The lungs are the major component of the respiratory system (Figures 1A and 1B). A good way to understand the workings of the lungs is to consider their structure, or anatomy (Figure 1A). The human respiratory system begins at the nose and includes the nasal passages, which direct air to the back of the throat and into the wind-pipe, or trachea. The trachea sits below the voice box (larynx), and can be felt in the front of your neck as it descends behind the breastbone (sternum) into the upper chest. The trachea ends and divides into two branches: the right mainstem bronchus and the left mainstem bronchus.
The right mainstem bronchus leads air to and from the right lung, and the left mainstem bronchus leads air to and from the left lung. The area where the trachea divides into the right and left mainstem bronchi is called the carina. After the split, the right and left mainstem bronchi leading to each lung subdivide further into smaller and smaller tubelike passages, via the branching tracheobronchial tree. As the bronchi continue to subdivide into successively narrower and narrower bronchi, they ultimately end in the tiniest subdivision, the bronchiole. Each bronchiole leads to the lung air sacs, the alveoli. The alveoli are highly specialized lung structures.
They are the gas-exchanging lung units. They ensure that fresh, oxygen-rich (O2) inhaled air enters the body at the same time that oxygen-poor, carbon dioxide-rich (CO2) air exits (Figure 1B). Oxygen (O2) is required for life; oxygen deprivation is rapidly fatal. As inhaled oxygen is provided to the body’s organs via the lungs, “used” air— composed mostly of carbon dioxide (CO2)—is excreted by exhalation. Carbon dioxide is produced by the body’s metabolism and is considered a “waste product.”
Abnor-mal accumulation of carbon dioxide in the body and the bloodstream is detrimental to health and is responsible for certain forms of respiratory failure. The process that is responsible for the body’s oxygen uptake and its carbon dioxide removal (or excretion) is called respiration. Respiration is the primary, crucial function of the lungs and of the respiratory system. Physicians occasionally refer to respiration as “gas exchange.” The exchanged gases are oxygen (O2) and carbon dioxide (CO2). Exchange means that CO2 gas is given up by the body and replaced by a fresh supply of O2. The exchange takes place in the deepest lung, at the level of the alveoli.
Oxygen and carbon dioxide exchange takes place along a specialized zone where each air sac (alveolus) is in intimate contact with fine, minute blood vessels called capillaries. The capillary bed com-pletely encircles the alveoli along the alveolar–capillary membrane. Because of the structure of the alveolar– capillary membrane, the inhaled oxygen (O2) easily passes from the alveolus into the capillary blood that then sends it to our organs. Similarly, the body’s CO2 is carried through the bloodstream into the blood supply of the alveolar–capillary membrane where it is given up to the alveolus, and from there, exhaled by our lungs with each breath.
A normal resting adult breathes approximately 12–18 times a minute, children about 20 times a minute, and babies and infants even more frequently. That number, the number of breaths a person breathes in 1 minute, is termed the respiratory rate.We should be unaware of our breathing in health, as respiration should be automatic, effortless, and of course, pain-less.
Increases in the respiratory rate, sometimes perceived as a kind of breathless sensation, can represent a normal process as during exercise or sports and allows for increased oxygen delivery to the body. A respiratory rate increase can also indicate the onset of a medical concern; it may in particular be a sign of increasing asthma symptoma-tology. That is why measurement of respiratory rate, along with pulse (or heart rate), blood pressure, and temperature measurements are collectively referred to as vital signs in medical terminology!