What is the functionality of Lungs in human biology?
The right lung consists of three lobes, and the left lung, two lobes. Both lungs hold between 5 to 6 L of gas, called the total lung capacity. The maximum amount of air a person can inhale and exhale is called the vital lung capacity. Vital lung capacity increases with exercise, making more oxygen available for the tissues. In a single normal breathing cycle breathing at rest, most persons inhale and exhale only about 0.5 L of gas, called the tidal volume. The respiration rate in humans is about 16 to 18 cycles per minute; this increases with physical activity or in stress situations.
The trachea divides into two air ducts called bronchi, one to each lung, and these in turn branch into progressively smaller bronchioles, ending in tiny globular sacs called alveoli. These grape-like clusters of alveoli consist of very thin cells surrounded by a network of capillaries, also made up of very thin cells. The space between the air and the blood for diffusion is only about 0.2 micrometers. You will remember that the diameter of a red blood cell is about 7 micrometers. Some estimates place the number of alveoli in an average lung at about 300 million. Based upon this figure, if you could spread out all of the alveoli - together, their surface area for gas exchange would be approximately 70 square meters!
Several protective mechanisms keep the lungs from contamination by foreign particles. First, hairs inside the nostrils filter out larger particles. The walls of the nasal cavity and pharynx are lined with mucus and cilia that warm and moisten the air and trap particles that are swept by the cilia into the pharynx and swallowed. During swallowing of food, the larynx moves up against a small flap of cartilage, the epiglottis, sealing off the respiratory system so that food normally cannot pass in to the respiratory tract. During inhalation, the epiglottis lifts and air enters the trachea, where beat of the ciliated lining propels a layer of sticky mucus containing trapped bacteria and foreign particles from inhaled air back up into the pharynx, from where the mucus is swallowed. Loss or inactivation of cilia in the bronchial system due to smoking or exposure to toxins from other sources can eventually result in emphysema, chronic bronchitis, or lung cancer.