Respiratory System

The respiratory system is a biological system responsible for exchanging gases — primarily oxygen (O₂) and carbon dioxide (CO₂) — between an organism and its environment. This process is essential for cellular respiration, where cells produce energy (ATP). All living organisms, from simple unicellular forms to complex animals, require oxygen for metabolism and must eliminate carbon dioxide. The structure and mechanism of respiration vary among species, from simple diffusion in unicellular organisms to specialised organs like gills, tracheae, and lungs in multicellular animals.

1.0Anatomy of the Respiratory System

The respiratory system can be divided into two main parts: the conducting portion and the respiratory portion.

• The Conducting Portion: This pathway directs air into the lungs. It includes the nose, pharynx, larynx, trachea, bronchi, and bronchioles. These structures not only transport air but also filter, warm, and humidify it.
• Nose and Nasal Cavity: The primary entry point for air. It contains hair and mucus to trap dust and other particles.
• Pharynx: A common passage for both food and air. It connects the nasal cavity to the larynx.
• Larynx (Voice Box): Contains vocal cords that produce sound. The epiglottis, a flap of cartilage, prevents food from entering the trachea.
• Trachea (Windpipe): A tube supported by C-shaped cartilage rings that extends from the larynx into the chest cavity, where it divides into the primary bronchi.
• Bronchi and Bronchioles: The trachea branches into two bronchi, which further divide into smaller and smaller tubes called bronchioles.
• The Respiratory Portion: This is where the actual gas exchange occurs. It consists of the respiratory bronchioles, alveolar ducts, and alveoli.
• Alveoli: Tiny, grape-like sacs at the end of the bronchioles. They are the primary sites of gas exchange. The walls of the alveoli are extremely thin and are surrounded by a network of capillaries, facilitating the rapid diffusion of gases.

2.0The Mechanism of Breathing

Breathing, also known as ventilation, is the process of inhaling and exhaling air. It involves the coordinated action of muscles and changes in pressure.

• Inspiration (Inhalation): This is an active process. The diaphragm contracts and flattens, and the intercostal muscles lift the rib cage. This increases the volume of the thoracic cavity, causing a decrease in pressure inside the lungs. As a result, air from the higher-pressure external environment rushes into the lungs.
• Expiration (Exhalation): This is usually a passive process. The diaphragm and intercostal muscles relax, decreasing the volume of the thoracic cavity. This increases the pressure inside the lungs, forcing the air out. Forced expiration, however, involves the contraction of abdominal muscles.

3.0Respiratory System Diagram

The respiratory system diagram shows the flow of air through various structures — nasal passages → pharynx → trachea → bronchi → lungs → alveoli, where the exchange of gases takes place.

Respiratory System Parts and Functions

The respiratory system organs differ among groups of organisms but serve the same core purpose — efficient gas exchange.

How it Works 

The process of respiration involves:

1. Breathing (Ventilation): Inhaling oxygen-rich air and exhaling carbon dioxide-rich air.
2. External Respiration: Exchange of gases between alveoli (or similar structures) and the blood.
3. Internal Respiration: Exchange of gases between blood and body tissues.
4. Cellular Respiration: Breakdown of glucose with oxygen to produce energy (ATP), releasing carbon dioxide and water.

Gas Exchange and Transport

Gas exchange is the core function of the respiratory system and occurs at two levels:

• External Respiration: The exchange of gases between the alveoli and the blood in the capillaries. Oxygen diffuses from the alveoli into the blood, while carbon dioxide diffuses from the blood into the alveoli to be exhaled.
• Internal Respiration: The exchange of gases between the blood in the capillaries and the body cells. Oxygen diffuses from the blood into the cells, and carbon dioxide diffuses from the cells into the blood.

Gas Transport:

• Oxygen Transport: The majority of oxygen (approx. 97%) is transported by haemoglobin, a protein found in red blood cells. Each haemoglobin molecule can bind up to four oxygen molecules, forming oxyhaemoglobin.
• Carbon Dioxide Transport: Carbon dioxide is transported in three ways:
• As dissolved CO₂ in plasma (7%)
• Bound to haemoglobin as carbaminohemoglobin (23%)
• As bicarbonate ions () in plasma (70%), this is the most significant method. The reaction is facilitated by the enzyme carbonic anhydrase inside red blood cells.

4.0Respiratory System Organs in Different Organisms

• In Plants:  Gas exchange occurs through stomata and lenticels.
• In Aquatic Animals:  Gills extract dissolved oxygen from water.
• In Insects: The tracheal system directly delivers oxygen to tissues.
• In Amphibians: Both skin and lungs function as respiratory organs.
• In Mammals and Birds: Lungs serve as the primary respiratory organs, supported by specialized air sacs (in birds).