What is the structure of the respiratory system? 

Written by Jarixa Balbuena

Reviewed by Reuben J C. Los Baños, Ph.D.

What is the structure of the respiratory system? The respiratory system is a network of different organs that help you breathe. These organs work together to help with gas exchange, necessary for a functional body. It is also composed of structures that connect the organs. The respiratory system is also surrounded by layers of tissues and blood vessels. These structures help protect and support the respiratory system. 

The general function of the respiratory system is to bring oxygen to the body. Along with this process, waste products like carbon dioxide are also expelled. This complex process happens whenever you breathe in and out. Other functions of the respiratory system include: 

• Gas exchange. This refers to the swapping of oxygen and carbon dioxide in the bloodstream. The lungs exchange gases. This way, it delivers oxygen to the body while eliminating waste carbon dioxide. 

• Air filtration. The mucus and cilia in the respiratory tract filter out dust and particles from the air. This ensures that the air we breathe in is free from pathogens and is safe for circulation. 

• Sound production. The larynx or voice box contains the vocal cords that vibrate with air. Respiration enables air to pass along the larynx. The vibration of the vocal cords produces speech and vocal sounds. 

• Olfaction. When we breathe air in, tiny odor particles float up to our nose. Special smell sensors (olfactory nerves) in the nose send a message to your brain. The brain interprets this signal, telling us what the smell is like. 

• Regulation of blood pH. Excess carbon dioxide levels in the blood make it acidic. Through respiration, carbon dioxide exits the body. This mechanism helps regulate blood pH back to safe pH levels for the body. 

A variety of organs, muscles, and tissues make up the respiratory system. We can classify the structures of the system based on its functions and locations. Based on functionality, the system has two components: 

• The respiratory system conducts air through a series of passages. This pathway starts with the nasal cavities and pharynx. It continues down the trachea, bronchi, and then the bronchioles and terminal bronchioles. These structures make up the conducting component of the system. 

• The second component of the system is the respiratory component. This part is where gas exchange occurs firsthand. The respiratory bronchioles, ducts, and alveoli make up this part. 

The system also divides into upper and lower regions based on location. 

• The upper respiratory tract is the first part of our breathing journey. It starts with the nose and mouth where air comes in. After that, air travels through the nasal cavity, sinuses, and larynx. These structures help moisten, warm, and filter air before it proceeds inside the body. 

• The lower respiratory tract includes the trachea, bronchi, and lungs. These organs all work together to take the air to the alveoli in the lungs. It is in the alveoli where gas exchange occurs. 

The system also has a group of muscles that aid in the respiration process. These groups of muscles are the muscles of respiration. The diaphragm, intercostal muscles, and some accessory muscles make up this muscle group. They control the movement of air in and out of the body as you breathe. 

• The diaphragm is a dome-shaped muscle below the lungs. It separates the chest cavity from the abdominal cavity. When it flattens and expands the chest cavity, it creates a vacuum. This vacuum pulls air into the lungs in a process called inhalation. As the diaphragm relaxes, the chest cavity also relaxes. The lungs push the air out from the body, known as exhalation. 

• Intercostal muscles are present between the ribs. These muscles lift the ribs during inhalation to accommodate the lung’s expansion. When the lungs relax during exhalation, these muscles help pull the ribcage downward. 

• Accessory muscles also aid in speech, singing, and breathing. The sternocleidomastoid in the neck helps during heavy breathing. This muscle also aids in forced exhalation such as in coughing or singing. 

What is the respiratory lining of the nasal cavity? 

The epithelium throughout most of the respiratory system is not a passive barrier. It plays a crucial role by being the first line of defense against germs and infections. It also helps keep the airways moist and prevent irritation. 

There are various histological layers within the respiratory system. 

Specialized linings and cells take charge of the respiratory tract. These cells ensure that the inhaled air is clean and safe. Some of these cells help us smell. Others ensure a healthy cell layer in the respiratory tract. Some cells aid in the functioning of the organs in the system. 

Pseudostratified columnar epithelium lines most of the respiratory tract. But, they are not found in the larynx and pharynx. This special lining acts like a double shield. It blocks germs and dust from entering. It also helps prevent infections and irritation. This epithelium has three types of cells present: 

• Goblet cells are responsible for mucus production. Mucus helps trap particles and pathogens. It also keeps a healthy level of moisture in the airways. This moisture prevents the respiratory tract from drying out and irritation. 

• Basal cells undergo constant division to replace other epithelial cells lining the airways. This ensures a healthy and functional layer of cells throughout the respiratory tract. 

• Cilia are millions of tiny hairs on the surface of respiratory airways. They beat in a constant wave-like motion. The rhythmic beating of cilia propels mucus and trapped particles towards the throat. This allows you to cough it out or swallow it. The mucus that reaches your stomach gets broken down. 

Alveolar epithelial cells (AECs) line the tiny, spongy air sacs throughout your lungs. These air sacs are the alveoli. The cells in this area take part in the easy diffusion of gases for gas exchange. There are two types of AECs: 

• Alveolar Epithelial Cell I (AEC I) covers the majority of the alveolar surface. These cells are vital in the gas exchange process around the alveoli. 

• Alveolar Epithelial Cell II (AEC II) contributes to lung defense. This alveolar epithelial cell is also subject to further studies. They also have regenerative potential. 

What is the main function of the respiratory epithelium in the nasal cavity? 

The respiratory epithelium acts as a guardian to the rest of the respiratory system. It filters, warms, and humidifies the air we breathe in before it reaches the lungs. It is important to warm the air we breathe in. 

The lungs work best when they receive air that is close to our body temperature. Cold air may cause our airways to constrict. Warming the air before gas exchange prevents this constriction. This ensures efficient oxygen delivery to the cells of the body. 

Warm and moist air prevents irritation to the delicate tissues lining the airways. The tissues moisten and warm the air to protect the respiratory linings from damage. The vasculature of the alveoli and capillaries is extra delicate. Warming the air lessens the likelihood of causing harm to these structures. 

The respiratory epithelium also relies on mucus and cilia to trap particles. This mucociliary clearance system is most effective at a specific consistency. Cold air dries out the mucus, making it thicker and stickier. This hinders the cilia’s ability to propel particles. 

These functions of the respiratory epithelium contribute to respiratory health. 

What is the histology of the olfactory mucosa? 

The olfactory mucosa has a specialized histological structure to detect smells. It is on the roof of the nasal cavity. This structure covers the superior nasal concha and the upper part of the nasal septum. The olfactory mucosa has two histological components: 

The olfactory epithelium is a pseudostratified columnar epithelium. It has three main cell types: 

• Olfactory Receptor Neurons (ORNs) are bipolar neurons. They extend to the epithelial surface and end in the cilia. These cilia contain receptors for smell molecules. 

• Supporting (Sustentacular) cells are columnar cells. They provide structural support, nourishment, and insulation to the olfactory receptor neurons. They also help detoxify harmful substances through their metabolic activities. 

• Basal cells are stem cells located at the base of the epithelium. They can differentiate into new olfactory receptor neurons or supporting cells. These cells maintain the regenerative capacity of the olfactory epithelium. 

The lamina propria is a connective tissue beneath the olfactory epithelium. It contains the following structures: 

• Bowman’s Glands. These glands produce mucus that is secreted onto the olfactory epithelium. The secreted mucus helps pick up the smell. It also flushes out old smells to detect new smells. 

• Blood Vessels and Nerves. The lamina propria is a vascularized structure. The blood vessels provide nutrients and support to the epithelial cells. 

This histological structure enables the olfactory mucosa to detect and process odorant molecules. This mechanism plays a crucial role in the sense of smell. 

What are the histological layers of the trachea? 

The trachea is also known as the windpipe. This tubular structure provides a passageway for air to enter and exit the lungs. The histological layers of the trachea are: 

• Mucosa. The mucosa is the innermost lining of the trachea. It has pseudostratified ciliated columnar epithelium. This layer also contains goblet cells and the lamina propria. 

• Submucosa. This layer has a dense connective tissue compared to the lamina propria. It contains glands that produce a mixture of serous (watery) and mucous (viscous) fluids. These secretions help to moisten the air and trap particles to protect the airways. 

• Cartilaginous layer. The trachea has C-shaped rings of hyaline cartilage. The cartilage rings provide structural support. They prevent the trachea from collapsing while allowing flexibility. 

• Adventitia. The outermost layer of the trachea is the adventitia. This layer consists of loose connective tissue. This layer also has blood vessels, nerves, and adipose tissue. It anchors the trachea to adjacent tissues and organs in the neck and thorax. 

What epithelium lines the bronchioles? 

Bronchioles are tiny branching tubes from the bronchi in your lungs. They are smaller than bronchi. This structure handles the delivery of air to the alveoli for gas exchange. The epithelium lining the bronchioles varies depending on the size of the bronchioles. 

Ciliated simple columnar epithelium lines the larger bronchioles. This epithelium type moves mucus and trapped particles upward. Once the trapped particles are in the bronchi and trachea, they expel these particles. 

Smaller bronchioles ( or terminal bronchioles) have ciliated simple cuboidal epithelium. The cilia continue to play a role in moving mucus and trapped particles. However, cilia in this part of the respiratory system are shorter. 

The respiratory bronchioles are the smallest branch. Simple cuboidal to simple squamous epithelia line this structure. The transition to simple epithelium facilitates gas exchange.

Where are the alveoli found? 

The alveoli are also known as air sacs. They are in alveolar ducts, sacs, and the terminal parts of the respiratory tract. Alveoli are also present in the lungs within the lung parenchyma. They are the primary sites for gas exchange between oxygen and carbon dioxide. 

The lung parenchyma is a functional tissue of the lung involved in gas exchange. The lungs have respiratory bronchioles, alveolar ducts, and alveolar sacs. All these structures have alveoli. 

There are also alveoli found at the terminal portions of the respiratory tract. This is the area where the bronchioles transition to alveolar ducts and sacs. Alveolar sacs are a group of alveoli clustered together. They open into alveolar ducts. 

Alveolar ducts are the passages that connect respiratory bronchioles to alveolar sacs. They also contain many alveoli along their walls. 

How many alveoli are in the lungs? 

The human lungs have about 300 to 500 million alveoli. This extensive network of alveoli provides a large surface area for gas exchange. An efficient functioning alveoli is crucial for acquiring oxygen and eliminating CO2 waste. 

The main function of the alveoli is to help the exchange of oxygen and carbon dioxide in the body. The large number of alveoli provides a bigger surface area for this exchange to occur. Alveoli in the lungs has an estimated surface area of 70 to 100 square meters. These numbers are about the size of a tennis court. The large surface area allows for rapid diffusion of gases. 

Also, the vast number of alveoli helps the lungs adapt to sudden respiratory changes. The millions of alveoli ensure the body has a reserve to compensate for damaged ones. 

When many alveoli get damaged, surface area decreases. This leads to a decrease in the efficiency of gas exchange. Very few alveoli counts could also result in low O2 levels in the blood, known as hypoxemia. 

The body may attempt to compensate for low oxygen by increasing respiration. However these mechanisms may not be enough to maintain normal gas exchange over the long term. 

The alveoli in the lungs are essential for maximizing gas exchange. It also ensures efficient respiratory function. Decreased alveolar number or function can lead to impaired gas exchange. Respiratory insufficiency could also happen. This insufficiency could contribute to the development of various respiratory diseases and symptoms. 

Conclusion 

The respiratory system is a complex network of organs and tissues. It works day in and day out to keep us alive. Its primary function is to bring in oxygen and expel carbon dioxide, a waste product. This gas exchange happens in the millions of tiny air sacs called alveoli within the lungs. 

A healthy respiratory system is the foundation of our well-being. Neglecting it through unhealthy choices can lead to a lifetime of struggle. 

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