4. Circulatory and Respiratory system

Lower respiratory tract

The lower respiratory tract refers to the parts of the respiratory system that lie below the cricoid cartilage and vocal cords, including the inferior part of the larynxtracheobronchial tree and lungs.

4. Circulatory and Respiratory system


Larynx; Image: Begoña Rodriguez


Following the laryngopharynx, the next and last portion of the upper respiratory tract is the superior part of the larynx. The larynx is a complex hollow structure found anterior to the esophagus. It is supported by a cartilaginous skeleton connected by membranes, ligaments and associated muscles. Above the vocal cords, the larynx is lined with stratified squamous epithelium like the laryngopharynx. Below the vocal cords, this epithelium transitions into pseudostratified ciliated columnar epithelium with goblet cells (respiratory epithelium). 

Besides its main function to conduct the air, the larynx also houses the vocal cords that participate in voice production. The laryngeal inlet is closed by the epiglottis during swallowing to prevent food or liquid from entering the lower respiratory tract.

If you want to learn more about the anatomy and function of the larynx, take a look at the study unit below!

4. Circulatory and Respiratory system


Nasopharynx (Pars nasalis pharyngis); Image: Begoña Rodriguez

NasopharynxPars nasalis pharyngis1/6Synonyms: Nasal part of pharynx, Nasopharynx,

After passing through the nasal cavity and paranasal sinuses, the inhaled air exits through the choanae into the pharynx. The pharynx is a funnel-shaped muscular tube that contains three parts; the nasopharynx, oropharynx and laryngopharynx. 

  • The nasopharynx is the first and superiormost part of the pharynx, found posterior to the nasal cavity. This part of the pharynx serves only as an airway, and is thus lined with respiratory epithelium. Inferiorly, the uvula and soft palate swing upwards during swallowing to close off the nasopharynx and prevent food from entering the nasal cavity.
  • The oropharynx is found posterior to the oral cavity and communicates with it through the oropharyngeal isthmus. The oropharynx is a pathway for both the air incoming from the nasopharynx and the food incoming from the oral cavity. Thus, the oropharynx is lined with the more protective non-keratinizing stratified squamous epithelium.
  • The laryngopharynx (hypopharynx) is the most inferior part of the pharynx. It is the point at which the digestive and respiratory systems diverge. Anteriorly, the laryngopharynx continues into the larynx, whereas posteriorly it continues as the esophagus. 
4. Circulatory and Respiratory system

Paranasal sinuses

Paranasal sinuses (Sinus paranasales); Image: Begoña Rodriguez

Paranasal sinusesSinus paranasales1/5

Several bones that form the walls of the nasal cavity contain air-filled spaces called the paranasal sinuses, which are named after their associated bones; maxillaryfrontalsphenoidal and ethmoidal sinuses.

The paranasal sinuses communicate with the nasal cavity via several openings, and thereby also receive the inhaled air and contribute to its humidifying and warming. In addition, the mucous membrane and respiratory epithelium that lines both the nasal cavity and the paranasal sinuses traps any harmful particles, dust or bacteria.

4. Circulatory and Respiratory system

Upper respiratory tract

The upper respiratory tract refers to the parts of the respiratory system that lie outside the thorax, more specifically above the cricoid cartilage and vocal cords. It includes the nasal cavityparanasal sinusespharynx and the superior portion of the larynx. Most of the upper respiratory tract is lined with the pseudostratified ciliated columnar epithelium, also known as the respiratory epithelium. The exceptions are some parts of the pharynx and larynx.

Nasal cavity

Nasal cavity (Cavitas nasi); Image: Begoña Rodriguez

Nasal cavityCavitas nasi1/6Synonyms: Nasal cavity proper, Cavum nasi

The upper respiratory tract begins with the nasal cavity. The nasal cavity opens anteriorly on the face through the two nares, and posteriorly into the nasopharynx through the two choanae. The floor of the nasal cavity is formed by the hard palate, while the roof consists of the cribriform plate of the ethmoid bone posteriorly, and the frontal and nasal bones anteriorly. The nares and anterior portion of the nasal cavity contain sebaceous glands and hair follicles that serve to prevent any larger harmful particles from passing into the nasal cavity. 

The lateral walls of the nasal cavity contain three bony projections called nasal conchae (superior, middle and inferior), which increase the surface area of the nasal cavity. The nasal conchae also disrupt the laminar flow of air, making it slow and turbulent, thereby helping to humidify and warm up the air to body temperature. 

The roof of the nasal cavity contains the olfactory epithelium which consists of specialized sensory receptors. These receptors pick up airborne odorant molecules and transform them into action potentials that travel via the olfactory nerve to the cerebral cortex, allowing the brain to register them and provide a sense of smell.

Another pathway for the entry of air is the oral cavity. Although it is not classified as a part of the upper respiratory tract, the oral cavity provides an alternative route in the case of obstruction of the nasal cavity. The oral cavity opens anteriorly on the face through the oral fissure, while posteriorly, it opens into the oropharynx through a passage called the oropharyngeal isthmus. 

4. Circulatory and Respiratory system

Respiratory system

The respiratory system, also called the pulmonary system, consists of several organs that function as a whole to oxygenate the body through the process of respiration (breathing). This process involves inhaling air and conducting it to the lungs where gas exchange occurs, in which oxygen is extracted from the air, and carbon dioxide expelled from the body. The respiratory tract is divided into two sections at the level of the vocal cords; the upper and lower respiratory tract.

The lungs are most often considered as part of the lower respiratory tract, but are sometimes described as a separate entity. They contain the respiratory bronchiolesalveolar ductsalveolar sacs and alveoli

This article will discuss the anatomy and function of the respiratory system.

Upper respiratory tractNasal cavity, paranasal sinuses, pharynx and larynx above the vocal cords
Lower respiratory airwaysLarynx below the vocal cords, trachea, bronchi, bronchioles and lungs
FunctionsUpper respiratory tract: conduction, filtration, humidification and warming of inhaled air
Lower respiratory tract: conduction and gas exchange
4. Circulatory and Respiratory system

Do arteries always carry oxygenated blood?

For the most part, yes. The exceptions are pulmonary arteries and veins. Pulmonary arteries carry deoxygenated blood to the lungs. Pulmonary veins return the oxygenated blood to the heart.

A note from Cleveland Clinic

Your circulatory system plays a critical role in keeping you alive. Blood vessels carry blood to the lungs for oxygen. Then your heart pumps oxygen-rich blood through arteries to the rest of the body. Your veins help your body get rid of waste products. Conditions like high blood pressure, high cholesterol and atherosclerosis can affect the health of your circulatory system. If you have one of these conditions, talk to your healthcare provider about steps you can take to protect your cardiovascular health.

4. Circulatory and Respiratory system

What is red blood and blue blood?

All blood is red. Hemoglobin, an iron-rich protein in red blood cells, mixes with oxygen to give blood its red color. Blood that’s rich in oxygen is known as red blood.

Your veins carry oxygen-poor blood. This is sometimes called blue blood because your veins can look blue underneath the skin. The blood is actually red, but the low oxygen levels give veins a bluish hue.

4. Circulatory and Respiratory system

How big is the circulatory system?

Your body has more than 60,000 miles of blood vessels that circulate about 1.5 gallons of blood every day.

4. Circulatory and Respiratory system

How can I prevent circulatory system problems?

These steps can protect the health of your circulatory system:

  • Aim for at least 150 minutes of physical activity every week.
  • Eat a heart-healthy diet rich in vegetables and fiber and low in saturated fats and processed foods. Consider a Mediterranean-style diets or plant-based diet, as they appear to be the most heart healthy.
  • Find healthy ways to ease stress.
  • Maintain a healthy weight.
  • Manage conditions like diabetes, high blood pressure and high cholesterol.
  • Get help to quit smoking.