Human Respiratory System: Anatomy & Functions

Human Respiratory System: Definition, Diagram and Examples

Edited By Irshad Anwar | Updated on Jul 02, 2025 06:40 PM IST

Respiratory System Definition: The human respiratory system facilitates the exchange of gases, providing oxygen to the body and removing carbon dioxide. It includes organs such as the nose, pharynx, larynx, trachea, bronchi, and lungs. This system plays a vital role in maintaining cellular respiration and energy production. In this article, respiration, the anatomy of the human respiratory system, gas exchange and transport, and respiratory system functions are discussed. The Human Respiratory System is a topic of the chapter Breathing And Exchange Of Gases in Biology.

This Story also Contains

What is Respiration?
Anatomy of the Human Respiratory System
Gas Exchange and Transport
Respiratory System Functions

Human Respiratory System: Definition, Diagram and Examples

What is Respiration?

Breathing is a fundamental life-sustaining mechanism by which people and all other living things metabolise oxygen and release carbon dioxide, a by-product into the environment. Lungs, the trachea, and the diaphragm are used to help facilitate the exchange of gases. Being one of the main regulatory and metabolic functions that are associated with every cell, the respiratory system is a vital component of human physiology. In respiration, air is directly taken into the lungs then the oxygen in it is transported to the various cells for metabolic purposes. At the same time, the carbon dioxide resulting from metabolism is transported back to the lungs to be expelled through exhalation maintaining the health of all tissues/organs in the body.

Anatomy of the Human Respiratory System

The human respiratory tract has the following parts:

Nose and Nasal Cavity

Structure and function: The principal pathways for air are the following: the external opening made of the nostrils, internal concha, and nasal septum.
Role in filtering and humidifying air: This structure prevents the entry of dust and pathogens by using nasal hair and mucous membranes. They also heat and moisten the incoming air to increase protection for the highly sensitive structures of the respiratory tract.

Pharynx and Larynx

Structure and function: The Pharynx is a muscular passage that forms a common wall of the nasal and oral cavities and it also connects to the larynx. The larynx, also known as the voice box is located below the pharynx and is found to contain the vocal cords.

Role in directing air to the lungs: The pharynx helps guide the air from the nose and mouth to the larynx and then to the trachea so that a proper distribution to the lungs is facilitated.

Trachea and Bronchi

Structure and function: The Trachea is a tubular structure made of cartilage that begins at the larynx and moves down to the chest where it branches off to the main bronchi.

The division into bronchi and bronchioles: The trachea branches into two parts of the bronchus the left and right which continue into the lungs branching out into smaller bronchial tubes which enable the distribution of the air within the lungs.

Upper respiratory tract

Lungs

Structure of the lungs: Lungs are two large spoke air-containing organs located in the thoracic cavity which is sheltered by ribs.

Lobes of the lungs: The right lung has three lobes; the right upper lobe, the right middle and the right lower lobe while the left lung has only two lobes; the left upper lobe and the left lower lobe because of the presence of the heart.

Alveoli

Structure and function: Alveoli are enormously small, these are the terminal branches of bronchial tubes which are again covered by a web of capillaries.

Gas exchange process: This is of significance in regards to the process of respiration as well as oxygenation of the alveoli tissues, they facilitate the exchange of oxygen and carbon monoxide in the air to the bloodstream or vice versa where needed most of the time.

Lungs and alveoli

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Gas Exchange and Transport

This involves the following process:

Gas Exchange in the Alveoli

Diffusion of oxygen and carbon dioxide: This process of gas exchange in alveoli is conducted through the process of diffusion in which the oxygen content has a freedom of movement from the alveolar surface to blood at the capillary walls while, on the other hand, the carbon dioxide from blood has the freedom of movement and get accumulated at the alveolar surfaces.

Partial pressures of gases: The diffusion process is stated through the partial pressure for oxygen and carbon dioxide; oxygen has a higher partial pressure in the alveoli than in the blood while carbon dioxide has a higher partial pressure in the blood than in the alveoli.

Transport of Oxygen

Binding to hemoglobin in red blood cells: When oxygen gets in the blood it combines with the hemoglobin of the red blood cells to form what is known as oxyhemoglobin, which is then distributed to various tissues of the body.

Transport of Carbon Dioxide

Conversion to bicarbonate ions: Carbon dioxide generated by the cells moves out of the cells into the plasma; the major portion of it is changed to bicarbonate owing to the enzyme carbonic anhydrase present in the red blood cells.

Transport in plasma and red blood cells: The CO2 is transported in 3 forms in blood; First dissolved in blood plasma, secondly, bound to the haemoglobin forming carbaminohemoglobin, and finally as bicarbonate ions in the blood which is the most common form of CO2 that is transported back to the lungs for exhalation.

Respiratory System Functions

The functions of the respiratory system are:

Inhalation and Exhalation

The respiratory system helps in inhalation whereby the diaphragm muscle and the muscles between the ribs expand to take in air during breathing and exhalation whereby the diaphragm muscle and the muscles between the ribs contract and force out air from the lungs during the process of breathing.

Exchange of Gases between Lungs and Bloodstream

Oxygen penetrates alveolar walls to access the capillaries on the same note, carbon dioxide that is within the blood gets to pass through the alveoli to get eliminated through respiration thus in the process helping regulate the concentration of these gases in the body.

Exchange of Gases between Bloodstream and Body Tissues

Blood containing oxygen gets transported to the several tissues of the body where oxygen is utilised in several activities and the carbon dioxide which is the end product of any metabolism is transported back to the lungs.

The Vibration of the Vocal Cords

It is a part of the respiratory system and also contains vocal cords, which vibrate and create sound as the air is breathed out. This function can allow for talking and phonic communication in one way or another.

Olfaction or Smelling

The nasal cavity has olfactory receptors that can detect chemicals floating in the air which gives the sense of smell. These receptors transport some impulses to the brain and they help in the feeling of scent.

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Frequently Asked Questions (FAQs)

1. What are the main parts of the human respiratory system?

The human respiratory system can be composed of the following; nasal cavity and nose, throat or pharynx, voice box or larynx, windpipe or trachea, bronchi, and lungs. The bronchi then subdivide into smaller branches called the bronchioles and end in small round structures that are called the alveoli where the exchange of gases takes place.

2. How does gas exchange occur in the alveoli?

In the alveoli’ there are also two processes, which take place and these involve diffusion. Oxygen present in the inhaled air moves to the alveolar walls and enters the neighbouring capillaries, while carbon dioxide in the blood also moves across the alveolar walls to be expelled. This process is a result of variations in partial pressures of oxygen and carbon dioxide.

3. What is the role of the diaphragm in breathing?

The diaphragm is a muscle in the form of a dome fixed at the lower level in the area of the lungs’ position. During inspiration, the diaphragm relaxes but becomes thinner and this in turn expands the thoracic cavity to allow a low pressure to suck in the air into the lungs. When breathing out, the diaphragm gets flattened and does not supply the thoracic cavity with more space, thus, air is expelled from the lungs.

4. What are common respiratory disorders and their symptoms?

Other respiratory illnesses are asthma, COVID-19, pneumonia, bronchitis, and lung cancer. The symptoms are many but the common ones are difficulties in breathing, wheezing sound, cough that does not stop, pain in the chest and mucus. These are diseases that require early diagnosis and treatment since they are chronic.

5. How can I maintain a healthy respiratory system?

The following measures are recommended to have a healthy respiratory tract; refrain from smoking and exposure to cigarette smoke, decrease exposure to pollutants, undertake regular exercises, practice cleanliness to minimise get infections, and regular check-ups. Supplementing the diet with antioxidants also helps in good respiratory health and equal intake of water.

6. How does the structure of the trachea contribute to its function?

The trachea's structure supports its functions in several ways:

7. What is the significance of the pleural membranes, and how do they relate to pneumothorax?

The pleural membranes consist of two layers (visceral and parietal) that surround the lungs. They produce pleural fluid, which reduces friction during breathing and helps maintain negative pressure in the pleural cavity. This negative pressure keeps the lungs inflated. In pneumothorax, air enters the pleural space, disrupting the negative pressure and causing partial or complete lung collapse.

8. How does the diaphragm facilitate breathing, and what type of muscle is it?

The diaphragm is a dome-shaped skeletal muscle that separates the chest cavity from the abdominal cavity. It facilitates breathing by contracting and flattening during inhalation, which increases the volume of the chest cavity and creates negative pressure, drawing air into the lungs. During exhalation, it relaxes and returns to its dome shape, helping to expel air from the lungs.

9. How does the autonomic nervous system regulate breathing rate and depth?

The autonomic nervous system regulates breathing through:

10. What is the role of pulmonary surfactant, and how does its absence affect premature infants?

Pulmonary surfactant is a mixture of lipids and proteins that:

11. How does the structure of alveoli contribute to efficient gas exchange?

Alveoli are tiny air sacs in the lungs with several structural features that enhance gas exchange:

12. What is the difference between external and internal respiration?

External respiration refers to the exchange of gases between the air in the alveoli and the blood in the pulmonary capillaries. Internal respiration, on the other hand, is the exchange of gases between the blood in systemic capillaries and the body's cells. Both processes involve the diffusion of oxygen and carbon dioxide across membranes.

13. How does the countercurrent flow in the gills of fish compare to human lung function?

In fish gills, water and blood flow in opposite directions (countercurrent flow), maximizing the oxygen concentration gradient and allowing for more efficient gas exchange. Human lungs use a different strategy with their alveolar structure, but both systems aim to maximize gas exchange efficiency. The countercurrent system in fish is particularly well-suited for extracting oxygen from water, which has a lower oxygen concentration than air.

14. What is the role of hemoglobin in oxygen transport, and how does it differ from myoglobin?

Hemoglobin is a protein in red blood cells that binds to oxygen in the lungs and transports it throughout the body. It can bind up to four oxygen molecules and releases them in areas with low oxygen concentration. Myoglobin, found in muscle tissue, has a higher affinity for oxygen than hemoglobin and acts as an oxygen storage molecule, releasing oxygen during periods of intense muscle activity.

15. How do the partial pressures of oxygen and carbon dioxide change as blood flows through the pulmonary and systemic circuits?

In the pulmonary circuit:

16. What is the primary function of the human respiratory system?

The primary function of the human respiratory system is to facilitate gas exchange between the air we breathe and our bloodstream. This process involves taking in oxygen from the air and expelling carbon dioxide produced by cellular respiration, ensuring that our body's cells receive the oxygen they need to function and removing waste gases.

17. What is the role of surfactant in the lungs, and what would happen without it?

Surfactant is a mixture of lipids and proteins that coats the inner surface of alveoli. It reduces surface tension, preventing alveoli from collapsing during exhalation. Without surfactant, the alveoli would stick together and collapse, making it difficult to breathe and potentially leading to respiratory distress syndrome.

18. What is the role of carbonic anhydrase in CO2 transport and pH regulation?

Carbonic anhydrase is an enzyme that catalyzes the reversible reaction between carbon dioxide and water to form carbonic acid. This reaction is crucial for:

19. How does the Bohr effect influence oxygen delivery to tissues?

The Bohr effect describes how hemoglobin's affinity for oxygen decreases in the presence of higher carbon dioxide levels or lower pH. This effect enhances oxygen delivery to active tissues, where CO2 levels are higher and pH is lower due to increased metabolic activity. As a result, hemoglobin releases more oxygen where it's needed most.

20. What is the significance of the respiratory quotient (RQ), and how does it vary with different nutrients?

The respiratory quotient (RQ) is the ratio of CO2 produced to O2 consumed during respiration. It indicates which type of nutrient is being metabolized:

21. What is the role of negative pressure in breathing, and how does it relate to Boyle's law?

Negative pressure breathing relies on creating a pressure difference between the atmosphere and the lungs. When the diaphragm contracts and the rib cage expands, it increases the volume of the thoracic cavity. According to Boyle's law, as volume increases, pressure decreases. This creates a negative pressure in the lungs relative to the atmosphere, causing air to flow in. The reverse occurs during exhalation, with the elastic recoil of the lungs helping to expel air.

22. How does the oxygen cascade explain the progressive decrease in oxygen partial pressure from air to cells?

The oxygen cascade describes the step-wise decrease in oxygen partial pressure (PO2) as oxygen moves from the atmosphere to the mitochondria:

23. How does the oxygen-hemoglobin dissociation curve illustrate the relationship between oxygen saturation and partial pressure?

The oxygen-hemoglobin dissociation curve shows the relationship between the partial pressure of oxygen (PO2) and the percentage of hemoglobin saturated with oxygen. The curve is S-shaped, indicating:

24. What is the role of the Hering-Breuer reflex in breathing regulation?

The Hering-Breuer reflex is a protective mechanism that prevents over-inflation of the lungs. When the lungs are stretched during deep inhalation, stretch receptors in the airways send signals to the brain's respiratory center. This triggers the switch from inhalation to exhalation, helping to regulate breathing depth and rate. The reflex is particularly important in infants but plays a lesser role in adult breathing regulation.

25. How does acclimatization to high altitudes affect the respiratory system?

Acclimatization to high altitudes involves several respiratory adaptations:

26. What is the significance of dead space in the respiratory system, and how does it affect ventilation?

Dead space refers to air that doesn't participate in gas exchange. It includes:

27. How do obstructive and restrictive lung diseases differ in their effects on lung function?

Obstructive lung diseases (e.g., asthma, COPD) and restrictive lung diseases (e.g., pulmonary fibrosis) affect lung function differently:

28. How does the respiratory system contribute to acid-base balance in the body?

The respiratory system plays a crucial role in acid-base balance by regulating the amount of CO2 in the blood. CO2 forms carbonic acid when dissolved in blood, so changes in breathing rate can quickly affect blood pH:

29. What is the significance of bronchial tone, and how is it regulated?

Bronchial tone refers to the degree of constriction in the airways. It's significant because it affects airflow resistance and distribution of air in the lungs. Bronchial tone is regulated by:

30. What is the role of intercostal muscles in breathing, and how do they work with the diaphragm?

Intercostal muscles are located between the ribs and play important roles in breathing:

31. How does the chloride shift contribute to carbon dioxide transport in the blood?

The chloride shift, also known as the Hamburger shift, is a process that occurs in red blood cells during CO2 transport:

32. What is ventilation-perfusion matching, and why is it important for efficient gas exchange?

Ventilation-perfusion matching refers to the coordination between air flow (ventilation) and blood flow (perfusion) in the lungs. Ideally, areas of the lung should receive air and blood in proportions that allow optimal gas exchange. Mismatches can occur due to:

33. How does the structure of the pulmonary capillaries enhance gas exchange?

Pulmonary capillaries have several structural features that enhance gas exchange:

34. What is the role of carbaminohemoglobin in CO2 transport, and how does it differ from oxyhemoglobin?

Carbaminohemoglobin is formed when CO2 binds directly to amino groups on the hemoglobin molecule. This method accounts for about 5-10% of CO2 transport in the blood. Unlike oxyhemoglobin, where O2 binds to the heme groups, CO2 binds to the globin chains. This allows hemoglobin to carry both O2 and CO2 simultaneously, though in different ways. The formation of carbaminohemoglobin is reversible, allowing for CO2 release in the lungs.

35. How does the respiratory system respond to exercise, and what mechanisms are involved?

The respiratory system responds to exercise through several mechanisms:

36. What is the significance of residual volume, and how does it relate to lung function?

Residual volume (RV) is the amount of air remaining in