CHAPTER 23 RESPIRATORY SYSTEM

 

Chemical reactions in cells constantly use oxygen and release carbon dioxide, which is toxic to cells and must be quickly removed. The respiratory system and the cardiovascular system work closely together to carry out these processes. Failure of either system quickly causes disruption of homeostasis, with death of cells from oxygen starvation and toxic buildup.

 

Organs of the respiratory system can be divided several ways:

     1. Structural classification

         a) Upper respiratory system—nose, pharynx, and associated structures

         b) Lower respiratory system—larynx, trachea, bronchi, and lungs

     2. Functional classification

         a) Conducting zone—cavities and tubes that carry air

         b) Respiratory zone—tissues where gas exchange occurs

 

 

ORGANS OF THE SYSTEM

1. NOSE

   a. External portion--framework of bone and hyaline cartilage covered with skin and lined with mucous membrane. Bones involved include the nasal bones, the frontal bone, and the maxillae. Two openings to the outside are the external nares or nostrils.

   b. Internal portion--large cavity in the skull behind the external nose. Communicates posteriorly with the nasopharynx through the internal nares (choanae). Ducts from the paranasal sinuses and the nasolacrimal duct also open into the internal nose. The floor of the internal nose is the hard palate. The ethmoid bone forms the roof.

 

The term nasal cavity refers to the space within the internal nose. It is divided into right and left halves by the nasal septum, formed by cartilage, the vomer, and the perpendicular plate of the ethmoid bone. The anterior portion of the nasal cavity is the vestibule.

 

Functions:

   a. Incoming air is warmed, moistened and filtered (by hairs in the vestibule). The nasal conchae are 3 shelves of bone that extend out of the lateral walls of the cavity and divide each side of the nasal cavity into the superior, middle and inferior meatuses, which twist and turn and give the air further to travel (more time for warming and moistening).

   b. Olfactory stimuli are received—olfactory epithelium located in superior portion of the nasal cavity

   c. Hollow resonating chamber for speech sounds

 

2. PHARYNX (throat)--funnel-shaped tube that starts at the internal nares and extends to the cricoid cartilage of the larynx. Its wall is of skeletal muscle and it is lined with mucous membrane. It functions as a passageway for air and food and as a resonating chamber for speech. 3 portions:

   a. Nasopharynx--posterior to the nasal cavity, extends to the soft palate. The internal nares open into it and also the lower openings of the Eustachian tubes. The pharyngeal tonsil is embedded in the posterior wall. It is lined with pseudostratified ciliated epithelium which helps move mucus downward. This portion is a passageway for air (not normally for food).

   b. Oropharynx--posterior to oral cavity. Extends from soft palate to level of the hyoid bone. The fauces is the opening from the mouth. It is lined with nonkeratinized stratified squamous epithelium and is both respiratory and digestive in function. It contains the palatine and lingual tonsils.

   c. Laryngopharynx--extends downward from the hyoid bone and opens into the esophagus & larynx. It is also both respiratory and digestive and lined with non-keratinized stratified squamous epithelium.

 

3. LARYNX (voice box)--connects pharynx and trachea. The opening at the top is called the rima glottidis. The lining is pseudostratified ciliated epithelium (these cilia sweep mucus upward). The larynx is composed of 9 pieces of cartilage:

   a. Thyroid cartilage (Adam's apple)--single hyaline cartilage which forms the anterior wall of the larynx. Larger in males.

   b. Epiglottis--large leaf-shaped elastic cartilage. The stem is attached to the anterior rim of the thyroid cartilage and the rest is unattached. The epiglottis forms a lid over the glottis during swallowing.

   c. Cricoid cartilage--ring of hyaline cartilage that attaches the bottom of the larynx to the first tracheal cartilage

   d. Arytenoid cartilages (paired)--located at the posterior cricoid. Attach to the vocal cords and pharyngeal muscles and help move the vocal cords. These cartilages are a very important factor in speech. 

   e. Corniculate cartilages (paired)—horn-shaped and located at the apex of each arytenoid

   f. Cuneiform cartilages (paired)—located anterior to the corniculates and support the vocal folds

 

Voice production--the mucous membrane of the larynx forms 2 pairs of folds:

 

   Ventricular folds (false vocal cords)--upper pair—can be brought tightly together & function in holding the breath against pressure in the thoracic cavity. The space between is called the rima glottidis.

 

   Vocal folds (true vocal cords)--lower pair--consist of bands of elastic ligaments covered with mucous membrane and stretched between the thyroid and arytenoid cartilages like guitar strings. Intrinsic muscles of the larynx attach to the cartilage and the vocal cords and change the amount of stretch by contracting or relaxing. We push air upward between the vocal folds and start them vibrating, setting up sound waves (speech).

 

   Intensity (loudness)--greater the pressure of the air the louder the sound

   Pitch--length, thickness and degree of elasticity determine the range possible. Within this range the amount of tension adjusts pitch. The tighter they are pulled the more rapidly they vibrate, giving a higher pitch. With less stretch the vibrations are slower, giving a lower pitch. Males have a lower range than females because the vocal cords are thicker and longer.

 

Speech sounds originate from the vocal cords, but the pharynx, mouth, nasal cavity & paranasal sinuses all act as resonating chambers, giving individual qualities to speech. Muscles of the pharynx, face, tongue, and lips also contribute to clear speech.

 

4. TRACHEA (wind-pipe)--air passage located anterior to the esophagus, extending from the bottom of the larynx to the point where it divides into right and left primary bronchi. The wall consists of mucous membrane, sub-mucosa, hyaline cartilage and an outer layer of areolar CT, the adventitia. The epithelial lining is pseudostratified columnar ciliated. There are 16-20 C-shaped rings of hyaline cartilage which give support and hold the air passage open. The open part of the C faces posteriorly and the esophagus fits there. Smooth muscle fibers and elastic CT extend across the opening.

 

Where the trachea divides into right and left primary bronchi, the last tracheal ring forms a ridge called the carina, which is covered by very sensitive mucous membrane associated with the cough reflex.

 

5. BRONCHI--primary bronchi are formed when the trachea divides. The right primary bronchus is wider, shorter and more vertical than the left. The primary bronchi also contain cartilage rings. After entering the lungs, the bronchi divide to form smaller secondary (lobar) bronchi, one to each lobe of the lung. The secondary bronchi continue to branch, forming tertiary bronchi and then bronchioles. Bronchioles continue to divide into smaller & smaller bronchioles. This branching is called the bronchial tree.

     Bronchoscopy--a bronchoscope is passed through the trachea into the bronchi. The interior can be viewed.

 

Larger bronchi are lined with pseudostratified ciliated epithelium, which gradually changes to simple cuboidal. Cartilage rings taper off & disappear. As the amount of cartilage decreases the amount of smooth muscle increases. Parasympathetic impulses and histamine (released in allergic reactions) cause contraction of the smooth muscle and narrowing of the bronchioles, even to the point of closing them. Sympathetic impulses and epinephrine cause relaxation of the muscle and dilation of the bronchioles.

 

6. LUNGS--paired organs located in the thoracic cavity, enclosed and protected by the pleural membrane.

   Parietal pleura lines the wall of the thoracic cavity

   Visceral pleura adheres to the lungs themselves

 

Between the 2 layers is a potential space, the pleural cavity, which contains a small amount of lubricating fluid. The right and left pleural cavities are completely separate. The pleural cavity can fill with air (pneumothorax), blood (hemothorax), or with pus (empyema) or fluid (pleural effusion). Pleurisy is inflammation of the pleura.

 

The lungs extend from the diaphragm to just above the clavicles and each contains medially a region called the hilum, through which the primary bronchus, pulmonary vessels, lymphatic vessels and nerves enter and leave. The medial aspect of the left lung contains a concavity, the cardiac notch, in which the heart lies.

 

The lungs are divided into lobes by grooves called fissures. The right lung has 3 lobes (superior, middle and inferior) and the left lung has 2 (superior & inferior). Each lobe receives its own secondary (lobar) bronchus. From these arise tertiary bronchi (10 per lung), which each supply an area of lung tissue called a bronchopulmonary segment. Each bronchopulmonary segment contains many smaller lobules, supplied by terminal bronchioles. Terminal bronchioles branch to form respiratory bronchioles, which in turn branch into alveolar ducts. An alveolus is a cup-shaped outpouching of lung tissue where the actual exchange of gases occurs. The lungs contain approximately 300 million alveoli.

 

Alveolar walls consist of:

     Type I alveolar cells (simple squamous pulmonary epithelium)—these are the main lining cells

     Type II alveolar cells (septal cells)--scattered among the type I cells. They secrete a fluid containing surfactant, which keeps the lungs moist and prevents collapse of the alveoli

     Alveolar macrophages (dust cells)--remove inhaled dust & debris

     Elastic fibers

 

Each alveolus is surrounded by a dense network of capillaries.

 

Gas exchange occurs across the respiratory membrane. This consists of the following layers:

   1. Alveolar epithelium wall (mainly simple squamous Type I cells)

   2. Epithelial basement membrane              often fused                        

   3. Capillary basement membrane     

   4. Capillary endothelial cells (simple squamous)

 

The membrane is very thin (about .5 µm or 1/50,000 of an inch) and allows rapid movement of respiratory gases by simple diffusion.

 

Deoxygenated blood enters the lungs through the pulmonary arteries and oxygenated blood returns to the left atrium  through the 4 pulmonary veins. The lungs also receive some oxygenated blood through the bronchial arteries, which branch off the aorta. This blood mainly reaches the walls of the bronchi and bronchioles.