���CHAPTER 22�BREATHING AND EXCHANE OF GASES�By SATHYASEELAN ASARI C�PGT BIOLOGY JNV UDUPI�KARNATAKA��

Human Respiratory system

• The human respiratory system
• • In humans and other
• mammals, air enters in to respi

ratory system

• through the nasal cavity
• It passes through the
• pharynx and larynx into

the trachea

• The trachea forks to

form two bronchi

• Each bronchus branches
• into numerous bronchioles

• The bronchioles end in
• clusters of tiny sacs
• called alveoli
• Alveoli form the

respiratory surface of the

• Lungs
• Oxygen diffuses

through the thin walls of

the alveoli intothe blood

• The part starting with external nostrils up to the terminal bronchioles constitute the conducting part.
• The alveoli and their ducts form the exchange part.
• Exchange part is the site of actual diffusion of O 2 and CO 2 between blood and atmospheric air

• Thoracic chamber is formed dorsally by the vertebral column, ventrally sternum, laterally ribs and on the lower side by diaphragm

VARIOUS STAGES OF RESPIRATION

• i)Breathing or pulmonary ventilation
• ii)Diffusion of gases across alveolar membrane.
• iii)Transport of gases by blood
• iv)Diffusion of gases between blood and tissues.
• v) Utilisation of O2 by cells and release of CO 2

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BREATHING

• Two stages: Inspiration and expiration
• Movement of air into and out of the lungs is effected by the pressure gradient between the lungs and atmosphere.
• Inspiration occurs when the intra pulmonary pressure is less than the atmospheric pressure.
• Expiration takes place when the intra pulmonary pressure is higher than the atmospheric pressure.

• The diaphragm and external and internal inter costal muscles helps in the generation of such gradients.
• Contraction of diaphragm----> increase in volume of thoracic chamber in the antero- posterior axis.
• Cotraction of intercostal muscle----->lifts the ribs and sternum------> increase in volume of thoracic chamber in dorso-ventral axis.
• Over all increase in volume ----> increase in pulmonary volume-----> decrease in intra pulmonary pressure lower than atmospheric pressure----> Air move in to the lungs, inspiration.

• Relaxation of diaphragm and intercostal muscles------> diaphragm and sternum to their normal position---> reduction in thoracic volume-----> decrease in pulmonary volume----> increase in intra pulmonary pressure above the atmospheric pressure-----> expulsion of air from lungs, expiration.
• On an average a healthy human breaths 12-16 times/ min.

RESPIRATORY VOLUMES

• Tidal volume
• Inspiratory reserve volume
• expiratory reserve
• residual volumes
• The tidal volume is the volume of air inspired or expired with each normal breath(500ml)
• The inspiratory reserve volume is the extra volume of air that can be inspired over and above the normal(2500-3000ml)

• The expiratory reserve volume is the maximum extra volume of air that can be expired by forceful expiration after the end of a normal tidal expiration(1000-1100ml)
• The residual volume is the volume of air remaining in the lungs after the most forceful expiration(1100-1200ml)
• The inspiratory capacity equals the tidal volume plus the inspiratory reserve volume.
• The functional residual capacity equals the expiratory reserve volume plus the residual volume

LUNG CAPACITY

• The vital capacity equals the inspiratory reserve volume plus the tidal volume plus the expiratory reserve volume.
• The total lung capacity is the maximum volume to which the lungs can be expanded with the greatest possible. It is equal to the vital capacity plus the residual volume.

EXCHANGE OF GASES

• Primary sites of gas exchange is alveoli
• Exchange also occur between blood and tissues.
• Exchange of gases in these sites take place by simple diffusion based on concentration gradient.
• Solubility of the gases and thickness of the membrane involved also affect the rate of diffusion.

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• Solubility of CO 2 is20-25 times higher than that of O 2 .
• Thus the amount of CO 2 that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of O 2 .
• The diffusion membrane is made up of three layers
• i)squamous epithelium of alveoli
• ii)endothelium of alveolar capillaries and
• iii) Basement membrane.

• Total thickness is much less than a millimeter.
• All these factors are favorable for diffusion of O 2 from alveoli to tissue and that of CO 2 from tissue to alveoli.

GAS EXCHANGE IN THE BODY

TRANSPORT OF GASES IN THE BODY

• Blood transports the respiratory gases, with
• hemoglobin carrying the oxygen
• • The heart pumps oxygen-poor blood to the lungs
• – In the lungs it picks up O2 and drops off CO2(external respiration)
• – In the tissues, cells pick up O2 and drop off CO2-(Internal respiration)
• – Gases diffuse down pressure gradients in the lungs
• and the tissues

TRANSPORT OF OXYGEN

• ⦿ Blood transports gases between the lungs & body tissues.
• ⦿ When O2 and CO2 enter the blood, certain chemical reactions
• occur that aid in gas transport & gas exchange.
• OXYGEN TRANSPORT
• ⦿ Oxygen does not dissolve easily in water, so only about 1.5% of
• inhaled O2 is dissolved in blood plasma, which is mostly water.
• ⦿ About 98.5% of blood O2 is bound to hemoglobin in red blood cells.
• ⦿ The heme portion of hemoglobin contains four atoms of

iron, each

• capable of binding to a molecule of O2.

• Oxygen & hemoglobin bind in an easily reversible reaction to form oxyhemoglobin.
• Hb + O2 ↔ Hb − O2
• (Reduced Hb) (Oxygen) (Oxyhemoglobin)
• ⦿ The 98.5 % of the O2 that is bound to hemoglobin is trapped inside RBCs, so only the dissolved O2 (1.5%) can diffuse out of tissue capillaries into cells.
• ⦿ The binding & dissociation of oxygen from

haemoglobin depends on

⮚ Partial pressure of oxygen

• ⦿ The higher the PO2 ,the more O2 combines with Hb.

• ⮚ Acidity(pH)
• ⦿ As acidity increases(pH decreases), the affinity of hemoglobin for O2 decreases & O2 dissociates more readily from haemoglobin.
• ⮚ Temperature
• ⦿ If the body temperature increases, oxygen dissociates more readily from hemoglobin.
• Partial pressure of carbon dioxide
• ⦿ CO2 also can bind to hemoglobin. As PCO2 rises, hemoglobin release O2 more readily.
• ⦿ PCO2 & pH are related factors because low blood pH(acidity)
• result from high PCO2.

OXYGEN DISSOCIATION CURVE

• When we plot the percent saturation of haemoglobin against partial pressure of oxygen in a graph, we get a sigmoid curve .
• It is called oxygen dissociation curve.
• Shows that as partial pressure of O2 increases the ability of Hb, combining with O2 increases and reaches a maximum value at a particular partial pressure of O2.(100mmHg)
• After this level there is no increase with increase in PO2

TRANSPORT OF CARBON DIOXIDE

• CO2is transported in the blood in three forms:
• i) Dissolved Carbon dioxide
• ⦿ The smallest percentage(7%) is dissolved in blood
• plasma. On reaching the lungs, it diffuse into alveolar
• air & is exhaled
• ⦿ ii) Carbamino Haemoglobin : about 23%, combines

with the amino group of haemoglobin to form carbamino

haemoglobin.

• The main CO2 binding sites are the terminal aminoacids in the two alpha & two beta chains.
• ⦿ Haemoglobin that has bound CO2 is termed as
• carbaminohemoglobin( Hb–- CO2)

• iii) Bicarbonate Ions: Most of the CO2( about 70%) is transported as bicarbonate ions in the blood.
• ⦿ As CO2 diffuses into capillaries & enters red blood cells, it reacts with water in the presence of the enzyme Carbonic anhydrase to form carbonic acid, which dissociates into H+ & HCO3.
• CO2 + H2O H2CO3 H+ + HCO3

⦿ Thus as blood picks up CO2 ,HCO3- accumulates

inside RBCs. Some moves out into the blood plasma,

in exchange Cl- ions move from plasma into the

RBCs.

• The net effect of these reactions is that CO2 is removed

from tisssue cells & transported in blood plasma as

HCO3- .As blood passes through pulmonary capillaries

in the lungs, all these reactions reverse & CO2 is exhaled

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• Most of co 2 is transported to the lungs as bicarbonate ions

REGULATION OF RESPIRATION

• Respiration has 2 control mechanisms :
• Nervous and Chemical
• NERVOUS :The respiratory center in medulla and pons consist of 1) Dorsal respiratory group :
• Located in the dorsal portion of medulla, activated when co2 con. remain high
• Mainly causes Inspiration
• 2) Ventral respiratory group :Located in the

ventrolateral portion of medulla

• Causes Inspiration or expiration
• 3) Pneumotaxic centre:
• Located in the dorsal portion of pons
• Mainly limits Inspiration

• CHEMICAL :
• Chemoreceptors are located in the carotid and aortic bodies
• Excess CO2 and H+ stimulate the respiratory

center and accordingly alter inspiratory

activities

• and expiratory signals to the respiratory muscles and regulate the ventilation of lungs

DISORDERS OF RESPIRATORY SYSTEM

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• Asthma: a severe allergic reaction in which contraction of

the bronchioles makes breathing difficult

• Bronchitis: an inflammation of the lining of the bronchial

tubes. The passageways to the alveoli become swollen and

clogged with mucus

• Emphysema: lungs lose their elasticity, deterioration of the

lung structure

• Pneumonia: alveoli become filled with fluid. Caused by

bacterial or viral infection

• Lung Cancer: a disease in which tumors form in the lungs

OCUPATIONAL LUNG DISEASES

• This group of lung diseases is caused by inhaling atmospheric pollutants at work place
• To cause disease, particles must be so small

that they are carried in inspired air to the

level of the respiratory bronchioles and

alveoli. The common examples are

1. SILICOSIS: This may be caused by long-term exposure

to dust containing silicon compounds.

High-risk industries are quarrying, granite, slate,

sandstone-mining, stone masonry, sand blasting and

glass and pottery work. silicosis appears to predispose to

the development of tuberculosis.

• 2.ASBESTOSIS:
• Asbestosis is caused by inhaling asbestos
• fibres, usually develops after 10 to 20 years
• exposure, but sometimes after only 2 years.
• Asbestos miners and workers involved in
• making and using some products containing
• asbestos are at risk.
• There are different types of asbestos, but blue
• asbestosis is associated with the most serious
• disease.