Important Diagrams: Breathing and Exchange of gases | Biology Class 11 - NEET PDF Download

Human Respiratory System

The human respiratory system starts with external nostrils leading to the nasal chamber, pharynx, larynx (sound production), and trachea, which splits into bronchi and further divides into bronchioles ending in alveoli within the lungs, encapsulated by pleura within the thoracic chamber; this structure supports breathing, gas exchange, and temperature regulation of inhaled air.

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Mechanism of Inspiration and Expiration

Inspiration: Inspiration occurs when the diaphragm and external intercostal muscles contract, enlarging the thoracic cavity and reducing intra-pulmonary pressure below atmospheric levels, allowing air to flow into the lungs.

Expiration: Expiration follows as the diaphragm and intercostal muscles relax, decreasing thoracic volume and increasing intra-pulmonary pressure above atmospheric levels, which forces air out of the lungs.

Exchange of Gases 

• Alveoli are crucial for gas exchange, where oxygen (O₂) and carbon dioxide (CO₂) transfer between air and blood through diffusion based on pressure gradients, gas solubility, and membrane thickness. 
• The diffusion is driven by partial pressures, with CO₂ being more soluble than O₂, allowing for more efficient diffusion despite the same pressure gradient. 
• The diffusion membrane in the respiratory system consists of three layers: the alveolar squamous epithelium, the endothelium of alveolar capillaries, and the basement substance between them, which is very thin, altogether less than a millimeter thick. 
• This minimal thickness optimizes the diffusion process, allowing efficient oxygen transfer from alveoli to tissues and carbon dioxide transfer in the opposite direction.

Partial Pressures (in mm Hg) of Oxygen and Carbon dioxide at Different Parts Involved in Diffusion in Comparison to those in Atmosphere

Exchange of gases at the alveolus and the body tissues with blood and transport of oxygen and carbon dioxide

A Diagram of a section of an alveolus with a pulmonary capillary.

Oxygen-Dissociation Curve

The Oxygen Dissociation Curve is a graphical representation showing the relationship between the saturation of haemoglobin with oxygen (as a percentage) and the partial pressure of oxygen (pO₂) in the blood. The curve is sigmoidal, which indicates that haemoglobin's affinity for oxygen increases as more oxygen molecules bind to it. At high oxygen levels, such as those in the lungs, haemoglobin binds oxygen efficiently, while at lower levels, such as in metabolically active tissues, it releases oxygen more readily.

Key influencing factors that shift the curve include:

• Increased CO2 levels, acidity, or temperature: These conditions shift the curve to the right (known as the Bohr effect), enhancing oxygen release in tissues.
• Decreased CO2 levels, lower acidity, or cooler temperatures: These shift the curve to the left, promoting oxygen uptake in the lungs.

Oxygen dissociation curve

Diagram Based Question NEET

Q1: The partial pressures (in mm Hg) of oxygen (O2) and carbon dioxide (CO2) at alveoli (the site of diffusion) are :     [NEET 2021] 
(a) pO2 = 95 and pCO2 = 40 
(b) pO2 = 159 and pCO2 = 0.3 
(c)  pO2 = 104 and pCO2 = 40 
(d) pO2 = 40 and pCO2 = 45 
Ans: (c) 

Option (c) is correct because pO2 in alveoli is 104 mm Hg and pCO2 in alveoli is 40 mmHg.  In atmosphere, pO2 is 159 mm Hg and pCO2 is 0.3 mm Hg.  In deoxygenated blood, pO2 is 40 mmHg and pCO2 is 45 mmHg.  In oxygenated blood, pO2 is 95 mmHg and pCO2 is 40 mmHg. 

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