Gaseous Exchange | First Year – Class Biology Notes

Gaseous Exchange 

RESPIRATORY ORGANS OF COCKROACH TRACHEAL SYSTEM

Cockroach has evolved a special type of invaginated respiratory system called Tracheal system, especially adopted for terrestrial mode of life and high metabolic rate of insects.

STRUCTURAL CONSTITUENTS OF TRACHEAL SYSTEM

1. TRACHEA

2. SPIRACLES

3. TRACHEOLES

1.TRACHEA

Tracheal system consists of number of internal tube called Trachea which are the connection between the spiracles and tracheal fluid.

2. SPIRACLES

Laterally, trachea open outside the body through minute, slit like pores called as spiracles.

There are 2 pairs of spiracles on lateral side of cockroach.

2 lie in thoracic segments and 8 in first abdominal segments.

3. TRACHEOLES

On the other side, trachea ramify throughout the body into fine branches or tracheols.
Tracheoles, finally end as blind, fluid filled fine branches which are attached with cells of tissue.

Both the trachea and tracheoles are lined internally by thin layer of cuticle.

MECHANISM OF RESPIRATION “INFLOW OF OXYGEN”

The cockroach takes in air directly from the atmosphere into the trachea through spiracles. This air diffuses directly into fluid filled tracheoles through which diffuses into the cells of tissues. Hence the blood vascular system of cockroach is devoid of haemoglobin.

OUTFLOW OF CARBONDIOXIDE

Removal of CO2 from cells of body is largely depended upon plasma of blood, which takes up CO2 for its ultimate removal through body surface via the cuticle.

RESPIRATORY SYSTEM OF FISH

MAIN RESPIRATORY ORGAN

In fish, main respiratory organs are “Gills”. They are out growth of pharynx and lie internally with in the body so that they are protected from mechanical injuries.

INTERNAL STRUCTURE OF GILLS

Each gill is highly vascularized structure. It is composed of

1. Filaments

2. Gill bar or Gill arch

3. Lamella

1. FILAMENTS

Each gill is composed of two rows of hundreds of filaments, which are arranged in V-shape.

2. GILL BAR OR GILL ARCH

Filaments are supported by a cartilage or a long curved bone the gill bar or gill arch.

3. LAMELLA

Lamella is a plate like structure which is formed by infolding of filaments. Lamella greatly increase the surface area of the gill. Each lamella is provided by a dense network of capillaries.

OPERCULUM (IN BONY FISHES)

Gills are covered on each side by gill cover called “operculum”

MECHANISM OF VENTILATION

In bony fishes, ventilation is brought about by combined effect of mouth and operculum.

§  Water is drawn into the mouth. It passes over the gills through pharynx and ultimately exists at the back of operculum through open operculur valve.

§  Water is moved over the gills in a continuous unidirectional flow by maintaining a lower pressure in operculur cavity than in buccopharynx cavity.

COUNTER CURRENT FLOW OF WATER AND BLOOD

Gaseous exchange is facilitated in gills due to counter current flow of H2O and blood.

In the capillaries of each lamella, blood flows in direction opposite to the movement of water across the gill. Thus the most highly oxygenated blood is brought to water that is just entering the gills and has even high O2 content than the blood. As the H2O flows over the gills, gradually loosing its oxygen to the blood, it encounter the blood that is also increasingly low in oxygen. In this way a gradient is establishment which encourages the oxygen to move from water to blood

IMPORTANCE

Counter current flow is very effective as it enables the fish to extract upto 80–90% of the oxygen from water that flows over the gills.

RESPIRATORY SYSTEM OF MAN

MAIN FUNCTION OF RESPIRATION

The main function of respiratory system is inflow of O2 from the atmosphere to the body and removal of CO2 from body to the atmosphere.

COMPONENTS OF RESPIRATORY SYSTEM

(1) PAIRED LUNGS

The respiratory (gas exchange) organs.

(2) AIR PASSAGE WAYS

Which conduct the air

(3) THORACIC CAVITY

Which lodges the lungs

(4) INTERCOSTAL MUSCLES AND DIAPHRAGM

Which decreases and increase the diameters of thoracic cavity

(5) RESPIRATORY CONTROL CENTRES

Areas in brain which control the respiration.

DETAILS OF COMPONENTS

+ THORACIC CAVITY

Paired lungs with in the pleural sacs are situated in the thoracic cavity. Separating the thoracic cavity from the abdominal cavity is a dome-shaped musculo-tendinuous partition called as Diaphragm.

BOUNDARIES OF CAVITY

Thoracic cavity is supported by bony cage (thoracic cage) which is made up of

§  Sternum -> in front

§  Vertebral column -> at the back

§  12 pairs of ribs -> on each side

§  Ribs are supported by Intercostal muscles

FUNCTION
Increase in thoracic cavity diameter is responsible for inspiration. While decrease in diameter is responsible for expiration.

AIR PASSAGE WAYS

Air is drawn into the lungs by inter-connected system of branching ducts called as “Respiratory tract” or “Respiratory passage ways”

Air passage ways consists of

AIR CONDUCTING ZONE(which only conducts the air)

1. Nostrils

2. Nasal Cavity

3. Pharynx (nasopharynx and oropharynx)

4. Larynx

5. Trachea

6. Bronchi

7. Bronchioles (also called terminal Branchioles)

RESPIRATORY ZONE(Where gaseous exchange takes place)

8. Respiratory Bronchioles

9. Alveolar duct

10. Alveolar sacs or alveoli

GENERAL FUNCTIONS OF CONDUCTING AIR PASSAGES

1. Conduction of air from atmosphere to the lungs

2. Humidification of inhaled dry air.

3. Warming / cooling of air to body temp.

4. The injurious particles are entrapped by mucous and removed by ciliary

movements.
5. Lymphoid tissues of pharynx provide immunological functions

6. Cartilages prevent the passages from collapse but are not present in Bronchioles which remains expanded by same pressure that expand the alveoli.

CONDUCTING ZONE

1. NASAL CAVITY

Atmospheric air enters the respiratory tract through a pair of openings called external nares (Nostrils), which lead separately into nasal cavity. Nasal cavity opens into naso pharynx through posterior nares (choanae).
Nasal cavity is lined internally by Pseudostratified columnar ciliated epithelium containing mucous secreting cells.
Hairs, sweat and sebaceous glands are also present.

SPECIALIZED FUNCTIONS

§  Warming of air

§  Humidification or moistening of air

§  Filteration of air with the help of hairs

§  All these together called as Air conditioning function of upper respiratory passages

§  Olfaction ( sense of smell)

2. PHARYNX

Air enters from Nasal cavity into pharynx through internal nostrils. The openings of nostrils are guarded by soft palate. It is internally lined by Pseudostratified ciliated epithelium, mucous glands are also present.

FUNCTION
Pharynx is responsible for conduction of air as well as food

3. LARYNX (VOICE BOX)

Pharynx leads air into larynx through an opening called glottis. Glottis is guarded by flap of tissue called epiglottis. During swallowing, soft palate and epiglottis close the nostrils opening and glottis respectively so that food is prevented to go either into nasal cavity or glottis. Larynx, a small chamber consists of pair of vocal cords

FUNCTION
During speech, vocal cords move medially and their vibration produce sound

4. TRACHEA (WIND PIPE)

Larynx leads the air into a flexible air duct or trachea. It bears C-shaped tracheal cartilages which keep its lumen patent during inspiration. Its internal lining is pseudostratified columnar ciliated epithelium containing mucous secreting goblet cells.

FUNCTION
Conduction of air
Due to mucous and upward beating of cilia, any residues of dust and germs are pushed outside the trachea towards the pharynx.

5. BRONCHI

“At its lower end, trachea bifurcates into two smaller branches called Principle Bronchi↑ which leads the air into lung of its side. They are also supported by C-shaped cartilage rings upto the point where they enter the lungs”.
In all areas of trachea and bronchi, not occupied by cartilage plates, the walls are composed mainly of smooth muscles.

6. BRONCHIOLES

On entering the lungs, each bronchus divide repeatidly. As the bronchi become smaller, U-shaped bars of cartilage are replaced by irregular plates of cartilages. The smallest bronchi divide and give rise to Bronchioles (less than 1.5 mm in diameter).

7. TERMINAL BRONCHIOLES

Bronchioles divide and give rise to terminal bronchioles (less than 1 mm in diameter). Walls possess no cartilages and are almost entirely the smooth muscles. These are the smalled airways without alveoli.

RESPIRATORY ZONE

In this zone of respiratory tract, gaseous exchange between capillary blood and air takes place.

1. RESPIRATORY BRONCHIOLES

Terminal bronchioles show delicate outpouchings from their walls, which explains the name Respiratory Bronchioles (less than 0.5 mm in diameter). They bear the pulmonary alveoli.

2. ALVEOLAR DUCTS AND SACS

Each respiratory bronchioles terminates at a tiny hollow sac like alveolar duct that lead into tabular passages with numerous thin walled out pouchings called Alveolar sacs.

3. PULMONARY ALVEOLI

The alveolar sacs consists of several alveoli openings into a single chamber. Alveoli are the site of exchange of respiratory gases so they are considered as Respiratory surfaces of lungs. Each alveolus is surrounded by a network of blood capillaries.

INTERNAL STRUCTURE OF ALVEOLI

The alveolar lining cells consists of

1. Type I cells

2. Type II cells

They are also called pneumocytes.

“Bifurcation of trachea is called Carina”.

TYPE I PNEUMOCYTES

Squamous shaped cells which form the epithelial lining of alveoli

TYPE II PNEUMOCYTES

Irregular and cuboidal shaped cells which secretes a substance called Surfactant

SURFACTANT

The internal area of an alveoli is provided with a thin layer of fluid called as Surfactant secreted by type II cells.

FUNCTION OF SURFACTANT

1. It reduces the internal surface tension of alveoli which prevent it collapsing during expiration.

2. It increases the compliance.

3. It stabilize the alveoli.

4. It also helps to keep the alveoli dry.

LUNGS

Lungs are paired, soft, spongy, elastic and highly vascularized structures, which occupy most of thoracic cavity. In child they are pink, but with age they become dark and mottled due to inhalation of dust.

RIGHT LUNG

Partitioned into 3 lobes by two fissures.

LEFT LUNG

Divided into 2 lobes by one fissures.

PLEURAL MEMBRANES

Each lung is enclosed by two thin membranes called as Visceral and parietal pleural membranes.

PLEURAL CAVITY

In between the membranes there is a narrow cavity, the pleural cavity filled with pleural fluid which acts as lubricant.

FUNCTION OF CAVITY

1. Cardinal function is to exchange gases.

2. Phagocytosis of air borne particles

3. Temperature regulation

4. Removal of water

5. Maintainence of acid-base balance (by elemination of CO2)

6. Acts as Reservoir of blood.

BREATHING

DEFINITION
“Breathing is the process of taking in (inspiration or inhalation) and giving out of air (expiration or exhalation) from the atmosphere up to the respiratory surface and vice versa”

TYPES OF BREATHING

There are two types of Breathing

Negative pressure Breathing

Positive pressure Breathing

NEGATIVE PRESSURE BREATHING

Normal breathing in man is termed as negative pressure breathing in which air is drawn into the lungs due to negative pressure (decrease in pressure in thoracic cavity in relation to atmospheric pressure).

POSITIVE PRESSURE BREATHING

“In this kind of breathing, lungs are actively inflated during inspiration under

positive pressure from cycling valve”.

EXAMPLES
Frog uses positive pressure breathing.

PHASES OF BREATHING

1. INSPIRATION OR INHALATION

2. EXPIRATION OR EXHALATION

(1) INSPIRATION

DEFINITION
“Inspiration is an energy consuming process in which air is drawn into the lungs due to negative pressure in thoracic cavity”

MECHANISM
During inspiration volume of thoracic cavity increases which creates a pressure (intra thoracic) that sucks the air into the lungs.

INCREASE IN VOLUME OF THORACIC CAVITY

Volume of thoracic cavity increases due to

1. Inc. in Anterio-posterior diameter

2. Inc. in Vertical diamter.

INCREASE IN ANTERIO-POSTERIOR DIAMETER During contraction of external intercostals muscle, the ribs as well as the sternum move upward and outward, which causes the increase in anterior-posterior diameter of thoracic cavity.

INCREASE IN VERTICAL DIAMETER

Vertical diameter of thoracic cavity inc. due to Contraction (descent) of

Diaphragm which makes it flat.

As a consequence thoracic cavity enlarges and the pressure is developed inside the thoracic cavity and ultimately in the lungs. So the air through the respiratory tract rushes into the lungs upto the alveoli where gaseous exchange occurs.

(2)EXPIRATION

DEFINITION
“It is reserve of inspiration. The passive process in which air is given out of lung due to increased pressure in thoracic cavity is called “Expiration”

MECHANISM
During expiration, elastic recoil of pulmonary alveoli and of the thoracic wall expels the air from the lungs.

DECREASE IN VOLUME OF THORACIC CAVITY

Volume of thoracic cavity ↓ due to

1. DECREASE IN ANTERIO-POSTERIOR DIAMETER

2. DECREASE IN VERTICAL DIAMETER

(1) DECREASE IN ANTERIO-POSTERIOR DIAMETER

It is caused by relaxation of external intercostals muscles and contraction of internal intercostals muscles which moves the ribs and sternum inward and downward.

(2) DECREASE IN VERTICAL DIAMETER

It is caused by relaxation of diapharagm which makes it dome shaped thus reducing the volume of thoracic cavity.
As a consequence, the lungs are compressed so the air along with water vapours is exhaled outside through respiratory passage.

CONTROL OF RATE OF BREATHING

Rate of breathing can be controlled by two modes.

VOLUNTARY CONTROL

INVOLUTARY CONTROL

VOLUNTARY CONTROL

Breathing is also under voluntary control by CEREBRAL CORTEX

EXAMPLES
We can hold our breath for short time or can breath faster and deeper at our will.

INVOLUNTARY CONTROL

Mostly, rate of breathing is controlled automatically. This is termed as Involuntary control which is maintained by coordination of respiratory and cardio-vascular system.

TWO MODES OF INVOLUNTARY CONTROL

A. NERVOUS CONTROL (through respiratory centers in brain)
B CHEMICAL CONTROL (through chemoreceptors)

(A) NERVOUS CONTROL

§  Control of rate of breathing by nervous control is through the Respiratory centers in Medulla oblongata which are sensory to the changes in Conc. of CO2 and H+ present in the cerebro-spiral fluid (CSF).

RESPIRATORY CENTRES IN MEDULLA

Two center are present

(1) DORSAL GROUP OF NEURONS

Medulla contains a dorsal group (Inspiratory group) of neurons responsible for inspiration

FUNCTION
In response to increase conc. of CO2 and H+ (decreased pH), it sends impulses to the intercostals muscles to increase the breathing rate

(2) VENTRAL GROUP OF NEURONS

Another area in the medulla is ventral (expiratory) group of neurons.

FUNCTION
It inhibits the dorsal group and mainly responsible for expiration

(B) CHEMICAL CONTROL

Chemical control of rate of breathing is through chemoreceptors.

LOCATION OF CHEMORECEPTORS

AORTIC BODIES

CAROTID BODIES

AORTIC BODIES

The peripheral chemoreceptors which are located above and below the arch of aorta are called Aortic bodies. It sends impulses to medulla through Vagus nerve.

CAROTID BODIES

Chemoreceptors which are located at the bifurcation of carotid arteries are called Carotid bodies. It sends impulses to medulla through Glossopharyngeal nerve.

FUNCTION
Inc. in concentration of CO2 and H+ in blood are basic stimuli to increase the rate of breathing which are monitered by these chemoreceptors and then send the impulses to medulla oblongata which produce action potential in inspiratory muscles.

DISORDERS OF RESPIRATORY TRACT

(1) LUNG CANCER (BRONCHIAL CARCINOMA)

CAUSES

§  Smoking is a major risk factor either acitively or passively.

§  Asbestos, nickel, radioactive gases are associated with increased risk of bronchial cascinoma

PHYSIOLOGICAL EFFECTS

+ LOSS OF CILIA

The toxic contents of smoke such as nicotine and SO2 cause the gradual loss of cilia of epithelical cells so that dust and germ are settled inside the lungs.

+ ABNORMAL GROWTH OF MUCOUS GLANDS

Tumor arises by uncontrolled and abnormal growth of bronchial epithelium mucous glands. The growth enlarges and some times obstruct a large bronchus.
The tumours cells can spread to other structures causing cancer.

SYMPTOMS

§  Cough- due to irritation

§  Breath lessness – due to obstruction.

(2)TUBERCLOSIS (KOCH’S DISEASE)(INFECTIOUS DISEASE OF LUNG)

CAUSE
Caused by a Bacterium called as “MYCOBECTERIUM TUBERCLOSIS”

PHYSIOLOGICAL EFFECTS

§  Tuber Bacili causes

§  Invasion of infected region by macrophages

§  Fibrosis of lungs thus reducing the total amount of functional lung tissues

These effects cause

§  Increased work during breathing

§  Reduced vital and breathing capacity

§  Difficulty in diffusion of air from alveolar air into blood.

SYMPTOMS

§  Coughing (some time blood in sputum)

§  Chest pair

§  Shortness of breath

§  Fever

§  Sweating at night

§  Weight loss

§  Poor apetite

PREVENTION
A live vaccine (BCG) provides protection against tuberclosis.

3.COPD-(CHRONIC OBSTRUCTIVE PULMONARY DISEASE)

They include

A. Emphysema

B. Asthma

(3-A)EMPHYSEMA

CAUSES
It is a chronic infection caused by inhaling Smoke and other toxic substances such as Nitrogen dioxide and Sulphur dioxide

PHYSIOLOGICAL EFFECTS

§  Long infection – Irritants deranges the normal protective mechanisms such as loss of cilia, excess mucus secretion causing obstruction of air ways

§  Elasticity of lung is lost

§  Residual volume increases while vital capacity decreases.

§  Difficulty in expiration due to obstruction

§  Entrapment of air in alveoli

§  All these together cause the marked destruction of as much as 50-80% of alveolar walls.

§  Loss of alveolar walls reduces the ability of lung to oxygenate the blood and remove the CO2

§  Oxygen supply to body tissues especially brain decreases.

SYMPTOMS

§  Victim’s breathing becomes labored day by day.

§  Patient becomes depressed, irritable and sluggish.

§  Concentration of CO2 increases which may cause death.

(3-B) ASTHAMA

“Respiratory tract disorder in which there are recurrent attacks of breathlessness, characteristically accompanied by wheezing when breathing out.”

CAUSES
It is usually caused by Allergic hypersensitivity to the plant pollens, dust, animal fur or smoke or in older person may be due to common cough.
Heridity is major factor in development of Asthma.

PHYSIOLOGICAL EFFECTS

§  Localized edema in walls of small bronchioles.

§  Secretion of thick mucus.

§  Spastic Contraction of bronchial smooth muscles (so the resistance in air flow increases).

§  Residual volume of lung increases due to difficulty in expiration.

§  Thoracic cavity becomes permanently enlarged.

SYMPTOMS

§  The asthmatic patient usually can inspire quite adequately but has great difficulty in expiring.

LUNG CAPACITIES

1. TOTAL AVERAGE LUNG CAPACITY

DEFINITION

“It is the maximum volume in which the lung can be expanded with greatest possible inspiratory efforts.”

Or

“Total lung capacity is the combination of residual volume and vital capacity.

VALUE
Total lung capacity = 5000 cm3 or 5 lit of air.

2. TIDAL VOLUME

“The amount of air which a person takes in and gives out during normal breathing is called Tidal Volume.”

VALUE
450cm3 to 500 cm3 (1/2 litre)

3. INSPIRATORY RESERVE VOLUME

DEFINITION
‘“Amount of air inspired with a maximum inspiratory effort in excess of tidal volume.”

VALUE
200 cm3 or 2 lit. (Average value)

4. EXPIRATORY RESERVE VOLUME

DEFINITION
“Amount of air expelled by an active expiratory effort after passive expirations.”

VALUE
1000 cm3 or 1 litre.

5. VITAL CAPACITY

DEFINITION

“After an extra deep breath, the maximum volume of air inspired and expired is called Vital capacity.”

Or

“It is the combination of inspiratory reserve volume, expiratory reserve volume and tidal volume.”

VALUE
Averages about 4 litre.

6. RESIDUAL VOLUME

DEFINITION

“Amount of air which remains in lung after maximum expiratory effort is called Residual volume.”

VALUE
Approximately 1 litre or 1000 cm3.

IMPORTANCE OF LUNG CAPACITY

§  Residual volume prevent the lung from collapsing completely.

§  Responsible for gaseous exchange in between breathing.

§  It is not stagnant since inspired air mixes with it each time.

§  Aging or Emphysema, etc can increase the residual volume at the expense of vital capacity.

HAEMOGLOBIN

INTRODUCTION
“Haemoglobin is an iron containing respiratory pigment present in the red blood cells of vertebrates and responsible for their red colour.”

STRUCTURE

Haemoglobin consists of

1. Heme

2. Protein (globin like chains)

1. HEME

One Haemoglobin molecule consists of 4 molecules of Heme. Each Heme molecule contains an iron (Fe++) binding pocket. Thus one molecule of Haemoglobin can combine with 4 iron atoms.

2. GLOBIN

Each Hb molecule contains four globin like chains (Two α chains and Two β chains).

ROLE OF HB DURING RESPIRATION

Two major functions are performed by Hb.

1. Transport of O2 from lung to tissues.

2. Transport of CO2 from tissues to lungs.

1. “TRANSPORT OF O2 FROM LUNGS TO TISSUES”

“Nearly 97% of O2 is transported from the lungs to the tissues in

combination with Hb.”

ATTACHMENT OF O2 WITH HB

It is the iron of Hb molecule which reversibly binds with oxygen. One Hb molecule can bind 4 molecules of O2. Thus due to Hb, blood could carry 70 times more oxygen than plasma.

MECHANISM OF TRANSPORT

§  Due to high O2 concentration in alveolar air, the O2 moves from air to the venous blood where O2 concentration is low.

§  It combines loosely with Hb to form Oxyhemo Globin.

§  In this form, O2 is carried to the tissues where due to low oxygen concentration in tissues, oxy Hb dissociates releasing oxygen, which enters in tissues.

Whole process can be represented by following equation.

2. “TRANSPORT OF CO2 FROM TISSUES TO LUNGS”

“Haemoglobin is also involved in 35% of transport of CO2 from tissues to alveolar blood capillaries in alveoli.”

ATTACHMENT OF CO2 WITH HB

CO2 binds reversibly with NH2 group of Hb to form loose compound called “Carboamino Haemoglobin.”

MECHANISM OF TRANSPORT

§  Carbon dioxide due to its higher concentration in tissue diffuses out into the blood where it combines with Hb to form Carboamino Hb.

§  In the alveoli it breaks and CO2 diffuses out into the Alveoli from where it is expired.

MYOGLOBIN

INTRODUCTION
“Myoglobin is a heme protein, smaller than Hb, found in muscles and giving red colour to them.

STRUCTURE

Myoglobin consists of one heme molecule and one globin chain. It can combine with one iron (Fe++) atom and can carry one molecule of O2.

FUNCTION OF MYOGLOBIN

§  Myoglobin has high affinity for O2 as compared to Haemoglobin so it binds more tightly.

§  It stores the O2 within the muscles.

§  It supplies the O2 to the muscles when there is severe oxygen deficiency (During exercise)

It can be represented as follows:

Mb + O2 ↔ MbO2

TRANSPORT OF GASES

Oxygen and carbondioxide are exchanged in, Alveoli by Diffusion.

O2 TRANSPORT

Blood returning into the lungs from all parts of body is depleted from oxygen. This deoxygenated blood is dark maroon in colour to appear bluish through skin. It becomes oxygenated in the lungs.

TWO FORMS OF O2 IN BLOOD

O2 is transported in the blood in two forms:

§  Dissolved form (3%)

§  Combination with Hb (97%) ® Oxyhaemoglobin

MECHANISM OF O2 TRANSPORT

+ DIFFUSION OF O2 FROM ALVEOLUS INTO PULMONARY BLOOD

The air inhaled into the lungs has high concentration of oxygen while venous blood in pulmonary capillaries has low in concentration. Due to this difference in concentration across the respiratory surface, oxygen diffuses into the blood flowing into capillaries around the Alveoli. Now blood becomes oxygenated which is bright red in colour.

+ DIFFUSION OF O2 FROM CAPILLARIES INTO CELLS

Concentration of O2 in the arterial end of capillaries is much more greater

than concentration of O2 in the cells. So O2 diffuses from the blood to the body cells. Since the blood takes in oxygen much more rapidly than water. Thus it can transport enough oxygen to the tissues to meet their demand.

CO2 TRANSPORT

Blood returning from tissues contain excess of CO2 as a respiratory by-product, which is eliminated from the body during expiration in the lungs.”

THREE FORMS OF CO2 IN BLOOD

§  Dissolved form (in plasma) – 5%

§  In form of HCO3- (in RBC’s) – 60%

§  In combination with Hb (Carboamino Hb) – 35%

+ DISSOLVED FORM

Only 5% of CO2 is transported in dissolved form in plasma. Here it combines with H2O of plasma to form H2CO3. But this reaction is very slow as plasma does not contain Carbonic Anhydrase to accelerate this reaction.
Reactions can be represented by following equations.

CO2 + H2O ↔ H2CO3

H2CO3 ↔HCO3- + H+

HCO3- + k+ ↔ KHCO3

+ IN FORM OF HCO3-

60% of CO2 is transported in the blood in form of HCO3- in RBC’s. Here it combines with water to form H2CO3. But this reaction occurs rapidly in

RBC’s due to presence of Carbonic Anhydrase.

Reactions can be represented by following equations

CO2 + H2O ↔ H2CO3

H2CO3 ↔ HCO3- + H+

HCO3- + Na+ ↔ NaHCO3

+ IN COMBINATION WITH HB

As discussed previously in role of Hb.

MECHANISM OF CO2 TRANSPORT

+ DIFFUSION OF CO2 FROM CELLS INTO CAPILLARIES

CO2 is continuously synthesizing in the tissues as a result of metabolism. Thus due to its higher concentration. CO2 diffuses from the tissues into blood, which becomes deoxygenated.

+ DIFFUSION OF CO2 FROM PULMONARY BLOOD INTO ALVEOLUS
Blood returning from tissues contain high concentration of CO2. This blood is brought to lungs, where CO2 diffuses from the blood into alveolus where its concentration is lower.

FACTORS EFFECTING THE TRANSPORT OF GASES

Following are some factors, which influence the transport of respiratory gases across the alveolar wall.

1. Concentration Gradient

2. Presence of competitor such as CO

3. Moisture

4. Surfactant

5. pH

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