The Respiratory System:

The primary function of the lungs is to exchange gases, taking in oxygen and expelling carbon dioxide. During inhalation, the diaphragm contracts and chest muscles expand, creating a vacuum-like effect that draws air into the lungs. During exhalation, the muscles relax and the lungs return to their normal size, pushing out the air. When you inhale, air enters through the nostrils and passes through the nasal cavity, which is lined with mucus-producing cells. This mucus contains lysozymes, enzymes that help kill bacteria. The mucus also coats nose hairs, which can trap dust, pollen, and bacteria, forming small clumps known as boogers.

The nasal cavity is linked to four sinuses, namely the frontal, ethmoid, sphenoid, and maxillary sinuses. These sinuses are air-filled spaces within the surrounding bones of the nose. They serve the purpose of allowing inspired air to circulate, providing time for it to become warm and moist. Additionally, the paranasal sinuses act as small echo-chambers, amplifying the sound of your voice. This is why your voice sounds different when the sinuses are congested with mucus during a cold. Once the air is relatively clean, warm, and moist, it moves from the nasal cavity into the nasopharynx, which is the region connecting the throat. The oropharynx, on the other hand, connects the pharynx to the oral cavity. The soft palate, located behind the hard part of the roof of your mouth, and the uvula work together to form a flap or valve. This flap closes off the nasopharynx when you eat, preventing food from entering that area.

The laryngopharynx, which connects to the larynx, marks the point where the paths of food and air diverge. The epiglottis, a spoon-shaped cartilage flap located at the top of the larynx, acts as a seal to prevent food from entering the airway while eating. If anything, other than air enters the larynx, the cough reflex is triggered to expel it. Once air enters the larynx, it continues its journey through the trachea, also known as the windpipe, which then splits into the two mainstem bronchi at a point called the carina. These bronchi lead into the lungs, with the right lung having three lobes and the left lung having two. The right mainstem bronchus is wider and more vertical, making it more susceptible to foreign objects entering the lung. As the mainstem bronchi divide further, they become smaller bronchi. The trachea and the initial three generations of bronchi are supported by cartilage rings and also contain a layer of smooth muscle with nerves from the autonomic nervous system.

The autonomic nervous system consists of two types of nerves: sympathetic nerves, which are responsible for the "fight or flight" response, and parasympathetic nerves, which promote the "rest and digest" mode. When running from a turkey, the sympathetic nerves stimulate beta 2 adrenergic receptors in the airways, increasing their diameter. On the other hand, parasympathetic nerves can stimulate muscarinic receptors, causing a decrease in airway diameter.

The larger airways are lined with ciliated columnar cells and goblet cells that secrete mucus. This mucus helps trap particles, and the ciliated columnar cells work together to move the mucus and trapped particles towards the pharynx. This mechanism, known as the mucociliary escalator, allows the particles to be either spit out or swallowed.

As the airways progress into the bronchioles, they become narrower and do not require cartilage for support. These bronchioles, also known as conducting bronchioles, continue to conduct air through smaller and smaller passages for about 15-20 generations. Within the smooth muscle layer of these bronchioles, there are nerves of the autonomic nervous system.

The conducting bronchioles are lined by ciliated columnar cells, mucus-secreting goblet cells, and club cells. Club cells secrete glycosaminoglycans to protect the bronchiolar epithelium and can transform into ciliated columnar cells for regeneration. The terminal bronchioles lead to the respiratory bronchioles, which have alveoli outpouchings. There are approximately 500 million alveoli in the lungs. The respiratory bronchioles end when only alveoli remain, forming the alveolar duct. The alveolar wall is lined by thin epithelial cells called pneumocytes, with type II pneumocytes secreting surfactant to decrease surface tension and keep the alveoli open.

The type II pneumocytes, like the club cells, have the ability to transform into type I pneumocytes, aiding in the regeneration and replacement of damaged cells. Additionally, alveolar macrophages can engulf and transport tiny particles from the lungs to the conducting bronchioles, where they can be expelled through coughing or swallowed. Once the inhaled air is free from particles, it enters the alveolus, which is primarily composed of type I pneumocytes. On the opposite side of the pneumocytes, there are endothelial cells that line the capillary walls, where deoxygenated blood from the pulmonary arteries flows. The alveolar wall, basement membrane, and capillary wall together form the blood-gas barrier, separating the air from the blood. Here, carbon dioxide diffuses out of the deoxygenated blood into the alveoli, which is then exhaled. Conversely, with each inhalation, oxygen freely diffuses from the alveoli into the blood. The oxygenated blood then travels to the pulmonary veins, the heart, and eventually to the body's tissues.

To summarise, the respiratory system enables the exchange of gases. Inhaled oxygen travels through various parts of the respiratory system, including the pharynx, larynx, trachea, airways, bronchioles, and alveoli, before reaching the body's tissues through capillaries. On the other hand, carbon dioxide follows the opposite path and is eventually exhaled.