Trachea Anatomy

Trachea Gross Anatomy

• The trachea is a long cartilaginous tube and a component of the lower respiratory tract.
• It links the lower part of the larynx to the lungs' bronchi.
• The trachea starts at the sixth cervical vertebra, at the lower border of the larynx.
• The trachea concludes at the fifth to seventh thoracic vertebrae, where it divides into the bronchi.
• The cricotracheal ligament joins the cricoid cartilage to the trachea.
• The cricotracheal ligament's strength is important, as it connects the upper and lower respiratory tracts.
• The trachea stretches 9-15 cm long.
• It is composed of a chain of cartilages.
• Tracheal cartilages are incomplete and C-shaped, with 15-20 rings.
• Each ring contains hyaline cartilage, which provides structural support.
• Annular ligaments are circular fibrous bands that hold the tracheal rings in position.
• These ligaments lend support to the tracheal wall.
• The posterior membranous wall makes up roughly 1/3 of the ring diameter with longitudinal elastic fibers.
• These fibers provide flexibility, allowing trachea stretching and inferior movement during inhalation.
• Tracheal glands are located in the submucosa and are exocrine glands.
• These glands produce watery and mucous secretions.
• Watery secretions moisten inhaled air, whereas mucus traps airborne particles.
• Cilia then transport these particles to the pharynx, keeping lungs clear of particles and germs.
• The carina can be found at the base of the trachea.
• It is a ridge that separates the openings of the main bronchi.
• The mucous membrane of the carina is part of the cough mechanism.
• The trachea divides into the two main bronchi at the carina.
• It is located at the sternal angle around the 4th or 5th vertebral body.
• The aortic arch slightly displaces the trachea to the right on its left side.
• The main bronchi enter the root of each lung, passing through the hilum after the bifurcation.

Bronchi Gross Anatomy

• There are three levels of bronchi: main, lobar, and segmental.
• The right main bronchus is shorter and larger than the left, with a more vertical orientation.
• It reaches the right lung at the fifth thoracic vertebra.
• The left main bronchus is almost 5 cm long, smaller, longer, and more horizontal compared to the right.
• It enters the left lung at the sixth thoracic vertebra.
• Lobar bronchi are intrapulmonary and enter the lungs with three on the right (superior, middle, inferior).
• Two lobar bronchi are on the Left (superior, inferior).
• Segmental bronchi supply a bronchopulmonary segment, a lung division separated by connective tissue.
• There are 10 segmental bronchi on the right and 8 on the left.
• Segmental bronchi divide further into bronchioles, then alveoli.
• The bronchial wall has the same mucous membrane as the trachea.
• The bronchial wall has layers of smooth muscle.
• Cartilage rings reduce as the bronchi divide and have ciliated epithelium.

Bronchi

• Main bronchi are the first branches of the trachea leading to a specific lung.
• Each lobar bronchus goes to a lobe of the lung.
• The right lung has three lobar bronchi, and the left has two.
• Segmental bronchi are branches of the lobar bronchi.

Bronchioles

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

Alveoli

• Alveoli are the endpoint of the respiratory tract where gas exchange occurs.
• The alveolar sac is inside the alveolus.
• Pores of Kohn connect alveoli, allowing communication between adjacent alveoli.
• Cell types include:
  • Type I pneumocytes:
    • Forms 90% of the surface area lining the alveoli, 40% of cells overall.
    • Very thin to facilitate gas exchange.
  • Type II pneumocytes
    • Forms 10% of surface area lining the alveoli, 60% of cells overall.
    • Round shape and secrete surfactant.
    • Cytoplast rich in mitochondria, rough and smooth endoplasmic reticulum
    • Surfactant lowers surface tension and prevents alveolar collapse during expiration, assisting walls to expand during inspiration.
  • Alveolar macrophages
    • Phagocytose bacteria, toxic particles, or other foreign bodies.
• Bronchial nerves carry parasympathetic fibers, causing bronchial tree constriction.
• The sympathetic nerves cause bronchodilation.

Vascular Structures

• The pulmonary artery transports deoxygenated blood away from the heart, branching into smaller pulmonary arterioles.
• Capillaries connect arterioles to the venous system.
• Thin walls allow gas exchange through passive diffusion.
• Oxygen diffuses from alveoli into capillaries, while carbon dioxide diffuses from capillaries into alveoli.
• Pulmonary venules are formed by the joining of capillaries, transporting blood to the pulmonary veins.
• Bronchial arteries arise from the thoracic aorta.
• Deliver oxygen and nutrient-rich blood to the bronchi and lung's connective tissue.
• These branch similarly to bronchi, supplying cells in these structures along the way.
• Bronchial veins collect deoxygenated blood from the bronchial tree.
• Some drain into pulmonary veins and some drain into azygous or hemizygous veins.

Pleura

• A serous membrane covers the lung surface and extends into the fissures between its lobes.
• Visceral pleura attaches directly to the lungs.
• Parietal pleura attaches to the thoracic cavity.

Lungs

• The right lung possesses three lobes: superior, middle, and inferior.
• The left lung possesses two lobes: superior and inferior.

Lung Fissures

• The horizontal fissure is only in the right lung.
• The horizontal fissure separates the superior lobe from the middle lobe of the right lung and follows the line of the 4th rib.
• Oblique fissures are in both lungs.
  • In the right lung, it separates the middle lobe from the inferior lobe.
  • In the left lung, it separates the superior lobe from the inferior lobe.
• The pulmonary apex extends to the thoracic inlet and extends beyond the first rib.

General (Pleura)

• The pleura is a thin membrane covers each lung and surrounds the pulmonary cavity.
• There are two layers: visceral and parietal.
• Visceral pleura adheres to the lungs.
• Parietal pleura lines the pulmonary cavity.
• The pleural cavity is the space between the layers that contains a thin fluid that lubricates the pleural surfaces for smooth sliding during breathing.

One Continuous Layer (Pleura)

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

Histology of the Trachea

• The trachea contains 16-20 C-shaped tracheal cartilages composed of hyaline cartilage on the anterior and lateral walls.
• The posterior wall consists of the trachealis muscle.
• The four layers of the trachea include:
  • Mucosa
    • Ciliated, pseudostratified epithelium with goblet cells, also known as respiratory epithelium.
    • Lamina propria.
  • Submucosa
    • Connective tissue.
    • Contains glands.
  • C-shaped cartilages
    • Made up of hyaline cartilage.
    • Surrounded by perichondrium.
  • Adventitia
    • Connective tissue that binds the trachea to surrounding structures.
    • With blood vessels and nerves.

Trachealis Muscle

• A fibroelastic membrane spans the gap between the posterior ends of the hyaline cartilage which contains the trachealis muscle.
• Most trachealis muscle fibers insert into perichondrium which covers the hyaline cartilage.

Cilia

• Ciliated cells are the most numerous cell types and project from the apical surface.
• Each cell possesses around 250 cilia.
• Cilia provide a sweeping motion of mucous from the bottom airways up to the pharynx which removes inhaled particles from the lower areas and lungs.

Histology of the Bronchi

• Initially, bronchi share a similar histologic structure to the trachea.
• As bronchi enter the lungs the wall structure changes where cartilage rings are replaced with cartilage plates.
• As the bronchi decrease in size from branching, the cartilage plates get smaller and less numerous.
• Smooth muscle that forms a complete circumferential layer gets added around the bronchus.
• Airways can be identified by the cartilage plates and a circular layer of smooth muscle.
• The layers of the bronchus are:
  • Mucosa
    • Respiratory epithelium consisting of pseudostratified ciliated columnar cells with goblet cells.
  • Muscularis
    • Continuous layer of smooth muscle.
    • Contraction regulates the airway diameter.
  • Submucosa
    • Loose connective tissue that blends with the adventitia.
    • Bronchial glands are scattered throughout.
  • Cartilage plates
    • Made of hyaline cartilage.
    • Perichondrium covers each cartilage plate.

Histology of the Bronchioles

• Bronchioles measure 1 mm or less in diameter.
• Larger bronchioles branch from the segmental bronchi, repeatedly branching to form smaller terminal bronchioles.
• Terminal bronchioles then form respiratory bronchioles.
• There are no cartilage plates and glands in bronchioles.
• Larger bronchioles start with a ciliated, pseudostratified columnar epithelium that turns to a simple ciliated columnar epithelium.
• The thick smooth muscle layer is present in the wall of all bronchioles.

Terminal Bronchioles

• The terminal bronchioles show the smallest conducting passageways for air.
• There is a simple columnar epithelium lining, possibly simple cuboidal in the smallest ones.
• A well-developed smooth muscle layer with mucosal folds surrounds the smooth muscle contractions.
• Terminal bronchioles are surrounded by interalveolar septa which surround lung alveoli.
• There is an absence of cartilage plates, glands, and goblet cells

Respiratory Bronchioles

• The respiratory bronchioles come from terminal bronchioles and form part of the respiratory part of the respiratory system.
• The wall of the respiratory bronchiole consists of a simple cuboidal epithelium.
• Thin layer of smooth muscle surrounds epithelium
• Alveolar outpocketings exist in the wall of each respiratory bronchiole.
• Each respiratory bronchiole forms an alveolar duct with alveoli.

Histology of the Alveoli

• Adjacent alveoli contain an interalveolar septum.
• The interalveolar septa include:
  • Type I pneumocytes, thin and squamous cells.
  • Capillaries.
  • Fine connective tissue fibers.
  • Fibroblasts.
• Type II pneumocytes are interspersed among the type I pneumocytes.
• The alveolar also contains macrophages.
• Alveoli are the terminal air spaces and actual site of gas exchange between air and blood and are surrounded and separated by the interalveolar septum containing blood capillaries.
• Approximately 150–250 million alveoli are in each adult lung, resulting in a combined internal surface area of approximately 75 m2.
• The alveolar epithelium forms a vulnerable interface that has continuous exposure to inhaled particles, pathogens, and toxins.

Type I Pneumocytes

• Comprise 40% of the cells overall but line 95% of the alveolar surface and are very thin squamous cells
• These cells connect one another by occluding junctions, forming a barrier between the air space and the septal wall.

Type II Pneumocytes

• These are secretory cells accounting for 60% of the alveolar-lining cells covering 5% of alveolar air surface.
• Cytoplasm contains granules with phospholipids, lipids, and proteins, secreted by exocytosis into the alveoli as surfactant.
• Surfactant lines the alveoli walls, decreases surface tension, and removes foreign materials, preventing alveolar collapse with each exhalation.

Alveolar Macrophages

• Removes inhaled particulate matter from the air spaces, scavenging surfaces and removing inhaled matter and phagocytosing infectious organisms.

Gas Diffusion

• Gases move across the interalveolar septum consisting of thin layer of surfactant.
• A type I epithelial cell and its basal lamina, and a capillary endothelial cell and its basal lamina.
• These basal laminae are usually fused together.

Nervous Supply of the Lungs

• Nerves from the sympathetic and parasympathetic divisions of the autonomic nervous system mediate reflexes to modify air passages and blood vessels by contracting smooth muscle.
• The autonomic nervous system controls glandular secretion of the respiratory mucosa.

3 Mechanisms of Carbon Dioxide Transport

• Dissolved in plasma which accounts for 5% of total CO2 transported by the blood.
• CO2 binds to amino acids on globin chains, accounting for 3% of total CO2.
• Each hemoglobin can bind 4 molecules of CO2.
• The product of hemoglobin bound to CO2 = Carbaminohemoglobin, decreases hemoglobin’s affinity for oxygen.
• Oxygen is unloaded and dropped off in tissues full of CO2.
• Causes a shift to the right in the oxygen-hemoglobin dissociation curve.
• CO2 turns into a bicarbonate ion (HCO3-) and accounts for 90% of total CO2.
• These reactions are reversible.
• The enzyme carbonic anhydrase in RBC produces a large amount of HCO3- and H+.
• Bicarbonate ions move into the plasma with Cl- coming into the RBC in exchange for HCO3- going into the plasma during the chloride shift.
• The HCO3- plays a key role in maintaining physiologic H+ levels in the blood following LeChatelier’s principle.
  • When hydrogen ion concentration gets low, the equation moves to the right which makes more bicarbonate and hydrogen ions.
  • Hydrogen ions (when in high concentrations) binds to bicarbonate and becomes carbonic acid splitting into water and carbon dioxide.
  • The concentration of hydrogen ions balances out preventing pH shifts.

Oxygen-Hemoglobin Dissociation Curve

• Shows how hemoglobin saturation with oxygen (SO2) is related to the partial pressure of oxygen in the blood (PO2).
• The main factor that influences oxygen saturation is the partial pressure of oxygen in the blood.

Binding of Hemoglobin to Oxygen

• Each hemoglobin protein can bind 4 molecules of oxygen.
• Hemoglobin isn’t always 100% saturated or bound by oxygen.
• Deoxyhemoglobin lacks bound oxygen with 0% saturation.
• Oxyhemoglobin has bound oxygen.
• Hemoglobin has an increasing affinity for O2 as the number of bound O2 molecules goes up.
• Around 60mmHg, vast majority of hemoglobin subunits have bound oxygen leveling off the curve.
  • Arterial blood: has partial pressure of oxygen at 100mmHg with hemoglobin fully saturated with oxygen.
  • Venous Blood: has partial pressure of oxygen at 40mmHg with hemoglobin 75% saturated with oxygen causing 25% of oxygen to unload into the tissues.

Factors that Change Hemoglobin’s Affinity for Oxygen

• CO2 produces low pH which causes oxygen to release to tissues from hemoglobin.
  • CO2 in the blood plasma produces increased PCO2 that enters RBCs and activates carbonic anhydrase to create more HCO3- and H+ which reduces pH.
  • Stabilizing the T-state of hemoglobin which reduces hemoglobin's affinity for oxygen and unloads oxygen to tissues most in need.
• Red blood cells produce 2,3-diphosphoglyceric acid (2,3-DPG).
  • In hypoxic conditions 2,3-DPG production increases.
  • 2,3-DPG binds to RBCs and stabilizes T-state of hemoglobin decreasing oxygen affinity and forcing offloading of O2 within tissues.
• Temperature as higher temperatures cause stronger effects on bond between iron and oxygen favoring T-state of hemoglobin.
  • Increasing temperature results in enhanced offloading of O2 meeting the muscle's increasing metabolic needs.
• There are varying levels of hemoglobin
  • Hemoglobin A (HbA) has lower affinity for oxygen than fetal hemoglobin (HbF)

Shifting of the Curve

• Reflects if there is a trend to unload oxygen or not

Shifts to the Right

• Hemoglobin affinity for oxygen decreases.
• Higher PO2 is needed to saturate hemoglobin.
• Shifts from higher PCO2, higher temperatures, and ↑ 2,3-DPG

Shifts to the Left

• Hemoglobin affinity for oxygen increases.
• Lower PO2 is needed to saturate hemoglobin.

Ventilation-Perfusion Ratios

• Alveolar ventilation (V) is amount of air that reaches alveoli in the lungs measured in liters/minute (L/min) which normally equals 4 L/min.
• Perfusion (Q) is pulmonary blood flow that reaches the capillaries surrounding the alveoli measured in L/min and is reflective of cardiac output which normally equals 5 L/min.
• Ventilation/Perfusion (V/Q) Ratio is at 0.8 with upright lungs at rest.

Zones of Lungs

• Blood flow and ventilation are lowest at the apexes and highest at the bases with an upright position due to gravity.
• The V/Q ratio progressively decreases from apex to base.
• Overall V/Q is an average of three zones equal to 0.8 with the zones:
  • Zone 1
    • Apexes Location
    • Lowest Alveolar ventilation
    • Lowest Blood flow
    • 3.6 V/Q
    • 130 mmHg PaO2
    • 28 mmHg PaCO2
  • Zone 2
    • Middle Location
    • V=Q Alveolar ventilation
    • V=Q Blood flow
    • 1 V/Q
    • 108 mmHg PaO2
    • 39 mmHg PaCO2
  • Zone 3
    • Bases Location
    • Highest Alveolar ventilation
    • Highest Blood flow
    • 0.6 V/Q
    • 88 mmHg PaO2
    • 42 mmHg PaCO2

Influence of V/Q Ratio

• The ratio influences how efficiently gases are exchanged in the lungs.
• V/Q ratio in healthy lungs measure:
  • PAO2 = 100 mmHg
  • PACO2 = 40 mmHg
  • PaO2 = 95 mmHg (slightly lower than what’s on the alveolar side)
  • PaCO2 = 40 mmHg (same as what’s on the alveolar side)
• Since Zone 3 has more blood flowing it accounts for more gas exchange

Regulation of Pulmonary Blood Flow

• The direction of blood withing the right ventricle through pulmonary circulation goes:
  • Right ventricle → pulmonary trunk → pulmonary arteries→ pulmonary arterioles → pulmonary capillaries that surround the alveoli

Pulmonary Capillaries

• Drain into small veins connecting the two pulmonary veins exiting each lung that delivers oxygen enriched blood into the left atrium

Pulmonary Blood Flow

• Volume of blood is pumped out of right ventricle over time, (usually 1 minute.
• It is = Cardiac output of the right ventricle. (mL per heartbeat x heart rate (beats per minute).
• Directly proportional to the difference in pressure between the pulmonary artery and the left atrium = ∆P.
• Inversely proportional to the resistance of the pulmonary vasculature (R).
• The blood pressure and resistance in the pulmonary circulation is normally much lower than the systemic blood pressure.

Resistance of Vasculature

• Resistance is controlled by the diameter of the arterioles.
• Decreasing diameter of arterioles causes an increase in resistance to decrease blood flow.
• Increase diameter of arterioles causes an decrease in resistance to increase blood flow.

Total Resistance of Pulmonary Vasculature

• Sum of the resistance of alveolar blood vessels and extra-alveolar blood vessels where:
• Alveolar vessels: tiny capillaries surrounding the alveoli
• Extra-alveolar vessels: those that located further away (arterioles)

Alveolar Blood Vessel Resistance

• Depend on alveolar air pressure by sharing with a basement membrane with alveoli.
  • High alveolar air pressure closes alveolar blood vessel.
  • Low alveolar air pressure opens alveolar blood vessel.

Extra-Alveolar Blood Vessel Resistance

• Depends on pressure in the pleural space.
  • With maximum inspiration of lungs since pleural pressure is negative lung can expands outwards pulling open extra-alveolar vessels decreasing extra-alveolar blood vessel resistance.
  • End of maximum expiration since pleural pressure becomes less negative the extra-alveolar blood vessels are compressed increasing extra-alveolar blood vessel resistance.

Total Pulmonary Resistance

• The sum of alveolar vessel and extra-alveolar resistance.
• Maximum inspiration causes high alveolar resistance.
• Maximum expiration causes high extra-alveolar resistance.
• Lowest total pulmonary resistance results at points just before expiration or inspiration occurs

Regulation of Pulmonary Blood Flow Pulmonary Arteriole Constriction and Dilation

• Vasoconstriction and vasodilation are controlled by the smooth muscle that wraps around the arterioles.
• Vasoconstriction increases resistance due to decreases to the partial pressure of o2 within the alveoli to trigger smooth muscle contraction called hypoxic vasoconstriction
  • In response to alveolar O2 decrease smooth muscles in the arterioles upstream of alveoli vasoconstrict redirecting blood away from damaged area and towards healthy tissue that allow for more effiecient oxygenation
  • Widespread Pulmonary arteriole produces increased pulmonary vascular resistance forcing the right side of the heart has to generate increased pressure
• Vasodilation decreases resistance.
• Metabolites from the blood that affect muscle cells are:
  • Thromboxane A2: vasoconstrictor produced via leukocytes in lung tissues.
  • Prostaglandin I2: vasodilator produced via lung endothelial cells.
  • Nitric oxide: vasodilator.