Human Respiratory System


Dr. M.Sabir, MD

‘Preliminary knowledge of structure & functioning of the organ system of body is important for understanding mechanism of disease development in human organs , its diagnosis and treatment. All possible attempt have been made to simplify the contents so that a non-medical individual can understand basic structure & functioning of human respiratory system; but still there may be some difficulty in understanding some of the contend. Authors regret for the inconvenience.’ Editor

Primary function of respiratory system is to obtain oxygen from environment for use by body cells & eliminate carbon dioxide that is produced (by cells) as a result of energy production and other chemical reactions necessary for normal functioning of human body.

During act of respiration fresh air (rich in oxygen) is inhaled (taken in to the lungs) through nasal cavities (also through oral cavity), pharynx, larynx, trachea, branches of bronchi and through bronchioles, than ultimately to alveoli (site of gas exchange). After giving up oxygen and taking up carbon dioxide air is exhaled out of lungs through the same path.

For performing these functions the human respiratory system has different parts with specific structure & functions; can be divided into three major regions:

A. Nasopharynx Region
B. Tracheobronchial Region(wind pipes)
C. Pulmonary (Alveolar) Region


Nasopharynx Region
The head region, including the nose, mouth, pharynx (throat) and larynx – receive air from environment and pass it to wind pipe.

Nose consists of:
Visible external nose (nostrils) and the internal nasal cavity, divided into right and left cavities by nasal septum. Nasal cavities funnel into two (left and right) internal nares to be communicated to pharynx. Nose plays important role in removing impurities from the air. Hair, mucus, blood capillaries, and cilia that line the nasal cavity filter, moisten, warm, and eliminate debris from the passing air and are thrown out during sneezing.


Pharynx (throat) consists of:

Nasopharynx (nasal part) receives the incoming air from the two internal nares. Two auditory (Eustachian) tubes connecting ears with throat that equalize air pressure in the middle ear also enter here. The pharyngeal tonsil (adenoid) prevents (to some extent) bacteria from entering lungs lies at the back of the nasopharynx.

Oropharyrnx receives air from the nasopharynx and food from the oral cavity. Tonsils are located here. Laryngopharynx passes food to the esophagus (food pipe) and air to the larynx.

Larynx receives air from the laryngopharynx. It consists nine pieces of cartilage are joined by membranes and ligaments. Epiglottis, the first piece of cartilage of the larynx, which prevents entrance of food in the wind pipe during swallowing. A forward projection of the cartilage (thyroid cartilage) appears as the Adam’s apple.

Vocal cord (apparatus for producing voice) contain elastic ligaments that vibrate when skeletal muscles move them into passage of air. Various sounds, including speech, are produced in this manner.

Tracheobronchial Region (wind pipes)

It includes the trachea, bronchi (primary- right & left, secondary, tertiary), and bronchioles.

Trachea(windpipe) is a flexible tube, 10 to 12 cm (4 inches) long and 2.5 cm (1 inch) in diameter contains 16 to 20 C-shaped cartilaginous rings, helps in maintaining its lumen.

Bronchial tree– its primary job is to spread the air received through trachea over a very wide area in the lungs Primary bronchiare two tubes that branch from the trachea to the left and right lungs. In lungs, each primary bronchus divides repeatedly into branches(like an inverted tree) of smaller and smaller diameters, forming secondary lobar bronchi & tertiary (segmental) bronchi.

Bronchopulmonary segments are the division of lung separated by connective tissue septums, so eachbronchopulmonary segment can surgically removed without affecting other segments. There are ten segments in right lung and eight in left lung.

Segmental bronchi divide into many primary bronchioles (1 mm or less in diameter), which further divide into terminal bronchioles (0.5 mm in diameter).

Each of terminal bronchioles which gives rise to several respiratory bronchioles, which go on to divide into two to 11 alveolar ducts (tterminal part of the bronchial tree)

The wall of the bigger bronchi has structure similar to trachea, but as the branches of the tree get smaller, the cartilaginous rings are replaced by smooth muscle and fibrous tissue.

  1. Pulmonary (Alveolar) Region

The exchange of oxygen and carbon dioxide through the process of respiration occurs in the alveolar region.

  • Alveolar sacs– five or six alveolar sacs are associated with each alveolar duct.
  • Alveolusis the basic anatomical unit of gas exchange in the lung. This bubble like thin

walled air sacs at the end of each alveolar duct, and a cluster of adjoining alveoli is called an alveolar sac. Some adjacent alveoli are connected by alveolar pores.

  • Respiratory membraneis the one cell alveolar and capillary walls where gas exchange (mainly for oxygen & carbon oxide) occurs.
    Lungs (an overview)

The lungs are the essential organs of respiration; they are two in number, placed one on either side within the chest (thorax), and separated from each other by the heart, major blood vessels and other organs located in middle (mediastinum) of the chest.

It is the primary organ of the respiratory system, comprising the majority of the airways ( lower part of trachea, branches of bronchi, bronchioles) and alveoli, network of blood vessels & lymphatic with supporting connective tissues (elastic & other fibers etc) .

The substance of the lung is of a light, porous, spongy texture; it floats in water, owing to the presence of air in the alveoli; it is also highly elastic. The surface of the lungs is smooth, shining, and marked out into numerous polyhedral areas, indicating the lobules of the organ.

At birth the lungs are pinkish white in color; in adult life the color is a dark slaty gray, with dark patches because of deposition of substance rich in carbon; and as age advances, this becomes darker more so in males, smokers and persons exposed to environmental pollution.

Among male lungs are heavier (about 625 gm) than in the female (about 567 gm.).

Lungs are very moist, which makes it vulnerable for infections by bacteria and other organisms.

Lobes of Lungs – lungs consist of sets of lobes: three lobes make up the right lung and two lobes make up the left lung. Each lobe is further

divided in lobules & segments. The left lung is smaller in order to accommodate the heart, which is situated under the left lung and behind the rib cage.

Each lung has branches of the bronchial tree that splits off from the windpipe. With further branching, bronchi become smaller as they extend into the lung lobes, becoming bronchioles.

Between the alveoli is elastic connective tissue, which is important to accommodate the expansion and collapse of the tissue.

The pulmonary artery and pulmonary vein transport blood between the heart and lungs for gas exchange, with a network of capillaries (smallest branches of blood vessels) surrounding each alveolus(air cell).

Lung Defense – airways in the respiratory system are lined with various types of specific cells having mechanical as well as chemical (immunological) properties to defend lungs from offending material inhaled with the air. Some of these cells (epithelia) are equipped with hair-like fibers called cilia that move in a wave-like motion to sweep debris out of the lungs for expulsion.


  • Primary function is to obtain oxygen for use by body’s cells & eliminate carbon dioxide that cells produces as a result of energy production and other chemical reaction necessary for normal functioning of human body.
  • During act of respiration air is inhaled (taken in to the lungs), e.g. passing through nasal cavities (or oral cavity), pharynx, larynx, trachea, primary bronchi (right & left), secondary bronchi, tertiary bronchi, bronchioles than ultimately to alveoli (site of gas exchange).
  • Lungs have about 300 million alveoli with a total surface area of about 75 square meters where it comes in contact with the blood in flowing the capillaries (smallest blood tubes) spread over it.
  • From the air cells (alveoli), oxygen from inhaled air is transferred to small blood vessels (capillaries) where it is to be bounded to the hemoglobin present in the red blood cells in blood.
  • This oxygen rich blood goes to heart, through the network of bigger blood vessels known as pulmonary veins, for onward supply to peripheral organ of the body through aorta & network of arteries & capillaries.

Breathing is the process that moves air in and out of the lungs. It is an automatic process controlled from the brain stem and is so strong a force that one can not stop breathing for any length of time. It is an active process – requiring the contraction and relaxation of muscles of respiration (skeletal muscles around the chest).

Primary muscles of respiration include the external intercostal muscles (located between the ribs) and the diaphragm.

Diaphragm is a sheet of strong muscle & fibrous tissues located between the thoracic & abdominal cavities completely separating them (with few openings).

Inspiration (act to take air in the lungs):
The coordinated contraction of intercostals and the diaphragm brings about inspiration

Contraction of external (outer layer) intercostal muscles causes elevation of ribs(like bucket handle) & sternum to cause increased front- to-back dimension of thoracic cavity(chest) to cause decrease in pressure in the thoracic cavity(chest), lowers air pressure in lungs, so air moves into lungs.

Contraction of diaphragm causes it to moves downward to increases vertical dimension of thoracic cavity adding further to lowers air pressure in lungs, so air moves into lungs.

 (act to bring out air from lungs):
Relaxation of external intercostals muscles & diaphragm causes return of diaphragm, ribs, & sternum to resting position leading to increase in pressure in the lungs so as to move air out of lungs to atmosphere (exhalation).

At rest, breathing out, or exhaling, is a combination of passive and active processes powered by the elastic recoil of the alveoli, similar to a deflating balloon.

Respiratory rate– On an average a healthy adult (above 18 yrs) in resting conditions breaths for 10 to 20 times per minute (new born-30 to 40 times per minute). Respiratory rate normally increases during physical exertion, emotional changes & pregnancy. Discomfort and pain due to any reason and numerous diseases of the body can cause increase in respiratory rate.

Control of breathing
Breathing is one of the few bodily functions which, within limits, can be controlled both consciously and unconsciously.

Conscious control
Human speech is common example of conscious control of breathing. It is also common in many forms of meditation, yoga, swimming, fitness exercise.

Unconscious control
Unconsciously, breathing is controlled by specialized centers in the brainstem, which automatically regulate the rate and depth of breathing depending on the body’s needs at any time.

Carbon dioxide levels in the blood plays important role in controlling breathing through chemoreceptors in the carotid and aortic bodies (associated to big blood tubes) in the blood system.

When carbon dioxide levels increase in the blood, it reacts with the water in blood, producing carbonic acid. Carbonic acid and other acids produced during exercise makes blood acidic e.g. lowers pH (term pH indicates weather body fluids are acidic or alkaline).

The drop in the blood’s pH stimulates chemoreceptors in the carotid and aortic bodies in the blood system to send nerve impulses(message) to the respiration centre in the medulla oblongata and pons in the brain. These, in turn send nerve impulses through the phrenic and thoracic nerves to the diaphragm for increasing breathing.

During rest, the level of carbon dioxide is lower, so breathing rate is lower, while exercising, the level of carbon dioxide in the blood increases due to increased metabolism in the muscles, which activates carotid and aortic bodies and the respiration center, which ultimately cause a higher rate of respiration.

It is not possible for a healthy person to voluntarily stop breathing indefinitely. If a healthy person were to voluntarily stop breathing (i.e. hold his or her breath) for a long enough amount of time, he or she would loseconsciousness, and the body would resume breathing on its own. Because of this one cannot commit suicide with this method, unless one’s breathing was also restricted by something else (e.g. hanging & other ways of strangulation, drowning etc).

Components of breathing gases.

– Oxygen is the essential component of all breathing gases.

– The air we inhale contains (approximately) 78% Nitrogen, 21% Oxygen, 0.96%, Argon and 0.04% Carbon dioxide, helium and water,

– All the gases in air we exhale are roughly 4% to 5% more carbon dioxide and 4% to 5% oxygen than was inhaled.

– In addition water vapor (5%) and small amounts of hydrogen, carbon monoxide, ammonia, acetone,methanol, ethanol alcohol (unless ingested, in which case much higher concentrations would occur in the breath)etc are also present in the air we bring out from lungs(exhaled air).

Amount of exhaled oxygen and carbon dioxide varies according to the physical fitness, exercise, energy expenditure and diet of the particular person.

Oxygen & carbon dioxide transport
With each inspiration about 250 to 500 ml. of fresh air (rich in oxygen) from the environment reaches to alveolus through respiratory passage (airways).

This oxygen rich blood goes to heart (through the network of bigger blood vessels known as pulmonary veins, for onward supply to peripheral organ of the body through bigger blood tubes (blood vessels – aorta & network of arteries & capillaries).

Oxygen – major portion of oxygen in the blood remains bound to hemoglobin ( 98.5% ), small amount (1.5%) is dissolved in the plasma (liquid part of blood).

Because almost all oxygen in the blood is transported by hemoglobin, the relationship between the concentration (partial pressure) of oxygen and hemoglobin saturation (the % of hemoglobin molecules carrying oxygen) is an important one.

Carbon dioxide – transported from the body cells back to the lungs as:

  • Bicarbonate (60%) – formed when carbon dioxide (CO2) released as the result of cell metabolism, combines with water (due to the enzyme in red blood cells called carbonic anhydrase).
  • Carbaminohemoglobin ( 30%) formed when CO2 combines with hemoglobin (hemoglobin molecules that have given up their oxygen)
  • Dissolved in the plasma ( 10% )

Pleura are continuous thin serous membrane, folds back to cover both the lungs & inner surface of chest cage.

Pleural cavity is the thin space between the two pleural layers that surrounds the lungs on both sides in chest cage. It normally contains small amount of pleural fluid which allows the pleurae to move smoothly against each other during breathing. Parietal pleura is the outer layer of pleura is attached to the chest wall. Visceral pleura is the inner layer of pleura covers the lungs and adjoining structures, viz. blood vessels, bronchi andnerves.

Pleura & pleural cavity plays important role in act of respiration and also add to protection provided by chest cage (ribs, vertebrae, muscles and skin) The parietal pleuron is highly sensitive to pain, while the visceral pleuron is not, due to absence of sensory nerves. In most cases pain due to lung diseases are because of irritation of this part of pleura.

Pleural fluid is produced (few milliliters) and reabsorbed continuously. Up to forty times of normal production of larger quantities of fluid can accumulate in the pleural space and can be reabsorbed by increasing the reabsorption rate up to 40 times the normal rate (as an increase in physiological response to accumulating fluid).

Profound increase in the production of pleural fluid, or some blocking of the reabsorbing lymphatic system is required for fluid to accumulate in the pleural space which is called pleural effusion.

The process of breathing goes on through entire life, from birth to death. On an average in resting conditions an individual breaths 12 to 20 times in a minute, inhaling about 8,000 to 9,000 liters of air, sufficient to provide oxygen to about 9,000 liters of blood passing through lungs per day.

Dr. M. Sabir,
Vice President 2010-11, ICS
Editor, Public Forum, ICS-Website
Former Member, Faculty Council,Indian College of Physician, API
Prof. & Head, Department of Medicine,MAMC, Agroha, Hisar
Former Professor & Head,Respiratory Division, Department of Medicine,
S.P.Medical College, Bikaner