Human Physiology Class 11: Complete Guide to Respiratory System

Overview of Human Physiology and the Respiratory System

Human physiology studies the functions of the human body systems. The respiratory system is vital as it provides oxygen to cells and removes carbon dioxide, a waste product. In Class 11 NCERT biology, this system is divided into two main parts:

• Conducting part: Includes nostrils, nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles. It transports, filters, warms, and humidifies air.
• Respiratory part: Comprises alveoli and alveolar ducts where gas exchange occurs.

This system ensures that oxygen reaches the bloodstream and carbon dioxide is expelled efficiently, maintaining homeostasis.

Structure and Function of Respiratory Organs

The respiratory organs work together to facilitate breathing and gas exchange:

• Nostrils and Nasal Cavity: Air enters through nostrils and is filtered by nasal hairs. The nasal cavity warms and moistens the air.
• Pharynx and Larynx: The pharynx is a passage for air and food. The larynx contains vocal cords and the epiglottis, which prevents food from entering the airway.
• Trachea: A tube supported by cartilaginous rings that conducts air to the lungs.
• Bronchi and Bronchioles: The trachea splits into right and left bronchi, which further divide into smaller bronchioles.
• Alveoli: Tiny sacs with thin walls surrounded by capillaries where oxygen diffuses into blood and carbon dioxide diffuses out.

Each organ has a specific role in ensuring air reaches the alveoli in optimal condition for gas exchange.

Mechanism of Breathing: How Air Moves In and Out

Breathing or pulmonary ventilation involves two phases:

• Inhalation: The diaphragm contracts and moves downward, and the rib cage expands, increasing thoracic cavity volume. This lowers pressure inside the lungs, causing air to flow in.
• Exhalation: The diaphragm relaxes and moves upward, and the rib cage contracts, decreasing thoracic cavity volume. This increases lung pressure, pushing air out.

The pleura, a double-layered membrane around the lungs with pleural fluid, reduces friction during these movements. Changes in thoracic volume directly affect lung volume, enabling efficient air flow.

Formula for Lung Volume Change:

$$ \Delta V = V_{inhaled} - V_{exhaled} $$

Where $\Delta V$ is the change in lung volume.

Gas Exchange Process in Alveoli

Gas exchange occurs exclusively in the alveoli due to their thin walls and rich capillary network. Oxygen from inhaled air diffuses across the alveolar membrane into blood, while carbon dioxide diffuses from blood into alveoli to be exhaled.

Why only alveoli?

• Thin walls (one cell thick)
• Large surface area (millions of alveoli)
• Close contact with capillaries

Partial pressures:

Oxygen diffuses from high to low partial pressure (alveoli to blood), and carbon dioxide moves from blood to alveoli for removal.

Transport of Gases in Blood

Oxygen and carbon dioxide are transported in blood by different mechanisms:

• Oxygen Transport:
• Mostly bound to hemoglobin forming oxyhemoglobin (~98.5%)
• A small amount dissolved in plasma (~1.5%)

• Carbon Dioxide Transport:

1. Dissolved in plasma (7-10%) 2. Bound to hemoglobin as carbaminohemoglobin (20-23%) 3. As bicarbonate ions (HCO$_3^-$) in plasma (70%)

Bicarbonate Formation Reaction:

$$ CO_2 + H_2O \xrightleftharpoons[carbonic~anhydrase]{} H_2CO_3 \rightarrow H^+ + HCO_3^- $$

This reversible reaction helps maintain blood pH and efficiently transports CO$_2$ from tissues to lungs.

Lung Volumes and Capacities: Understanding Vital Capacity

Lung volumes and capacities are important indicators of respiratory health:

Vital Capacity (VC) is the maximum air exhaled after a maximum inhalation:

$$ VC = TV + IRV + ERV $$

It reflects lung efficiency and health. A healthy adult typically has a VC of about 4600 ml.