Breathing and Exchange of Gases | Plus 1 | Exam Capsule Notes (Web and PDF)

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BREATHING & EXCHANGE OF GASES: CHAPTER AT A GLANCE
HUMAN RESPIRATORY SYSTEM 

1. Air passages

External nostrils → nasal passage → nasal chamber → pharynx → glottis → larynx → trachea → primary bronchi → secondary bronchi → tertiary bronchi → bronchioles → terminal bronchioles → alveoli.

Epiglottis closes glottis to prevent entry of food into larynx. 


2. Lungs

Lungs are covered by double-layered pleura.

Alveoli (air sacs) are the structural and functional units of lungs.

Spirometer: To measure respiratory rate.

Normal respiratory (breathing) rate: 12-16 times/min. 

MECHANISM OF BREATHING 

a. Inspiration 


Diaphragm & External intercostal muscles contract → thoracic volume increases → pulmonary volume increases → intra-pulmonary pressure decreases → air into lungs.

b. Expiration


Intercostal muscles & diaphragm relax → thoracic volume decreases → pulmonary volume decreases → intra-pulmonary pressure increases → air moves out.

Respiratory volumes/capacities: 
  • Tidal volume (TV): Volume of air inspired or expired during a normal respiration. 500 ml.
  • Inspiratory reserve volume (IRV): Additional volume of air that can inspire by forceful inspiration. 2500-3000 ml.
  • Expiratory reserve volume (ERV): Additional volume of air that can expire by a forceful expiration. 1000-1100 ml.
  • Residual volume (RV): Volume of air remaining in lungs after a forcible expiration. 1100-1200 ml.
  • Inspiratory capacity (IC): Total volume of air inspired after a normal expiration (TV+IRV). 3000-3500 ml.
  • Expiratory capacity (EC): Total volume of air expired after a normal inspiration (TV+ERV). 1500-1600 ml.
  • Functional residual capacity (FRC): Volume of air in lungs after normal expiration (ERV+RV). 2100-2300 ml.
  • Vital capacity (VC): Volume of air that can breathe in after a forced expiration or Volume of air that can breathe out after a forced inspiration (ERV + TV + IRV). 3500-4500 ml.
  • Total lung capacity (TLC): Volume of air in lungs after a maximum inspiration (RV + ERV + TV + IRV or VC + RV). 5000-6000 ml.
GAS EXCHANGE 

Gas exchange occurs by simple diffusion between 1. Alveoli & blood 2. Blood & tissues

Alveoli are the primary sites of gas exchange. 

Factors influencing gas exchange are:

Pressure/ concentration gradient

Respiratory gas

pO2 (in mm Hg)

pCO2 (in mm Hg)

Atmospheric air

159

0.3

Alveoli

104

40

Deoxygenated blood

40

45

Oxygenated blood

95

40

Tissues

40

45

Solubility of gases: Solubility of CO2 is 20-25 times higher than that of O2.

Thickness of diffusion membranes: 3 layers- Squamous epithelium of alveoli + Endothelium of capillaries + Basement substance. Its total thickness is very less → easy gas exchange. 


Surface area: Presence of alveoli increases surface area → gas exchange increases.

GAS TRANSPORT (O2 TRANSPORT & CO2 TRANSPORT) 

1. O2 TRANSPORT (from lungs to various tissues)

a. By blood plasma (3%): O2 + plasma → tissues.
b. As oxyhaemoglobin (97%): O2 + haemoglobin (Hb) → oxyhaemoglobin.


In alveoli: high pO2, low pCO2, lesser H+ ion and lower temperature → formation of oxyhaemoglobin.

In tissues: low pO2, high pCO2, high H+ ions and high temperature → Hb4O8 dissociates to release O2.

Oxygen-haemoglobin dissociation curve


It is a sigmoid curve obtained when percentage saturation of Hb with O2 is plotted against the pO2.

It is used to study the effect of factors like pCO2, H+ concentration etc., on binding of O2 with Hb.

2. CO2 TRANSPORT (from tissues to lungs)

a. As carbonic acid (7%): CO2 + plasma water → carbonic acid → lungs → CO2 releases.
b. As carbamino-haemoglobin (20-25%): CO2 + Hb → carbamino-haemoglobin → lungs → CO2 dissociates.
c. As bicarbonates (70%):

REGULATION OF RESPIRATION 

Respiratory centres in Brain:
  • Respiratory rhythm centre: In medulla oblongata. It regulates respiratory rhythms.
  • Pneumotaxic centre: In Pons. It moderates functions of respiratory rhythm centre.
  • Chemosensitive area: Seen adjacent to the rhythm centre. Increase in the concentration of CO2 and H+ activates this centre.
DISORDERS OF RESPIRATORY SYSTEM 
  • Asthma: Difficulty in breathing due to inflammation of bronchi and bronchioles.
  • Emphysema: Damage of alveolar walls → decreases respiratory surface. Major cause is cigarette smoking.
  • Occupational respiratory disorders: Exposure of industrial dusts → fibrosis of lungs → lung damage. 

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