Respiratory System

The respiratory system has the primary function of exchanging the gases O2 and CO2 between the blood and the atmosphere. Along with the integument and the digestive tract, it is a major area of contact between the body and the environment. In fact, it is the major area of exposure, in terms of the volume of environmental material contacted. Besides infectious illness, the respiratory system is susceptible to gaseous and particulate pollutants, ranging from irritants to carcinogens.

The upper respiratory system includes the nose and nasal cavity, the nasal sinuses, and the pharynx. The sinuses are open chambers in the face, connected to the nasal cavity by passages. Their function is to lighten the head, add timbre to the voice, and to produce mucus to moisten and lubricate the surfaces of the nasal cavity. Air passing through the nasal cavity is warmed, humidified, and filtered of large particles. The pharynx is the part of the respiratory system shared with the digestive system, extending from the back of the mouth down to the larynx.

The lower respiratory system includes the larynx (voice box), trachea (windpipe), and the bronchi, bronchioles, and alveoli of the lungs. The glottis is a narrow passageway between the pharynx and the trachea. It can open and close much like a pair of sliding doors. In speech, air passing through the glottis vibrates ligaments at the inner edge of the glottis, the vocal cords. The larynx is a group of cartilaginous structures that surround and protect the glottis. At the top is a spoon-shaped cartilage called the epiglottis. When you swallow, the larynx is pushed up, forcing the epiglottis to fold down and seal off the trachea. At the same time, the glottis closes. If particles get past the epiglottis, they fall on the glottis and stimulate the coughing reflex. In a cough, the glottis is held closed against air pressure from the lungs and then is opened abruptly to eject any material.

Air is then conducted down through the trachea, which then branches into two bronchi and enters each of the lungs (Figure 9.10). The trachea and each bronchus has a similar tubelike structure surrounded by C-shaped cartilage rings. These branch successively into smaller and smaller bronchi. At the smallest level they form bronchioles, with an inside diameter of 0.3 to 0.5 mm. Bronchioles lack cartilage. Like arterioles, they can expand and contract under the influence of the autonomic nervous system to control flow.

Carina

Right primary bronchus

Right superior lobe

Right inferior lobe

Right middle lobe

Trachea

Carina

Right primary bronchus

Right superior lobe

Right inferior lobe

Right middle lobe

Trachea

Lower Respiratory System Inferior Lobe
Figure 9.10 Trachea, bronchi, and lungs. (From Van De Graaff and Rhees, 1997. © The McGraw-Hill Companies, Inc. Used with permission.)

Asthma is a spasm of the bronchioles that greatly restricts the air-exchange capability of the lungs. The bronchioles lead to groups of sacs arranged like clusters of grapes. These are dead-end chambers called alveoli, which are the site of actual mass transfer of oxygen and CO2. Each alveolus is surrounded by capillaries. The gases need to diffuse as little as 0.1 mm to cross between the lumen of the alveoli and the blood. Both oxygen and CO2 are lipid soluble, so diffusion is relatively easy.

Most of the epithelium of the respiratory tract, from the nasal passages to the bronchi, secretes sticky, viscous mucus onto the surface. The epithelial cells are lined with cilia that sweep continuously the mucus toward the pharynx, where it can be swallowed, along with trapped particles. This mechanism, called the mucociliary escalator, traps particles larger than 10 mm. The mechanism is damaged by tobacco smoking, as evidenced by "smokers' cough,'' which is needed to replace the mechanism's throat-clearing effect. The mucociliary escalator does not cover bronchioles or alveoli. However, special macrophages called dust cells roam about and consume particles of size 1 to 5 mm that may be deposited in the alveoli. Particles smaller than about 0.5 mm remain suspended and pass out of the lung with expiration. In the genetic disease cystic fibrosis the mucus is unusually thick and cannot be removed by the cilia, resulting in lung blockage and frequent infections.

The atmosphere, with its total pressure of 760 mmHg, is 20.9% oxygen (a partial pressure of 159 mmHg). It has 400 ppmv CO2 (0.04%, or 0.3 mmHg) and about 2 to 20 mmHg of water vapor, depending on the humidity. The air we breathe out is saturated with water at physiological temperature: 6.2% or 47 mmHg. Oxygen has dropped to about 15.3% (116 mmHg) and CO2 increased to 3.7% (28 mmHg). Oxygen-depleted blood arriving in the lungs absorbs oxygen due to the mass transfer driving force between the air and the blood. For the same reason, it releases carbon dioxide.

Another mechanism enhances this transfer. The capacity of hemoglobin for oxygen depends on the pH of the blood. This, in turn, is affected by how much CO2 is being carried by the blood. The hemoglobin carries some CO2, but most is dissolved in the blood plasma. Carbon dioxide reacts with water to form carbonic acid, H2CO3, which dissociates at a normal blood pH range of 7.2 to 7.6. Since CO2 is being produced in the tissues, its concentration increases there. As blood passes through, its pH drops, from the formation of carbonic acid. As a result, the capacity of hemoglobin for oxygen also drops, forcing the hemoglobin to release oxygen. The reverse situation occurs in the lungs.

The force for inspiration (inhalation) comes from the diaphragm, a sheet of muscle that spans the chest under the lungs, plus other muscles that lie over the ribs. Expiration may occur passively by elastic rebound or by contraction of muscles under the ribs and of the abdomen. A resting adult breathes at a frequency, f, of about 12 to 18 min-1. The volume inhaled or exhaled with each breath under resting conditions is called the tidal volume, VT. The respiratory flow, Q, is the product f x VT. For example, a typical tidal volume for an adult is 0.5 L. At a frequency of 12 breaths/min, the flow would be 6.0 L/min. The U.S. Environmental Protection Agency uses a respiratory flow rate of 20 m3/day (about 13.9 L/min) to estimate exposure to atmospheric toxins for risk assessment purposes.

Physicians use an instrument called a spirometer to measure tidal volume and other respiratory air volumes, as shown in Figure 9.11. The vital capacity is measured by having the patient draw in a very deep breath and then blow out as completely as possible into a spirometer. This measurement is used to detect harm in populations exposed to air pollutants.

Figure 9.11 Spirogram, showing respiratory air volumes. (Based on Van de Graaff and Rhees, 1997.)

Breathing is controlled by centers in the medulla oblongata of the brain. Two groups of nerves fire alternately to stimulate inspiration and expiration. Of course, the brain can control these centers voluntarily as well as involuntarily. The brain is very poor at detecting low oxygen concentration and will not respond to oxygen deprivation by increasing the breathing rate. However, it strongly senses CO2 accumulation in the blood and will increase the breathing rate in response to CO2 buildup.

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