Respiratory SystemDifferences b/w Breathing & RespirationS. No. Breathing ( Ventilation ) Respiration 1. It is the first step of respiration & is a

physical process.It is a biochemical process & catabolic process.

2. It involves inspiration of fresh air ( O₂ ) to the respiratory organs & expiration of foul air from the respiratory organs ( CO₂ ).

It involves exchange of gases ( i.e., O₂ & CO₂ ) & step-wise oxidation of food with the incoming oxygen; & elimination of carbon dioxide produced in the cells during oxidation.

3. It does not release any energy. It releases energy that is stored in ATP during oxidation of food.

4. It is an extracellular process. It is an intracellular process.5. It does not involve enzyme action. It involves a number of enzymes in oxidation.6. Breathing mechanism varies in different

animals.Respiratory mechanism is similar in all animals.

7. It is confined to certain organs only. It occurs in all cells of the body

Respiratory Surface The surface at which exchange of gases ( CO₂ & O₂ ) occurs is termed respiratory surface. This surface must have enough area for gas exchange to meet the metabolic needs of the

organism.

Respiratory Medium Most of the earth’s oxygen occurs in the air, but some is dissolved in water. Thus, air or

water may serve as the source of oxygen for the animals. The source of oxygen is called respiratory medium. The respiratory medium supplies oxygen to the body at the body’s respiratory surface. The body’s respiratory surface may be the general body surface or some specialized area

such as lung or gill. Most of the animal’s cells lie some distance from the respiratory surface , & obtain oxygen

from the tissue fluid, which bathes all the cells. Blood brings oxygen from the respiratory surface to the tissue fluid & carries CO₂ from the tissue fluid in the reverse direction.

Exchange of gases across a respiratory membrane occurs by diffusion, & diffusion can distribute substances over a short distance only, about 0.5 mm. Therefore, the animals

having cells more distant than 0.5 mm from the respiratory surface require circulatory system to transport the gases b/w the respiratory surface & the tissue fluid present around the cells.

Differences b/w Anaerobic & Aerobic RespirationS. No. Anaerobic Respiration ( Fermentation ) Aerobic Respiration1. It does not use molecular O₂. It uses molecular O₂.2. It may or may not release CO₂ It always releases CO₂.3. It does not produce H₂O. It produces H₂O.4. It provides less energy ( only 5% of that is

available in glucose ).It provides much more energy ( whole of that is available in glucose ).

5. It consists of 2 steps :- glycolysis & incomplete breakdown of pyruvate.

It consists of 5 steps :- glycolysis, pyruvate oxidation, TCA cycle, ETS & chemiosmotic ATP synthesis.

6. It yields organic end product with or without an inorganic one.

It yields inorganic end products only.

7. It occurs in the cytoplasm only. It occurs partly in the cytoplasm & partly in the mitochondria.

8. It is found in a few organisms such as yeasts, some bacteria & parasitic worms.

It is found in the majority of animals & plants.

The organisms which carry on aerobic respiration are termed aerobes whereas those which carry on anaerobic respiration are termed anaerobes.

Certain cells, such as mammalian red blood corpuscles, lack mitochondria for aerobic respiration & must depend on anaerobic respiration.

Anaerobic respiration is also occurs in certain tissues such as skeletal muscles during vigorous movements, by producing lactic acid from glucose.

Functions of Respiration1. Metabolic Role ( Energy Production ) :- Respiration provides O₂ for aerobic metabolism

to release energy for creating ATP molecules that later power the body activities.2. Excretion :- It excretes CO₂ & volatile substances such as ammonia, ketone bodies,

alcohol, water vapour, etc.3. Maintenance of Acid-Base Balance :- This is done by adjusting the amount of CO₂

elimination.4. Maintenance of Temperature :- Large amount of heat is lost in expired air.5. Return of Blood & Lymph :- During inspiration, the intrathoracic pressure falls & the

intraabdominal pressure rises. This facilitates the return of blood & lymph.6. Detoxification :- At the respiratory surface of the lungs, the toxic substances are oxidized

to less harmful materials.7. Cytochrome P 450 :- This cytochrome is used as a carrier protein in facilitated diffusion.

It also oxidizes the toxic substances to make them less harmful to the body. Availability

of oxygen at the lung surface enables the cytochrome P 450 to oxidize foreign materials. Thus, like the liver, lungs too are detoxifying organs.

Human Respiratory System The human respiratory system may be divided into two major components :- respiratory

tract ( conducting portion ) & respiratory organs.

Respiratory Tract

Respiratory tract serves as a passageway for the fresh air to flow from outside to the lungs & for the foul air to return from the lungs to the exterior.

Gas exchange does not occur in the respiratory tract. The respiratory tract consists of external nares, nasal chambers, internal nares, pharynx,

larynx, trachea, bronchi & bronchioles. Of these, some bronchi & all bronchioles lie within the lungs, other parts lie outside the lungs.

1. External Nares ( Nostrils ) :- The external nares are a pair of slits at the lower end of the nose.

They open into the nasal chambers.

2. Nasal Chambers :- The nasal chambers are a pair of passages in the head above the palate. The two chambers are separated from each other by a median partition, the nasal septum. Each chamber shows three regions :- lower vestibular, middle respiratory & upper olfactory.

(a) Vestibular Region:- It lies just within the external naris. It is very short & lined with skin. It bears hair, & contains some sweat & oil glands. The hair act as a sieve to check the entry of large dust particles.

(b) Respiratory Region:- It is the middle region of the nasal chamber. It is lined with respiratory epithelium. This is a ciliated, pseudostratified columnar epithelium

rich in gland cells. The gland cells include mucous & serous cells. The mucous cells secrete a viscid fluid called mucus. The serous cells produce a watery fluid. The respiratory epithelium is highly vascular & appears pink or reddish. The respiratory region acts as an air-conditioning & filtering unit in breathing.

(c) Olfactory Region:- It is the upper region of the nasal chamber. It is lined with olfactory epithelium. This epithelium is confined to the upper part of the nasal

chamber & the superior nasal concha. It looks yellowish-brown. The olfactory region acts as an organ of smell. It detects the odour of the inspired air. If the

odour is offensive or pungent, the air is not allowed to pass in. Thus, the organ of smell helps in selecting the air for inspiration.

Nasal Conchae:-

Arising from the wall of each nasal chamber are 3 shallow bony ridges called nasal conchae. These are individually named superior, middle & inferior nasal conchae according to their position.

The conchae are covered with mucous membrane. They greatly increase the surface area of the nasal chambers.

Paranasal Sinuses:-

These are cavities in the frontal, maxillary, ethmoid & sphenoid bones.

Goblet cells in their lining epithelium produce mucus that drains into the nasal chambers.

3. Internal Nares ( Choanae ) :- The nasal chambers open behind into the nasopharynx by internal nares.

4. Pharynx :- The pharynx is a short, vertical, about 12 cm long tube behind the buccal cavity. The food &

air passages cross here. Its upper part is called nasopharynx. The internal nares open into it. The middle part is called oropharynx. The oropharynx receives the buccal cavity. The lower part is called laryngopharynx. It leads into two tubes :- the one at the front is the

windpipe or trachea & the one at the back is the foodpipe or oesophagus. The pharynx is lined with nonkeratinized stratified squamous epithelium which is ciliated in

the nasopharynx. Mouth serves as an alternate route for air when nasal chambers are blocked. It leads into

the oropharynx through the buccal cavity.

Differences b/w Pharynx & Larynx

S. No. Pharynx Larynx 1. Located in the head at the back of the

buccal cavity.Located in the upper part of the neck.

2. Lined with nonkeratinized, stratified squamous epithelium.

Lined with pseudostratified, ciliated columnar epithelium.

3. Wall not supported by cartilages. Wall supported by cartilages held together by fibroelastic connective tissue.

4. Buccal cavity, internal nares, glottis & eustachian tubes open into the pharynx.

Pharynx opens into it via glottis & it itself leads into the trachea.

5. Both food & air pass through it. Only air passes through it.6. It is never closed. It is closed by epiglottis during deglutition.7. It does not act as a sound box. It acts as a sound box.8. Pharynx is about 12 cm long. Larynx is much shorter.9. Shows no enlargement after puberty. After puberty, it grows & become more

prominent in males.

5. Larynx :- The larynx is often called the Adam’s apple, & is more prominent in men than in women. Before puberty, the larynx is inconspicuous & similar in both sexes. The larynx is the upper part of the trachea. It is a short, tubular chamber supported by a cartilaginous framework. It opens into the laryngopharynx by a slit-like aperture, the glottis. The glottis always

remains open except during swallowing. The glottis bears a leaf-like cartilaginous flap, the epiglottis, at its anterior margin.

The epiglottis projects into the pharynx opposite the uvula. During swallowing, the larynx moves upward to meet the epiglottis. This closes the glottis to

prevent the entry of food into it. Besides forming a part of the respiratory tract, it also serves as the voice box. It is lined with ciliated epithelium, the cilia of which beat toward the pharynx. The larynx allows nothing but air to pass into the lungs. A laryngeal spasm ( an involuntary muscular contraction ), in the form of a cough, results if

any substance enters the larynx. Entry of food into the respiratory tract can be fatal. The larynx is followed by trachea.

6. Trachea :- The trachea is a thin-walled tube, about 11 cm long & 2.5 cm wide. It is also called windpipe. It extends downward through the neck.

7. Primary Bronchi :- In the middle of the thorax, trachea divides into two tubes, the major or primary bronchi. One major bronchus enters the right lung, & the other enters the left lung. The right primary bronchus further divides into three lobar or secondary bronchii which

extend separately into the three lobes of the right lung. The left primary bronchus divides into two lobar or secondary bronchii that likewise pass

into the two lobes of the left lung. The secondary bronchi subdivide into smaller tertiary bronchi which divide into still smaller

bronchioles. The small terminal bronchioles give off respiratory bronchioles which divide into alveolar

ducts. The alveolar ducts enter the alveolar sacs. The respiratory passageways within the lungs are referred to as the bronchial tree. Wall of trachea, bronchi & bronchioles is composed of fibromuscular tissue & is lined by

pseudostratified ciliated columnar epithelium rich in mucus-secreting cells. The mucus holds the dust & bacteria, which are swept by cilia toward the pharynx from

where they are thrown out or swallowed. This ensures clean & bacteria-free air to the lungs. Terminal bronchioles are lined with simple ciliated columnar epithelium without mucous

cells. The mucus, if present, may block these narrow tubules. A short distance down the respiratory bronchioles, epithelium changes to low nonciliated

cuboidal type. The alveolar ducts also have thin, nonciliated cuboidal epithelium. Cartilaginous rings, incomplete behind, support the walls of the trachea & the bronchi to

prevent their collapsing. Thus, the trachea & bronchi remain permanently open even during negative pressure created by exhalation. This normally allows easy passage for air.

The rings gradually become thinner, & finally disappear over the bronchioles. The bronchioles have smooth muscle controlled by autonomic nervous system so that air-

flow is adjusted to suit metabolic needs.

Respiratory Organs

The respiratory organs provide the surface for the exchange of gases. In human beings, the respiratory organs are a pair of lungs.

Mechanics of Pulmonary Respiration

( Breathing Mechanism ) Lungs have little musculature & cannot expand or contract of their own. Breathing is brought about by alternate expansion & contraction of the thoracic cavity

wherein the lungs lie. This leads to intake of fresh air called inspiration ( inhalation, breathing in ) & elimination of foul air called expiration ( exhalation, breathing out ) respectively.

Inspiration & expiration are together referred to as respiratory movements. Lungs can be expanded or contracted in two ways :-

1. By downward & upward movement of the diaphragm to expand or contract the chest cavity.

2. By raising or lowering of the ribs to increase or decrease the diameter of the chest cavity.

1. Inspiration :- Inspiration is an active process. It is brought about by diaphragm muscles & external intercostal muscles. Therefore, these

muscles are called inspiratory muscles. The abdominal muscles play a passive role in this process.

(a) Diaphragm :- The diaphragm is convex upward & has a peripheral muscle attached to the ribs & vertebral

column. This muscle contracts & lowers the diaphragm, making it flat. This pushes the abdominal

viscera downward & enlarges the thoracic cavity vertically.

(b) External Intercostal Muscles :- These muscles slant forward & downward b/w the ribs. Their contraction pulls the ribs & sternum upward & downward. This enlarges the thoracic

cavity from front- to- back as well as from side- to- side.

(c) Abdominal Muscles:- These muscles relax & allow compression of abdominal organs by the diaphragm.

Movement of Fresh Air into the Lungs :-

As the lungs are held tightly against thoracic wall, enlargement of the thoracic cavity results in expansion of the lungs. This reduces the pressure of air in the lungs below atmospheric pressure by a few ( -2 to -6 ) mm Hg. In other words, lungs come to have a negative pressure.

Air always moves from a place of higher pressure to a place of lower pressure. Hence, the fresh air from outside, where pressure is higher, rushes into the lungs through the respiratory passage till the pressure of air in the lungs becomes equal to the atmospheric pressure.

On reaching the lungs, the fresh air is distributed by the bronchi, bronchioles & alveolar ducts to the alveoli.

The fresh air follows the following route :- External Nares Nasal Chambers Internal Nares Pharynx Glottis Larynx Trachea Bronchi Bronchioles Alveolar Ducts Alveoli.

With the expansion of the thoracic cavity & the lungs, additional blood is drawn into the blood vessels of the lungs & the thorax.

2. Conditioning & Filteration of Air :- The air passing through the nasal chambers is subjected to four important processes :-(i) Warming:- Blood vessels of the conchae radiate heat like hot water pipes, & warm the

air passing over them to the body temperature. Very cold winter air, if inhaled as such, may freeze the lung tissues.

(ii) Moistening:- Fluid secreted by nasal mucous membrane is slowly evaporated & the vapours are added to the passing air, which gets almost saturated with moisture. Dry air would kill the lung cells.

(iii) Sterilization:- Mucus is sticky & antiseptic. It hold & kills the bacteria of the air.(iv) Cleaning:- Hair in the vestibular region entrap the coarse dust particles from the ingoing

air. A large inhaled dust particle stimulates a sensory cell in the nasal chamber. This cell signals the brain to cause a sneeze that forcefully throws out the particle. Mucus also holds dust particles of the passing air. The cilia of the nasal chambers by their ceaseless beating ‘‘sweep’’ the dust & germs trapped in the mucus to the exterior via external nares.

Advantages of Nasal Breathing :-

The above processes occurring in the nasal chambers prove the advantages of nasal breathing over the mouth breathing. The vital functions of the nose are disrupted by cigarette smoking.

The cilia lining the trachea & bronchi also clean the air by sweeping the tiny foreign particles held in mucus toward the pharynx, where they are disposed of by swallowing or throwing out.

3. Exchange of Gases in Alveoli :- Exchange of gases takes place in the alveoli. Oxygen in the alveoli has a higher partial pressure or concentration than that in the blood.

Hence, oxygen diffuses from the alveoli into the blood through the alveolar epithelium & capillary endothelium.

Oxygen passes first into the blood plasma & then combines with the haemoglobin in the red blood corpuscles to form oxyhaemoglobin.

Carbon dioxide in the lung capillaries has a higher partial pressure or concentration than that in the alveoli. Therefore, carbon dioxide diffuses from the blood into the alveoli. Thus, the air of the alveoli becomes foul & needs renewal.

The alveolar capillaries are narrower than the red blood corpuscles so that the RBCs are squeezed through the capillaries by blood pressure. This exposes more of their surface area to the gaseous exchange surface of the alveolus, allowing greater uptake of oxygen.

The progress of the corpuscles is also slow, thus increasing the time available for gaseous exchange to occur.

When the blood leaves the alveolus, it has almost the same partial pressure of O₂ & CO₂ as the alveolar air.

4. Expiration :- Expiration is normally a passive process as it simply involves relaxation of the inspiratory

muscles, i.e., peripheral muscle of diaphragm & external intercostal muscles.

(a) Diaphragm :- The peripheral muscle of the diaphragm relaxes. With this, the abdominal viscera,

compressed during inspiration, push the diaphragm upward, making it convex.

(b) External Intercostal Muscles :- These muscles also relax. This brings the ribs & the sternum to their original position. This is

aided by the elastic recoil ( contraction ) of the lungs & thoracic wall which are stretched during inspiration.

With the above two events, the thoracic cavity becomes smaller.

(c) Abdominal & Internal Intercostal Muscles :- In forced breathing, as during exercise, abdominal muscles & internal intercostal muscles

come into action, making expiration also an active, energy-consuming process. These muscles contract & decrease the thoracic cavity further. The contraction of abdominal muscles presses the abdominal viscera against the diaphragm,

bulging it further upward. This shortens the thoracic cavity vertically. The abdominal muscles are practically inactive during normal breathing, but become active

during coughing & sneezing. Contraction of internal intercostal muscles moves the ribs downward & inward. This reduces

the thoracic cavity from front- to- back & also from side- to- side. The abdominal & internal intercostal muscles are called expiratory muscles.

Movement of Foul Air out of the Lungs :-

All round decrease in the thoracic cavity, reduces the lungs & raises the pressure of air in the lungs above atmospheric pressure by +3 to +4 mm Hg. This positive pressure in the lungs pushes out the foul air from the lungs until the air pressure in the lungs falls to that of the atmosphere.

On its return journey, the air again passes through the respiratory passage. Much air stays in the alveoli after expiration. This air keeps the walls of the alveoli from

sticking together & collapsing.

The foul air follows the following route :-

Alveoli Alveolar Ducts Bronchioles Bronchi Trachea Larynx Glottis Pharynx Internal Nares Nasal Chambers External Nares Atmosphere

Respiratory Rate :-

Each respiration consists of one inspiration & one expiration. At rest, breathing occurs about 14 – 18 times per minute in a normal human beings. Alternate inspiration & expiration are due to the rhythmic arrival & suspension of nerve

impulses from the brain to the inspiratory muscles.

Advantage of Negative Pressure Breathing :-

Mammals have negative pressure breathing, i.e., the lungs draw air due to reduction in pressure within them. This allows them to eat & breathe at the same time.

If air were to be forced into the lungs, it might carry food particles into the trachea & block it.

Negative pressure breathing gently moves air which is less likely to carry food particles into the wind pipe.

Pulmonary Air Volumes & Capacities The quantities of air the lungs can receive, hold or expel under different conditions are called

pulmonary or lung volumes. Pulmonary capacity refers to a combination of two or more pulmonary volumes.

1. Tidal Volume ( TV ) :- It is the volume of air normally inspired or expired in one breath without any effort. It is about 500 ml for an average adult human male. Actually, only about 350 ml of air enters the lung alveoli for the exchange of gases, the remaining 150 ml fills the respiratory passage. The respiratory passage is often called dead air space because no exchange of gases takes place here. Thus, the tidal volume consists of about 350 ml of alveolar air & some 150 ml of dead space volume. During excitement & activity, the tidal volume increases 4 to 10 times.

2. Inspiratory Reserve Volume ( IRV ) :- It is the extra amount of air which can be inhaled forcibly after a normal inspiration. It is about 2000 to 2500 ml.

3. Expiratory Reserve Volume ( ERV ) :- It is the extra amount of air which can be exhaled forcibly after a normal expiration. It is about 1000 to 1500 ml.

4. Vital Capacity ( VC ) of Lungs :- It is the amount of air which one can inhale with maximum effort & also exhale with maximum effort. It is about 3.5 – 4.5 litres in a normal adult person. It is equal to the sum of the tidal, inspiratory reserve & expiratory

reserve volumes of air ( 500 + 2000 + 1500 = 4000 ml ). Vital capacity represents the maximum amount of air one can renew in the respiratory system in a single respiration. The vital capacity is higher in athletes & sportsmen than in others, in mountain dwellers than in plains dwellers, in males than in females, & in the young than in the old persons. Smoking reduces the vital capacity of the lungs & decreases the capacity for strenuous muscular activity.

5. Residual Volume ( RV ) :- Some air always remains in the lungs even after forcible

expiration. This amount of air is called residual volume. It is about 1500 ml. The residual air enables the lungs to continue exchange of gases even after maximum exhalation or on holding the breath. In other words, exchange of gases goes on in the lungs uninterrupted during inspiration as well as expiration.

6. Inspiratory Capacity ( IC ) :- It is the total volume of air which can be inhaled after a normal expiration. It is equal to the tidal volume plus the inspiratory reserve volume ( IC = TV + IRV ). It is about 2500 to 3000 ml.

7. Functional Residual Capacity ( FRC ) :- It is the sum of residual volume & the expiratory reserve volume ( FRC = RV + ERV ). It is about 2500 to 3000 ml.

8. Total Lung Capacity ( TLC ) :- It is the sum of vital capacity & the residual volume ( 3500 – 4500 + 1500 = 5000 – 6000 ml ). It is about 5000 – 6000 ml.

Respiratory Quotient ( RQ ) :- The ratio of the volume of CO₂ produced to the volume of O₂ used in a unit time is called respiratory quotient ( RQ ). It varies with different foods utilized in respiration. For glucose, RQ is 1 ( RQ = 6CO₂/6O₂ = 1 ), for fats about 0.7, & for proteins about 0.85. The RQ indicates the type of food oxidized in the animal body during respiration.

Composition of Inspired, Expired & Alveolar Airs

During normal breathing, a part of the inspired air is left in the respiratory tract, the so called ‘‘dead space’’, where no gaseous exchange occurs. This ‘‘dead space’’ air is expelled at the next expiration.

Thus, the expired air contains fresh air from the ‘‘dead space’’ & foul air from the lungs. Therefore, the alveolar air has less oxygen & more carbon dioxide than the expired air.

A part of the expired air is also left in the dead space. This air enters the lungs at the next inspiration.

Some air is also left in the lungs after expiration as residual air. The fresh inspired air is mixed up in the lungs with the foul air from the ‘‘dead space’’ & the

stale residual air. Therefore, the alveolar air has less O₂ & more CO₂ than the inspired air also.

The inspired air has the composition of the atmospheric air.

Exchange of Gases Exchange of gases occurs in the lungs & the body tissues partly by diffusion & partly by

facilitated diffusion. The facilitated diffusion uses a carrier protein, a cytochrome, which speeds up diffusion,

allowing the blood to take up oxygen faster. The kinetic motion of the molecules provides the energy required for this diffusion of gaseous molecules.

Diffusion is the net flow of a substance from a region of higher concentration to a region of lower concentration.

The diffusion is directly proportional to :-(i) Partial pressure gradient ( difference ) of gases on the two sides of a membrane b/w

them.(ii) Thinness of the membrane.(iii) Surface area of the membrane.(iv) Permeability of the membrane.

Moisture is also necessary because both O₂ & CO₂ are more easily exchanged in cells when in a liquid medium.

Partial Pressure :-

Partial pressure of a gas is the pressure it exerts in a mixture of gases, & is equal to the total pressure of the mixture divided by percentage of that gas in the mixture.

In other words, the partial pressure of a gas is proportional to its concentration in the mixture.

1. Lungs :- In the lungs, exchange of gases takes place b/w the air in the alveoli & the blood in the

capillaries around the alveoli. In this exchange, the blood takes up O₂ from the alveolar air & releases CO₂ to the alveolar air.

(a) Uptake of Oxygen by Blood :- The Po₂ ( partial pressure of oxygen ) of the alveolar air is higher than the Po₂ of blood in

alveolar capillaries.

Due to a Po₂ difference b/w air & blood, oxygen diffuses rapidly from the alveolar air into the blood of alveolar capillaries.

Gases always diffuse from a region of higher partial pressure ( concentration ) to a region of lower partial pressure ( concentration ).

(b) Release of Carbon Dioxide by Blood :- The Pco₂ ( partial pressure of carbon dioxide ) of blood reaching the alveolar capillaries is

higher than the Pco₂ of alveolar air. Therefore, carbon dioxide diffuses from the blood of alveolar capillaries into the alveolar air.

The exchange of gases in the alveoli that raises the Po₂ of blood & lowers its Pco₂ is the

external respiration. The blood oxygenated by this respiration is returned from the lungs by pulmonary veins to the left side of the heart. The heart supplies the oxygenated blood to the body tissues.

2. Tissues :- In the tissues, exchange of gases occurs b/w the blood & the tissue cells. This exchange

occurs via tissue fluid that bathes the tissue cells.

The blood reaching the tissue capillaries has Po₂ higher than that in the tissue cells & Pco₂ lower than that in the tissue cells.

The tissue cells constantly use oxygen in oxidation that produces carbon dioxide. Therefore, they always have lower Po₂ & higher Pco₂ than the blood coming to them.

Because of Po₂ & Pco₂ differences b/w blood & tissue cells, oxygen separates from oxyhaemoglobin & diffuses from the blood into the tissue fluid & thence into the tissue cells; & carbon dioxide diffuses from the tissue cells into the tissue fluid & thence into the blood in the tissue capillaries.

Gases mostly diffuse through the tissue fluid as such, only small amounts dissolve in it. Exchange of gases in the tissues that lowers the Po₂ of the blood & raises its Pco₂ is called

internal respiration. The blood deoxygenated by this respiration returns to the right side of the heart that sends it to the lungs for reoxygenation.

Transport of Gases in Blood Blood carries oxygen from the respiratory organs to the tissue cells for oxidation, & carbon

dioxide from the tissue cells to the respiratory surface for elimination.

Transport of Oxygen :- Oxygen is carried by the blood in two forms :- solution & oxyhaemoglobin.

(i) Solution :- Being slightly soluble in water, a small amount of oxygen ( about 3% ) travels as its solution

in the plasma.

(ii) Oxyhaemoglobin :- Bulk of oxygen ( about 97% ) diffuses from the plasma into the red blood corpuscles. Here it loosely joins with the Fe²⁺ ions of haemoglobin ( Hb ) to form bright red

oxyhaemoglobin ( HbO ). The process is called oxygenation. Haemoglobin molecule has four Fe²⁺ ions, each of which can combine with one oxygen

molecule. Thus, an oxyhaemoglobin molecule may carry 1 to 4 oxygen molecules, depending on its degree of saturation with oxygen.

Hb₄ + 4O₂ Hb₄O₈

Only about 0.3 ml of O₂ can dissolve in 100 ml of plasma, about 20 ml of O₂ is carried by haemoglobin in 100 ml of blood.

Special Feature/Role of Haemoglobin :-

Haemoglobin is a wonderful pigment. It readily combines with oxygen when exposed to high Po₂ in the respiratory organs, &

releases oxygen equally readily when exposed to low Po₂ in the tissues. The affinity of haemoglobin for oxygen increases with a fall in the Pco₂ of the blood resulting

from the diffusion of CO₂ from the blood into the alveoli of the lungs. Simultaneous exposure of haemoglobin to high Po₂ & low Pco₂ in the lung alveoli enables it

to take up a large amount of oxygen. Because of its role of carrying oxygen, haemoglobin is called respiratory pigment.

Release of Oxygen :-

In the tissue capillaries, low Po₂ & high Pco₂ favour dissociation of oxyhaemoglobin to darker purplish red deoxygenated haemoglobin ( reduced haemoglobin ) & molecular oxygen.

Oxygen diffuses from the blood into the tissue fluid & then into the cells where it is used in oxidation.

Haemoglobin is returned to the lungs for reuse in oxygen transport.

Oxygen Dissociation Curve of Haemoglobin :-

A more active tissue has much lower Po₂ & much higher Pco₂ than a less active tissue. Therefore, much more oxygen is released from oxyhaemoglobin in a more active tissue than in a less active tissue.

The relationship b/w the percentage saturation of the haemoglobin in the blood & the oxygen tension Po₂ of the blood depicts oxygen dissociation curve of haemoglobin.

The curve is sigmoid.

From the figure, it is clear that as Po₂ increases, there is progressive increase in the percentage of the haemoglobin that is bound with oxygen.

Infact, haemoglobin is 97% saturated with oxygen ( at Po₂ ≈ 100 mm Hg ) as it leaves the pulmonary capillaries.

On the other hand, the blood which has returned from peripheral tissues to the lungs & enters the pulmonary capillaries has Po₂ of only 40 mm Hg with haemoglobin only 70% saturated with oxygen.

When fully saturated, each gram of haemoglobin combines with nearly 1.34 ml of oxygen. Taking 14.5 g/dl of blood as the concentration of haemoglobin, the total amount of oxygen that can be transported as oxyhaemoglobin would be 14.5 X 1.34 = 19.43 ml/dl of blood.

The sigmoidal pattern of oxygen dissociation curve of haemoglobin is the result of two properties which play significant role in the transport of oxygen. These two properties are :-

1. Minimal loss of oxygen from haemoglobin occurs above Po₂ 70-80 mm Hg despite significant changes in tension of oxygen beyond this. This is depicted by relatively flat portion of the curve.

2. Significant change of the dissociation curve below Po₂ of 40 mm Hg ensures that with any further decline in Po₂ there will occur a disproportionately greater release of oxygen from the haemoglobin.

Factors Affecting Oxygen Dissociation Curve of Haemoglobin :-

1. H⁺ concentration.

2. Carbon dioxide tension.3. Temperature.4. Erythrocyte concentration of 2,3 diphosphoglycerate ( DPG ).

Increase in these factors brings rightward shift of the curve thereby decreasing the affinity of haemoglobin for oxygen.

Bohr Effect Carbon dioxide reacts with water to form carbonic acid that lower the pH in active tissue &

induces oxyhaemoglobin to give up more of its O₂. This phenomenon is called Bohr Effect. The Bohr effect states that haemoglobin’s oxygen binding affinity is inversely related both to

acidity & to the concentration of carbon dioxide, i.e., an increase in blood CO₂ concentration, which leads to a decrease in blood pH, will result in a lower affinity of haemoglobin for oxygen.

Conversely, a decrease in carbon dioxide provokes an increase in pH, which results in haemoglobin picking up more oxygen.

Since carbon dioxide reacts with water to form carbonic acid, an increase in CO₂ results in a decrease in blood pH.

Bohr effect plays a crucial role in enhancing oxygenation of the blood in the lungs & also in the release of oxygen in the tissues. From a tissue capillary bed, the deoxygenated blood enters the veins as venous blood.

Mechanism

Decreases in blood pH, meaning increased H⁺ concentration, are likely the direct cause of lower haemoglobin affinity for oxygen.

Specifically, the association of H⁺ ions with the amino acids of haemoglobin appear to reduce haemoglobin’s affinity for oxygen.

Because changes in the carbon dioxide partial pressure can modify blood pH, increased partial pressures of carbon dioxide can also result in right-ward shifts of the oxygen-haemoglobin dissociation curve.

The relationship b/w carbon dioxide partial pressure & blood pH is mediated by carbonic anhydrase which converts gaseous carbon dioxide to carbonic acid that in turn releases a free hydrogen ion, thus reducing the local pH of blood.

Significance

The Bohr effect allows for enhanced unloading of oxygen in metabolically active peripheral tissues such as exercising skeletal muscle. Increased skeletal muscle activity results in localized increases in the partial pressure of CO₂ which in turn reduces the local blood pH.

Because of the Bohr effect, this results in enhanced unloading of bound oxygen by haemoglobin passing through the metabolically active tissue & thus improves oxygen delivery.

Importantly, the Bohr effect enhances oxygen delivery proportionally to the metabolic activity of the tissue.

As more metabolism takes place, the carbon dioxide partial pressure increases thus causing larger reductions in local pH & in turn allowing for greater oxygen unloading. This is especially true in exercising skeletal muscles which may also release lactic acid that further reduces local blood pH & thus enhances the Bohr effect.

Transport of Carbon dioxide Carbon dioxide is carried by the blood in three forms :- physical solution, bicarbonate ions &

carbaminohaemoglobin.

(i) Physical Solution:- A very small amount of carbon dioxide ( about 7% ) dissolves in the plasma & is carried as a

physical solution.

(ii) Bicarbonate Ions:- About 70% of carbon dioxide released by respiring tissue cells diffuses into the plasma &

then carried into the red blood corpuscles. Here, CO₂ combines with water to form carbonic acid. The reaction is catalyzed by a zinc-

containing enzyme carbonic anhydrase. Carbonic acid dissociates into bicarbonate & hydrogen ions. Hydrogen ions are picked up by proteins & bicarbonate ions are joined by bases ( Na⁺, K⁺ ) to

maintain pH of the blood.

CO₂ + H₂O Carbonic Anhydrase H₂CO₃

Carbonic Acid

H₂CO₃ H⁺ + HCO₃⁻

Carbonic Acid Hydrogen Ion Bicarbonate Ion

A small amount of bicarbonate ions is transported in the RBCs, whereas most of them diffuse into plasma to be carried by it.

Chloride Shift It is also known as the Hamburger’s shift or Hamburger’s phenomenon. Exit of bicarbonate ions considerably changes ionic balance b/w the plasma & the

erythrocytes. To restore the ionic balance, the chloride ions diffuse from the plasma into the erythrocytes. This movement of chloride ions is known as Chloride Shift.

It is a process which occurs in a cardiovascular system & refers to the exchange of bicarbonate ( HNO₃¯ ) & chloride ( Cl¯ ) across the membrane of red blood cells ( RBCs ).

The movement of chloride ions from the plasma into red blood cells as a result of the transfer of carbon dioxide from tissues to the plasma, a process that serves to maintain blood pH.

Bicarbonate & chloride ions are transported across the red blood cell membrane in opposite directions by the bicarbonate-chloride carrier protein.

The chloride shift is extremely rapid, occurring within 1 second. The chloride shift results in the chloride content of venous blood being greater than that of

arterial blood.

Mechanism

Carbon dioxide ( CO₂ ) generated in tissues passively diffuses into capillaries via the interstitial fluid ( lymph ). Once in circulation, CO₂ enters red blood cells ( RBCs ) to balance the intracellular & extracellular CO₂ partial pressures.

RBCs contain appreciable quantities of carbonic anhydrase, an enzyme which catalyzes the conversion of CO₂ to carbonic acid & which is not highly expressed in interstitial fluid & plasma.

RBC carbonic anhydrase catalyzes the conversion of dissolved CO₂ & intracellular water to carbonic acid ( H₂CO₃ ), which spontaneously dissociates to form bicarbonate ( HCO₃¯ ) & a hydrogen ion ( H⁺ ) in response to the fall of intracellular pCO₂, more CO₂ passively diffuses into the cell.

Red blood cell membranes are impermeable to hydrogen ions but are able to exchange bicarbonate ions for chloride ions using the anion exchanger protein Band 3.

The rise in intracellular bicarbonate leads to bicarbonate export & chloride intake. The term ‘‘chloride shift’’ refers to this exchange.

As a result, plasma chloride concentration is lower in systemic venous blood than in systemic arterial blood :- high venous pCO₂ leads to bicarbonate production in RBCs, which then leaves the RBC in exchange for chloride coming in.

The opposite process occurs in the pulmonary capillaries of the lungs when the pO₂ rises & pCO₂ falls, & the Haldane effect occurs ( release of CO₂ from haemoglobin during oxygenation ). This releases hydrogen ions from haemoglobin, increases H⁺ concentration within RBCs, & shifts the equilibrium towards CO₂ & water formation from bicarbonate .

The subsequent decrease in intracellular bicarbonate concentration reverses chloride-bicarbonate exchange :- bicarbonate moves into the cell in exchange for chloride moving out.

Inward movement of bicarbonate via the Band 3 exchanger allows carbonic anhydrae to convert it to CO₂ for expiration.

The chloride shift may also regulate the affinity of haemoglobin for oxygen through the chloride ion acting as an allosteric effector.

(iii) Carbaminohaemoglobin:- About 23% of CO₂ entering the RBCs loosely combines with the amino group ( -NH₂ ) of the

reduced haemoglobin ( Hb ) to form carbaminohaemoglobin. The reaction releases oxygen from oxyhaemoglobin.

HbO₂ + CO₂ HbCO₂ + H⁺ + O₂

Haemoglobin as a Buffer :-

Addition of hydrogen ions would make the blood very acidic. However, most of the hydrogen ions are neutralized by combination with haemoglobin,

which is negatively charged, forming acid haemoglobin. This reduces the acidity of the blood, & also releases additional oxygen.

HbO₂⁻ + H⁺ HHb + O₂

If the blood becomes too basic, acid haemoglobin dissociates, releasing hydrogen ions. Thus, the haemoglobin also acts as a buffer, a substance that keeps the pH from fluctuating.

HHb H⁺ + Hb

Release of Carbon Dioxide :-

The venous blood loaded with carbon dioxide in the tissues returns to the lungs via heart. Here, it is oxygenated.

Its oxygenation helps in the release of carbon dioxide from it. This CO₂ is eliminated from the lungs during expiration.

Haldane Effect Binding of oxygen with haemoglobin tends to displace carbon dioxide from the blood. This

phenomenon is called Haldane Effect. It results from the simple fact that combination of oxygen with haemoglobin causes it to

become a stronger acid which, in turn, displaces carbon dioxide from the blood. Haldane effect plays far important role in promoting carbon dioxide transport than is the

Bohr effect in promoting oxygen transport.

Mechanism

Carbon dioxide is less soluble in arterial blood than in venous blood. Therefore, some carbon dioxide diffuses from the blood plasma of the lung capillaries into the lung alveoli.

Oxyhaemoglobin is a stronger acid than deoxyhaemoglobin. Therefore, it donates hydrogen ion ( H⁺ ) which joins bicarbonate ion ( HCO₃⁻ ), forming carbonic acid ( H₂CO₃ ). Then, the carbonic acid is split into water & carbon dioxide by the enzyme carbonic anhydrase.

H₂CO₃ Carbonic Anhydrase H₂O + CO₂

The bicarbonate ions pass into the RBCs for hydrolysis by carbonic anhydrase .

High Po₂ in the pulmonary capillaries due to oxygenation of haemoglobin favours separation of carbon dioxide from carbaminohaemoglobin. Thus, the reactions that bind carbon dioxide into bicarbonate & carbaminohaemoglobin in the tissue capillaries are reversed in the pulmonary capillaries & the free carbon dioxide.

The carbon dioxide diffuses into the lung alveoli because the Pco₂ of venous blood is higher than that of alveolar air.

Hypoxia It is a respiratory disorder. Hypoxia is a condition of oxygen shortage in the tissues. It is of two types :-

(i) Artificial Hypoxia :- It results from the shortage of oxygen in the air as at high ( over 2400 m ) altitudes. It causes mountain sickness characterised by breathlessness, headache, dizziness, nausea, vomiting, mental fatigue & bluish tinge on the skin & mucous membranes.

(ii) Anaemic Hypoxia :- It results from the reduced oxygen carrying capacity of the blood due to anaemia ( decreased haemoglobin content in blood ) or carbon monoxide poisioning ( some haemoglobin occupied by CO ). In both cases, less haemoglobin is available for carrying O₂.