Pleura: Visceral and Parietal Layers

Pleura General Information

• The pleura is a thin membrane covering each lung and surrounding the pulmonary cavity.
• It is subdivided into two layers: the visceral pleura and the parietal pleura.
• The visceral pleura adheres to the lungs.
• The parietal pleura lines the pulmonary cavity.
• The Pleural cavity is a space between the two pleural layers
• It is normally filled with a thin film of fluid that lubricates the pleural surfaces.
• The fluid allows the pleural layers to slide smoothly over each other during each breath.

One Continuous Pleural Layer

• The visceral pleura is continuous with the parietal pleura at the hilum of the lung, forming one continuous layer but are located on different surfaces.
• The visceral pleura covers and adheres to the entire lung surface and adheres to the fissures of each lung.
• The parietal pleura lines and adheres to the pulmonary cavity, thoracic wall, mediastinum, and diaphragm, and is thicker than the visceral pleura.

Trachea General Information

• The trachea is a long cartilaginous tube and a part of the lower respiratory tract.
• It connects the inferior part of the larynx with the bronchi of the lungs.
• The trachea begins at the lower border of the larynx, at the level of the sixth cervical vertebra.
• It ends at the level of the 5th-7th thoracic vertebrae, where it bifurcates into the bronchi.
• The cricotracheal ligament connects the cricoid cartilage to the trachea, connecting the upper and lower respiratory tracts together.
• The trachea is 9-15 centimeters long and made up of a series of cartilages.

Tracheal Cartilages and Posterior Wall

• The tracheal cartilages are incomplete and C-shaped.
• The trachea is comprised of 15-20 tracheal rings, each made from hyaline cartilage, providing structural rigidity.
• Annular ligaments are circular horizontal fibrous bands that keep the tracheal rings together and provide further support to the tracheal wall.
• The posterior membranous wall of the trachea covers around 1/3 of the ring's diameter and consists of longitudinal elastic fibers.
• The trachea's posterior membranous wall makes the trachea a flexible tube, enabling it to stretch and shift inferiorly during inspiration.

Tracheal Glands and Carina

• Tracheal glands are located in the submucosa of the trachea and are exocrine glands.
• Tracheal glands secrete a combination of water and mucus through narrow ducts.
• Watery secretions humidify the inspired air and mucus traps particles.
• Trapped particles are transported upwards towards the pharynx by the cilia on the epithelium, keeping the lungs free of particles and bacteria.
• The carina is a ridge located at the base of the trachea that separates the openings of the right and left main bronchi.
• The carina's mucous membrane is one of the sites that contributes to the cough mechanism.
• At the level of the carina, the trachea bifurcates into the two main bronchi.
• The carina is located at the level of the sternal angle or the 4th or 5th vertebral body.
• The trachea is displaced slightly to the right by the arch of the aorta on its left.
• After bifurcation, the two main bronchi each enter the root of each lung passing through the hilum.

Bronchi General Information

• The main bronchi (primary bronchi) are the first branches of the trachea and the first component destined for a specific lung.
• The right main bronchus is wider and shorter than the left, takes a more vertical course through the root and the hilum, and enters the right lung at the fifth thoracic vertebra.
• The left main bronchus is smaller and longer than the right main bronchus, almost 5 centimeters long, more horizontal, and enters the left lung at the sixth thoracic vertebra.

Lobar and Segmental Bronchi

• Lobar bronchi (secondary bronchi) are intrapulmonary and enter the lungs where each one travels to a lobe of the lung.
• The right side has three lobar bronchi for the right superior, right middle, and right inferior lobes of the right lung which has 3 lobes.
• The left side has two lobar bronchi for the left superior and left inferior lobes. The left lung has two lobes.
• Segmental bronchi (tertiary bronchi) are branches of the lobar bronchi and travel to segments of the lung lobes and supply a bronchopulmonary segment.
• A bronchopulmonary segment is a division of the lung separated by connective tissue.
• The right lung has 10 segmental bronchi and the left lung has 8 segmental bronchi
• The segmental bronchi divide into smaller branches called bronchioles.

Bronchial Wall Characteristics

• The mucous membrane of the bronchial wall and trachea wall are the same.
• The bronchial wall has layers of smooth muscle.
• The rings of cartilage decrease with increasing division of the bronchi.
• The bronchial walls have ciliated epithelium.

Lung and Pleura

• The Pleura is a serous membrane that covers the surface of the lung and deep into the fissures between its lobes.
• The visceral pleura is the part of the pleura that is attached directly to the lungs.
• The parietal pleura is the part of the pleura attached to the thoracic cavity.

Lung Lobes and Fissures

• The right lung has 3 lobes: superior, middle, and inferior.
• The left lung has 2 lobes: superior and inferior.
• The horizontal fissure is only in the right lung, separates the superior lobe from the middle lobe of the right lung and extends along the line of the 4th rib
• The oblique fissure is in both lungs.
• In the right lung, the oblique fissure separates the middle lobe from the inferior lobe.
• In the left lung, the oblique fissure separates the superior lobe from the inferior lobe.

Pulmonary Apex Information

• The pulmonary apex of the lung extends to the thoracic inlet and extends beyond the first rib.

Bronchioles

• Bronchioles are beyond the tertiary bronchi.
• Bronchioles continue to branch and branch 20-25 times before they end.
• Conducting bronchioles transport air.
• Terminal bronchioles are the last and smallest conducting bronchioles.
• Respiratory bronchioles are distal to terminal bronchioles, terminate with alveoli, and are not considered conducting bronchioles.

Alveoli Information

• Alveoli are the end of the respiratory tract and the unit that facilitates gas exchange.
• The alveolar sac is inside of alveolus.
• Pores of Kohn are connections between alveoli that allow for direct communication between adjacent alveoli.
• There are multiple cell types in the alveoli including Pneumocytes and Alveolar macrophages

Type I and II Pneumocytes

• Type I Pneumocytes form 90% of the surface area lining the alveoli and 40% of the total cells and are very thin
• Type I facilitate gas exchange
• Type II Pneumocytes form 10% of the surface area lining the alveoli and 60% of the total cells. Type II Pneumocytes are round shaped
• Type II Pneumocytes have cytoplast rich in mitochondria, rough and smooth endoplasmic reticulum
• Type II Pneumocytes secrete surfactant, which lines the alveolar walls.
• Surfactant lowers surface tension to prevent the alveoli from collapsing during expiration and expand during inspiration.
• Alveolar Macrophages phagocytose bacteria, toxic particles, or other foreign bodies.

Bronchial Nerves Information

• Bronchial nerves carry fibers from the parasympathetic nervous system, which causes contraction of the bronchial tree
• Bronchial nerves carry fibers from the sympathetic nervous system which causes bronchodilation

Vascular Structures

• Pulmonary arteries carry deoxygenated blood away from the heart and branch into smaller pulmonary arterioles, which are smaller and thinner.
• The capillaries are the smallest and most abundant type of blood vessel.
• Capillaries connect arterioles to the venous system
• Capillary walls are very thin to allow for gas molecules to pass through via passive diffusion.
• Oxygen diffuses from alveoli into the capillaries and carbon dioxide diffuses from capillaries into the alveoli.

Pulmonary Venules and Bronchial Arteries

• Pulmonary venules are created by the joining of 2+ capillaries and blood travels towards the pulmonary veins from them.
• Bronchial arteries arise from the thoracic aorta, bring oxygen and nutrient-rich blood to the bronchi and connective tissue of the lungs, and branch in a similar pattern to the bronchi and bronchioles.
• Bronchial arteries give off small branches along the way to supply the cells of these structures.

Bronchial Veins

• Bronchial veins collect deoxygenated blood from the bronchial tree.
• Some bronchial veins may drain into the pulmonary veins, adding a bit of deoxygenated blood into that oxygen-rich blood.
• Some bronchial veins drain into the azygous or the hemizygous veins.