respiratory structures.
• Bronchioles are innervated by both parasympathetic and sympathetic nervous systems.
• Sympathetic input from T1-T5 generally causes bronchodilation, thus increasing airflow.
• Parasympathetic input from the Vagus nerve causes bronchoconstriction, decreasing airflow.
• The Vagus nerves (2) are also main sources of nerve innervation to the larynx.
• Damage to one of the Vagus nerves going to the larynx can cause a person to develop a monotone or permanently hoarse voice.

Pleural Membranes and Pleural Cavity

• The pleura has a Visceral pleura, directly against lungs.
• The pleura has a Parietal pleura, on internal thoracic walls, lateral surface of mediastinum and superior surface of diaphragm.
• The Pleural cavity is located between the visceral and parietal serous membranes.
• When lungs are inflated, the pleural cavity is considered a potential space because both layers are almost in contact with each other.

Clinical View: Pleural Effusion vs Pleurisy

• Pleural effusion is excess fluid in the pleural cavity that causes shortness of breath and chest pain.
• Potential causes are systemic factors such as failure of the left side of the heart, pulmonary embolism, and lung infections or cancer.
• Pleurisy is inflammation of the pleural membranes, causing severe chest pain with breathing.
• Inflamed membranes have increased friction between visceral and parietal pleura, usually only on one side.

How Lungs Stay Inflated

• The chest wall is configured to expand outward in order for the lungs to stay inflated.
• Lungs cling to chest wall due to serous fluid's surface tension.
• Elastic tissue of lungs pulls inward when the chest expands.
• Lungs experience an outward pull from the chest wall and inward pull from the elastic tissue.
• This creates a suction (vacuum) effect in the pleural cavity, resulting in the intrapleural pressure (pressure within pleural cavity) being less than the intrapulmonary pressure (pressure within the lungs).
• This difference in pressure keeps the lungs inflated.

Lungs and Atmospheric Pressure

• Transpulmonary pressure is the difference between 760 mm Hg - 756 mm Hg = 4 mm Hg.
• Intrapleural pressure reads 756 mm Hg (-4 mm Hg).
• Intra-alveolar pressure is 760 mm Hg (0 mm Hg).

Clinical Views: Pneumothorax and Atelectasis

• When air enters the pleural space, the lung collapses, which is called pneumothorax if a total collapse is observed.
• If only part of the lung is affected or collapsed, it is referred to as atelectasis.
• Alveoli tend to collapse in atelectasis, whereas penumothorax refers to presence of air in the pleural cavity.
• Atelectasis is caused by obstruction of the airways either by a foreign body, mucus plugs or tumors.