The lower respiratory system begins below the epiglottis in the larynx or voice box. The trachea, or windpipe, is a cartilaginous tube extending from the larynx that provides an unobstructed path for air to reach the lungs. The trachea bifurcates into the left and right bronchi as it reaches the lungs. These paths branch repeatedly to form smaller and more extensive networks of tubes, the bronchioles. The terminal bronchioles formed in this tree-like network end in cul-de-sacs called the alveoli. These structures are surrounded by capillary networks and are the site of gas exchange in the respiratory system. Human lungs contain on the order of 400,000,000 alveoli. The outer surface of the lungs is protected with a double-layered pleural membrane. This structure protects the lungs and provides lubrication to permit the lungs to move easily during respiration.

The larynx is a cartilaginous structure inferior to the laryngopharynx that connects the pharynx to the trachea and helps regulate the volume of air that enters and leaves the lungs. The structure of the larynx is formed by several pieces of cartilage. Three large cartilage pieces—the thyroid cartilage (anterior), epiglottis (superior), and cricoid cartilage (inferior)—form the major structure of the larynx. The thyroid cartilage is the largest piece of cartilage that makes up the larynx. The thyroid cartilage consists of the laryngeal prominence, or “Adam’s apple,” which tends to be more prominent in males. The thick cricoid cartilage forms a ring, with a wide posterior region and a thinner anterior region. Three smaller, paired cartilages—the arytenoids, corniculates, and cuneiforms—attach to the epiglottis and the vocal cords and muscle that help move the vocal cords to produce speech.

The larynx, commonly called the voice box or glottis, is the passageway for air between the pharynx above and the trachea below. It extends from the fourth to the sixth vertebral levels. The larynx is often divided into three sections: sublarynx, larynx, and supralarynx. It is formed by nine cartilages that are connected to each other by muscles and ligaments.

The larynx plays an essential role in human speech. During sound production, the vocal cords close together and vibrate as air expelled from the lungs passes between them. The false vocal cords have no role in sound production, but help close off the larynx when food is swallowed.

The thyroid cartilage is the Adam's apple. The epiglottis acts like a trap door to keep food and other particles from entering the larynx.

The epiglottis, attached to the thyroid cartilage, is a very flexible piece of elastic cartilage that covers the opening of the trachea. When in the “closed” position, the unattached end of the epiglottis rests on the glottis. The glottis is composed of the vestibular folds, the true vocal cords, and the space between these folds. A vestibular fold, or false vocal cord, is one of a pair of folded sections of mucous membrane. A true vocal cord is one of the white, membranous folds attached by muscle to the thyroid and arytenoid cartilages of the larynx on their outer edges. The inner edges of the true vocal cords are free, allowing oscillation to produce sound. The size of the membranous folds of the true vocal cords differs between individuals, producing voices with different pitch ranges. Folds in males tend to be larger than those in females, which create a deeper voice. The act of swallowing causes the pharynx and larynx to lift upward, allowing the pharynx to expand and the epiglottis of the larynx to swing downward, closing the opening to the trachea. These movements produce a larger area for food to pass through, while preventing food and beverages from entering the trachea.

Continuous with the laryngopharynx, the superior portion of the larynx is lined with stratified squamous epithelium, transitioning into pseudostratified ciliated columnar epithelium that contains goblet cells. Similar to the nasal cavity and nasopharynx, this specialized epithelium produces mucus to trap debris and pathogens as they enter the trachea. The cilia beat the mucus upward towards the laryngopharynx, where it can be swallowed down the esophagus.

Stay hydrated:

• Drink plenty of water, especially when exercising.
• If you drink caffeinated beverages or alcohol, balance your intake with plenty of water.
• Take vocal naps—rest your voice throughout the day.
• Use a humidifier in your home. This is especially important in winter or in dry climates. Thirty percent humidity is recommended.
• Avoid or limit use of medications that may dry out the vocal folds, including some common cold and allergy medications. If you have voice problems, ask your doctor which medications would be safest for you to use.

Maintain a healthy lifestyle and diet:

• Don't smoke, and avoid second-hand smoke. Smoke irritates the vocal folds. Also, cancer of the vocal folds is seen most often in individuals who smoke.
• Avoid eating spicy foods. Spicy foods can cause stomach acid to move into the throat or esophagus, causing heartburn or GERD.
• Include plenty of whole grains, fruits, and vegetables in your diet. These foods contain vitamins A, E, and C. They also help keep the mucus membranes that line the throat healthy.
• Wash your hands often to prevent getting a cold or the flu.
• Get enough rest. Physical fatigue has a negative effect on voice.
• Exercise regularly. Exercise increases stamina and muscle tone. This helps provide good posture and breathing, which are necessary for proper speaking.
• If you have persistent heartburn or GERD, talk to your doctor about diet changes or medications that can help reduce flare-ups.
• Avoid mouthwash or gargles that contain alcohol or irritating chemicals.
• Avoid using mouthwash to treat persistent bad breath. Halitosis (bad breath) may be the result of a problem that mouthwash can't cure, such as low grade infections in the nose, sinuses, tonsils, gums, or lungs, as well as from gastric acid reflux from the stomach.

Use your voice wisely:

• Try not to overuse your voice. Avoid speaking or singing when your voice is hoarse or tired.
• Rest your voice when you are sick. Illness puts extra stress on your voice.
• Avoid using the extremes of your vocal range, such as screaming or whispering. Talking too loudly and too softly can both stress your voice.
• Practice good breathing techniques when singing or talking. Support your voice with deep breaths from the chest, and don't rely on your throat alone. Singers and speakers are often taught exercises that improve this kind of breath control. Talking from the throat, without supporting breath, puts a great strain on the voice.
• Avoid cradling the phone when talking. Cradling the phone between the head and shoulder for extended periods of time can cause muscle tension in the neck.
• Consider using a microphone when appropriate. In relatively static environments such as exhibit areas, classrooms, or exercise rooms, a lightweight microphone and an amplifier-speaker system can be of great help.
• Avoid talking in noisy places. Trying to talk above noise causes strain on the voice.
• Consider voice therapy. A speech-language pathologist who is experienced in treating voice problems can teach you how to use your voice in a healthy way.