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RESPIRATORY SYSTEM

An Introduction to the Respiratory System

An efficient respiratory system must do more than merely move air. Cells need energy for maintenance,

growth, defense, and division. Our cells obtain that energy mainly through aerobic mechanisms that require

oxygen and produce carbon dioxide, the major end product of oxidative metabolism. A unicellular organism

can exchange oxygen and carbon dioxide directly with the external environment, but this is obviously

impossible for most cells of a complex organism like a human being. Therefore, the evolution of large

animals required the development of specialized structures to exchange oxygen and carbon dioxide for the

entire animal with the external environment. In humans (and other mammals), the respiratory system

includes the lungs, the series of tubes leading to the lungs, and the chest structures responsible for moving

air into and out of the lungs during breathing. The cardiovascular system is the link between interstitial

fluids and the exchange surfaces of lungs. Circulating blood carries oxygen from the lungs to peripheral

tissues. Blood also accepts and transports the carbon dioxide generated by those tissues, delivering it to the

lungs.

Functions of the Respiration

1. Respiration helps to exchange oxygen from the lungs to the tissues and carbon dioxide from the tissues to

the lungs through the blood.

2. The body consumes oxygen and produce carbon dioxide continuously. As the carbon dioxide levels in

the body increase acidity increases which is toxic to the cells. But the carbon dioxide is continuously

eliminated from the body through the lungs during expiration. Thus, respiration helps to maintain

homeostasis in the body.

3. It also helpful to maintain PH of the blood and other body fluids by eliminating carbon dioxide.

4. Respiration also helps to maintain temperature of the body as some heat is lost through exhaled air.

5. The process of respiration also acts as a means for expressing human emotions such as crying, laughing,

sighing and sobbing.

Organization of the Respiratory System

We can organize the respiratory system from either an anatomical or a functional perspective. Anatomically,

we can divide the system into an upper respiratory system and a lower respiratory system. The upper

respiratory system consists of the nose, nasal cavity, paranasal sinuses, and pharynx (throat). These

passageways filter, warm, and humidify incoming air, protecting the more delicate surfaces of the lower

respiratory system. They also cool and dehumidify outgoing air. The lower respiratory system includes the

larynx (voice box), trachea (windpipe), bronchi, bronchioles, and alveoli of the lungs.

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The term respiratory tract refers to the passageways that carry air to and from the exchange surfaces of the

lungs. The conducting zone of the respiratory tract begins at the entrance to the nasal cavity and extends

through the pharynx, larynx, trachea, bronchi, and larger bronchioles. The respiratory zone of the tract

includes the smallest, most delicate bronchioles and the associated alveoli. To meet the metabolic

requirements of peripheral tissues, the surface area for gas exchange in the lungs must be very large. It is

about 35 times the surface area of human body. Estimates of the surface area involved in gas exchange

range from 70 m2 to 140 m2.

Filtering, warming, and humidifying inhaled air begin at the entrance to the respiratory tract and continue as

air passes along the conducting portion. By the time air reaches the alveoli, most foreign particles and

pathogens have been removed, and the humidity and temperature are within acceptable limits. The success

of this “conditioning process” is due to the respiratory mucosa.

The Respiratory Defense System

Debris or pathogens in inhaled air can severely damage the delicate exchange surfaces of the respiratory

system. A series of filtration mechanisms that make up the respiratory defense system prevent such

contamination. Along much of the respiratory tract, mucous cells in the epithelium and mucous glands in the

lamina propria produce sticky mucus that bathes exposed surfaces. In the nasal cavity, cilia sweep that

mucus and any trapped debris or microorganisms toward the pharynx. There it is swallowed and exposed to

the acids and enzymes of the stomach. In the lower respiratory system, the cilia beat toward the pharynx,

moving a carpet of mucus in that direction and cleaning the respiratory surfaces. This process is often called

a mucus escalator.

Filtration in the nasal cavity removes virtually all particles larger than about 10µm from the inhaled air.

Smaller particles may be trapped by the mucus of the nasopharynx or by secretions of the pharynx. The rate

of mucus production in the nasal cavity and paranasal sinuses speeds up upon exposure to unpleasant

stimuli, such as noxious vapors, large quantities of dust and debris, allergens, or pathogens. The familiar

signs and symptoms of the “common cold” appear when any of more than 200 types of viruses invades the

respiratory epithelium.

The upper respiratory system (Located outside the thoracic cavity):

The upper respiratory system consists of the nose, nasal cavity, paranasal sinuses, and pharynx.

The nose is the primary passageway for air entering the respiratory system. Air normally enters through the

paired external nares, or nostrils, which open into the nasal cavity. The nasal vestibule is the space

contained within the flexible tissues of the nose. The epithelium of the vestibule contains coarse hairs. Large

airborne particles, such as sand, sawdust, or even insects, are trapped in these hairs and prevented from

entering the nasal cavity. The nasal septum divides the nasal cavity into left and right portions. The mucous

secretions produced in the paranasal sinuses (sinuses of the frontal, sphenoid, ethmoid, and paired maxillary

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and palatine bones) help keep the surfaces of the nasal cavity moist and clean. The tears draining through the

nasolacrimal ducts do so as well.

To pass from the vestibule to the internal nares, air tends to flow between adjacent conchae. These are

narrow grooves rather than open passageways. This results in turbulence and serves several purposes. As the

air swirls, small airborne particles are likely to come into contact with the mucus that coats the lining of the

nasal cavity. In addition, the turbulence provides extra time for warming and humidifying incoming air. It

also creates circular air currents that bring olfactory stimuli to the olfactory receptors. The nasal cavity opens

into the nasopharynx through a connection known as the internal nares.

The Pharynx

The pharynx, or throat, is a chamber shared by the digestive and respiratory systems. We can divide the

pharynx into the nasopharynx, the oropharynx, and the laryngopharynx.

1. The nasopharynx is the superior portion of the pharynx. It is connected to the posterior portion of the

nasal cavity through the internal nares. The soft palate separates it from the oral cavity. The left and right

auditory tubes open into the nasopharynx on either side of this tonsil.

2. The oropharynx is the posterior portion of the oral cavity, communicates directly with the oropharynx.

3. The narrow laryngopharynx is the inferior part of the pharynx. It includes that portion of the pharynx

between the hyoid bone and the entrance to the larynx and esophagus. Like the oropharynx, the

laryngopharynx is lined with a stratified squamous epithelium that resists abrasion, chemical attack, and

invasion by pathogens.

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Larynx

The larynx, or voice box, is a short passageway that connects the laryngopharynx with the trachea. It lies in

the midline of the neck anterior to the esophagus. The wall of the larynx is composed of nine pieces of

cartilage. Three occur singly (thyroid cartilage, epiglottis, and cricoid cartilage), and three occur in pairs

(arytenoid, cuneiform, and corniculate cartilages). Of the paired cartilages, the arytenoids cartilages are the

most important because they influence changes in position and tension of the vocal folds (true vocal cords for

speech). The extrinsic muscles of the larynx connect the cartilages to other structures in the throat; the

intrinsic muscles connect the cartilages to one another.

Trachea

The trachea, or windpipe, is a tubular passageway for air that is about 12 cm (5 in.) long and 2.5 cm (1 in.)

in diameter. It is located anterior to the esophagus and extends from the larynx to the superior border of the

fifth thoracic vertebra (T5), where it divides into right and left primary bronchi. It consists of dense regular

connective tissue and smooth muscle reinforced with 15–20 C-shaped pieces of hyaline cartilage. The

cartilages support the anterior and lateral sides of the trachea. They protect the trachea and maintain an open

passageway for air.

Bronchi

At the superior border of the fifth thoracic vertebra, the trachea divides into a right primary bronchus,

which goes into the right lung, and a left primary bronchus, which goes into the left lung. The right primary

bronchus is more vertical, shorter, and wider than the left. On entering the lungs, the primary bronchi divide

to form smaller bronchi—the secondary bronchi, one for each lobe of the lung. (The right lung has three

lobes; the left lung has two.) The secondary bronchi continue to branch, forming still smaller bronchi, called

tertiary bronchi, that divide into bronchioles. Bronchioles in turn branch repeatedly, and the smallest ones

branch into even smaller tubes called terminal bronchioles. This extensive branching from the trachea

resembles an inverted tree and is commonly referred to as the bronchial tree.

As the branching becomes more extensive in the bronchial tree, several structural changes may be noted.

The terminal bronchioles divide to form respiratory bronchioles, which have a few attached alveoli. Alveoli

are small, air-filled chambers where gas exchange between the air and blood takes place. There are

approximately 300 million alveoli in the two lungs. As the respiratory bronchioles divide to form smaller

respiratory bronchioles, the number of attached alveoli increases. Approximately seven generations of

branching occur from the terminal bronchioles to the alveolar ducts.

Lungs

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The lungs are paired cone-shaped organs in the thoracic cavity. Its tip, or apex, points superiorly. The apex on

each side extends superior to the first rib. The broad concave inferior portion, or base, of each lung rests on

the superior surface of the diaphragm. The left and right lungs are surrounded by the left and right pleural

cavities, respectively. They are separated from each other by the heart and other structures in the

mediastinum, which divides the thoracic cavity into two anatomically distinct chambers. As a result, if trauma

causes one lung to collapse, the other may remain expanded.

Alveoli

Around the circumference of the alveolar ducts are numerous alveoli and alveolar sacs. An alveolus is a cup

shaped out pouching lined by simple squamous epithelium and supported by a thin elastic basement

membrane; an alveolar sac consists of two or more alveoli that share a common opening. The walls of alveoli

consist of two types of alveolar epithelial cells. The more numerous type I alveolar cells are simple

squamous epithelial cells that form a nearly continuous lining of the alveolar wall. Type II alveolar cells, also

called septal cells, are fewer in number and are found between type I alveolar cells. The thin type I alveolar

cells are the main sites of gas exchange. Type II alveolar cells, rounded or cuboidal epithelial cells containing

microvilli, secrete alveolar fluid, which keeps the surface between the cells and the air moist. Included in the

alveolar fluid is surfactant, a complex mixture of phospholipids and lipoproteins. Surfactant lowers the

surface tension of alveolar fluid, which reduces the tendency of alveoli to collapse. Roaming alveolar

macrophages, or dust cells, patrol the epithelial surface. They phagocytize any particles that have eluded

other defenses.

The exchange of O2 and CO2 between the air spaces in the lungs and the blood takes place by diffusion across

the alveolar and capillary walls, which together form the respiratory membrane. It contains

1. A thin layer of fluid lining the alveolus

2. The alveolar epithelium composed of simple squamous epithelium

3. The basement membrane of the alveolar epithelium

4. A thin interstitial space

5. The basement membrane of the pulmonary capillary endothelium

6. The pulmonary capillary endothelium composed of simple squamous epithelium

At the respiratory membrane, only a very short distance separates alveolar air from blood. The total distance

can be as little as 0.1 µm, but averages about 0.5 µm. Diffusion proceeds very rapidly across the respiratory

membrane because the distance is short and both oxygen and carbon dioxide are small, lipid-soluble

molecules.

Pleura

The lungs are contained within the thoracic cavity, but each lung is surrounded by a separate pleural cavity

formed by the pleural serous membranes. The parietal pleura cover the inner thoracic wall, diaphragm, and

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mediastinum. The parietal pleura is continuous with the visceral pleura, which covers the surface of the

lung. The pleural cavity is filled with pleural fluid, which is produced by the pleural membranes. The pleural

fluid does two things:

(1) It acts as a lubricant, allowing the parietal and visceral pleural membranes to slide past each other as the

lungs and the thorax change shape during respiration, and

(2) It helps hold the parietal and visceral pleural membranes together.

The pleural fluid acts like a thin film of water between two sheets of glass (the visceral and parietal pleurae);

the glass sheets can slide over each other easily, but it is difficult to separate them.

The Blood Supply to the Lungs

Two circuits nourish lung tissue. One supplies the respiratory portion of the lungs. The other perfuse the

conducting portion. The respiratory exchange surfaces receive blood from the pulmonary arteries. The

pulmonary arteries carry deoxygenated blood. They enter the lungs at the hilum and branch with the bronchi

as they approach the lobules. Each lobule receives an arteriole and a venule, and a network of capillaries

surrounds each alveolus as part of the respiratory membrane. Oxygen-rich blood from the alveolar capillaries

passes through the pulmonary venules and then enters the pulmonary veins, which deliver the blood to the

left atrium. In addition to providing for gas exchange, the endothelial cells of the alveolar capillaries are the

primary source of angiotensin-converting enzyme (ACE), which converts circulating angiotensin I to

angiotensin II. This enzyme plays an important role in regulating blood volume and blood pressure.

The tissues of conducting passageways of your lungs receive oxygen and nutrients from capillaries supplied

by the bronchial arteries, which branch from the thoracic aorta. The venous blood from these bronchial

capillaries empties into bronchial veins and then into azygos vein and then empties into superior venacava.

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Physiology of respiration

The process of exchange of gases (O2 and CO2) between body cells and the environment is known as

respiration. The process of respiration involves:

1. Breathing or pulmonary ventilation

2. Exchange of gases

a. External respiration

b. Internal respiration

1. Breathing or pulmonary ventilation: It is the inward and outward movement of air between the

environment and the alveoli of the lungs. Breathing supplies oxygen to the lungs and eliminates carbon

dioxide. Breathing involves two phases:

a. Inspiration or inhalation: Breathing in is known as inspiration. When the diaphragm and

external intercoastal muscles are contracted, contraction of these muscles elevates the ribs. The size of

the thoracic cavity increases. When the diaphragm contracts, its dome-shaped structure tends to get

flattened which eventually increases the volume of the thoracic cavity. Under normal conditions the

descent of the diaphragm by 1cm creates a pressure difference of about 1-3 mmHg due to which a

quantity of about 500 ml of air enters into the lungs. During quite breathing, inhalation lasts for about 2

seconds. However, under stressful conditions, the diaphragm descends by 10 cm, the pressure difference

being even more i.e. 100 mmHg and therefore inhalation of about 2-3 liters of air.

b. Expiration or exhalation: Breathing out is known as expiration or exhalation. Inspiration is

followed by relaxation of the diaphragm and the external intercoastal muscles.

Relaxation of diaphragm makes it acquire its characteristic dome-shaped structure while relaxation of

the external intercoastal muscles depresses the ribs. As a result, the thoracic cavity decreases followed

by a decrease in the volume of the lungs. This results in an increased pressure within the alveoli of the

lungs than the atmospheric pressure. As a result, air flows from the alveoli to the atmosphere. During

quite breathing, exhalation lasts for about 3 seconds.

2. Exchange of gases: Exchange of gases takes place by diffusion across semi permeable membrane from a

region of higher concentration to a region of lower concentration until equilibrium is reached. Exchange of

gases takes place at two levels:

a. External respiration: The exchange of gases (O2 and CO2) between the alveoli of the lungs and the

blood in the pulmonary capillaries is known as external respiration. In this process, the oxygen from the

Partial Pressure:It is the pressure exerted by a gas in a mixture of gases. According to Dalton’s law,

in a mixture of gases, the part of the total pressure resulting from each type of gas is determined by

the percentage of the total volume represented by each gas type. For example, if the total pressure of

all gases in a mixture of gases is 760 millimeters of mercury (mm Hg), which is the atmospheric

pressure at sea level, and 21% of the mixture is made up of oxygen, then the partial pressure for

oxygen is 160 mm Hg (0.21 × 760 mm Hg = 160 mm Hg).

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alveoli passes into the pulmonary capillaries while the diffusion of CO2 occurs in the reverse direction.

Because the PO2 of venous blood in the pulmonary arteries is only 40 mm Hg, as opposed to a PO2 of

approximately 104 mm Hg in the alveoli, a steep oxygen partial pressure gradient exists, and O2 diffuses

rapidly from the alveoli into the pulmonary capillary blood. Equilibrium—that is, a PO2 of 104 mm Hg on

both sides of the respiratory membrane—usually occurs in 0.25 second, which is about one-third the time

a red blood cell is in a pulmonary capillary. Carbon dioxide moves in the opposite direction along a much

gentler partial pressure gradient of about 5 mm Hg (45 mm Hg to 40 mm Hg) until equilibrium occurs at

40 mm Hg. Carbon dioxide is then expelled gradually from the alveoli during expiration. Even though

the O2 pressure gradient for oxygen diffusion is much steeper than the CO2 gradient, equal amounts of

these gases are exchanged because CO2 is 20 times more soluble in plasma and alveolar fluid than O2.

b. Internal respiration: In internal respiration, the partial pressure and diffusion gradients are reversed

from the situation described for external respiration and pulmonary gas exchange. However, the factors

promoting gas exchanges between the systemic

capillaries and the tissue cells are essentially

identical to those acting in the lungs. Tissue

cells continuously use O2 for their metabolic

activities and produce CO2. Because PO2 in the

tissues is always lower than that in the systemic

arterial blood (40 mm Hg versus 100 mm Hg),

O2 moves rapidly from the blood into the

tissues until equilibrium is reached, and CO2

moves quickly along its pressure gradient into

the blood. As a result, venous blood draining

the tissue capillary beds and returning to the

heart has a PO2 of 40 mm Hg and a PCO2 of 45

mm Hg.

Transport of gases in blood:

Blood transports oxygen and carbon dioxide

between the alveoli of the lungs and the tissues.

1. Transportation of oxygen: Oxygen is

carried in the blood mainly by haemoglobin.

Haemoglobin combines with oxygen to form

oxyhaemoglobin. The binding of oxygen and

haemoglobin is a reversible reaction. The

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oxyhaemoglobin is a reversible reaction.

Hb + O2 HbO2

The oxyhaemoglobin is unstable and under certain conditions, it loses its oxygen to form

deoxyhaemoglobin (reduced haemoglobin). The binding and dissociation of oxygen from haemoglobin

depends on:

a. Partial pressure of oxygen (Po2): If Po2 is high, haemoglobin binds with more amount of oxygen. If

Po2 is low, haemoglobin binds with only small amount of oxygen.

b. PH: when decreases i.e. acidity increases (where hypoxia conditions), oxygen readily dissociates from

haemoglobin. Therefore, more oxygen is available to the metabolically active tissues which produce

acids. (Ex: lactic acid, carbonic acid).

PH also increases with decrease in Po2. Carbon dioxide is converted to carbonic acid in the RBC by an

enzyme carbonic anhydrase. Carbonic acid (H2CO3) dissociates into H+ and HCO3 ions. As the

concentration of H+ ions increases, the PH of blood decreases. Therefore, when Pco2 increases blood PH

decreases i.e. oxygen dissociates more readily from haemoglobin and so, more oxygen is available for

tissues.

c. Body temparature: If the body temparature increases, oxygen dissociates more readily from

haemoglobin. Hence, more amount of oxygen is available for active tissues which liberate heat due to

metabolic reactions.

2. Transportation of carbon dioxide: Carbon dioxide produced by the body cells as a metabolic waste

product is transported in the blood in three forms.

a. Bicarbonate ions(HCO3): most of the CO2 (about 70%) is transported as bicarbonate ions in the blood.

When CO2 enters RBCs, it combines with water to form carbonic acid. The carbonic acid dissociates into

H+ and HCO3- ions. Now HCO3

- ions moves into the plasma (due to concentration gradient) while Cl- ions

in the plasma moves into the RBCs. This is known as chloride shift. When the blood reaches the

pulmonary capillaries in the lungs, HCO3- ions get converted to CO2 by reverse of the above reaction. The

CO2 thus formed diffuses into the alveoli and is exhaled by the lungs during expiration.

b. Carbaminohaemoglobin: when CO2 enters the RBCs, some (about 23%) of it loosely combines with the

amino group of haemoglobin to form carbaminohaemoglobin. When the blood reaches the pulmonary

capillaries, carbaminohaemoglobin splits and the CO2 diffuses into the alveoli.

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c. A small amount of CO2 gets dissolved in the blood plasma to form carbonic acid. When the blood

reaches the pulmonary capillaries, carbonic acid dissociates into CO2 and diffuses into the alveoli.

Mechanism of respiration:

The respiratory muscles(diaphragm and intercoastal muscles) are the skeletal muscles which contract and

relax upon receiving nerve impulses from the respiratory center located in the medulla oblongata and pons of

the brain stem. The respiratory centre contains two areas:

1. Inspiratory center and

2. Expiratory center

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The respiratory center sends the nerve impulses via phrenic nerve and intercoastal nerve to the diaphragm and

external intercoastal muscles respectively resulting in their contraction. As a result, the thoracic cavity

increases followed by an increase in the pleural cavity and the volume of the lungs. This results in a decrease

in the pressure within the alveoli of lungs (alveolar pressure) to subatmospheric pressure. Hence, air is drawn

into the lungs.

When the atmospheric pressure equilizes the alveolar pressure, the impulses from the respiratory center

ceases. This results in the relaxation of the diaphragm and external intercoastal muscles and so the volume of

the thoracic cavity decreases. This is followed by an increase in the intrapleural pressure and alveolar pressure

greater than the atmospheric pressure. Hence, the air is expelled from the lungs.

During forceful inspiration, accessory muscles of respiration (sternocleidomastoid muscles, scalene muscles

and pectoralis minor muscles) also contract in addition to the diaphragm and external intercoastal muscles to

increase pressure in the abdominal region and thorax.

Regulation / Control of respiration:

At rest, about 200 mL of O2 used each minute by body cells. During strenuous exercise, however, O2 use

typically increases 15- to 20-fold in normal healthy adults, and as much as 30-fold in elite endurance-trained

athletes. Several mechanisms help match respiratory effort to metabolic demand.

Usually respiration is under autonomic control (involuntary). Respiration is regulated by voluntary

control during activities such as speaking, singing, etc. the autonomic system system of the respiration is

located in the pons and medulla oblongata and is collectively referred to as the respiratory center.

Respiratory center:

The respiratory center is divided into three areas based on their function as,

1. Medullary rhythmicity center

2. Pneumotaxic center

3. Apneutic center

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1. Medullary rhythamicity center is formed by a group of neurons in the medulla oblongata whichcontrol

the rate and rhythm of respiration. It consists of an inspiratory area and an expiratory area. The nerve

impulses generated in the inspiratory center passes to the diaphragm and external intercoastal muscles via

phrenic nerve and intercoastal nerve respectively. The diaphragm and external intercoastal muscles contract

for about 2 seconds wherein they receive the impulses and inspiration occurs. When the nerve impulses

from the inspiratory center stops, the diaphragm and the external intercoastal muscles relax for 3 seconds

resulting in expiration.

During normal (quite) breathing, the expiratory

center remains inactive. But, during forceful

breathing, the expiratory center is activated and sends

impulses to the abdominal and internal intercoastal

muscles resulting in their contraction. As a result, the

thoracic cavity decreases and causes expiration to

occur forcefully.

2. Pneumotaxic center is located in the upper pons

of the brain stem. After inspiration, it transmitts the

nerve impulses to inhibit the inspiratory center,

thereby allowing expiration. Therefore the

pneumotaxic center prevents the complete filling of

lungs with air. The breathing rate is directly

influenced by the activity of pneumotaxic center,

greater activity, rapid the breathing rate.

3. Apneustic center is located in the lower pons of the brain stem. It stimulates the inspiratory center to

become active and prolongs the inhalation i.e. long and deep inhalation.

Regulation of the Respiratory Center

The basic rhythm of respiration set and coordinated by the inspiratory area can be modified in response to

inputs from other brain regions, receptors in the peripheral nervous system, and other factors.

a) Cortical Influences on Respiration

Because the cerebral cortex has connections with the respiratory center, we can voluntarily alter our pattern

of breathing. We can even refuse to breathe at all for a short time. Voluntary control is protective because it

enables us to prevent water or irritating gases from entering the lungs. The ability to not breathe, however, is

limited by the buildup of CO2 and H+ in the body. When PCO2 and H+ concentrations increase to a certain

level, the inspiratory area is strongly stimulated, nerve impulses are sent along the phrenic and intercostal

nerves to inspiratory muscles, and breathing resumes, whether the person wants it to or not. Nerve impulses

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from the hypothalamus and limbic system also stimulate the respiratory center, allowing emotional stimuli

to alter respirations as, for example, in laughing and crying.

b) Chemoreceptor Regulation of Respiration

Chemoreceptors respond to changes in the levels of oxygen and carbon dioxide in the blood and other body

fluids. There are two types of chemoreceptors.

i.Central chemoreceptors located in the medulla oblongata respond to the changes in Po2 and H+ ion

concentrations in CSF. When Pco2 in the arterial blood increases, the central chemoreceptors get stimulated

and activate the respiratory center to increase breathing. Due to this, the arterial P co2 decreases to normal.

ii.Peripheral chemoreceptors located in the carotid bodies and in aortic bodies (located in arch of aorta)

responds to the changes in Pco2, Po2 and H+ ions in the blood. When the arterial Pco2 rises, the peripheral

chemoreceptors are also stimulated to activate the respiratory center.

Peripheral chemoreceptors also respond to changes in Po2 in addition to the changes in Pco2. When the

arterial Po2 decreases, the peripheral chemoreceptors stimulate the respiratory center.

c) Stretch receptors are located in in the walls of the lungs. When the lungs are inflated, they send

inhibitory impulses to the inspiratory center via the vagus nerve. Thereby, the inspiratory center is inhibited

and the air is exhaled from the lungs. After the air is exhaled, inspiration stars again. This mechanism is

known as inflation reflex which is a protective mechanism to prevent over inflation of lungs.

Pulmonary Volumes and Capacities

Spirometry (spiro, to breathe + metron, to measure) is the process of measuring volumes of air that move

into and out of the respiratory system, and a spirometer is a device used to measure these pulmonary

volumes. The four pulmonary volumes and representative values for a young adult male follow:

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1. Tidal volume is the volume of air inspired or expired with each breath. At rest, quiet breathing results in

a tidal volume of approximately 500 ml.

2. Inspiratory reserve volume is the amount of air that can be inspired forcefully after inspiration of the

tidal volume (approximately 3000 ml at rest).

3. Expiratory reserve volume is the amount of air that can be forcefully expired after expiration of the tidal

volume (approximately 1100 ml at rest).

4. Residual volume is the volume of air still remaining in the respiratory passages and lungs after the most

forceful expiration (approximately 1200 ml).

The tidal volume increases when a person is more active. An increase in tidal volume causes a decrease in

the inspiratory and expiratory reserve volumes because the maximum volume of the respiratory system does

not change from moment to moment.

Pulmonary capacities are the sum of two or more pulmonary volumes. Some pulmonary capacities follow:

1. Inspiratory capacity is the tidal volume plus the inspiratory reserve volume, which is the amount of air

that a person can inspire maximally after a normal expiration (approximately 3500 ml at rest).

2. Functional residual capacity is the expiratory reserve volume plus the residual volume, which is the

amount of air remaining in the lungs at the end of a normal expiration (approximately 2300 ml at rest).

3. Vital capacity is the sum of the inspiratory reserve volume, the tidal volume, and the expiratory reserve

volume, which is the maximum volume of air that a person can expel from the respiratory tract after a

maximum inspiration (approximately 4600 ml).

4. Total lung capacity is the sum of the inspiratory and expiratory reserve volumes plus the tidal volume

and the residual volume (approximately 5800 ml).

RESPIRATORY DISORDERS

Hypoxia is a pathological condition in which the body as a whole (generalized hypoxia) or a region of

the body (tissue hypoxia) is deprived of adequate oxygen supply. Variations in arterial oxygen

concentrations can be part of the normal physiology, for example, during strenuous physical exercise. A

mismatch between oxygen supply and its demand at the cellular level may result in a hypoxic condition.

Hypoxia in which there is complete deprivation of oxygen supply is referred to as anoxia.

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Asphyxia (from Greek a-, "without" (sphygmos), "heartbeat") is a condition of severely deficient supply

of oxygen to the body that arises from being unable to breathe normally. An example of asphyxia is

choking. Asphyxia causes generalized hypoxia, which primarily affects the tissues and organs.

Dysbarism refers to medical conditions resulting from changes in ambient pressure. Various activities

are associated with pressure changes. Scuba diving is the most frequently cited example, but pressure

changes also affect people who work in other pressurized environments (for example, caisson workers),

and people who move between different altitudes.

Ambient pressure is the pressure in the water around the diver (or the air, with caisson workers etc). As a

diver descends, the ambient pressure increases. At 10 meters (33 feet) in salt water, it is twice the normal

pressure on land at sea level. At 40 meters (the recommended safety limit for recreational diving) it is 5

times the pressure at sea level.

Pressure decreases as we rise above sea level, but less dramatically. At 3000 feet altitude (almost 1000

meters), the ambient pressure is almost 90% of sea level pressure. Ambient pressure does not drop to

50% of sea level pressure until 20,000 feet or 6,000 meters altitude.

Oxygen therapy is the administration of oxygen as a medical intervention, which can be for a variety of

purposes in both chronic and acute patient care. Oxygen is essential for cell metabolism, and in turn,

tissue oxygenation is essential for all normal physiological functions.

Room air only contains 21% oxygen, and increasing the fraction of oxygen in the breathing gas increases

the amount of oxygen in the blood. It is often only required to raise the fraction of oxygen delivered to

30–35% and this is done by use of a nasal canula. When 100% oxygen is needed, it may be delivered via

a tight-fitting face mask, or by supplying 100% oxygen to an incubator in the case of infants. Oxygen can

be administered in other ways, including specific treatments at raised air pressure, such as hyperbaric

oxygen therapy.

High blood and tissue levels of oxygen can be helpful or damaging, depending on circumstances and

oxygen therapy should be used to benefit the patient by increasing the supply of oxygen to the lungs and

thereby increasing the availability of oxygen to the body tissues, especially when the patient is suffering

from hypoxia and/or hypoxemia.

Cardiopulmonary resuscitation (CPR) is an emergency procedure for people in cardiac arrest or, in

some circumstances, respiratory arrest. CPR is performed both in hospitals and in pre-hospital settings.

CPR involves physical interventions to create artificial circulation through rhythmic pressing on the

patient's chest to manually pump blood through the heart, called chest compressions, and usually also

involves the rescuer exhaling into the patient (or using a device to simulate this) to ventilate the lungs and

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pass oxygen in to the blood, called artificial respiration. Some protocols now downplay the importance of

the artificial respirations, and focus on the chest compressions only (CCR).

Despite its name, CPR is unlikely to restart the heart; its main purpose is to maintain a flow of

oxygenated blood to the brain and the heart, which are both the most essential organs to life and the most

vulnerable to damage from lack of oxygen (hypoxia). Effective CPR helps by delaying tissue death and

extending the brief window of opportunity for a successful resuscitation without permanent brain

damage. Advanced life support, including intravenous drugs and defibrillation (the administration of an

electric shock to the heart) is usually needed to restore a viable or "perfusing" heart rhythm--one which

will support life. This only works for patients in certain heart rhythms, namely ventricular

fibrillation or pulseless ventricular tachycardia, rather than the 'flat line' asystolic patient, although CPR

can help induce a shockable rhythm in an asystolic patient.