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Human Respiratory System: Respiratory Tract, Structure of Lungs, Practice Problems and FAQs

How long can you hold your breath? While most of us are not able to hold our breath for more than a few seconds, some people are trained to hold their breath for up to a few minutes. But can any human survive without breathing indefinitely? Definitely not! Breathing is an integral body function which is so crucial that the body doesn’t even need our permission or willpower to perform it. We breathe involuntarily even when we are asleep or unconscious. Breathing helps us to take in oxygen-rich air and expel the waste carbon dioxide from the body. But why is breathing so crucial and which organ system of our body helps us breathe?

Our body contains about 15 trillion cells. For each of these cells to do their jobs and keep you alive and healthy, they need to take up oxygen along with the nutrients they need to function.

However it would be very inconvenient and next to impossible for each and every cell to handle its own oxygen needs. Thus multicellular life has come up with an ingenious way to take in large amounts of oxygen at once. Yes, we are talking about our respiratory system.

Our respiratory system is in charge of supplying oxygen to our body. Their are different types of respiratory organs existing in different forms of life - gills, surface diffusion, skin breathing, spiracles and of course lungs. These systems are adapted to different environments and modes of living.

In this article, we will mainly look at the human respiratory system.

Table of contents

  • Human respiratory system
  • Respiratory passage
  • Lungs
  • Steps of breathing and gaseous exchange
  • Practice problems
  • FAQs

Human respiratory system

The human respiratory system comprises the respiratory tract, respiratory organ, i.e, lungs and the structures involved with breathing movements. The breathing movements are produced by the expansion and contraction of the thoracic cavity brought about by the contraction and relaxation of the diaphragm, intercostal muscles and abdominal muscles. The respiratory tract or passage helps to transport oxygen-rich air from the environment to the respiratory organ, that is, lungs.

Respiratory passage

It is a passage that takes air from outside to the respiratory surface of lungs. It consists of

  • A pair of nostrils
  • Nasal cavity
  • A pair of internal nares
  • Pharynx
  • Larynx
  • Trachea
  • Bronchi and their branches
  • The respiratory surface of lungs

The respiratory tract can be divided into an upper respiratory tract which comprises the organs up to larynx and the lower respiratory tract which includes organs starting from the trachea to the respiratory surface of lungs.

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Fig: Respiratory tract

External nares or nostrils

A pair of nostrils represents the first part of respiratory passage. These are two ellipsoidal or pear shaped apertures present inferiorly at the apex of the nose. This leads to the nasal chamber of the nasal cavity.

Fig: Nostrils

Nasal cavity

It is a large cavity present above the buccal cavity, lying between the cranium and the palate. It is divided into two nasal chambers with the help of a vertical partition known as the nasal septum. Each nasal cavity is divided into three regions - nasal vestibule, respiratory region, olfactory region.

Fig: Nasal cavity

Nasal vestibule

It is the anterior part of the nasal chamber which bears coarse hair which traps particulate matter from incoming air. The surface of this region has oil glands and sweat glands.

Fig: Nasal vestibule

Respiratory region or conditioner

It is the middle large division of the nasal chamber. This area contains 3-4 bony elevations known as nasal conchae or turbinates or turbinals - superior concha, middle concha, inferior concha and highest concha. The highest concha might be absent in humans. Cavities known as paranasal sinuses also open into the nasal chambers.

The epithelial lining of the conditioner region is also called the respiratory epithelium and consists of pseudostratified ciliated columnar epithelium, non-ciliated columnar epithelium, cuboidal epithelium and goblet cells (mucous secreting cells). It also has serous and mucous glands and is richly supplied with blood vessels. Hence, it appears pinkish in colour.

This region acts as the air conditioner and brings the temperature of the incoming air equal to that of the body temperature. It also makes the incoming air moist. The mucous released from the goblet cells helps to trap particulate matter present in the inhaled air. The serous glands secrete watery secretions having antimicrobial chemicals such as lysozyme, lactoferrin, immunoglobulin A (IgA) which help to eliminate germs in the inspired or inhaled air. The cilia of the columnar epithelium beat continuously towards the nasopharynx.

Fig: Respiratory region

Olfactory region

It lies in the upper part of the nasal chamber which also contains the superior nasal concha. The epithelial lining of this region contains sensory cells along with non-sensory supporting cells. It is also known as the olfactory epithelium and helps in perceiving the sensation of smell.

Fig: Olfactory region

Internal nares

These are the posterior openings of the nasal cavity that leads into the nasopharynx.

Pharynx

It is a vertical tube that is differentiated into three parts - nasopharynx, oropharynx and laryngopharynx. The nasopharynx is the upper part into which the internal nares open and it lies in level with the soft palate. It also receives the openings of the eustachian tubes from both ears.

Oropharynx is the middle part which is the continuation of the buccal cavity. Laryngopharynx is the lower part that serves as a common passage for both food and air. It has an opening to the oesophagus, known as gullet, towards the backside and glottis, the opening to the larynx, in front.

Fig: Pharynx

Larynx

The pharynx opens into the trachea through the larynx. Larynx helps in the production of sound, that's why it is also known as sound box. It is present at the upper part of the trachea.

Fig: Larynx

The larynx is made up of a number of cartilages which are supported by ligaments, muscles and fibrous membranes. The unpaired cartilages are - thyroid cartilage (known as Adam’s apple in men), cricoid cartilage and epiglottis. The paired ones are arytenoid cartilages, corniculate cartilages and cuneiform cartilages.

Fig: Cartilages of larynx

The upper part of the larynx has an opening called glottis. The glottis is covered by epiglottis, during swallowing. Epiglottis is a leaf-shaped cartilaginous structure made up of elastic cartilage. It is the covering of glottis, which prevents the entry of food into the larynx at the time of swallowing.

GIF: The epiglottis closes the glottis while swallowing

Sound production in larynx

The laryngeal cavity has two paired mucosal folds. The upper folds are called the vestibular folds or false vocal cords while the lower ones are called the true vocal cords. The true vocal cords are composed of yellow elastic tissue covered by stratified squamous epithelium for bearing mechanical stress. They are thicker and larger in men than in women. The vocal cords enclose a passage named rima glottidis.

At rest, the vocal cords remain unstretched and lie at an angle so as to keep the rima glottidis wide enough to allow the passage of air. While producing sound, the vocal cords are stretched with the help of muscles which brings them closer and parallel to each other, thus narrowing down the rima glottidis. As air passes through the rima glottidis, it causes the vocal cords to vibrate, thus producing sound. The pitch of the sound depends on the thickness of the cords and the tension in them. Males have a low-pitched voice due to thicker vocal cords. The sound is converted to speech due to modulation and resonation brought about by the teeth, lips, tongue, nasal cavity, etc.

Fig: Vocal cords

Trachea

Trachea is also known as windpipe. It develops from the posterior border of the larynx and extends up to the mid-thoracic cavity. It divides into right and left primary bronchi at the level of the fifth thoracic vertebra and enters the lungs. It is lined by 16-20 C-shaped cartilaginous rings, which prevent it from collapsing in the absence of air. The mucosal lining of the trachea is formed of ciliated columnar epithelium and also has goblet cells. The cilia beat the mucous released by the goblet cells towards the larynx and pharynx.

Fig: Trachea

Bronchi

The trachea divides into right and left primary bronchi at the level of the fifth thoracic vertebra. The point of branching is known as carina. The right primary bronchus is nearly vertical, wider and shorter than the left one. Due to the short length and vertical arrangement of the right primary bronchus, the right lung receives more influx of foreign particles and microbes.

The left primary bronchus is narrower, longer and slightly bent.

The primary bronchi branches into the secondary lobar bronchi - three in the right lung (right superior, middle and right inferior) and two in the left lung (left superior and left inferior). The lobar bronchi branch into tertiary or segmental bronchi which ultimately branch into terminal bronchi. Just like the trachea, cartilaginous rings are also present around the bronchi but they become thinner and narrower with subsequent branching.

The terminal bronchi branch into initial bronchioles which also possess cartilaginous rings but as they branch further, the cartilaginous rings disappear. Initial bronchioles branch into terminal bronchioles which further divide into respiratory bronchioles each of which form an alveolar duct which leads to the atrium which further opens into the alveolar sacs. Each alveolar sac consists of a number of alveoli or air sacs. This branching and re-branching of the respiratory passageways is called the respiratory tree.

Fig: Bronchi

The bronchi and bronchioles have a similar pseudostratified ciliated columnar epithelial lining as that of the trachea. The bronchioles have a comparatively lesser number of ciliated cells and the cells are also shorter in height. The goblet cells are absent in the bronchioles.

Alveoli

Bronchioles give rise to very thin, irregular walled balloon-like structures called alveoli which are richly supplied with blood vessels and lie in close contact with blood capillaries. Each lung has around 300 million alveoli which are held together with the help of highly vascular connective tissue. The average respiratory surface area of the human lungs is believed to be around 100 m2.

Fig: Alveoli

The alveoli are not closed sacs. They are connected to adjacent alveoli with the help of Kohn pores to eliminate the effect of any blockage in the respiratory pathway. The walls of alveoli are very thin and made up of a single layer of squamous epithelial cells that are 0.05-0.2 μm thick. The barrier between the alveolar air and the capillary blood is very thin and is known as the respiratory membrane.

Fig: Respiratory membrane

During breathing, oxygen present in the inhaled air filled in the alveoli diffuses across the respiratory membrane into the capillaries and the carbon dioxide present in the capillary blood diffuses out into the alveoli to be exhaled. The CO2 carried by blood is the waste product of cellular respiration. The oxygen that diffuses into the blood is carried by the haemoglobin present in the blood and it reaches the tissues via blood vessels. The cells in the tissues take up the oxygen from the blood to generate energy by cellular respiration. The carbon dioxide that they produce as a waste product of cellular respiration is released into the blood and is carried to the lungs to be expelled out.

Fig: Gaseous exchange in alveoli

Lungs

In humans, a pair of lungs is present in an airtight chamber called thoracic cavity. Thoracic cavity is formed :

  • Dorsally by the vertebral column
  • Ventrally by the sternum
  • Laterally by the ribs
  • Closed below by diaphragm
  • Pectoral girdle and neck on the upper side

Fig: Thoracic cavity

Each lung is enclosed by extremely thin double- membrane sacs called pleural sacs. The outer membrane of the pleural sac is called the parietal pleura and the inner membrane is called the visceral pleura. The parietal pleura is in close contact with the thoracic cavity and the visceral pleura is in close contact with the lungs.

The narrow space present between the two membranes is known as pleural cavity, which is filled with pleural fluid. Pleural fluid lubricates the pleural membranes, to avoid friction during breathing. It also keeps the lungs moist and acts as a shock absorber.

Fig: Pleural membranes

The pleural sacs of the two lungs meet in the middle to form a septum. They separate to create a cavity called mediastinum in the lower part. The heart is located in this cavity.

External structure of lungs

Lungs are somewhat conical shaped sacs which are soft, spongy and elastic. Each lung has a narrow apex towards the neck, a broad and concave semilunar base that lies above the diaphragm, a convex costal surface facing the ribs and a concave median surface facing the mediastinum. Each lung has a triangular depression known as hilum which is the point of entry and exit of structures such as blood vessels, bronchi, etc entering or leaving the lungs.

Fig: External structure of lungs

Right lung

It is shorter by 2.5 cm, with respect to the left lung, due to the raised position of the diaphragm on the right side. It has two fissures, horizontal and oblique, which dive the right lung into 3 lobes - right superior, right middle and right inferior.

Left lung

It is longer and narrower than the right lung, as it contains a cardiac notch for accommodating an asymmetrically placed heart. It is divided into two lobes (left superior and left inferior) by an oblique fissure.

Internal Structure of lungs

Internally the lungs are divided into lobes which are divided into segments which in turn are further divided into lobules. The right lung has eight segments while the left one has ten. The segments are provided with tertiary or segmental bronchi and the lobules are supplied with the terminal bronchi. The terminal bronchi branch further to form bronchioles and terminal bronchioles. Starting from the trachea to the terminal bronchioles, the entire stretch of respiratory passageway constitutes the conducting zone. The conducting part transports the atmospheric air to the alveoli, clears it from foreign particles, humidifies and also brings the air to body temperature.

The terminal bronchioles divide into respiratory bronchioles which divide 1-3 times and produce 2-3 alveolar ducts. Each alveolar duct expands to form an atrium which further leads to an air sac or alveolus. The respiratory passageway from the respiratory bronchioles to the alveoli constitute the respiratory zone and is the region of gaseous exchange in the lungs.

Fig: Conducting and respiratory zones of the respiratory tract

Contraction and expansion of lungs

The lungs are incapable of performing independent movements. Due to the presence of a common pleural sac lining the thoracic cavity and covering the lungs, the expansion and contraction of the thoracic cavity automatically brings about the expansion and contraction of the lungs during breathing. The lungs expand during inspiration or inhalation and contract during expiration or exhalation. The breathing movements are assisted by the ribs, intercostal muscles, diaphragm and abdominal muscles.

Steps in Breathing and Cellular Respiration

  • Breathing or pulmonary respiration - It involves inhalation of atmospheric air and exhalation of carbon dioxide.
  • Diffusion of gases across alveolar membrane - Diffusion of O2 and CO2 takes place across the alveolar membrane to blood capillaries.
  • Transport of gases by blood - Blood transports O2 to the body cells from alveoli and CO2 from the body cells to alveoli.
  • Diffusion of gases between blood and tissues - O2 diffuses from blood to tissues and CO2 diffuses from tissues to blood.
  • O2 utilisation and release of CO2 and energy by cells.

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

Practice problems

1. If you were travelling in a miniaturised ship through the respiratory system, from the pharynx to an alveolus, which structures would you pass along the way, and in what order?

  1. trachea, larynx, bronchi, and bronchioles
  2. larynx, trachea, bronchi, and bronchioles
  3. bronchioles, bronchi, trachea, and larynx
  4. bronchioles, trachea, bronchi, and larynx

Solution: Respiratory passage takes air from outside to the respiratory surface of lungs. It consists of a pair of nostrils, nasal cavity, a pair of internal nares, pharynx, larynx, trachea, bronchi, bronchioles and the respiratory surface of lungs. So from pharynx to the alveolus following structures will be seen- larynx, trachea, bronchi, and bronchioles. Hence option (b) is correct.

Fig: Respiratory tract

2.Which one of the following is not correct regarding the larynx?

  1. It houses the vocal cords.
  2. The opening to the larynx is covered by the glottis during swallowing.
  3. It is made up of a number of cartilages which are supported by ligaments, muscles and fibrous membranes.
  4. It connects the pharynx to the trachea

Solution: The laryngeal cavity has two paired mucosal folds. The upper folds are called the vestibular folds or false vocal cords while the lower ones are called the true vocal cords. The vibration of the vocal cords caused due to passage of air helps to produce sound, which is why the larynx is also known as sound box.

The larynx is made up of cartilages which are supported by ligaments, muscles and fibrous membranes. It has three unpaired cartilages (thyroid, cricoid and epiglottis) and three paired ones (arytenoid, corniculate and cuneiform).

It is present at the upper part of the trachea and connects the pharynx to the trachea. Its upper part has an opening called glottis. The glottis is covered by epiglottis, during swallowing. Epiglottis is a leaf-shaped cartilaginous structure made up of elastic cartilage. It is the covering of glottis, which prevents the entry of food into the larynx at the time of swallowing.

Thus, the correct option is b.

3. Which of the following is not a part of the respiratory zone of the respiratory tract?

  1. Alveolar duct
  2. Air sacs
  3. Respiratory bronchioles
  4. Terminal bronchioles

Solution: Starting from the trachea to the terminal bronchioles, the entire stretch of respiratory passageway constitutes the conducting zone as it conducts air to the lungs and there is not gaseous exchange occurring at these regions of the respiratory tract.

The terminal bronchioles divide into respiratory bronchioles which divide 1-3 times and produce 2-3 alveolar ducts. Each alveolar duct expands to form an atrium which further leads to an air sac or alveolus. The respiratory passageway from the respiratory bronchioles to the alveoli constitute the respiratory zone. Hence option d is correct.

4. Choose the incorrect statement.

(a) The respiratory region of the nasal cavity warns and adds moisture to the inhaled air before it enters the lungs

(b) The right lung consists of three lobes while the left lung has only two lobes.

(c) The primary bronchi branch out into lobar bronchi which further branch out into segmental bronchi.

(d) The visceral pleura lies in direct contact with the wall of the thoracic cavity.

Solution: The respiratory region of the nasal cavity acts as the air conditioner and brings the temperature of the incoming air equal to that of the body temperature. It also makes the incoming air moist.

The right lung is divided into three lobes by horizontal and oblique fissures, but the left lung has only two lobes created by an oblique fissure.

The trachea branches out into two primary bronchi, each of which enter a lung and further branch out into secondary or lobar bronchi and tertiary or segmental bronchi.

Each lung is enclosed by extremely thin double- membrane sacs called pleural sacs. The parietal outer pleural membrane is in close contact with the thoracic cavity and the visceral or inner pleural membrane is in close contact with the lungs.

Hence option (d) is the correct option.

FAQs

1. What is the colour of lungs?

Answer: Due to rich supply of blood vessels, the lungs appear pinkish at birth but as we grow and breathe in air rich in pollutants and carbon particles, the deposition of carbonaceous matter turns the lungs grey and mottled in adults.

2. Why do we have two lungs?

Answer: The course of evolution has selected a specific lung capacity that can satisfy the oxygen needs of the body. To meet this lung capacity a specific quantity of lung tissue has to be organised. It makes more sense to equally distribute the lung tissue across both sides of the body rather than having all lung tissue on one side of the body. Having two lungs also has structural advantages because the trachea naturally bifurcates into two bronchi and hence having two separate lungs makes sense. Moreover it allows the space to fit in organs which are indivisible like the heart in between. Separation also reduces the chance of infections spreading from one lung to the other.

3.Why can lungs float on water?

Answer: Being filled with air, the volume of the lungs is much higher compared to their mass which reduces their density to less than 1 g/cm3 which is the density of water. Being less dense than water, allows them to float on water.

4. What prevents the alveoli and bronchioles from collapsing?

Answer: Unlike the trachea, bronchi and initial bronchioles, the respiratory bronchioles and alveoli are not surrounded by C shaped cartilaginous rings which can prevent these airways from collapsing. Hence the clara cells (non-ciliated cuboidal cells) lining the respiratory bronchioles and around 5% of the cells in the lining of the alveoli secrete a surfactant which reduces the surface tension in the walls and prevents the alveoli and the terminal and respiratory bronchioles from collapsing in the absence of air. The surfactant is made up of dipalmitoyl lecithin, cholesterol, polyunsaturated phospholipids, proteins and calcium ions.

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