Gas Exchange - Session 1

We inhale and exhale air. The oxygen from the inhaled air is used by our body. While the carbon dioxide is removed by our body. In simple words, the exchange of gases occur between our body and environment. How does this exchange of gases take place?.

In unicellular organisms, the exchange of gases occurs through the plasma membrane. But in human, the exchange of gases occurs through the respiratory system. Human respiratory system consists of several organs and structures that work together to facilitate breathing and gas exchange. Let us explore each component of the respiratory system.

The respiratory system begins with the nose and the nasal cavity. The nose serves as the main entry-point for air into the respiratory system. The nasal cavity is lined with mucous membranes. A mucous membrane contains mucus secreting cells and numerous small hair like structures called cilia.

When air enters the nasal cavity, it passes through these mucous membranes. The mucus traps dust, pollen, bacteria and other foreign particles in the air. The cilia then move in coordinated waves. They sweep the trapped particles toward the throat where they are either swallowed or expelled through coughing or sneezing. This filtration process helps to protect the delicate tissues of the respiratory tract from potential harm caused by inhaled particles.

Dry air can irritate the respiratory tract and cause discomfort. Mucous in the mucous membrane helps to moisten the incoming air. The extensive network of blood vessels within the mucous membranes helps in transferring heat from the body to the incoming air. This warms-up the air near to the body temperature. The warming process is crucial because cold air can potentially damage the sensitive tissues of the respiratory tract.

From the nasal cavity, the air travels downward in the pharynx. Pharynx is a common passage for both air and food. The larynx is located below the pharynx and houses the vocal cords. It plays a crucial role in producing sound for speech. It prevents food and liquids from entering the airway during swallowing.

After passing through the larynx, the air enters the trachea. The trachea is a rigid tube composed of cartilage rings. It extends from the larynx into the chest-cavity. The trachea serves as a conduit for air to pass into and out of the lungs. The trachea is also called the windpipe.

The trachea branches into two main bronchi. Each lung has its own bronchus. The right-lung has a right-main-bronchus and the left-lung has a left-main-bronchus. Bronchi also have cartilage rings. The primary function of the bronchi is to transport air to and from the lungs.

Bronchi serve as the main passageways through which air flows, distributing it to the smaller airways within the lungs. The lungs are the primary organs of the respiratory system responsible for the exchange of gases between the atmosphere and the bloodstream. They are located within the thoracic cavity, on either side of the heart. They are protected by the rib cage.

As the main bronchi enter the lungs, they divide into smaller bronchi and bronchioles. They form a branching structure known as the bronchial tree. Bronchioles lack the cartilage rings found in the larger bronchi. This allows them to be more flexible and responsive to changes in airflow.

Instead of cartilage, the bronchioles are surrounded by smooth muscle. These muscles can contract or relax to regulate the diameter of the airway. As the bronchioles become smaller, they eventually lead-up to clusters of air sacs called the alveoli. The alveoli are tiny, grape like structures at the end of the bronchioles.

The alveoli are surrounded by a network of blood vessels. This allows for efficient exchange of gases between the respiratory and circulatory systems. The structure of alveoli is highly specialized to facilitate gas exchange. They are lined with a thin layer of epithelial cells. These epithelial cells are surrounded by a dense network of capillaries. This thin barrier allows for rapid diffusion of gases across the alveolar wall.

Each lung is divided into lobes. The right-lung has three lobes. The left-lung has two lobes. The lungs are surrounded by a double layered membrane called the pleura. The outer layer is called the parietal pleura. It lines the chest-wall.

The inner layer is called the visceral pleura. It covers the surface of the lungs. The space between the two layers of the pleura is called the pleural cavity. Pleural cavity contains a small amount of pleural fluid that lubricates and reduces friction during breathing movements. Now lets see how the breathing mechanism takes place.

Breathing consists of inhalation and exhalation. Inhalation is the active process of breathing in air, which brings oxygen-rich air into the lungs. It begins with the contraction of the diaphragm. Diaphragm is a dome shaped muscle located below the lungs.

When the diaphragm contracts, it moves downward, increasing the volume of the thoracic cavity. Simultaneously, the external intercostal muscles between the ribs start contracting. This causes the rib cage to expand. As the thoracic cavity expands, the pressure inside the lungs decreases relative to the atmospheric pressure. This creates a pressure gradient. Air flows from an area of higher pressure into an area of lower pressure.

Exhalation is the passive process of breathing out air. It removes carbon dioxide rich air from the lungs. It occurs when the diaphragm and external intercostal muscles relax. This allows the rib cage to return to its resting position. As the thoracic cavity decreases in volume, the pressure inside the lungs increases relative to the atmospheric pressure. Air flows from an area of higher pressure to an area of lower pressure. This expels carbon dioxide from the lungs.