Structure and Function of the Human Respiratory System

Structure and Function of the Human Respiratory System

The human respiratory system is a sophisticated network responsible for the intake of oxygen and the expulsion of carbon dioxide, vital processes for sustaining life. Its efficient function is crucial for cellular metabolism and proper physiological functioning. This article explores the anatomy, physiology, and intricate mechanisms behind the respiratory system, shedding light on how each component works in harmony to facilitate respiration.

Anatomy of the Respiratory System

The human respiratory system can be divided into the upper and lower respiratory tracts, each consisting of various structures that work collaboratively.

Upper Respiratory Tract

1. Nose and Nasal Cavity : The entry point for inhaled air, the nose, and its internal nasal cavity filter, warm, and humidify the air. The nasal cavity is lined with cilia and mucus, trapping dust, pathogens, and other particulate matter.
2. Pharynx : The pharynx, or throat, serves as a passageway for both air and food. It is divided into three regions:
– Nasopharynx : Located behind the nasal cavity.
– Oropharynx : Situated behind the oral cavity.
– Laryngopharynx : The region that opens into the larynx and esophagus.
3. Larynx : Known as the voice box, the larynx houses the vocal cords and is situated below the pharynx. It is crucial for speech and also acts as a protective mechanism by closing the airway during swallowing to prevent aspiration.

Lower Respiratory Tract

1. Trachea : The trachea or windpipe is a tube reinforced with cartilaginous rings that maintain its structure. It extends from the larynx down to the chest, where it bifurcates into the bronchi.
2. Bronchi and Bronchioles : The trachea splits into the left and right bronchi, each leading to a lung. The bronchi further divide into smaller bronchioles, resembling an inverted tree with multiple branches. These structures facilitate the conduction of air into the deeper regions of the lungs.
3. Lungs : The two lungs are the primary organs of the respiratory system, occupying much of the thoracic cavity. Each lung is divided into lobes – three in the right lung, and two in the left lung. The lungs are enveloped by a double-layered membrane known as the pleura, which secretes pleural fluid to reduce friction during respiration.
4. Alveoli : Tiny air sacs at the end of the bronchioles where gas exchange occurs. Alveoli are surrounded by a network of capillaries that facilitate the transfer of oxygen into the blood and the removal of carbon dioxide. The walls of the alveoli are extraordinarily thin, allowing for efficient gas diffusion.

See also  Effect of Insulin Hormone on Glucose Metabolism

Physiology of the Respiratory System

The Mechanics of Breathing

Inspiration (Inhalation) : During inspiration, the diaphragm contracts and moves downward, while the intercostal muscles of the ribs contract, expanding the chest cavity. This creates negative pressure within the thoracic cavity, causing air to flow into the lungs.

Expiration (Exhalation) : Expiration is typically a passive process where the diaphragm and intercostal muscles relax, reducing the volume of the chest cavity. This creates positive pressure, expelling air from the lungs. Forced expiration, such as during vigorous exercise, involves additional muscles like the abdominal muscles.

Gas Exchange

The primary function of the respiratory system is the exchange of gases, primarily oxygen and carbon dioxide, between the air and the blood.

1. External Respiration : Occurs in the alveoli where oxygen from the inhaled air diffuses across the alveolar membrane into the capillaries, while carbon dioxide from the blood diffuses into the alveoli to be exhaled.
2. Internal Respiration : This refers to the exchange of gases between the blood and body tissues. Oxygen transported by red blood cells diffuses into the cells, facilitating cellular respiration, while carbon dioxide, a metabolic waste product, diffuses from the cells into the blood.

Oxygen Transport

Once oxygen diffuses into the blood, it binds to hemoglobin molecules in red blood cells, forming oxyhemoglobin. This complex is transported through the circulatory system to body tissues. Hemoglobin releases oxygen in response to the lower oxygen concentration in tissues, ensuring that cells receive the necessary oxygen for metabolic processes.

Carbon Dioxide Removal

Carbon dioxide produced by cellular metabolism diffuses into the blood and is transported back to the lungs in three forms:
– Dissolved in Plasma : A small percentage of carbon dioxide is dissolved directly in the blood plasma.
– Carbaminohemoglobin : Carbon dioxide binds with hemoglobin to form carbaminohemoglobin.
– Bicarbonate Ions : The majority of carbon dioxide reacts with water in red blood cells to form carbonic acid, which quickly dissociates into bicarbonate and hydrogen ions. This reaction is catalyzed by the enzyme carbonic anhydrase. Bicarbonate ions travel in the plasma until they reach the lungs, where the reaction is reversed, allowing carbon dioxide to be expelled.

See also  Blood Pressure Regulation by the Renin-Angiotensin System

Regulation of Respiration

Respiratory control centers in the brainstem, primarily the medulla oblongata and the pons, regulate the rate and depth of breathing. These centers receive input from chemoreceptors that monitor levels of carbon dioxide, oxygen, and pH in the blood. Elevated levels of carbon dioxide or decreased pH stimulate an increase in respiratory rate, enhancing the expulsion of carbon dioxide and restoring homeostasis.


The human respiratory system is an intricate and vital structure, meticulously designed to facilitate the critical process of gas exchange. From the entry point of the nasal cavity to the microscopic alveoli, each component plays a strategic role in ensuring that oxygen enters the bloodstream and carbon dioxide is removed efficiently. Understanding the structure and function of this system underscores its significance in maintaining overall health and highlights the marvels of human physiology.

Print Friendly, PDF & Email

Leave a Comment

Discover more from PHYSIOLOGY

Subscribe now to keep reading and get access to the full archive.

Continue reading