Human Respiratory System - Anatomy and Physiology (Class XI)

Lungs are the main respiratory organs of the human respiratory system. The windpipe or trachea divides into two branches called bronchi, entering into each lung. Each bronchus in the lung further divides to form many bronchioles. These bronchioles end up in a sac-like structure called the alveoli.  Hence, the lungs are made up of bronchi, bronchioles and alveoli.

The lungs are protected by two membranes called the pleural membranes. The outer pleural membrane is called the pleura and the inner one is called the visceral or pulmonary layer. There is a space between the two layers called the intrapleural space. It is filled with fluid to keep the lungs protected from the shocks and injuries.

A. Ribs are attached to sternum with their anterior ends.

The trachea, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings. 

A column-shaped cell found in the respiratory and intestinal tracts, which secretes the main component of mucus. The mucus produced by the goblet cells traps the finest dust particles and filter out largest dust particles.

So, the correct option is 'II and III'.

Alveoli is the region in the lungs where exchange of gases takes place.According to histology of alveolar wall ,it contain 2types of cell

This is due to the presence of transformed elastic cartilage which gives support to the initial bronchioles and prevents it from collapsing. The incomplete cartilaginous rings that are present in the trachea and bronchi are absent in the bronchioles. They are supported by the transformed elastic cartilage.

The trachea, or windpipe, is reinforced by C-shaped rings of hyaline cartilage, which prevent it from collapsing and keep the airway permanently open.

Total lung capacity (TLC) is the maximum amount of air the lungs can hold when they are fully inflated. The average total lung capacity of an adult human male is about 6 litres of air. It can be calculated by using the formula TLC = TV + IRV + ERV + RV where TV is tidal volume(500 ml), IRV is inspiratory reserve volume(3100 ml), ERV is expiratory reserve volume (1200 ml) and RV (1200  ml) stands for residual volume.

Alveoli are the site of respiration inside the lungs. The human lungs have 300 million alveoli with a total area of about ${100 m^2}$ which is 50 times than that of the skin surface.

• The left lung is divided into two lobes, an upper and a lower, by the oblique fissure, which extends from the coastal to the mediastinal surface of the lung both above and below the hilum.
•  The left lung, unlike the right, does not have a middle lobe, though it does have a homologous feature, a projection of the upper lobe termed the "lingula". 
• Its name means "little tongue". Hence, the Lingula structure originates from the Superior side of the left lung.
  

Trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of 5th thoracic vertebra into a right and left primary bronchi. Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles. The tracheae, primary, secondary and tertiary bronchi and initial bronchioles are supported by incomplete cartilaginous rings, which prevents them from collapsing. Each terminal bronchiole gives rise to a number of very thin, irregular walled and vascularised bag-like structures called as alveoli. The branching network of bronchi, bronchioles and alveoli comprise the lungs.

The lungs of mammals including those of humans are soft, spongy, elastic organs with the smooth shining surface. Lungs are not hollow like balloons but are made of spongy, flexible tissue that inflates, when filled with air.

The walls of the alveoli are lined with simple squamous epithelial cells known as alveolar cells. This single layer of squamous cells is ideal for the diffusion of gases in the lungs.

In mammals, the nasal chambers are called as nasal vestibules. The nasal vestibules are formed by invagination of skin in embryonic stage and hence nasal vestibules have hair and sebaceous glands. In this manner dust particles cannot cross nasal vestibules. The remaining part of nasal chamber is surrounded by three types of bones- nasal, maxilla and ethmoids. At several places in the nasal passage these bones form outgrowths. These outgrowths are called turbinal bones. Due to these outgrowths, the nasal passage is long and spiral. This helps in making the temperature of air equal to body temperature before it reaches the lungs and does not allows unwanted particles like dust to reach lungs.

The trachea branches to give rise to two primary (main) bronchi. These then branch successively to give rise in turn to secondary and tertiary bronchi. These then branch to give rise to several orders of progressively smaller airways called bronchioles, the smallest of which are called terminal bronchioles. These are the last components of the conducting portion of the respiratory system. Terminal bronchioles give rise to respiratory bronchioles, which ultimately lead to the alveoli.

Alveoli are small pouches or sacs. Large number of alveoli occur inside each lung. The whole surface area of an alveolus functions as a respiratory surface. Due to large number of  alveoli in each lung, a very large area of respiratory surface becomes available (about $80{ m }^{ 2 }$) for exchange of gases. Thus, alveoli are designed to maximize the exchange of gases.

Trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of 5th thoracic vertebra into a right and left primary bronchi. Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles. The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings. Each terminal bronchiole gives rise to a number of very thin, irregular walled and vascularised bag-like structures called alveoli. The branching network of bronchi, bronchioles and alveoli comprise the lungs. Thus trachea and bronchi are present outside the lungs and alveoli are tiny bags of lungs.

Hiccups occur due to hasty eating or wrong posture during the meal which allows the food particles to enter the windpipe. The windpipe carries air from the nostrils to the lungs. It runs adjacent to the food pipe. But inside the throat, air and food share a common passage. During the act of swallowing a flap-like valve closes the passage of the windpipe and guides the food into the food pipe. If by chance,food particles enter the windpipe, hiccups can be experienced.

A.Bronchus - a passage of airway in the respiratory system that conducts air into the lungs.

B.Alveoli - any of the many tiny air sacs of the lungs which allow for rapid gaseous exchange.

C.Bronchioles - any of the minute branches into which a bronchus divides.

D.Hilum - a part of an organ where structures such as blood vessels and nerves enter the body.

So, the correct option is ‘Bronchioles’

Breathing involves expansion and contraction off lungs to adjust the partial pressure of relative gases for exchange. The process of inhalation occurs due to an increase in the lung volume (diaphragm contraction and chest wall expansion) which results in a decrease in lung pressure in comparison to the atmosphere. Thus, air rushes in the airway. The process of exhalation occurs due to an elastic recoil of the lung tissue which causes a decrease in volume, resulting in increased pressure in comparison to the atmosphere. Thus, air rushes out of the airway. 

A bronchus, also known as a main or primary bronchus, is a passage of airway in the respiratory tract that conducts air into the lungs. There is a right bronchus and a left bronchus and these bronchi branch into smaller secondary and tertiary bronchi which branch into smaller tubes, known as bronchioles. No gas exchange takes place in the bronchi. The trachea (windpipe) divides into two main or primary bronchi, the left and the right, at the level of the sternal angle and of the fifth thoracic vertebra or up to two vertebrae higher or lower, depending on breathing, at the anatomical point the carina of trachea. An alveolus is an anatomical structure that has the form of a hollow cavity. Thus, option B is correct.

The vertebral column, also known as the backbone or spine, is a bony skeletal structure found in vertebrates. Skull is present in head. The rib cage is formed by the vertebral column, ribs, and sternum and encloses the heart and lungs. In humans, the rib cage, also known as the thoracic cage, is a bony and cartilaginous structure which surrounds the thoracic cavity and supports the pectoral girdle (shoulder girdle).

A trachea is a connects the larynx and the lungs. It is made up of cartilaginous rings which prevent it from collapsing. It divides into bronchi which further separates to form bronchioles and terminates into air filled lobes known as alveoli.

• The process of breathing ) is divided into two distinct phases, inspiration and expiration occur simultaneously.
  
• Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure,
•  Inspiration is initiated by the contraction of a diaphragm which increases the volume of the thoracic chamber in the anteroposterior axis. The contraction of external inter-costal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorsoventral axis.
• The overall increase in the thoracic volume causes a similar increase in pulmonary volume. An increase in pulmonary volume decreases the intrapulmonary pressure to less than the atmospheric pressure which forces the air from outside to move into the lungs. Hence, When the pressure of air within the lungs is less than the atmospheric pressure, air will not flow out of the lungs.
  

• Carbon dioxide is the waste gas that is produced when carbon is combined with oxygen as part of the body's energy-making processes.
• The lungs and respiratory system allow oxygen in the air to be taken into the body, it also helps the body to get rid of carbon dioxide in the air breathed out.
• When the blood from the heart flows through the capillaries and collects oxygen from the alveoli. At the same time, carbon dioxide passes out of the capillaries and into the alveoli. These blood brings oxygen to the body cells and takes away their carbon dioxide.  Hence, the circulatory system and respiratory system eliminates carbon dioxide from the body.

The digestive system includes alimentary canal and associated glands to carry out the digestion of food and release the energy stored in these food substances. The primary function of the respiratory system is to allow atmospheric oxygen from the air to enter the blood and carbon dioxide from the blood to exit into the air. The endocrine system includes endocrine glands and maintains the homeostasis of the body through hormonal actions. Nervous system serves in the reception of sensory input followed by their integration in the brain and spinal cord and generation of motor output to produce the response. Thus, the correct answer is B.

The bronchioles or bronchioli are the passageways by which air passes through the nose or mouth to the alveoli (air sacs) of the lungs. The bronchus divides into smaller branches known as segmental bronchi, which divide again into bronchioles, and then again into terminal bronchioles. 

Bronchi are tubes that branch off from the trachea into each lung. Bronchioles are smaller tubes that branch off from each bronchus. There are only two bronchi compared with the hundreds of bronchioles.

The bronchioles or bronchioli are the passageways by which air passes through the nose or mouth to the alveoli (air sacs) of the lungs, in which branches no longer contain cartilage or glands in their submucosa.  The diameter of the bronchioles is often said to be less than 1 mm, though this value can actually range from 0.3 mm to 5 mm.

The cartilage rings in the trachea prevent the trachea from collapsing during the absence of air and also protect it. These cartilaginous rings are C-shaped.

(d) Some snakes have only one bronchus and the right lung. in mammalian lungs each alveolar duct ends in a passage, called atrium, which leads into a number of rounded alveolar sacs. Each alveolar sac is studded with a large number of air sacs or alveoli. Alveoli are the sites of respiration.

Alveoli are tiny, balloon-like structures that significantly increase the total surface area of the lungs, facilitating efficient gas exchange between air and blood.

Humans use a diaphragm and rib cage muscles for ventilation (negative pressure breathing), whereas frogs use a buccal pump mechanism and lack a diaphragm.

From the larynx, air moves into lower respiratory tract, starting with the trachea and down to the intersection that branches to form the right and left primary (main) bronchi. Each of these bronchi branch into secondary (lobar) bronchi that branch into tertiary (segmental) bronchi that branch into smaller narrow tube airways called bronchioles that eventually connect with tiny specialized structures called alveoli that function in gas exchange.

Ventilation in mammals is achieved by changing the volume of the thoracic cavity, which is controlled by the contraction and relaxation of the diaphragm and the intercostal muscles.

The large number of alveoli maximizes the surface area available for the diffusion of oxygen and carbon dioxide, which is essential for efficient respiration.

Air follows the pathway: Nostrils -> Pharynx -> Larynx -> Trachea -> Bronchi -> Bronchioles -> Alveoli.

The presence of alveoli allows for efficient gas exchange. Residual air (the air remaining in lungs after forced expiration) is a normal feature of mammalian lungs that prevents collapse.

Residual volume is the air that remains in the lungs even after a maximal expiratory effort, primarily contained within the alveoli.

Gas diffusion occurs from areas of higher partial pressure to areas of lower partial pressure. Oxygen diffuses from the alveoli (high pO2) into the blood (low pO2).

Tidal volume (approx 500ml) < Expiratory reserve volume (approx 1000-1200ml) < Residual volume (approx 1200ml) < Vital capacity (approx 4600ml).

Boyle's Law states that pressure is inversely proportional to volume (P = k/V). Thus, increasing the volume of the thoracic cavity decreases the pressure inside the lungs.

As the bronchial tree branches, the diameter of the airways decreases significantly. Respiratory bronchioles are the smallest of these structures before reaching the alveolar ducts.

The trachea (windpipe) begins immediately below the larynx (voice box).

Alveoli are the functional units of the lungs where gas exchange occurs.

Pulmonary surfactant is a mixture of lipids and proteins that reduces the surface tension of the fluid lining the alveoli, preventing them from collapsing during expiration.

Alveoli are the specific sites within the lungs where oxygen enters the blood and carbon dioxide leaves the blood via diffusion across the respiratory membrane.

The lining of the respiratory tract (bronchi and bronchioles) contains ciliated cells that help move mucus and trapped particles out of the airways.

In healthy human alveoli, the partial pressure of oxygen (pO2) is approximately 104 mm Hg and the partial pressure of carbon dioxide (pCO2) is approximately 40 mm Hg.

Lymph from the lungs drains into the thoracic duct, which is the largest lymphatic vessel in the body.

The intrapleural pressure within the pleural cavity is always negative relative to atmospheric pressure, acting as a partial vacuum that pulls outward on the lung walls and prevents the lungs from collapsing between breaths.

The respiratory system terminates in the alveoli which are sac-like structures and are enveloped by the capillary network for the exchange of gases.

Alveoli are the air sacs of the lungs. They have thin walls surrounded by blood capillaries (another layer of simple squamous epithelium). Gas exchange occurs in the alveoli. During inhalation, oxygen gas is in higher concentration in the alveoli than in the blood and so it diffuses into the blood through this thin layer of cells. During exhalation, carbon dioxide is in higher concentration in the blood than in the alveoli and so it diffuses into the alveoli through this thin layer of cells.

The two large branches of the airway that lead off the trachea at the tracheal bifurcation are the bronchi. They act as the main passageway into the lungs. Each bronchus enters one of the two (either the left or the right lung). Within the lungs, the bronchi branch off into smaller and smaller passageways, the smallest of which are called bronchioles and at the end of the bronchioles are tiny air sacs that are known as the alveoli. The re-oxygenized blood is sent through meta venules to the pulmonary vein. So, the correct option is 'bronchi'.

Alveoli are tiny balloon shaped structures in the respiratory system. The alveoli are only one cell thick and are surrounded by blood capillaries. This allows the relatively easy passage of oxygen and carbon dioxide (CO2) between the alveoli and the blood. Alveoli are therefore the site of gaseous exchange in the respiratory system. So, the correct option is 'alveoli'.

The trachea or the windpipe is an airway that is a cartilaginous tube that connects the pharynx and larynx to the lungs, allowing the passage of air. It connects the nose and the mouth with the pharynx and larynx. So, the correct option is 'pharynx; larynx'.

The trachea is also known as the windpipe and is part of the passageway that supplies air to the lungs, it is about 4.5 inches long. The trachea branches into the bronchi that act as the passageways that enter the lungs. The larynx connects the nasal and oropharynx with the trachea and functions in air conduction and vocalization. The pharynx is the part of the throat behind the mouth and nasal cavity. The trachea connects the pharynx and the larynx to the lungs. So, the correct option is 'trachea'.

Even after the most forcible expiration possible some amount of air remains in the lungs. This is known as the residual volume and is about 1200 ml. So, the correct option is 'False'.

In humans, gas exchange occurs in the lungs between the alveoli and the capillaries located on the walls of the alveoli. The inhaled oxygen moves from the alveoli to the blood in the capillaries, and carbon dioxide moves from the blood in the capillaries to the air in the alveoli. So, the correct option is 'alveolar sacs'.

Even after the most forcible expiration possible some amount of air remains in the lungs. This is known as the residual volume and is about 1200 ml. This keeps the alveoli open as some air is always present inside the lung alveoli even during expiration. So, the correct option is 'true'.

Simple squamous non-ciliated epithelia are found in capillaries, alveoli, and other tissues where rapid diffusion is required. Cells are flat with flattened and oblong nuclei. It is also called pavement epithelium due to its tile-like appearance. The bronchi are lined by ciliated, pseudostratified columnar epithelium and bronchioles are lined by simple cuboidal epithelium. So, the correct option is 'alveoli'.

Each lung is enclosed by a double-layered serous membrane, called the pleura. The visceral pleura is firmly attached to the surface of the lung. At the hilum, the visceral pleura is continuous with the parietal pleura that lines the wall of the thorax.

Prawn respires with gills (Branchial respiration) and insects respire by tracheae (Tracheal respiration).

The number of alveoli in the human lungs has been estimated to be approximately 300 million. This enormous number of alveoli are meant to increase surface area of lungs for diffusion of gases.

In humans, trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of 5th thoracic vertebra into a right and left primary bronchi. Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles. The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings.

Nasal cavity open into pharynx which is common passage for transfer of food and air. Pharynx leads to larynx which is also called the voice box. Tracheae carry air to lungs and branch into left and right bronchus for entering left and right lungs. Inside lungs each bronchus divides and subdivides into terminal bronchioles.

The primary function of the respiratory system is to exchange oxygen and carbon dioxide. Inhaled oxygen enters the lungs and reaches the alveoli. The layers of cells lining the alveoli and the surrounding capillaries are each only one cell thick and are in very close contact with each other. This barrier between air and blood averages about 1 micron  in thickness. Oxygen passes quickly through this air-blood barrier into the blood in the capillaries. Similarly, carbon dioxide passes from the blood into the alveoli and is then exhaled.

Trachea is a respiratory organ of insects. It is made up of a system of branching tubes that deliver oxygen to the tissues and remove carbon dioxide from the tissues.

The lungs which form the main respiratory system is a pair of spongy, air-filled organs which is located on either side of the chest in the rib cage. The inhaled air passes through the trachea (windpipe) into the lungs through its tubular branches known as bronchi which further divide into numerous smaller tubes known as the bronchioles. 

• During cellular respiration, glucose and oxygen are changed into energy and carbon dioxide. 
• Therefore, carbon dioxide is released into the atmosphere during the process of cellular respiration. Hence, respiration is responsible for the release of carbon dioxide by animals.

• The free oxygen in the atmosphere comes from photosynthesis by green plants, chiefly the phytoplankton in the ocean.
  
•  Animals and combustion of organic matter absorb the oxygen and release CO2.
• This Carbon dioxide is absorbed by green plants which are exhaled by animals.
•  It’s a dynamic equilibrium. Hence, By observing the interdependence of plants and animals and circulation of oxygen and carbon dioxide we can say that The level of oxygen and carbon dioxide in the atmosphere is kept constant.

• All plants and animals on earth engage in a process called respiration.
• Respiration combines oxygen and the food created during photosynthesis to produce usable energy. 
• One of the byproducts of respiration is CO2,  if plants stop to take up CO2 and stop releasing O2, then plants will stop to convert sunlight into carbohydrates, and die.
• The death of autotrophs would lead to a chain reaction to the death of all higher-order organisms, Hence, Plants and animals help in maintaining the balance of Oxygen and carbon dioxide. 

• All plants and animals on earth engage in a process called respiration.
• Respiration combines oxygen and the food created during photosynthesis to produce usable energy. 
• One of the byproducts of respiration is CO2,  if plants stop to take up CO2 and stop releasing O2, then plants will stop to convert sunlight into carbohydrates, and die.
•  The death of autotrophs would lead to a chain reaction to the death of all higher-order organisms. Hence, Animals will die from the lack of oxygen if plants stop releasing oxygen into the atmosphere.

• Respiration is the movement of oxygen from the outside environment to the cells within tissues and the transport of carbon dioxide in the opposite direction.
•  During cellular respiration, glucose is broken down in the presence of oxygen to produce carbon dioxide and water. 
• The energy released during the reaction is captured by the energy-carrying molecule ATP (adenosine triphosphate). Hence, The energy releasing process in living organisms is respiration.

Physiological respiration is the complete process of respiration which includes physical and biochemical pathway. In physical pathway, there is an exchange of gasses that takes place with the help of respiratory organ i.e., lungs. In biochemical pathway, cells are involved where oxidation of food i.e. glucose takes place in mitochondria and cytoplasm in the presence of several respiratory enzymes. 

Fish brought out of water die due to inability to respire. Fish gills are specialized for extracting oxygen from water and cannot effectively extract oxygen from air. The gills collapse and dry out outside water, making gas exchange impossible.