2.2.1 The Lungs

The Gas Exchange System:

• Trachea: Also known as the windpipe, the trachea is the tube through which air moves from the mouth and nose into the lungs.
• Intercostal Muscles: These muscles are located between the ribs and are responsible for ventilating the lungs by contracting and relaxing to move the ribcage during breathing.
• Bronchi: The trachea divides into two bronchi, each leading to one lung, where air is further directed into the lungs.
• Bronchioles: The bronchi split into smaller tubes called bronchioles, which distribute air throughout the lungs.
• Alveoli: At the ends of the bronchioles are the alveoli, small air sacs where gas exchange occurs. The alveoli are crucial for transferring oxygen into the blood and removing carbon dioxide.
• Diaphragm: The diaphragm is a large muscle that separates the lungs from the digestive organs. When it contracts and moves downward, it increases the volume of the thoracic cavity, allowing air to be drawn into the lungs.

Ventilation:

• Inhalation:

  1. The ribcage moves up and out, and the diaphragm moves down, increasing the volume of the chest cavity.
  2. The increased volume leads to lower pressure inside the chest.
  3. Air is drawn into the lungs from the environment, as air moves from areas of higher pressure (outside the body) to lower pressure (inside the lungs).

• Exhalation: 4. The ribcage moves down and in, and the diaphragm moves up, decreasing the volume of the chest cavity. 5. The decreased volume leads to higher pressure inside the chest. 6. Air is forced out of the lungs, moving from the higher pressure inside the lungs to the lower pressure outside.

Gas Exchange:

• Inhalation: 7. When you inhale, the alveoli fill with oxygen. 8. The blood in the capillaries surrounding the alveoli is deoxygenated (it has come from the pulmonary artery) and contains high levels of carbon dioxide, a waste product of respiration. 9. Oxygen diffuses down its concentration gradient from the alveoli into the blood, where the oxygen concentration is lower. 10. Carbon dioxide diffuses from the blood into the alveoli, down its concentration gradient, to be exhaled.

Adaptations of Alveoli for Gas Exchange:

• Large Surface Area: The alveoli are tiny and arranged in clusters, providing a large surface area to maximise diffusion.
• Rich Blood Supply: The surrounding capillaries provide a large blood supply, maintaining a concentration gradient that favours the diffusion of oxygen into the blood and carbon dioxide out of it.
• Thin Walls: The alveoli walls are very thin, creating a short diffusion pathway, which speeds up the process of gas exchange.

Calculating Breathing Rate:

• To calculate breathing rate, divide the number of breaths taken by the number of minutes. This gives you the number of breaths per minute, which is an important indicator of respiratory health.