NEET Physics: Waves – NCERT Masterclass & 100 MCQ Quiz

BREATHING AND EXCHANGE OF GASES

NCERT Masterclass & NEET Biology Study Module

1. Human Respiratory System

The respiratory system is responsible for the exchange of $O_2$ from the atmosphere with $CO_2$ produced by the cells.

• Pathway: External nostrils $\rightarrow$ Nasal passage $\rightarrow$ Pharynx (common passage for food and air) $\rightarrow$ Larynx (sound box) $\rightarrow$ Trachea $\rightarrow$ Primary, Secondary, Tertiary Bronchi $\rightarrow$ Bronchioles $\rightarrow$ Alveoli.
• Epiglottis: A cartilaginous flap that prevents the entry of food into the glottis (opening of the larynx) during swallowing.
• Trachea: A straight tube extending up to the mid-thoracic cavity, which divides at the level of the 5th thoracic vertebra into right and left primary bronchi. Supported by incomplete cartilaginous rings.
• Lungs: Covered by a double-layered pleura with pleural fluid between them to reduce friction. Lungs are situated in the air-tight thoracic chamber.

2. Mechanism of Breathing

Breathing involves two stages: Inspiration (drawing atmospheric air in) and Expiration (releasing alveolar air out). Movement of air depends on a pressure gradient.

🔥 The Pressure Game:

• Inspiration (Active process): Contraction of diaphragm and external intercostal muscles increases thoracic volume. Intrapulmonary pressure becomes LESS than atmospheric pressure (negative pressure). Air rushes in.
• Expiration (Passive process): Relaxation of diaphragm and external intercostals decreases thoracic volume. Intrapulmonary pressure becomes MORE than atmospheric pressure. Air is expelled.

Note: We have the ability to increase the strength of inspiration and expiration using additional muscles in the abdomen. A healthy human breathes 12-16 times/minute.

3. Respiratory Volumes and Capacities

Measured using a Spirometer, which helps in clinical assessment of pulmonary functions.

Key Capacities:
IC (Inspiratory Capacity) = TV + IRV
FRC (Functional Residual Capacity) = ERV + RV (Volume remaining in lungs after normal expiration).
VC (Vital Capacity) = ERV + TV + IRV (Max volume a person can breathe in after forced expiration).

4. Exchange of Gases

Alveoli are the primary sites of exchange of gases. Exchange of $O_2$ and $CO_2$ between blood and tissues occurs by simple diffusion based on pressure/concentration gradients.

• Partial Pressure ($pO_2$ and $pCO_2$): Pressure contributed by an individual gas in a mixture.
  • Atmospheric Air: $pO_2 = 159$, $pCO_2 = 0.3$
  • Alveoli: $pO_2 = 104$, $pCO_2 = 40$
  • Deoxygenated Blood: $pO_2 = 40$, $pCO_2 = 45$
  • Oxygenated Blood: $pO_2 = 95$, $pCO_2 = 40$
  • Tissues: $pO_2 = 40$, $pCO_2 = 45$
• Solubility: The solubility of $CO_2$ is 20-25 times higher than that of $O_2$. Therefore, the amount of $CO_2$ that can diffuse across the membrane per unit difference in partial pressure is much higher.
• Diffusion Membrane: Very thin (less than 1 mm). Made of 3 layers: Thin squamous epithelium of alveoli, endothelium of alveolar capillaries, and the basement substance in between.

5. Transport of Gases

Transport of Oxygen

About 97% of $O_2$ is transported by RBCs in the blood. The remaining 3% is dissolved in plasma. $O_2$ binds reversibly with haemoglobin (Fe-containing) to form oxyhaemoglobin. Each Hb molecule can carry a maximum of four $O_2$ molecules.

Every 100 ml of oxygenated blood can deliver around 5 ml of $O_2$ to the tissues under normal physiological conditions.

Transport of Carbon Dioxide

• 20-25% is carried by RBCs as Carbamino-haemoglobin.
• 70% is carried as Bicarbonate ($HCO_3^-$). The enzyme Carbonic anhydrase (highly concentrated in RBCs) facilitates the reaction: $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons HCO_3^- + H^+$.
• 7% is carried in a dissolved state through plasma.
• Every 100 ml of deoxygenated blood delivers approximately 4 ml of $CO_2$ to the alveoli.

6. Regulation of Respiration & Disorders

• Respiratory Rhythm Centre: Located in the Medulla. Primarily responsible for maintaining normal rhythm.
• Pneumotaxic Centre: Located in the Pons. Moderates the functions of the rhythm centre. A neural signal here can reduce the duration of inspiration.
• Chemosensitive Area: Adjacent to rhythm centre. Highly sensitive to $CO_2$ and hydrogen ions ($H^+$). Increase in these substances activates the centre. Note: The role of Oxygen in the regulation of respiratory rhythm is quite insignificant.

Disorders of Respiratory System:

• Asthma: Difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles.
• Emphysema: A chronic disorder where alveolar walls are damaged, decreasing respiratory surface area. Major cause: Cigarette smoking.
• Occupational Respiratory Disorders: In industries involving grinding/stone-breaking. Long exposure to dust leads to inflammation and fibrosis (proliferation of fibrous tissues). E.g., Silicosis, Asbestosis.

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🚀 NEET RESPIRATION MEGA QUIZ (100 MCQ)

Solve the 5 parts below to master Respiratory Volumes, $O_2$ Transport, and Partial Pressures.

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