Chapter 5

Taking a Deep Breath with the Respiratory System

IN THIS CHAPTER

Taking a look at the respiratory system

Getting to know the anatomy of the respiratory system

Seeing how different conditions affect your breathing

Vertebrates evolved in the sea from animals that obtained their oxygen by breathing water. Whatever it was that drove a fish to leave the watery environment, the challenge of breathing the gaseous atmosphere instead had to be met. All the major groups of land vertebrates (amphibians, reptiles, birds, and mammals) have succeeded by evolving the anatomy and physiology for breathing air. That anatomy and physiology makes up the respiratory system, which we discuss in this chapter.

Surveying the Duties of the Respiratory System

Air is the source of the oxygen your cells need for nearly all reactions. Your respiratory organ system manages the flow of air into and out of your body. It oversees a number of vital body functions, including

• Ventilating: Ventilation — the act of breathing — isn’t the same as respiration — the exchange of gases. You don’t have to think about performing either task, but ventilation involves the physical movement of your diaphragm muscle, rib cage, and lungs to draw air in and push air out of the body. See the “Discovering the Ins and Outs of Breathing” section later in this chapter for details.
• Exchanging gasses: The job of the respiratory system is to supply oxygen and remove carbon dioxide from the blood as it circulates through the body. Chapter 4 explains how the blood gets into and out of the lungs in the process of pulmonary circulation, which is sometimes called external respiration to distinguish it from cellular respiration. Gas exchange happens in the lungs, where the respiratory tissues and the circulatory tissues meet. Refer to the later section “Respiratory membrane” to read more.
• Regulating blood pH: Maintaining blood pH within the homeostatic range requires coordination between the respiratory and urinary systems, with help from the endocrine system and, of course, the circulatory system itself.
• Producing speech: The ability of humans to consciously control breathing permits speech and singing.

Nosing around Your Respiratory Anatomy

The respiratory tract is the path of air from the nose to the lungs. It’s divided into two sections: the upper respiratory tract (from the beginning of the airway at the nostrils to the pharynx) and the lower respiratory tract (from the top of the trachea to the diaphragm).

The respiratory tract is one of the places in the body where cells are replaced constantly throughout life.

We introduce you to each component of the human respiratory system in the sections that follow.

An entrance to the system: The nose

This is one time you can turn your nose up at your anatomy. Really. Point your nose up while looking in the mirror. (Yes, you have to put the book down.) See the two big openings? Those are your nostrils, and they’re two places where air enters and exits your respiratory system. Now, see all those tiny hairs in your nostrils? Those little hairs serve a purpose. They trap dirt, dust particles, and bacteria. Okay. You can put your head down now. The rest of your respiratory parts are way inside of your body, so you can’t see them in the mirror.

Just beyond your nostrils, the nasal septum separates your nasal cavities. Inside the nasal cavities, the three tiny bones of the nasal conchae provide more surface area inside the nose because they’re rolled up (like conch shells). The cells of the respiratory mucosa that lines the inside of the nasal cavity have tiny cilia that move the dirt-laden mucus toward the outside of the nostrils.

The lacrimal glands secrete tears that flow across the eye’s surface and drain through the openings in the corner of the eye (lacrimal puncta), into the nasolacrimal ducts and the nasal cavities. That’s why your nose runs when you cry.

Your sinuses are air spaces in your skull that lighten the weight of your head. They open into the nasal cavities so they can receive air as you breathe, and, like the nasal cavities, they’re lined with mucous membranes.

Passing through the pharynx

Air passes through your pharynx on its way to your lungs. Along the way, it passes through and by some other important structures, such as your larynx and tonsils.

Your pharynx is divided into three regions based on what structures open into it:

• Nasopharynx: This is the top part of your throat where your nasal cavities drain. If you press your tongue to the roof of your mouth, you can feel your hard palate. This bony plate separates your mouth (oral cavity) from your nose (nasal cavities). If you move your tongue backward along the roof of your mouth, you reach a soft spot. This spot is the soft palate. Beyond the soft palate is your nasopharynx, which is where your nasal cavities drain into your throat. Your soft palate moves backward when you swallow so the nasopharynx is blocked.

  Normally, the soft palate blocking the nasopharynx keeps food from going up into your nose. But when you’re laughing and eating or drinking at the same time, your soft palate gets confused. When you go to swallow, it starts to move back, but when you laugh suddenly, it thrusts forward, allowing whatever’s in your mouth to flow up into your nasal cavities and immediately fly out of your nostrils to the delight of everyone around you.

• Oropharynx: This is the middle part of your throat, but it’s frequently referred to as the back of the throat. The oropharynx extends from the uvula to the level of the hyoid bone. It’s the location of the epiglottis, a cartilage structure that guides materials passing though the mouth to the trachea or esophagus, as appropriate. This part of anatomy is the reason your food comes close to your windpipe but rarely goes in.

• Laryngopharynx: This is the lower part of your throat adjacent to your larynx. The larynx (or voice box) is triangular. At the apex of the triangle is thyroid cartilage, commonly known as your Adam’s apple. If you could look down your throat onto the top of your larynx, you’d see your glottis, the opening through which air passes. When you swallow, a flap of tissue called your epiglottis covers your glottis and blocks food from getting into your larynx.

  Inside your glottis are the vocal cords — gathered mucous membranes that cover ligaments. Your vocal cords vibrate when air passes over them, producing sound waves. Pushing more air over them increases the vibration’s amplitude, making the sound louder. When you tighten your vocal cords, the glottis narrows, and your voice has a higher pitch.

Winding through the windpipe, or trachea

Your trachea (windpipe) is a tube that runs from your larynx to just above your lungs. Just behind your sternum, your trachea divides into two large branches called primary bronchi (singular, bronchus) that enter each lung.

The trachea and bronchi are made of smooth muscle and cartilage, allowing the airways to constrict and expand.

The lungs: The pair that just don’t quit

Your lungs are large paired organs within your chest cavity on either side of your heart. Like the heart, they’re protected by the rib cage. The lungs sit on top of the diaphragm, a powerful muscle that’s fixed to the lower ribs, sternum, and lumbar vertebrae. The heart sits in a depression between the lungs, called the cardiac notch.

The right lung is somewhat larger than the left. Both lungs are separated into lobes (three on the right and two on the left). The lobes are further divided into segments and then into lobules, the smallest subdivision visible to the eye.

The following sections discuss two important lung-related structures.

Getting to know the pleural sac

Each lung is completely enclosed in the pleural sac. This sac is similar to the pericardial sac (which surrounds the heart) in that it’s made up of two membranes, the parietal pleura, attached to the thoracic wall, and the visceral pleura, attached to the lung’s surface, with the pleural cavity between them. The pleural cavity contains a lubricating fluid called the intrapleural fluid.

The intrapleural fluid completely surrounds the lungs. It keeps the pleural membranes moist and lubricated. Because this fluid has a pressure lower than atmospheric pressure (that is, little air is in the fluid), the lungs stay inflated.

Because of the adhesive force of the fluid interface between the parietal pleura and the visceral pleura, the lung is essentially attached to the chest wall. Thus, as muscles associated with the thoracic wall contract and relax and the chest rises and falls, the lungs expand and contract.

The visceral pleura surrounds the mediastinum, the region that separates the left and right lungs and houses the heart, thymus, and part of the esophagus.

Understanding the bronchial tree

After the primary bronchus enters the lung on each side, it splits into secondary and tertiary branches called bronchi. The tertiary bronchi divide into smaller branches called brionchioles. At the end of the smallest bronchioles are little structures that look like raspberries. These are the alveolar sacs, and each sac contains many alveoli (singular, alveolus). The alveoli’s walls are composed of a simple squamous epithelium (designed to facilitate rapid diffusion) and elastic tissue that alternately stretches and constricts as you breathe.

Each of the approximately 300 million alveoli is wrapped with capillaries, whose walls, like the alveoli’s walls, contains simple squamous epithelium, a tissue type adapted for the exchange of materials. The interface of the simple squamous epithelium of an alveolus and the simple squamous epithelium of a pulmonary capillary (along with its supporting connective tissue) is called the respiratory membrane. Gas exchange occurs in this membrane.

Exchanging gases at the respiratory membrane

The respiratory membrane refers to the area within the lung where the epithelial cells of the alveoli meet the capillaries and gas exchange takes place. Because of the folds and convolutions of the bronchioles and alveoli, the respiratory membrane has a large surface area. And because gas exchange for a large, active, warm-blooded animal requires a membrane with a large surface area, evolution has favored those with convoluted bronchioles and alveoli. Look to Figure 5-1 to get an understanding of the respiratory membrane’s structure.

FIGURE 5-1: Respiratory gases are exchanged by diffusion across alveolar and capillary walls.

The respiratory membrane is the location where blood is reoxygenated in the process of pulmonary circulation, which we describe in detail in Chapter 4. The process of gas exchange at the respiratory membrane is almost exactly the same as the capillary exchange process, also covered in Chapter 4. However, at the respiratory membrane, oxygen flows through the alveolar wall and into the blood, and carbon dioxide flows out of the blood and through the alveolar wall into the air space. This process proceeds in the opposite direction of the flow in the capillary beds.

The respiratory membrane is where carbon dioxide, the waste product of aerobic metabolism, is eliminated. It flows into the air in your lungs and you just breathe it out.

The comptroller of ventilation: The diaphragm

The thoracic diaphragm is a dome-shaped sheet of muscle separating the base of the lungs from the liver, and, on the left side, from the stomach and the spleen. The diaphragm pushes up beneath the lungs to control their contraction and expansion during ventilation. The motor fibers in the phrenic nerves signal to the diaphragm when to contract and relax. The diaphragm can also exert pressure on the abdominal cavity, helping with the expulsion of vomit, feces, or urine.

Discovering the Ins and Outs of Breathing

Breathing is essential to life, and thankfully, your body does it automatically. Air is alternately pulled into (inhaled) and pushed out of (exhaled) the lungs due to changes in the gas pressure in the alveoli. This change in pressure comes about because the alveoli are expanded when the chest cavity expands.

In the following sections, we take a look at how your body breathes under different conditions.

Breathing under normal conditions

When you’re sleeping, sitting still, and doing normal activities, your breathing rate is 12 to 20 inhalation/exhalation cycles per minute. That’s 17,000 breaths or more a day.

Normal breathing (eupnea) is involuntary, which is why you never really forget to breathe. Breathing continues during sleep. In many cases, breathing continues even during a coma. Impulses to the diaphragm come through a pair of spinal nerves, called the phrenic nerves. They initiate the regular alternating contraction and release of the diaphragm. The rhythm of the impulse is controlled by the autonomic system in the brain stem. (You can control your breath voluntarily, but doing so involves the cerebral cortex.)

Inspiration and expiration are the two processes that make up normal breathing. Here’s the lowdown on each:

• Inspiration: Inspiration (breathing in) is the result of the diaphragm’s contraction. The diaphragm moves downward into the abdomen, expanding the lungs as it does so. This, of course, decreases the air pressure in the lungs. (Remember: The diaphragm is attached to the base of the lungs at the bottom and the thoracic wall at the sides.) The skeletal muscles of the ribs (intercostal muscles) help to expand the lungs by pulling the ribs up and out. Air is pulled in at the top of the airway (nose and mouth) and all the way down to take up the expanded space in the alveoli.
• Expiration: Expiration (breathing out) is a passive process that occurs between impulses. Stretch receptors within the alveoli send nerve impulses to the respiratory center to release the contraction of the diaphragm. As the diaphragm relaxes, it moves back up out of the abdomen (because of abdominal muscle tone), the elastic tissue in the lungs snaps back (gets smaller), and the rib cage falls back (mostly because of gravity). This increases the air pressure in the lungs and pushes the air out (see Figure 5-2).

FIGURE 5-2: The inspiration (inhalation) and expiration (exhalation) processes.

Seeing how stress affects breathing

Breathing under stress is a bit different from breathing normally. Keep in mind that “stress” just means that an extra physiological demand is being placed on the body. Stress isn’t necessarily negative — whether physical or emotional, stress can be painful or pleasurable, and it’s often both.

No matter how much fun you are or aren’t having, stress increases metabolism. More oxygen is consumed, and more carbon dioxide is produced. Chemoreceptor cells in the carotid arteries and aorta detect an elevated level of carbon dioxide or hydrogen ions and alert your respiratory center. Both inspiration and expiration become active processes. You breathe more deeply and frequently. The intercostal muscles forcefully contract and push more air out of your lungs (this can’t occur during rest). These processes restore homeostasis and support the elevated metabolism.

The forcible exhalation involved in coughing or sneezing is aided by the sudden contraction of the abdominal muscles, raising the abdominal pressure. The rapid increase in pressure pushes the relaxed diaphragm up against the pleural cavity, forcing air out of the lungs.

Controlling your breathing

As far as has been determined, humans are the only animals who can bring their breathing under conscious control. Controlled breathing allows people to speak and sing as well as moderate other physiological systems. The following sections show you how.

Holding your breath

You can stop breathing (by holding your breath), at least for awhile, when unpleasant odors, noxious chemicals, or particulate matter is in the air around you, while you swim underwater, or just for the fun of it. The cerebral cortex sends signals to the rib muscles and the diaphragm that temporarily override the respiratory center signals.

Holding your breath long enough to cause damage to your own brain from a lack of oxygen isn’t possible. Metabolism and gas exchange continue as usual while you hold your breath. Carbon dioxide concentration increases in the blood. At a certain point, long before brain damage is even a possibility, the chemoreceptors that work with the respiratory center are stimulated to the point where they override the cerebral cortex. At the extreme, you lose consciousness. (The evolutionarily older brainstem puts the whippersnapper cerebral cortex into time out.) You exhale and the system rapidly returns to normal.

Speaking and singing

Speech, another uniquely human activity, requires breath control. The exhalation passes breath over the vocal cords, causing sound waves to be emitted, and the lips and tongue shape the sound waves into speech. The rate of exhalation is lower while you’re speaking and is controlled by the diaphragm, the intercostal muscles, and the abdominal muscles. Singing requires even more breath control than speaking.

Directing other systems

The relationship between the autonomic nervous system and the respiratory system appears to be two-way. For example, anxiety prompts hyperventilation, and hyperventilation produces symptoms of anxiety. Consciously controlling the rate and depth of breathing, mainly by achieving awareness and control of the diaphragm, has been demonstrated to decrease anxiety and sympathetic nervous system activation.

Controlled breathing is a feature of many religious, spiritual, and physical disciplines in all traditions. Clinical benefits have been demonstrated for meditation and controlled breathing in a wide variety of conditions of the neural, cardiovascular, and pulmonary systems.