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The Respiratory System

Okay. So every body is trying its hardest, in the face of internal and external stimuli, to maintain homeostasis (equilibrium). All of your systems do their damnedest to contribute to that. Your respiratory system, specifically, contributes to homeostasis by exchanging gases (commonly understood as O2 and C02, but others as well) between the atmospheric air, your blood, and the cells of your body's tissues.

Your respiratory system also helps adjust your body's pH to keep it at its normal, slightly alkaline 7.4. It also contains your olfactory receptors, and excretes small amounts of water and heat. (The heat mechanism is more noticeable in other mammals, such as dogs, whose skin does not assist in modulating their internal temperature: that's why they pant.)

The respiratory system is made up of several parts, divided into the Upper Respiratory Tract and the Lower Respiratory Tract. The URT contains your nose, and pharynx (and associated structures, like your tonsils); the LRT contains your larynx, trachea, bronchi, and lungs.

[[Side note: how to remember these? For U, NP (no problem); ... LTBG? Larynx, trachea, bronchi, gasbags? *LOL*]]

There are also two major zones in your respiratory tract. The Conducting Zone, predictably, conducts air; it turbinates the air in your nasal cavity in order to warm and moisten it before sending it down to your lungs. Meanwhile, there is the Respiratory Zone in your lungs; it is where the alveoli (small sacs of the lung, and the lung's basic functional unit) exchange gases with the capillaries surrounding them.

Basic Parts of the Nose

The nose has several relevant structures, such as the external nares (aka your nostrils), and its bony and cartilaginous framework. The relevant bones are your frontal bone (forehead), nasal bones (the two small bones that form the bridge of the nose where your glasses might sit), and the maxilla (the bone that holds your upper teeth and forms the hard palate of your mouth). The cartilaginous parts are the septal nasal cartilage (the dividing wall between the nostrils; the affected structure of a deviated septum), the lateral nasal cartilage (the squishy part of the triangle of your nose), and the alar cartilage (the 'wings' that form your nostrils - the bits that flare).

Then, within the nasal cavity, we have the vestibule, the empty, front part of the cavity which does very little except 'accept' the air. Further back, we have the superior, middle, and inferior conchae on the lateral walls, which perform that 'turbination' of air for moistening and heating.

Basic Parts of The Pharynx

Then we move on to the pharynx. The pharynx is about 13cm long, and extends from the internal nares (a space within the nasal cavity) and extends to the cricoid cartilage (part of the cartilage that makes up your throat - the lowest cartilage of the larynx). It is posterior to/behind your nasal and oral cavities (mouth & nose), above the larynx, and in front of your cervical spine. It is lined by a mucus membrane and has skeletal (voluntarily controlled) muscle. These muscles assist in swallowing. Your pharynx allows for food and air to enter the body, protects your system from outside invaders (due to the tonsils), and serves as a resonating chamber for the voice.

The pharynx is divided into three parts: the nasopharynx, the oropharynx, and the laryngopharynx.

The nasopharynx, as you can probably guess from the name, lies behind your nasal cavity and above your soft palate (an arch-shaped muscular partition that forms the back of the roof of your mouth). The nasopharynx is lined by pseudostratified ciliated columnar epithelium (PCCE) and a mucous membrane, the better to trap dust and other particles traveling with the inbound air.

There are five openings in the nasopharynx: the two internal nares at the start, the opening to the oropharynx at the end, and the openings to the two pharyngotympanic/eustachian tubes (which allow for air exchange with the middle ear, allowing your middle ear to equalize pressure with the atmosphere - when your ears pop as you go up in a plane, that's this working - and why going up in a plane with a stuffy nose is hellish).

The pharyngeal tonsil - the one we think of in tonsilitis - is located here. (There are others!)

The nasopharynx is followed by the oropharynx, which runs from the soft palate to the hyoid bone (the bone you can feel moving when you swallow). It only has one opening from the oral cavity, to which it lies posterior. The oropharynx has both digestive and respiratory functions, as both food and air will pass through it. It has two pairs of tonsils, the palatine and the lingual.

Because food passes through it, it is lined by nonkeratinized stratified squamous epithelium (NSSE), in order to provide a buffer against friction.

The oropharynx is followed by the laryngopharynx or hypopharynx. It is also lined by NSSE, and begins at the hyoid bone, ending at the opening to the larynx (anteriorly) and the esophagus (posteriorly). This is where food will go down the wrong way if you talk and eat at the same time. ;)

The Larynx

The larynx connects your pharynx to your trachea. It lies at the level of C4-C6 (so actually fairly high in your throat). It produces sound, and is made up of several muscles and cartilage.

From top to bottom, we have: the epiglottis, a single cartilage piece that attaches at the posterior of the larynx, and closes the larynx during feeding to direct food to the esophagus. (This is what malfunctions when you swallow something the wrong way.) This is followed by the thyroid cartilage, which does not fully surround the larynx and which is unpaired; this is what creates mens' Adam's apples. These are followed by the arytenoid cartilages (paired), which are 'horned' by the coniculate cartilages. The arytenoid cartilages influence changes in position and tension in the vocal cords. The paired cuneiform cartilages are also in this region. Following these is the cricoid cartilage, which fully surrounds the larynx.

There are also intrinsic and extrinsic muscles, which we're mostly not worried about.

More later...